AN ABSTRACT OF THE THESIS OF
Bruce Preston Finney
Oceanography
in
Title:
for the degree of
Doctor of Philosophy
July 16. 1986
presented on
Paleoclimatic Influence on Sedimentation and Manganese
Nodule Growth During the Past 400.000 Years at MANOP
Site H (Eastern Equatorial Pacific)
Abstract Approved:
Redacted for privacy
Sediments at MANOP Site H (6°33'N, 92°49'W) show marked downcore variations in the abundance of calcium carbonate, opal, organic
carbon and Mn.
The profiles of organic carbon and Mn indicate that
redox conditions in surface sediments at the site have varied continuously over the past 400 kyr.
The content of organic carbon in
surface sediments shows a consistent relation to the depth of the Mn
redox boundary.
Application of this relation to downcore organic
carbon data, indicates that the Mn redox boundary has varied from
depths of 5 to 25 cm over this time period.
reducing, on average, since 180 kyr ago.
The site has been more
Changes in the redox con-
ditions of surface sediments can affect the cycling and burial of Mn
and associated Ni and Cu; a rapid shallowing of the redox boundary
can result in the burial of enhanced concentrations of these transition metals.
Downcore variations in the relative proportions of
organic carbon, opal and calcium carbonate indicate that surface
productivity or the relative ability of bottom water to
degrade and dissolve these components, or both, have changed over
the past 400 kyr.
Concurrent increases in the contents of opal and
organic carbon suggest that variations in productivity have played a
major role in controlling the redox variations.
A suite of manganese nodules from the site show time-correlable
variations in texture, mineralogy and composition.
Mn-rich, minor
element-poor nodule bottoms have grown at rates greater than 150
nun/my for the past 160 kyr.
Prior to this time, the nodule bottoms
grew at average rates of about 50 mm/my, and were enriched in most
minor elements relative to Mn.
These variations are best explained
by time-variable nodule aecretionary processes.
The fast growth and
chemical composition of the nodule bottoms since 160 kyr ago
resulted from an increased supply of Mn from suboxic diagenesis in
the sediments.
This is consistent with more intense reduction of
sediments at the site since about this time.
The correspondence is
complicated, however, because a nodule may have to exceed a critical
size before its accretion can be dominated by the suboxic source.
Variations in bottom water composition and surface water productivity have affected the redox conditions of surface sediments
and the subsequent burial of transition metals at Site H for the
past 400 kyr.
Variations in the redox conditions of the sediments,
in turn, have produced changes in growth rates and chemical compositions of manganese nodules at the site.
All of these processes,
ultimately can be attributed to climatic changes during the Quaternary.
Paleoclimatic Influence on Sedimentation and Manganese
Nodule Growth During the Past 400,000 Years at
MANOP Site H (Eastern Equatorial Pacific)
by
Bruce Preston Finney
A THESIS
submitted to
Oregon State University
in partial fulfillment of
the requirements for
the degree of
Doctor of Philosophy
Completed July 16, 1986
Commencement June 1987
APPROVED:
Redacted for privacy
Professor of Oceanography in charge of major
Redacted for privacy
Dean of Col1eg,/of Oceanography
Redacted for privacy
Dean of Graduat
choo1
July 16. 1986
Date thesis is presented
Typed by
bruce Finney
and
Jackie Popletofl
For my Family and Friends
ACKNOWLEDGEMENTS
My sincere thanks go to my major professor, Ross Heath, for
always finding time in his busy schedule to offer valuable advice,
words of wisdom, and support.
The encouragement, friendship,
support and unique insights of Mitch Lyle were a great help to me.
Jack Dymond's door was always open, and our discussions were very
beneficial in guiding my research.
I also thank the rest of my
Committee, Erwin Suess, Bill Harrison, G. David Faulkenberry and
Roger Petersen, for help during this thesis work.
Andy Ungerer, Greg Campi, Nick Pisias, Jim Robbins, Cydne
Perhats and Bobbi Conard all helped out in the laboratory or at the
computer.
Dave Kadko generously offered his lab, and he and Dick
Kovar taught me the techniques and tricks of U-series analysis.
Thanks to Pat, Bob, Sharon, Marta, Dave and Scott for assistance
during the frantic final days.
Jackie Poppleton was a great help
during preparation of the final version of this thesis.
This work would have been much more difficult without the
direct and indirect aid of all of my friends.
The day and night
crews of Jameson and Extension (especially Dave Murray) kept me
going.
Pat and Bob Collier gave me a home when I needed one, and
enabled me to keep my sense of humor.
Terry Sloan, my good friend
and fishing partner, and Nancy, helped me to escape when I needed
to.
Robert Wood Chambers gave me strong friendship and support over
long distances.
advice.
Kathy Fisher always gave encouragement and good
Emily, Philip, Ian, John, Carol, Todd, Gina, Marta, Dave,
Annette, Chip, Sharon, Kris, John, Chris, Lennie, Tom, Katie, Kim,
Laurel, Curt, the Poker Gang, the Marysville Maulers and Pellets
Soccer Clubs, and the rivers and lakes of Oregon, all offered relief
along the way.
Love from Katy helped pull me through.
Thanks for your
confidence in me and for being patient to the end.
Finally, thanks to my family for their love, patience and
encouragement, and for not having any unrealistic expectations of
me.
This work was supported by the NSF through grants 0CE821720-Ol,
0CE8101845, and 0CE8217250.
Every so often one or another of these intelligent
explorations pays off; the fish rises and I feel that I
am really growing in understanding of fish and their
ways.
More often nothing happens at all and I am left
The laughing water
still floundering in my ignorance.
dances on over the sunny shallows and into the greener
depths, revealing nothing at all.
I comfort myself
then with the thought that what we fisherman attain
from our position above the surface is not so much
knowlege as a different degree of ignorance - which is
not without its special satisfactions in a day when
most specialists know so much.
Roderick Haig-Brown
"Fishernians Fall"
TABLE OF CONTENTS
CHAPTER I
GENERAL INTRODUCTION
1
GROWTH RATES OF MANGANESE-RICH NODULES AT
MANOP SITE H (EASTERN NORTH PACIFIC)
5
ABSTRACT
6
INTRODUCTION
7
GROWTH RATE ESTIMATES
11
Crustal Age
11
Size Distribution
11
Growth Since a Burial Event
17
Mn/Fe Ratios
20
Elemental Accumulation Rates
21
Uranium-Series Methods
22
Chemical/Radiochemical Model
25
DISCUSSION
36
CONCLUSIONS
37
ACKNOWLEDGEMENTS
CHAPTER II
SEDIMENTATION AT MANOP SITE H (EASTERN
EQUATORIAL PACIFIC) OVER THE PAST 400 KYR CLIMATICALLY INDUCED REDOX VARIATIONS AND
THEIR EFFECTS ON TRANSITION METAL CYCLING
40
ABSTRACT
41
INTRODUCTION
42
BACKGROUND AND OBJECTIVES
45
METHODS
48
TIMESCALE
62
RESULTS
75
Table of Contents - continued
DISCUSSION
Factors Controlling the Redox Condition
of Surface Sediments
79
Paleo-redox Changes at Site H
85
Downcore Manganese Variations
87
Factors Controlling the Organic Carbon
Variations
CHAPTER III
79
100
CONCLUSIONS
108
COMMON GROWTH HISTORIES IN MANGANESE
NODULES FROM MANOP SITE H (EASTERN
EQUATORIAL PACIFIC): CONTROL OF NODULE
ACCRETION BY SEDIMENTARY PROCESSES
110
ABSTRACT
ill
INTRODUCTION
113
SETTING
116
SAMPLING AND METHODS
118
RESULTS AND DISCUSSION
121
Physical and Textural Properties
121
Chemical Composition
123
Chemical Variations with Depth
133
Uranium-Series Distribution and
Growth Rates
135
Fluxes of Elements into Nodules
142
Mineralogy
145
Characteristics and identification
of minerals
145
Mineralogical variability within nodules
149
Variations in the Chemical Composition
of Site H Nodules with Time
161
Table of Contents - continued
Diagenetic processes
161
Variability in primary accretionary
processes
165
Changes in the intensity of redox
conditions
166
Nodule size
173
Turnover or change in orientation
175
Summary of Accretionary Control
176
REFERENCES
178
LIST OF FIGURES
CHAPTER I
Figure
I-i
1-2
Page
Location of MANOP Site H in the eastern North
Pacific in relation to major topographic
features.
9
Nodule occurrences in the eastern North Pacific
(data from Monget et al., 1976 and Scripps
Institution of Oceanography, unpublished data).
The isopleths are crustal ages in millions of
years.
1-3
(a) Histograms of the abundance of surface and
buried nodules as a function of size for Site H
cores recovered during the Vulcan I and Pluto III
expeditions. (b) The probability of a Site H
nodule being found buried as a function of size,
based on Figure I-3a.
1-4
12
14
Size distributions of nodules from Vulcan I and
Pluto III cores (closed circles) and Indonied
deeptow photographs (open circles). The slope
(m) for the Vulcan and Pluto nodules, calculated
over the interval for which the probability of
burial is independent of size, is -0.36 nun
1-5
1-6
The slope from the Indomed data is -0.077 nun
16
(a) Abundance of nodules as a function of subbottom depth in Vulcan I and Pluto III cores.
(b) Size distributions of Vulcan I and Pluto III
nodules as a function of subbottom depth.
18
Locations of box cores (squares) and gravity
cores (circles) recovered during the Vulcan I and
Pluto III expeditions. Cores with buried nodules
at subbottom depths of 40 to 50 cm are shown by
closed symbols.
1-7
1-8
19
Portion of a triangular plot showing the relative
abundances in Site H nodule samples of the three
compositional factors determined by Q-niode factor
analysis.
27
The strong negative correlation between Th and Mn
in Site H nodules.
29
Chapter I - continued
Figure
1-9
Page
230
Th
(solid lines) and Mn/Fe ratios (dashed
linesY plotte3gainst depth in nodule VulcanI-37BC.
The
has been normalized to
Th
Q-niode factor 2 byhe method described in the
text.
(a) Nodule top, (b) nodule bottom.
33
CHAPTER II
Figure
11-1
11-2
11-3
11-4
11-5
11-6
Page
Locations of the MANOP Sites relative to the
major sediment types of the Pacific (after
Horn et al., 1972).
46
Bathymetry of Site H and locations of the box
cores (squares) and gravity cores (stars)
referred to in this work. All cores
collected during the Vulcan I expedition
except for 33 which was taken during the
Indomed expedition.
49
Calcium carbonate abundances vs depth for the
Site H gravity cores. The age picks used for
the time-scale are indicated.
63
Variations over the past 400 kyr in the
Ti/Al, calcium carbonate, Al, organic carbon,
opal, Mn and ash contents of the Site H
gravity cores.
64
The inter-relations of Ni, Cu, Zn and Mn over
the past 400 kyr in core 49CC.
77
The depth to the Mn redox boundary as a
function of the organic carbon content in
surface sediments. R2 - .96.
83
Chapter II - continued
Figure
11-7
11-8
11-9
11-10
11-11
11-12
Page
Variations in the depth to the Mn redox
boundary at Site H over the past 400 kyr.
86
Nodule-free areas (white) in a dense nodule
field at Site H as revealed by side-looking
sonar records (after Spiess et al., 1977).
88
Organic carbon contents as a function of
depth below the seafloor for a nodule-free
bare patch (33BC) and a nodule-bearing area
(37BC).
89
The predicted variation in depth to the Mn
redox boundary for the two organic carbon
records in Figure 11-9 over the past 50 kyr.
90
Cartoon showing the proposed mechanism of Mn
peak trapping. A and B are two steady-state
examples at different redox intensities.
C
and D represent non-steady-state changes from
condition B to A and A to B, respectively.
The mechanism is described in the text (solid
lines - Mn, dashed lines - organic carbon,
sedimentation rate
0.65 cm/kyr).
93
Manganese variations and the rate of change
of organic carbon in the sediments (dC/dT, in
wt % organic carbon per kyr multiplied by 100)
vs depth in the Site H gravity cores.
101
CHAPTER III
Page
Figure
111-1
Locations of the Manganese Nodule Project
(MANOP) Sites.
117
Chapter III - continued
Figure
111-2
111-3
111-4
111-5
111-6
111-7
111-8
111-9
111-10
Page
Bathymetry of Site H and locations of the
cores discussed in this report.
119
Photographs of sections through the nodules
studied.
122
The strong negative correlations of Mn to
Fe, Ni, Cu, Al and Si. Box
Nodule 4OBC,
triangle
Nodule 48BC, star
Nodule 5OBC
and cross - Nodule 52BC.
127
Mn/Fe ratio vs distance above nodule bottom
for the four Site H nodules studied.
134
Mn/Ni, Mn/Fe and Mn/Cu vs distance from the
bottom of Nodule 4OBC.
136
230Th
vs depth into the nodule bottom.
The mhod used is described in the text.
(a) Nodule 48BC (the growth rate is 168±25
mm/my.
(b) Nodule 48BC (the growth rate is
150±28 mm/my.
140
X-ray diffractograms for the outermost
bottom sample of Nodule 4OBC (sample 40.1).
The upper diagram is for the sample before
dehydration, and the lower diagram shows
the effect of drying the sample at 105°C.
146
A representative suite of diffractograins
illustrating the mineralogical profile
through Nodule 4OBC. Samples 1 through 16
are for the nodule bottom, sample 26 is the
The peak
core, and sample 29 is the top.
at about 6 A is the internal standard,
boehmite.
150
The ratio of (a) the 7 A peak to
boebmite, (b) the 10 A to boehmite as a
function of distance above the nodule
bottom for Nodules 4OBC and 48BC.
153
Chapter III - continued
Page
Figure
111-11
111-12
111-13
111-14
The exponential decrease with time in the
ratio of the 7 A peak to boehnuite in
The decrease
nodule bottoms 4OBC and 48BC.
suggests that the diagenetic transformation
has a half-life of about 45 kyr.
156
Variations over the past 500 kyr in the
Mn/Fe ratio through five Site H nodules.
u1e bottoms 4OBC and 48BC dated by
Nodule 37BC dated by
230Thex (this study).
Thex (Finney et al.,, 1984). Nodule
bottoms 5OBC and 52BC dated by correlation
(horizontal dashed line) to the other
nodules. Other areas dated using equation of
Lyle (1982). Nodule bottoms - solid line,
nodule tops - dashed line.
162
The depth to the Mn redox boundary as a
function of the organic carbon content in
surface sediments (from Finney et al.,
l986a).
168
Variations in the depth to the Mn redox
boundary at Site H over the past 400 kyr
(modelled by Finney et al., 1986a).
170
LIST OF TABLES
Table
rage
CHAPTER :i:
I-I.
Core locations and nodules recovered at MANOP Site H.
10
1-2
Dry Bulk densities, Co content, Mn/Fe ratios and
calculated growth rates using the authigenic element
accumulation rate method (equation 4).
23
U-series growth rates estimates for Site H nodules
(from Huh, 1982).
24
1-4
Q-mode varimax factor scores for Site H nodules.
26
1-5
Normalized varimax factor socres for Vulcan I-37BC
1-3
nodule.
31
1-6
Mn, Fe and 23°Th data for Vulcan I-37BC nodule.
32
1-7
Growth rate estimates for Site H nodules.
38
CHAPTER ii:
11-1
11-2
11-3
11-4
(a) Depths, chemical compositions, ages and sample
(b) Chemical compositions
numbers for Site H cores.
(XRF) for samples listed in Table Il-la (Na, Mg, Al,
Si, P, S, K, Ca, Ti, Cr, Mn and Fe in wt %, Co, Ni,
Cu, Zn, Rb, Sr and Ba in ppm).
50
Chemical compositions of ash layers at Site H and
those of ash D and ash K in other cores.
72
Organic carbon and manganese contents and characteristics of the suboxic MANOP Sites H, M and C.
82
Burial concentrations of manganese under changing
redox conditions calculated from the box model.
99
List of Tables - continued
CHAPTER III
Table
Page
111-1
Chemical composition (AA Data) for Site H nodules.
124
111-2
U-series data for Site H nodules.
138
111-3
in
Fluxes of Mn, Fe, Ni, Cu, Co, Si and 230Th
ex
nodules and sediments at Site H.
143
111-4
Mineralogical and chemical data for Site H nodules.
154
111-5
Chemical and textural properties as a function of
nodule size at Site H.
174
PALEOCLIMATIC INFLUENCE ON SEDIMENTATION AND MANGANESE NODULE
GROWTH DURING THE PAST 400,000 YEARS AT MANOP SITE H
(EASTERN EQUATORIAL PACIFIC)
GENERAL INTRODUCTION
The climate of the earth has changed dramatically during the
Quaternary.
The periodic advances and retreats of continental ice
sheets are vivid land-based results of the climate changes.
In the
oceans, climatic variation has led to changes in factors such as:
sea level, surface and deep water circulation, water temperature
distribution, surface water productivity, chemical composition of
the water, and composition and distribution of planktonic assemblages.
These changes have affected sedimentation in the oceans
during the Quaternary by influencing both the composition flux of
source materials and the diagenesis of newly deposited sediments.
Internal layering is common to all deep-sea ferromanganese
concretions; the layers reflect differences in chemical composition
and mineralogy.
Many recent studies have shown that the chemical
composition and accretion rate of manganese nodules growing at the
sea floor reflect the sedimentary environment in which they grow.
Therefore, it seems reasonable that variations in growth rate and
chemical composition within a nodule should reflect, at least in
part, changes with time in the sedimentary environment in which the
nodule grew.
Because climate change has affected depositional pro-
cesses and sedimentary environments in the ocean, it seems likely
that accretion of manganese nodules has also been affected.
2
There have been few attempts to relate variations in chemical
composition within deep-sea nodules to changes in the sedimentary
environment in which they grew, however, because the growth rate of
most deep-sea nodules is a few millimeters per million years.
Thus,
samples less than a millimeter thick may still represent several
hundred thousand years of growth.
Clearly, it is difficult to
sample these nodules at a resolution comparable to Quaternary climate change.
The extent to which post-depositional diagenesis
degrades the primary chemical signal in slowly growing nodules at
these time scales is not well known.
In addition, typical pelagic
nodules usually grow in areas where sedimentation is complex;
erosion, redeposition and diagenesis make it difficult to unravel
the history of accumulation of the associated sediment.
Preliminary work at MANOP Site H indicated that it was well
suited for a study of the relation between sedimentation history and
nodule growth during the Quaternary.
Work on the growth rates of
Site H nodules indicated that they have grown at rates of up to 200
mm/my, much faster than typical pelagic nodules.
Thus, a milli-
meter-thick sample, an amount easily and accurately sampled, records
about 5,000 years of history, an interval short enough to record
Quaternary climate changes.
The sediment at Site H accumulates
fairly rapidly (0.65 cm/kyr) and at a fairly uniform rate without
significant post-depositional erosion or redeposition.
The sedi-
mentary record indicates that the site was strongly influenced by
Quaternary climatic changes.
1
The work of Dymond et al. (1984) and Finney et al. (1984) has
yielded a basic understanding of the processes which control nodule
growth at Site H.
Suboxic diagenesis in the sediments at the site
is important in controlling accretion of the nodule bottoms.
This
process results in the Mn-enriched, minor element-depeleted compositions and fast growth rates found in the Site H nodule bottoms.
Nodule tops at the site grow more slowly by oxic diagenesis and are
enriched in Cu and Ni and depleted in Mn relative to the bottoms.
The chemical compositions and growth rates of whole nodules, however, cannot be explained by steady-state growth in which the accretionary processes supply metals to the nodules at uniform rates.
Non-steady-state growth can best be explained by discontinuous deposition of the suboxic component.
Suboxic diagenesis is driven by the flux of organic carbon into
the sediments (Froelich et al., 1979).
Thus, climatically-induced
changes in the flux of organic carbon to the seafloor (e.g.,
Muller and Suess, 1979; Muller et al., 1983; Pederson, 1983),
may result in variations in nodule accretion at Site H.
Greater
organic carbon fluxes should result in more reducing conditions in
surface sediments.
Such conditions should result in a shallow Mn
redox boundary and enhanced supply of Mn to surface sediments and
nodules.
Unless conditions become so reducing that Mn is exported
from the sea floor, this should result in faster nodule growth due
to an increased suboxic component.
To test these ideas, the following objectives were established.
(1) To determine the history of sedimentation at Site H, par-
4
ticularly the way in which redox conditions in surface sediments
have varied over the past 400 kyr.
This interval spans the esti-
mated age of 5-6 cm nodules at the site.
(2) To define the growth
histories of a suite of nodules at the site.
By examining several
nodules, it should be possible to distinguish regional depositional
changes from changes due to singular events, such as burial or turnover.
(3) To assess the relation between the history of nodule
accretion and changes in the sedimentary environment over this time
period.
Chapter 1 of this thesis summarizes the preliminary work at
Site H which laid the groundwork for developing the hypotheses
tested and discussed in the last two chapters.
This work estab-
lished a method for determining growth rates of Site H nodules and
showed that they grow at rates much faster than most deep-sea
nodules.
In addition, this work and Dyinond et al. (1984) showed
that the growth rates and chemical compositions of these nodules
could be consistently attributed to distinct aceretionary processes.
Chapter 2 deals with the sedimentary history at the site.
The
thesis culminates in Chapter 3 where the growth histories of manganese nodules at Site H are described, and compared to the sedimentary history developed in Chapter 2.
Chapter 1 has been published in Ceochimica et Cosmochimica Acta
(1984, 48:911-919), with C.R. Heath and M.W. Lyle as co-authors.
Chapters 2 and 3 will be submitted for publication in the same
j ournal.
CHAPTER 1
GROWTH RATES OF MANGANESE-RICH NODULES AT MANOP SITE H
(EASTERN NORTH PACIFIC)
Reprinted with permission from:
Geochimica et Cosmochimica Acta, Vol. 48
Copyright 1984, Pergamon Press Ltd.
ABSTRACT
Estimates of the growth rates of ferromanganese nodules at
MANOP Site H (6°33'N, 92°49'W) lie between 7 and 1200 mm/my.
The
most probable range (estimated from nodule size distributions,
nodule chemistry, accumulation rates of authigenic elements and
U-series modeling) is 30-300 mm/my.
Such rates are much faster than
measured values for most pelagic nodules.
The distributions of
U-series and stable elements suggest that a typical nodule consists
of two compositónally distinct non-detrital components, one rich in
Mn, U and Sb, and the other in Fe, Co, REE, Ni, Cu and Th.
When the
uncorrected U-series data, which do not show simple decay patterns
with depth, are normalized to the relative proportion of the Fe-rich
component, the data yield well constrained growth rates of 55 mm/my
for a nodule top and 200 mm/my for its bottom.
The component
enriched in Mn, which is most abundant on nodule bottoms, appears to
accrete much faster than the component enriched in Fe.
7
INTRODUCTION
Most studies of the growth rates of pelagic ferromanganese
nodules have been based on radiochemical analyses of nuclei or the
surrounding oxyhydroxide layers (see for example, Ku's (1977]
review).
Other estimates have been based on: stratigraphic limits
on the initiation of nodule formation (Von Stackelberg, 1979), bio-
stratigraphy of included microfossils (Harada and Nishida, 1976;
Kadko and Burckle, 1980), hydration rind and K-Ar dating of volcanic
nuclei (Burnett and Morgenstein, 1976; Barnes and Dymond, 1967);
amino acid racemization (Bada, 1972), paleomagnetic stratigraphy
(Crecelius et al., 1973) and a growth-burial model based on on
nodule size distributions (Heath, 1979).
These methods yield accre-
tion rates for most deep-sea nodules in the range of 1 to 10 mm/my
(Heath, 1981).
In contrast, recent analysis of a nodule from the eastern South
Pacific (Peru Basin; 7°40.O'S, 9l°59.5'W, recovered from a depth of
4240 in) yields a uranium series derived growth rate of 168 mm/my
(Reyss et al., 1982).
The Mn/Fe ratio in this nodule is as high as
90, much higher than the values of 1 to 5 typical of deep-sea
nodules.
Such a difference is consistent with the findings by Lyle
(1982) and Piper and Williamson (1977) of a positive correlation
between radiochemically determined growth rates and Mn/Fe ratios.
These studies conclude that pelagic nodules grow faster at locations
where sedimentary diagenetic processes supply manganese and related
metals to the nodules.
MANOP Site H is centered at 6°33'N, 92°49'W in the Guatemala
Basin (Figure 1-1), where the water depth averages about 3600 m.
Nodules are abundant over most of the site with coverage ranging up
to 300 nodules/rn2 or 30 kg/rn2.
but range up to 20 cm.
common shapes.
Nodule diameters average about 4 cm
Spheres and flat
"hamburgers'1
are the most
The nodule surface textures can be smooth or rough;
bottoms are usually smooth whereas tops are rough.
Nuclei often are
hard to recognize, but the most common nucleus appears to be an old
nodule fragment.
Internal layering is ubiquitous; the most striking
feature is the boundary between the smooth arid rough textures.
Site
H nodules are highly enriched in Mn compared to those from the central equatorial North Pacific (Dymond et al., 1980).
Their Mn/Fe
ratios range up to 70 and their compositions generally are similar
to that of the fast-growing nodules described by Reyss et al.
<1982).
The site is in an area where sedimentary diagenetic pro-
cesses are expected to be an important source of metals to the overlying nodules (Klinkhammer, 1980).
The following sections summarize
determinations of the growth rate of Site H nodules (Table I-i) by
several independent methods.
0zco
Fractu,.
Zen.
Central America
15
15
'Axls of
Mtddt. Amarica
TrsncP
Cllpp.,ton
Ftactur. Zen.
10
10
Guatemala Basin
Site
ii M
Slqu,lto,
Fractur.
Zen.
Cr.,t of
East PscIflRls
s ite
H
0
0
5
5.
3300
0
1s00
1500
of Galap
C
lOSS
Figure I-i.
100
$pr.sdtng Cant.,
955
Galapagos Island.
.
-
. a
00
Location of MANOP Site H in the Eastern North Pacific
in relation to major topographic features.
'.0
10
Table I-i.
Core locations and nodules recovered at MANOP Site H.
Location
Long (W)
Core ID
Vulcan 1
Lat (N)
Number of
Surface Nodules
Number of
Buried Nodules
34CC
6°34.11'
92°47.25
3
0
36CC
6°33.87'
92°47.12'
1
1
37BC
6°33.97'
92°47.12'
20
3
38GC
6°34.16'
92°47.61'
1
0
4OBC
6°33.75'
92°47.31'
24
5
41CC
6°33.85'
92°47.29'
1
0
47CC
6°33.56'
92°47..03'
0
2
48BC
6°33.75'
92°47.05'
11
2
49CC
6°32.20'
92°46..27'
1
1
5OBC
6°31.81'
92°45.94'
40
1
52BC
6°32.33'
92°46.60'
40
4
54CC
6°32.76'
92°48.50'
0
1
5SBC
6°32.95'
92°48.93'
18
19
60CC
6°32..56'
92°48.55'
1
1
63GC
6°33.04'
92°48.84'
1
1
64CC
6°33.11'
92°48.56'
1
1
66BC
6°32.93'
92°48.57'
13
0
68BC
6°32.49'
92°47.64'
76
1
69BC
6°32.11'
92°48.16'
17
1
70CC
6°32.19'
92°47.34'
3
0
6BC
6°35.00'
92°49.76'
28
6
12BC
6°35.31'
92°47.84'
3
6
15BC
6°35.Ol'
92°46.12
17
1
Lander Red
6°33.91'
92°47.1l
11
2
PLUTO III
11
GROWTH RATE ESTIMATES
Crustal Age
The minimum average growth rate is obtained by assuming that
growth began as soon as the underlying oceanic crust had formed.
Nodules with radii of 10 cm are found at Site H, where the basement
is 14 my old (based on magnetic anomalies; Lynn and Lewis, 1976),
yielding a minimum growth rate of 7 mm/my.
Because nodules are
found only on the portion of the Cocos Plate older than 7 my (Figure
1-2), a more realistic minimum average growth rate is 14 mm/my.
Even these minimum rates indicate that Site H nodules grow faster
than most pelagic nodules.
Size distribution
Heath (1979) proposed a model to explain the size distribution
of surface nodules in terms of an equilibrium between growth and
burial rates.
The model assumes that the growth rate of a nodule
and its probability of burial are independent of size, and that the
rate of nucleation of nodules is constant over time.
This can be
expressed mathematically as:
1nN-lnNo----(D-Do)
2g
(1)
where N is the number of nodules of diameter D per unit area, b is
the burial rate of nodules, g is the nodule growth rate, and No is
the number of the smallest nodules (diameter Do) per unit area.
Such a model predicts an exponential decrease in the abundance of
nodules with increasing size, and identical size distributions for
12
N 20°
4
.
1
\
\
T4
110° W
Figure 1-2.
90
Nodule occurrences in the eastern North Pacific (data
from Monget et al.
unpublished data).
years.
100
,
1976 and Scripps Institution of Oceanography,
The isopleths are crustal ages in millions of
13
surface and buried nodules.
Where burial rates can be estimated for
two sites in the northern equatorial Pacific, Heath's model yields
growth rates similar to those based on radiochemical methods.
The size distributions of nodules at Site H are consistent with
those predicted by Heath's model except for the extreme size classes
(Figure I-3a).
The probability of burial, based on the ratio of the
number of buried nodules to the total number of nodules of a given
size class (Figure I-3b) is independent of size for nodules with
diameters of 2 to 8 cm, the size range of most nodules at the site.
For this size range, the surface and downcore size distributions are
similar and show the exponential decrease in abundance with increasing size predicted by Heath's (1979) model (Figure 1-4).
The higher
probability of burial for nodules smaller than 2 cm or larger than 8
cm is consistent with the hypothesis that bioturbation maintains
nodules at the sediment surface (Piper and Fowler, 1980).
Nodules
larger than 8 cm extend so far into the sediment that bioturbation
can no longer move them up, resulting in an increased probability of
burial.
This is supported by modeled rates of bioturbation at the
site (Kadko and Heath, 1984) which show a rapid downward decrease
within the upper few centimeters of sediment.
In contrast, nodules
smaller than 2 cm are not easily distinguished from the surrounding
sediment and thus are often mixed down and buried where larger
nodules are kept at the surface. Thus Do, the size of the smallest
nodules, is about 2 cm at the site; nodules do not behave like the
rest of the population until they reach this size.
14
10
[i
CI)
w
-J
0
z
U-
0 120
cc
w
ca
z
40
0-2
Figure 1-3.
2-4
4-6
6-8
8-10 10-12
SIZE RANGE (cm)
(a) Histograms of the abundance of surface and buried
nodules as a function of size for Site H cores recovered during
the Vulcan I and Pluto III expeditions.
(b) The probability of
a Site H nodule being found buried as a function of size, based
on Figure I-3a.
15
-S
>.
I-J
0
cc
0
4
2
6
NODULE DIAMETER (CM)
Figure 1-3. . . continued
8
10
16
100
0
.'.
1
(0
C,)
'I
m -0.077 mm1
-J
o
w
0
10
w
m 0.036 mni
0
04
-S
(I)
w
-J
'I
c
0
z
a
z
O
0.1
0
20
40
60
80
100
120
NODULE DIAMETER (M'K4)
Figure 1-4.
Size distributions of nodules from Vulcan I and Pluto
III cores (closed circles) and Indomed deep-tow photographs (open
circles).
The slope (m) for the Vulcan and Pluto nodules, calculated over the interval for which the probability of burial is
independent of size, is -0.36 mm''.
The slope from the Indomed
data is -0.077 mm.
17
Heath's model requires that the ratio of the growth rate (g) to
the burial rate (b) is proportional to the slope (m) of the surface
size distribution for a semi-logarithmic plot (Figure 1-4), or that:
b
(2)
For Site H, b, calculated from the abundance of surface and buried
nodules (Figure I-3a), and a sedimentation rate of 6.6 rn/my (Kadko
and Heath, 1984) is 3.40 my'.
The value of in, for nodules in cores
recovered by the Vulcan I and Pluto III cruises, is -0.036 mm,
yielding a growth rate of 47 mm/my.
The m value derived from
Indomed deep-tow bottom photographs is a factor of 2 smaller (Figure
1-4), probably due to underestimates of the numbers of large
nodules, many of which are partially covered by sediment.
Obser-
vation from ALVIN during the Pluto III cruise support this sug-
gestion; most large nodules were found to be almost buried and
covered by a thin layer of sediment.
If some buried nodules were
missed during processing of the box cores, the 47 mm/my growth rate
estimate would be low rather than high.
Growth since a "burial event"
A large population of nodules 6 cm or more in diameter found
buried at depths of 40-50 cm (Fig. I-5a,b) leads to a strati-
graphically constrained maximum estimate of their growth rate at
Site H.
The presence of this population throughout the site (Figure
1-6) suggests that it marks a "burial event" that took place 60,000
to 100,000 years ago.
The sediment above this horizon is lighter in
color and more carbonate-rich than the underlying deposits,
18
NUMBER OF NODULES
100
10
1
0%
1000
2
I
10-20
w
20-30
0
F-
F- 30-40
0
40-50
C,,
NODULE DIAMETER (CM)
0-2
2-4
4-6
6-8 8-10 10-12
0-10
62
147
63
31
3
10-20
1
1
4
2
2
20-30
1
1
o 30-40
1
2
=
I.-
0
w
2
1
a3
i)
40-50
Figure 1-5.
1
1
11
(a) Abundance of nodules as a function of subbottom
depth in Vulcan I and Pluto III cores.
(b) Size distributions
of Vulcan I and Pluto III nodules as a function of subbottorn
depth.
19
SITE H
6°3 6' N
I
(0
o
6 34
IU
dD
6032/
I
G515
?,600
I
92°50'
Figure 1-6.
6°30'
I
92°48'
I
-
92°46'W
Locations of box cores (squares) and gravity cores
(circles) recovered during the Vulcan I and Pluto III expeditions.
Cores with buried nodules at subbottom depths of 40 to
50 cm are shown by closed symbols.
20
indicating that it may mark a glacial-interglacial climate change
(Arrhenius, 1952).
If nodules above the burial horizon have grown
since the "burial event", their minimum growth rates are 1200 mm/my.
The rates could be even greater if growth did not begin immediately
after the event.
The presence of surface nodules in all of the
four-inch gravity cores which contain the burial horizon provides
some support for this estimate.
Mn/Fe ratios
Piper and Williamson (1977) and Lyle (1982) have reported
systematic relationships between the Mn and Fe contents and growth
rates of nodules.
Heye and Marchig (1977) also noted a correlation
between these parameters.
had been dated by
23O
Lyle (1982), using fifteen nodules that
or 10Re, found that the growth rates could
be described by the equation:
g - 16.0 Mn/Fe2 + 0.448
.
(3)
For Site H, this relation yields average growth rates of 30 (±l2)
rn/my for nodule tops and 1050 (±370 la) mm/my for nodule bottoms.
Although the equation is not well constrained for rates greater than
50 mm/my, it seems to indicate the general magnitude of fast growth
rates (Lyle, 1982).
The equation may become less accurate at growth
rates greater than 50 mm/my because of a different mode of accretion, in which diagenetically remobilized manganese from the underlying sediments is supplied to the growing nodule.
Thus the top/
bottom contrast in the growth rates of Site H nodules may indicate
21
addition of remobilized Mn to the nodule bottoms, as inferred by
Dyniond et al. (1984) from compositional relationships.
Elemental accumulation rates
Bender et al. (1966), Kraemer and Schornick (1974) and Somaya-
julu et al. (1971) have reported that elements such as Mn, Go, Ni
and Cu accumulate at similar rates in pelagic sediments and nodules.
If so, the growth rate of a nodule can be determined by measuring
the concentrations of a "constant rate" authigenic element in the
nodule and sediment, the dry bulk densities of the sediment and
The equation that
nodule and the accumulation rate of the sediment.
relates the nodule growth rate to these parameters (assuming that
the cross sectional area governs elemental uptake by the nodule)
is:
SpC
ss
4pC
(4)
nn
where:
g
radial growth rate of nodule in mm/my
S
sedimentation rate in mm/my
dry bulk density of sediment g/cm3
dry bulk density of nodule g/cm3
concentration of the element in the sediment
C
S
C
n
concentration of the element in the nodule.
Mn, Ni and Cu appear to be supplied to the nodules at Site
diagenetic reactions in the sediments (Dymond et al., 1984).
I-I by
Thus,
Co has been chosen as the "purest" authigenic element (Somayajulu
22
et al., 1971; and Turekian ,1968).
For Site H, this model suggests
that the average growth rate for the nodule tops is 83 (±30 lc)
mm/my and for the nodule bottoms is 260 (±128 la) mm/my (Table 1-2).
Because Bender et al. (1970), Kraemer and Schornick (1974) and
Somayajulu et al. (1971) found that the accumulation rate of authigenie elements in nodules is generally less than or equal to the
accumulation rate in sediments, estimates derived from equation (4)
will be maxima.
However, if authigenic elements supplied by sedi-
mentary diagenesis enrich nodules relative to sediment, the estimtes
will be minima.
A better understanding of the way in which elements
are incorporated into nodules is needed to establish confidence in
equation (4).
Uranium-Series Methods
The activities of U-series isotopes have been measured in six
Site H nodules (Huh, 1982).
Most of the 230Th, 230Th/232Th and
231Pa profiles, unlike those typical of deep-sea nodules, do not
decrease exponentially or even regularly with depth.
In none of the
Site H nodules analyzed do the profiles allow growth rates to be
determined by all three methods (Table 1-3).
nodules, the growth rates determined from
231
230
For most deep-sea
Th,
232
230
Th and
Th/
Pa profiles agree to within 20% (e.g. Moore et al., 1981; Ku,
1977).
Agreement between two of the methods is observed only in
Indomed-I-9BC27 (bottom) and Vulcan-I-37BC (top).
When decay pro-
files permit the determination of growth rates, they fall within the
range estimated by the methods discussed previously, but it is clear
23
Table 1-2.
Dry bulk densities, Co content, Mn/Fe ratios and
calculated growth rates using the authigenic element
accumulation rate method (equation 4).
p
dry
(-3)
cm
Co
(wt. %)
g or S ()
lil)
(w
%)
Sediment
0.300
0.0055
6600
il
Nodule top (aye)
l.3512.O2
0.0204
67-99
12
Nodule bottom (aye)
l.3512.O2
0.0065
209-3l0
50
Crust (aye)
l.3512.O2
0.121
11-l7
1.2
1Calculated from data in Greenslate (1977)
2Reported by Ku (1977) and Krishnaswami and Cochran (1978)
3Corrected for detrital Co using a Co/Al ratio for shale of .00024
Turekian and Wedepohl (1961)
4Kadko and Heath (1984)
5The range of g values (nodule growth rates) result from the average
of calculated dry bulk density values.
For purposes of discussion
in the text we have used the average of this range.
24
Table 1-3.
U-series growth rate estimates for Site H nodules (from
Huh, 1982).
Method
Sample
1M19 BC26 top
230
Th
230
ex
231
Th
ex
Pa
ex
18
n.d.
n.d.
1M19 BC26 bottom
n.d.
n.d.
n.d.
1M19 BC27 top
n.d.
n.d.
n,d.
1M19 BC27 bottom
n.d.
61
63
38
n.d.
15
1M19 BC34 bottom
n.d.
n.d.
n,d.
Vulcan I 37BC top
61
50
n.d.
Vulcan I 37BC bottom
n.d.
n.d.
n.d.
Pluto III 68C top
n.d.
26
n.d.
Pluto III 6BC bottom
- - -
Pluto III 6BC *top
n.d.
47
n.d.
Pluto III 6BC *bottom
n.d.
31
n.d.
1M19 BC34 top
not enough data
n.d. - growth rate cannot be determined -
*Buried nodule, 10 cm below sea floor
- - -
25
that the standard radiochemical dating techniques do not work well
for Site H nodules.
Cheniical/Radiochemjcal Model
We hypothesize that much of the variability in the U-series
profiles results from the accretion in the growing nodules of more
than one compositionally uniform component.
If each component has
different proportions of U, Pa and Th, the gross uranium-series
distribution will reflect not only isotopic growth and decay, but
also variations in the relative proportions of the several cornportents through the profile.
Therefore, knowledge of both the
chemical and radiochemical compositions of Site H nodules is
required to interpret the distribution of U-series isotopes.
We
have applied Q-mode factor analysis (Kiovan and Inibrie, 1971) to
define the components which comprise the Site H nodules.
The data
set analyzed includes the concentrations of Si, Al, Mg, Ca, Mn, Fe,
S
Co, Ni, Cu, Zn, Ba, La, Ce, Nd, Sb, Th and U in the top and bottom
3-5 nun and bulk samples of 26 nodules recovered throughout the site.
The results (Table 1-4, Figure 1-7) indicate that the chemical com-
position of the non-detrital component of the nodules is described
very well by the mixing of two end members.
One end member (Fl),
enriched in Mn, Mg, Ca, Sb and U and strongly depleted in Fe, Co,
Th, Ce, La, accounts for 53% of the variance in the data.
The other
(F2), with high loadings in Fe, Co, Th, Cu and the REE's, accounts
for 37% of the variance.
A third end member (F3), which accounts
for 9% of the variance, has high loadings in Si and Al and is
typical of Site H detritus.
Because of the strong negative factor
26
Table 1-4.
Q-niode varimax factor scores for Site H nodules.
Factor Scores
Element
Factor 1
Factor 2
Factor 3
Mg
1.13
.49
.03
Al
0.28
.02
1.70
Si
.08
-.69
2.87
Ca
1.29
.23
.17
Mn
2.07
- .01
- .49
Fe
- .19
1.01
.96
Ni
.94
1.33
-.79
Cu
.26
1.92
- .73
Zn
.99
1.27
-.75
Co
.01
1.69
- .19
Ba
.73
- .47
1.78
La
.20
1.11
.37
Ce
.04
1,18
.44
Nd
.22
1.02
.42
Th
-.06
.97
.85
U
2.75
-1.13
.03
Sb
2.63
- .62
- .42
Variance (%) accounted in nodule samples:
53.45
36.88
9.66
53.45
90.33
99.99
Cumulative variance:
= WHOLE
A= BOTTOM
= TOP
30% F3 (Si, Al, Ba)
'aI
I
kA
A
S
A
AA.A
A..
a
A
I
Fl (Mn, Ca, Mg, Sb, U)
Figure 1-7.
a.
(Ni, Cu, Fe, Co, REE, Th) F2
Portion of a triangular plot showing the relative
abundances in Site 1-1 nodule samples of the three compositional
factors determined by Q-mode factor analysis.
28
loading for Th in Fl, and the strong negative correlation between Mn
and Th for the nodules (Figure 1-8), Fl can be viewed as a Th-free
Consequently, the concentration
diluent for 230Th in the nodules.
of 230Th within the nodule will depend not only on the decay of
.
initial unsupported
230
Th and the growth of
230
Th from
234
U, but
also on the relative importance of the Fl and F2 end members.
In order to convert a
230
Th profile to a growth rate, both the
U-series nuclides and stable element concentrations for successive
layers of the nodule must be determined.
If the initial ratio of
230Th to F2 is constant through time and if Fl can be treated as a
230Th-free diluent, the corrected
content at some depth (x)
Th
in a nodule can be expressed as:
sup
ex
230
Th
ex
-
(x)
230Th
(coor)
tot
(1 - e
-
230Th
23°Th
ex
-A x/g)
sup
/(F2)
(5)
(6)
(7)
where A is the decay constant of 230Th, g is the nodule growth rate,
and F2 is the fraction of Factor 2 determined by Q-mode factor
analysis.
F3 is not included since the radioisotopes are measured
on a residue-free basis.
of
234
U-supported
230
Equation (5) takes account of the ingrowth
.
Th.
In pelagic nodules, this occurs so close
to the surface that a correction is rarely necessary.
At Site }{,
however, the rapid growth rates inferred in previous sections would
result in 234U-230Th djseciuilibrium to depths of centimeters into a
nodule.
Because the determination of 23°Th sup requires a knowledge
29
0
0
I-
2
o
Mn (wt. %)
Figure 1-8.
The strong negative correlation between Th and Mn in
Site H nodules.
of the nodule growth rate, 230Th
sup
and 230Th
ex(corr)
(from which
the growth rate is estimated) are determined iteratively (Reyss et
al., 1982; Koide et al., Huh, 1982).
Thus far, only one nodule from Site H (Vulcan-I-37BC) has been
analyzed on a layer by layer basis for both U-series isotopes and
stable elements.
The results (Tables 1-5 and 1-6; Figure I-9a,b)
indicate that the corrected 23°Th values yield good exponential
decay curves from which growth rates can be derived.
The growth
rate of 55 mm/my for the top of Vulcan-I-37BC is within the range of
values determined from uncorrected 230Th
files (Table 1-3).
and 230Th
/232Th pro-
The best fit to the data for the bottom of
Vulcan-I-37BC suggests a change in growth rate at a depth of about 8
mm, where there is an abrupt change in the Mn/Fe ratio of the nodule
(Figure I-9b).
The outer 8 mm, with a growth rate of 200 mm/my, has
a Mn/Fe ratio of more than 52 and is similar in composition to the
fast-growing nodule described by Reyss et al. (1982).
The growth
rate (50 mm/my) and composition (Mn/Fe ratio less than 34) of the
deeper layers are similar to those of the top of the nodule, and may
indicate the this nodule has turned over.
However, since the top
shows no change in growth rate during the time the bottom changed
rate, turning seems an unlikely explanation.
31
Table 1-5.
Normalized varimax factor scores for Vulcan I-37BC
nodule.
Nodule Sample
and Depth (mm)
Factor Score
Factor 2
Factor 1
Communality**
T*
0-5
0.25
0.75
0.99
T
5-10
0.44
0.56
0.96
T
10-14
0.30
0.71
0.99
T
14-24
(nucleus)
0.20
0.80
0.99
0.76
0.24
0.99
B
0-3.5
B
3.5-6
0.80
0.20
0.99
B
6-8
0.77
0.23
0.99
B
8-10
0.48
0.52
0.99
B
10-14
0.64
0.37
0.98
B
14-19
0.34
0.67
0.99
* T
Top; B = Bottom
** Total fraction of the variance accounted for
Table 1-6.
Mn, Fe and 230Th data for Vulcan-I-373C nodule.
Sample
Mn
Fe
Fe
234
Mn
U
230
Th
230
tot
Th
230
sup*
Th
ex(corr)*
T
0-5
32.8
4.05
8.1
3.00
34.7
1.11
44.91
T
5-10
39.8
2.01
19.8
3.70
13.8
2.78
19.79
T
10-14
37.8
3.11
12.2
3.59
10.8
3.20
10.78
T 14-24
(nucleus)
34.8
4.92
7.1
3.89
5.58
3.77
2.27
0-3.5
44.6
0.717
62.2
5.57
9.97
0.41
39.50
B 3.5-6
45.6
0.633
72.0
3.95
8.43
1.03
37.77
B
6-8
44.4
0.848
52.4
2.22
8.00
0.80
31.86
B
8-10
40.6
2.27
17.9
4.02
221
23.26
B
10-14
43.6
1.29
33.8
4.66
7.67
3.45
11.57
B
14-19
38.6
3.19
12.3
6.05
8.53
5.57
0.46
B
14.4
*Calculated using equations 5-7, and method discussed in text
N.)
33
Figure 1-9.
230Th
(solid lines) and Mn/Fe ratios (dashed lines)
plotted against depth in nodule Vulcan-I-37BC.
The 230Th0
has
been normalized to Q-mode factor 2 by the method described in the
text.
(a) Nodule top, (b) Nodule bottom.
34
E
a
0
a
0
4-
I-
0
U
K
0
I-
c)
30
('SI
20
z
-n
Ph
10
0
DEPTH (mm)
Figure I-9a.
35
40
20
a.
-U
0
U
I,
I-
0
U
0
0,
04
4
2
+VT1
m
20
0
5
10
15
DEPTH (mm)
Figure I-9b.
20
36
DISCUSSION
The modeled chemical data and derived growth rates suggest that
the Site H nodules are composed mainly of two components which
differ markedly in chemical composition and accumulate at very
different rates.
Furthermore, work by Dymond et al. (1984) suggest
that these components also have different mineralogical and textural
properties.
All growth rate estimates which take account of nodule
chemistry indicate that the bottom or Mn-enriched surfaces grow
faster than the nodule tops.
Analysis of nodule compositions
indicate that variations in the relative proportions of these two
components occur at scales from the sub-millimeter to several centimeter range.
It is best, therefore, to view any growth rate esti-
mate as an average integration of the processes which formed a
nodule, as it seems probable that major variations in the growth
rate mark changes in the orientation, degree of burial, input of
metal-bearing detritus, and intensity of diagenetic reactions in the
sediments underlying a nodule.
Observations from the submersible ALVIN suggest that repeated
burial and exhumation by benthic organisms are part of the normal
cycle of development of Site H nodules.
Dymond et al. (1984), on
the basis of compositional data and variations in the vertical flux
of particulates caught in sediment traps (Fischer et al., 1982;
Honjo, 1981), also conclude that the growth of these nodules cannot
be attributed solely to steady state processes.
37
CONCLUSIONS
A number of independent estimates of the growth rates of Site H
nodules indicate an extreme range of about 7 to 1200 mm/my (Table
1-7).
The more reliable techniques, based on nodule size distri-
butions, nodule chemistry, accumulation rates of authigenic elements
and U-series modeling, yield a most probable range of 30-300 mm/my.
The distributions of stable elements and U-series radionuclides
suggest that the nodules are comprised of two distinct non-detrital
components. Because the relative proportions of these two com-
ponents, which have very different compositions, vary with depth
into a nodule, the U-series data alone do not allow proper determinations of growth rates.
However, when the U-series data are
normalized on the basis of the stable element distributions, the
U-series data and stable element compositions become internally
consistent and yield interpretable growth rates.
A Mn-enriched
component, which contains Mg, Ca, U and Sb, is concentrated in the
nodule bottoms and appears to grow at least four times faster than
the nodule tops.
Co, Th and B.EE.
The nodule tops are enriched in Fe, Ni, Cu, Zn,
The nodules appear to grow at varying rates during
their lifetimes in response to changes in the sedimentary environment in which they form.
Disturbance by benthic organisms and
changes in the flux of metal bearing particles to the sea floor can
both have a profound influence on the chemical environments
immediately adjacent to the surfaces of a growing nodule.
38
Table 1-7.
Growth rate estimates for Site H nodules.
Method
Growth Rate
mm/my
Crustal Age
>7
Crustal Age (7 my initiation)
>14 mm/my
Heath Model (rate a dD/dlnN)
>47 mm/my
Lyle Model (rate a Mn/Fe2)
Co Accumulation Rate
(rate sed - rate nodule)
Standard Radiochemical Modeling
Chemical/Radiochemical Model
(Vulcan-I-37BC)
Growth since a late Pleistocene
"burial event"
30 mm/my
tops
bottoms
1050 mm/my
83 mm/my
tops
bottoms
26O mm/my
>15-60 mm/my
top
bottoms
<1200 mm/my
55 mm/my
200 mm/my
39
ACKNOWLEDGEMENTS
We thank Jack Dyniond for helpful discussions and for access to
some of his data, and David Z. Piper and William C. Burnett for
constructive reviews.
We are also grateful to Chih-An Huh and
Richard Ku for providing splits of samples which they analyzed for
U-series isotopes.
This research was supported by the NSF Manganese
Nodule Program (MANOP) through grants OCE-8101845 and OCE-8217250.
40
CHAPTER II
SEDIMENTATION AT MANOP SITE H (EASTERN EQUATORIAL PACIFIC) OVER
THE PAST 400 KYR - CLIMATICALLY INDUCED REDOX VARIATIONS
AND THEIR EFFECTS ON TRANSITION METAL CYCLING
41
ABSTRACT
Gravity cores recovered from MANOP Site H (6°33'N, 92°49'W)
show marked downcore variations in the abundance of calcium carbo-
nate, organic carbon, opal, manganese and other components deposited
over the past 400 kyr.
Variations in the downcore abundance of
organic carbon, which ranges from 0.2 to 1.0 %, can be used to hindcast redox conditions in the surface sediments over this time.
The
results indicate that the depth to the manganese redox boundary
varied from about 5 to 25 cm below the seafloor during four major
cycles.
Downoore variations in solid-phase Mn, Ni and Cu can be
produced by such changes in redox conditions.
A model which pre-
dicts that solid phase Mn can be trapped and buried when the Mn
redox boundary migrates rapidly upward, is consistent with the
observed organic carbon and Mn records and supports the reconstructed redox variations.
The history of sedimentation at Site H can be explained by
changes with time in surface water productivity or in the properties
of bottom water, or both.
Concurrent changes in opal and organic
carbon abundances imply that productivity variations were important
in controlling such changes and hence the redox conditions in the
surface sediments.
42
INTRODUCTION
Variations in the composition of pelagic sediments with
subbottoin depth are ubiquitous in the oceans.
Changes in the com-
position and fluxes of source materials, bioturbation and diagenesis
are the main factors controlling these variations.
All of these
factors may be climatically related since climate affects the flux
of calcium carbonate, opal, organic carbon and inorganic detritus to
the sea floor.
The flux of organic carbon to the sediments drives
bioturbation and early diagenesis of the sediments (Bender et al.,
1977; Froelich et al., 1979; Emerson et al., 1980).
Such diagenesis
strongly influences the near surface distribution of redox-sensitive
elements such as Mn (onatti et al., 1971; Klinkhammer, 1980; Sawlan
and Murray, 1983).
In addition, sedimentary diagenetic processes
are believed to play a major role in controlling the supply of
metals to ferromanganese nodules (Calvert and Price, 1977; Piper and
Williamson, 1977; Heath, 1981).
Therefore, temporal variations in
the flux of biogenic material to the sea floor may affect the intensity of diagenesis in the sediments and, in turn, influence the
growth rate and chemical composition of manganese nodules and the
cycling of redox sensitive elements in sediments.
Significant periodic variations of the earth's climate during
the Quaternary period are well documented (e.g., CLIMAP, 1976).
A
number of studies have shown that these climatic changes resulted in
variable deposition of many important components of eastern equatorial Pacific deep-sea sediments.
The cycles in abundance of cal-
cium carbonate common to this region (Arrhenius, 1952) reflect
43
climatically induced variations in productivity and carbonate dissolution (Berger, 1973; Adelseck and Anderson, 1978).
Rea et al.
(1986) found that the grain size and flux of eolian material varied
significantly at DSDP Site 503 in the eastern equatorial Pacific as
a result of climatically induced changes in wind patterns and
aridity of source regions.
Changes in the distribution of plank-
tonic assemblages in the sedimentary record in this region have been
strongly influenced by changes in oceanic circulation patterns
(Moore et al., 1980; R.omine and Moore, 1981; Romine, 1982).
Changes
in oceanic circulation also affect the biological productivity of
surface waters which may be recorded in the opal, organic carbon and
calcium carbonate contents of the underlying sediments (Pisias,
1976; Adelseck and Anderson, 1978; Pederson, 1983).
Such studies
have demonstrated that the composition and accumulation of sediments
in the eastern equatorial Pacific are strongly influenced by climatic change.
Because changes in the flux and composition of sedimentary
components have occurred throughout the Quaternary, redox conditions
in surface sediments may have also varied.
Several studies have
suggested that reducing conditions in surface sediments varied as a
result of the climatic change associated with the most recent
glacial-interglacial transition (Thomson et al., 1984; Gardner et
al., 1982; Berger et al., 1983).
The sedimentary record at many
sites indicates that changes in sedimentation, comparable to those
of the most recent glacial-interglacial transition, occurred at
earlier times during the Quaternary (see, for example, Hartmann et
44
al., 1976; Bonatti, 1971).
It is reasonable to conclude, therefore,
that redox conditions in surface sediments have varied throughout
this period.
45
BACKGROUND AND OBJECTIVES
Site H was chosen by the Manganese Nodule Project (MANOP) as
the type hemipelagic sediment site in a study of sedimentation and
nodule forming processes at five distinct sedimentary environments
in the Pacific (Figure Il-i).
The site is centered at 6°33'N,
92°49'W in the Guatemala Basin and has an average water depth of
3600 m.
The sediments are largely a mixture of opal, calcium carbo-
nate, Fe-Mn oxyhydroxides and aluminosilicates (mainly smectite)
which have accumulated at an average rate of 0.65 cm/kyr (Kadko and
Heath, 1984).
Primary productivity in the surface waters overlying
the site is relatively high and varies strongly with a seasonal
cycle (Fischer, 1983).
The sediments at the site are suboxic; Mn is
reduced at a depth of about 10 cm, but reduced Fe has not been
detected in porewaters (Klinkhanimer, 1980).
Manganese nodules are
abundant over most of the site except for a few scattered nodulefree "bare patches" which range from about 100 m to 1 km in diameter.
The chemical composition and growth rates of the nodules
reflect the influence of suboxic diagenesis.
Their Mn/Fe ratios
commonly exceed 60 and their growth rates range up to 200 mm/my;
values much greater than those for the typical pelagic nodules found
at sites where the sediments are oxidized to great depths (Dymond et
al., 1984; Finney et al., 1984).
In our preliminary work at Site H (Finney et al., 1983), we
found that sedimentary horizons enriched in preserved organic carbon
corresponded to fast-growing, Mn-enriched layers in a manganese
nodule.
We hypothesized that the depth to the Mn redox boundary
__________________________
1I I I I I
IiIIU
PUII
liti !'i!;flfli;1IiIj II, 1111111 II'fI;'I[I
111111111
i
30
110
II III
1111111
'1
20
10
:.:)..::.
111
LII
Figure 11-i.
inn
Locations of the MANOP Sites relative to the major
sediment types of the Pacific (after Horn et al., 1972).
1]
47
varied as the result of climatically-induced changes in the particulate organic carbon flux to the seafloor; a high organic flux
corresponding to a shallow Mn redox boundary and to accelerated
transfer of Mn to surface sediments and nodules.
Such periods
should be recorded in the growth rates and chemical compositions of
manganese nodules and in the cycling and burial of Mn and other
transition metals in the sediment.
We attempt to test this hypothesis by: (1) defining the history
of sedimentation at the site over the past 400 kyr, particularly the
variations in redox conditions, and (2) determining how the accretion of manganese nodules at the site varied during this time.
report here the results from the first part of our study.
We
In a
companion paper (Finney et al., 198Gb), we describe the growth his-
tories of a suite of manganese nodules from the site, and use the
results from this study to evaluate the influence of changes in the
sedimentary environment on nodule accretion.
48
METHODS
The cores used in this study were recovered during the Vulcan I
expedition to Site H in 1980.
Gravity cores 38CC, 49GC, 63CC and
70CC (Figure 11-2), which range in length from 190-270 cm, were
sampled at 6 cm intervals.
to a fine powder.
The samples were freeze-dried and ground
The concentrations of major and minor elements
(Table 11-1) were determined using a wavelength dispersive Phillips
PW 1600 simultaneous X-ray fluorescence (XRF) spectrometer.
Approximately 2.5 g of the dried ground sample was pressed into XRF
pellets at five tons of pressure.
The machine was calibrated using
a set of more than 60 U.S.G.S., N.B.S., international and "in house"
standards.
The precision for these determinations is within 2% and
the accuracy is within 5%.
Purified samples of two dispersed ash layers revealed by their
anomalous bulk elemental compositions were also analyzed by XRF.
Approximately 20 g of wet sediment containing the ash was first
sieved at 38 microns to remove the clay fraction.
Calcium carbo-
nate, opal and Fe-Mn oxyhydroxides were subsequently removed by
treatments with acetic acid, Na2CO3 and NH2OH.HCl leaches (see
Robbins et al., 1984 for leach methods).
Microscopic inspection
indicated that this procedure resulted in a very pure ash fraction.
The samples were ground to a fine powder and pressed into XRF
pellets at five tons of pressure using two drops of PVA cement to
aid in binding.
The amount of ash in a bulk sediment sample was
then determined by assuming that all of the Ti and Al in the sample
result from a mixture of ash and a more uniformly deposited detrital
49
SITE H
O361
38.
6°34
,,,( 9
3
N\L3s1
6°30
92°50
Figure 11-2.
92"45
Bathymetry of Site H and locations of the box cores
(squares) and gravity cores (stars) referred to in this work.
All cores collected during the Vulcan I expedition except for 33
which was taken during the Indomed expedition.
50
Depths, Chemical Compositions, Ages and
Table Il-la.
Sample Numbers for Site H Cores.
Vulcan 38GC
SAIIPLE.
tIP 020579
tIP 020580
tIP 020381
tIP 020582
tIP 020583
tip 020584
tIP 020585
NP 020306
tIP 020387
tIP 020569
NP 020589
tIP 020390
NP 020591
tiP 020592
NP 020593
NP 020594
tIP 020595
tIP 020596
NP 020597
tIP 020598
020599
NP 020600
NP
tIP 020601
lIP 020602
tIP 020603
tIP 020604
020605
tIP 020606
NP 020607
NP
tIP 020608
NP 020609
NP 020610
Subbotton
Age
Organic
Depth
(Kyr)
Carbon
(Vt 1)
(cn)
-
0
6
12
27
36
46
19
56
24
30
36
42
48
54
60
66
72
78
84
90
96
65
102
108
114
120
126
132
138
144
150
156
162
170
176
182
188
75
85
92
100
108
116
123
131
139
147
166
177
187
200
212
224
237
249
261
272
284
295
307
318
329
340
.60
.78
.454
.394
.384
.228
.231
Calcium
Carbonate
Opal
(Vt Z)
(Vt Z)
Volcanic
Aeb
(Vt
)
17.18
15.93
16.49
16.09
9.91
3.03
2.1
1.2
I. 56
0. 53
0. 47
. 5
51. 1
, 246
. 225
1. 3
3. 1
28. 0
.274
.310
.363
.622
.704
.567
0.49
0.53
0.49
3.01
8.41
10,97
0.2
0.0
2,8
10.4
7.0
6.2
7.63
3.2
3.6
11.0
10.25
0.
4.8
19.2
21.4
33.3
46.0
7. 9
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
5.0
9.2
24.72
20.37
8,3
8.2
8.0
7.4
8.2
6.54
7.25
6.45
7.23
6.9
6.4
10.1
10.8
. 441
22. 25
6. 1
13. 0
.422
10.08
7.8
. 414
7. 53
8. 1
.344
.306
.174
7.98
13.07
7.38
6.7
6.2
3.1
17.2
10.0
24.0
24.3
34.3
. 189
16. 61
0. 7
19. 3
53
9. 14
.534
.477
.453
.36
.386
.298
.312
.310
.426
12.14
9.38
3.94
1.00
4.83
14.23
20.49
'.1-0 u
' .-. P3 P3 P.)
CDU U U A U) .0 A U
'.4 '.4
0-0 -0'-lG'UAU
CU
.
- I
(U
U UO- P.) 0' 0 U '.1 OP.) .J.4 '.10' 0' 0 CD CD CU 0 U
'
._s)
CUN.4CU'..1O-.wopJO
CUUUUAOCUUO.00UW0'(J.PJ.U'..UU,.. .A-0 .AUOA'-IUOOCD
PP9pr.10 'Z'o
- - (U -
0 0U.i- U -
i
CUA(.3P.3
C
'
a
'
a
'-a..?
ppppppppp
CU--UUU)(U.00D'.-0UAA(UOOOOoOooO00000O.-cD(UUuUuCD(UUoO ,
CD
...
.AUU (.3UWUUU(.3U UUpJ.JJ
OUAUUM'.*0.0AUoAU
-00000000
U
U U (.3 U U U U U U U U U U U U U U (JU (.3U U (JU U
A UAUAUAUAUAU AUUUUU
NCUCUCUCUP3P3P3pjpj
..-,--. U U U (4(4 U U U U (4W (4 U
C)-
CD
C)
(a
C)
CD
.(..
D
C)
52
Table 11-la. . continued
Vulcan 70Cc
Subbottom
Depth
SAI'IPLE
NP 020611
NP 02O612
NP 020613
NP 020614
tIP 020615
NP 020616
NP 020617
NP 020618
NP 020619
tIP 020620
NP 020621
AP 020622
lIP 020623
rIP 020624
NP 020625
NP 020626
NP 020627
NP 020628
NP 020629
NP 020630
NP 020631
NP 020632
NP 020633
lIP 020634
NP 020635
tIP 020636
PIP 020637
NP 020638
PIP 020639
PIP 020640
NP 020641
NP 020642
NP 020643
Me
(Kyr)
(ca)
0
6
12
18
24
30
36
42
48
54
60
66
72
79
94
90
96
102
108
114
120
126
132
138
144
ISO
156
162
168
174
180
186
192
Organic
Ca4ciu.
Carbon
(Vt Z)
Carbonate
(Vt Z)
5
14
22
31
40
49
57
66
75
84
90
96
103
110
116
123
130
136
143
149
156
162
169
175
181
188
194
201
207
214
220
226
233
75
17. 19
4. 82
0. 0
1.0
11.0
16.5
24.6
43.7
.
1.61
,57
2.2
18.11
7,7
5.0
3.7
2.7
15
0. 32
0. 0
0.0
61. 8
0.31
26
0. 32
0. 0
0.0
0.1
23. 9
0.36
0.43
0,56
33
0. 46
2. 1
0. 7
9. 5
0. 0
14.47
12.84
8.33
7.76
11.58
10.0
0.0
0.0
0.0
0.0
0.0
2. 47
7. 1
0. 0
.20
.25
.35
.29
.29
.38
53
.60
58
.64
.58
.60
.55
.50
,
0.0
0.0
0.0
7.6
7.6
4,5
.
,
Ash
(wt Z)
5,00
.96
.63
.48
.42
.33
.
(wt Z)
.65
.65
.48
.
.
Volcanic
Opal
38
.36
.36
.40
,35
.29
.39
13.47
19.44
22.62
4.93
0.44
1,63
10. 65
14,08
16. 16
1,40
5.69
2.98
5.90
11.04
14.36
1.6
0.0
3.9
9,4
9,0
9.0
8.6
8.0
8.4
6.9
7.1
8.5
6.9
6.1
7.0
40.3
35.4
9.0
4.3
0.0
0.0
0.0
0. 0
0.0
0.0
0,0
0.0
0.0
0.0
Table Il-la.
continued
Vulcan 63CC
SA(IPLC
HP 020644
tiP 020645
tiP 020646
tIP 020647
Subbotton
Depth
Age
(ca)
(Kyr)
0
6
12
18
13
36
38
84
102
020649
020650
020651
020652
24
30
36
42
48
HP 020653
54
191
tiP 020654
tIP 020661
60
66
72
78
84
90
96
102
HP
HP
HP
HP
HP
108
114
120
126
132
208
226
244
262
280
297
315
333
351
369
HP 020648
tiP
tIP
tIP
tIP
HP 020655
tiP 020656
tIP 020657
HP 020658
NP 020659
NP 020660
020662
020663
020664
020665
020666
120
137
155
173
386
404
422
Table Il-lb.
*
Chemical Compositions
(XRF) for samples listed in Table Il-la.
(Na, Mg, Al, Si, P, S, K, Ca, Ti, Cr, Mn and Fe in wt %, Co, Ni
Cu, Zn, Rb, Sr and Ba in ppm).
SA,IPLE
MO
PIP 020579
PIP 020580
0. 6338
PIP 02051
0. 7265
0. 6647
0. 8061
tIP
PIP
PIP
PIP
PIP
PIP
020582
0205&3
0203A4
020583
020586
020587
0 3164
1. 0791
1. 2088
0. 9321
0. 8313
SAMPLE
YIP
PIP
PIP
PIP
PIP
020579
020500
020581
020382
020583
PIt' 020584
PIP 020585
tIP 020386
PIP 020587
5. 6720
52
23. 6570
5.7412
24.4907
0. 1639
0. 1608
5. 67H2
24. 8848
1.2604
0.1563
5.6168
0.2704
0,2614
23.1199
1. 1401
5. 894?
27. 0386
1.0914
1.0487
6.3634
0. 2387
0. 2640
30,0611
0. 1539
0. 1376
0. 1191
6. 4205
6. 97u?
7. 3000
30. 6804
30. 1194
29. 5080
1. 4910
1. 8314
Mn
CT.
0.
0.
0.
0.
0.
0.
0.
0.
0.
0085
0070
0081
0037
0031
0038
0039
0055
0082
AL
1. 43/5
1. 4201
1. 3498
3. 2624
1.9860
Fe
4.9932
1. 8247
1. 6308
5. 1423
4. 7079
4, 5455
1. 23'?5
4.2645
0. 8862
4. 1433
1.0263
3.8818
5.5316
6.8148
2. 1578
2. 6439
on a sal b-free I asis
Co
47
47
41
41
29
23
27
47
66
P
0.1180
0. 1446
0. 1392
Ni
530
393
401
333
240
134
212
399
538
5
0. 2703
0.2346
0.2328
0.2963
0. 3488
Cu
442
463
445
436
361
293
290
502
807
1.
1.
1.
1.
1.
2.
2.
2.
1.
K
2130
2809
4261
52t2
8232
1963
3457
8720
4694
26t5
3695
0397
6185
4310
7441
1956
8322
9279
Zn
450
450
365
332
260
223
219
322
417
Ti
Cia'
7.
6.
7.
6.
4.
1.
1.
0.
0.
0.
0.
0.
0.
0,
0.
0.
0.
0.
3270
3280
2038
2762
2603
2548
2453
3395
4123
Sr
57
39
55
62
70
95
106
78
48
662
639
849
644
574
470
442
496
541
8803
8351
8230
8088
8124
8009
7609
10498
12632
0
CD
0
4
00
0U0
0 '40
U 00
00 U
*
P4
0 U (A a
04
0
-0
RI
I)
*0 0 U
- 0 0 0 P40
-0
RI P4
Ri
0-
U
U
Ii
'4
Ri
RI
U
00
0 4 a U UU U
a-UO
a UU0-04*U*R00'IUt C'JW 00-OU
00WP4RI4'J
000.01.000
(An14000'4UaO0*uo-'4
O--000' 90400) 0-0-0-04400
- 0 0i 0
*
'44 a U 0 u u
*0CC
-0-iCR
UOPJ*RI0--lPU*4U0
0*0-14141440
0U14140-0D0UU**
U U
VP
P4 04
40 RI RI 4 0 0
__0,p009RIbURiU4000U00
RIRI0000000UO0 UUCP)U*0"
000
0
u
ORQ-40**4
0.404U4P404'0P44RI -4000.404
40U4400U00U4U0P4
0 a U
UU UUUUUU
UUU
U
000000000000000 00000000
(4
a
'
ej
- '4
,..
0 IU00
.uoUauo0.pJ41400
-
2
P4
01400
0-
-.4-
0140
000JOCUPJO4O 14U044 0
W0URIOPJUN44OUO .00'iP) -0PJ0
a
2
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Table Il-lb. . . continued
SAMPLE
tiP
SIP
lIP
NP
SIP
NP
NP
SIP
NP
NP
SIP
NP
lIP
SiP
NP
020620
020621
020622
020623
020624
020625
020626
020627
020628
020629
020630
020631
020632
020633
020634
020635
020636
020637
Ni
fig
AL
4263
0915
9910
1692
0330
0. 8999
I. 2805
6. 3032
6. 7063
1. 8071
1. 4860
1. 8740
6 8876
7.0799
0.7300
1.8252
7.3931
0.
0.
0.
0.
I.
1.
1.
I.
1.
7.
7.
7.
5.
5.
1.
1.
0.
1.
1.
5900
5916
7267
4754
0.4246
0.4173
0.4251
0.4946
0.4893
1.0188
7997
8029
9134
7678
7500
1.6621
1.7265
1.7883
1.9127
1.8820
7. 4240
5356
6468
4097
9828
7018
Si
30.
29.
29.
30.
28.
28.
29.
28.
29.
26.
25.
P
5031
8872
6918
1581
8355
7411
0866
4856
2281
2789
3585
0.
0.
0.
0.
0.
0.
0.
0.
0.
0.
0.
0982
1307
1509
1392
1629
1558
1600
1601
1696
1479
1408
5.5896
5.7010
5.9306
6.1843
24.5093
24.9172
0.1398
0.1401
25. 6061
0. 1483
27.0943
0.1541
27. 6527
27. 2906
28. 7170
0. 1544
0. 1427
0. 1573
0. 7453
0. 9468
1. 8821
1.9985
6. 5447
6. 4008
6. 9329
Cr
Mn
Fe
SIP
0. 0019
SIP
0.0052
0.0070
2. 5073
2. 8265
3. 0794
4. 5735
23
36
501
491
56
46
87
647
SIP
NP
SIP
SAMPLE
020620
020621
SIP 020622
lIP 020623
SIP 020624
tIP 020625
SIP 020626
NP 020627
tIP 020828
tIP 020829
SIP 020830
NP 020831
tIP 020832
SIP 020633
SIP 020634
SIP 020635
NP 020836
SIP 020837
Co
3.7562
5.6535
0. 0059
3. 0698
5. 0718
0.0080
0.0087
3.3456
2.4474
6.7981
6.8577
0. 0092
1. 6852
7. 0221
0.0090
0.0085
0.0077
0.0069
0.0056
2.0881
1.4828
0.6088
0.6283
0.7144
6.8812
8.9823
5.9536
5.7425
5.5986
0. 0057
0. 7480
5. 7313
0.0060
0.9179
5.9060
0. 0065
0. 9538
6. 2966
39
40
42
49
49
0.0069
1.1084
6.3807
51
0. 0054
0. 9029
6. 1253
0.0070
1.6115
6.6478
46
82
65
63
84
59
41
Ni
561
835
482
280
464
474
299
319
358
364
419
453
459
433
573
0.
0.
0.
0.
0.
Ca
K
S
1706
2492
3074
2784
3326
2. 5173
2. 0997
1. 6870
Ti
6789
7647
8514
7962
9131
0. 2091
0. 2908
0. 3450
1. 4129
0.
0.
0.
0,
0.
0.3430
1.4052
0.0946
0.4273
0.
0.
0.
0.
1.
1.
1.
1.
1.
0.
0.
1.
4.
6.
0.
0.
0.
0.
0.
3162
2833
2961
3043
0.3051
0.2918
0.3034
0.3115
0.3176
0.3137
0. 3122
0. 3362
1.9628
4242
4079
4390
3130
2712
8947
8971
3843
7237
0414
0.3258
0. 4167
4454
4540
4309
3417
3322
1.2274
1.2499
1.2810
1.3553
6.9469
6.0779
5.5410
3.8679
0.3189
0.3229
0.3321
0.2532
1. 3755
1, 3465
1. 4357
3. 7667
5. 0204
1. 6283
0. 3651
0. 3507
0. 3838
Cu
Zn
379
390
600
502
578
467
410
435
450
249
302
374
338
407
336
98
83
62
78
59
347
437
483
440
6503
9171
11040
9675
511
61
381
421
55
510
486
440
444
598
827
680
839
832
12024
11926
10962
331
378
402
478
555
585
645
620
669
476
470
484
491
488
498
543
489
468
479
Rb
62
82
57
58
53
63
62
63
67
65
63
Sr
571
803
823
545
9849
9991
10192
10014
9836
10023
10538
10873
11496
10626
12
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61
component.
The composition of this background detrital component
was assumed to be that of highest Ti/Al samples corrected for
dilution by calcium carbonate, opal and oxyhydroxides.
The contri-
bution of an element in a sample from the ash was calculated from
the amount of ash present and the chemical composition of the pure
ash.
Calcium carbonate and organic carbon abundances were determined
using the wet oxidation technique of Weliky et al. (1983).
The
method uses a LECO carbon analyzer to independantly measure calcium
carbonate and organic carbon from the CO2 evolved by successive
treatments with phosphoric and chromic acids.
Opal concentrations
were calculated from the XRF-measured Si and Al concentrations,
corrected for ash contribution where necessary, using equation (1).
Opal (%) - 2.47 (Si - 3.73 Al)
(1)
The value of 3.73 represents the ratio of non-opaline Si to Al, on
an ash-free basis, determined from Na2CO3 leaches of Site H sediment.
The factor of 2.47 converts opaline Si into percent opal.
We
assume that the Si to Al ratio of the detrital fraction at the site
has not changed with time.
62
TIMES GALE
Ash stratigraphy and downcore variations in the abundance of
calcium carbonate have been used to establish a timescale for the
Site H gravity cores.
The carbonate profiles (Figure 11-3) are
The
typical for equatorial Pacific sediments (Arrhenius, 1952).
profiles are similiar in all cores and indicate laterally uniform
sedimentation; there is no evidence for major erosional events,
hiati or large sedimentation rate changes.
Slight differences in
the carbonate-depth profiles (Figure 11-3) indicate that the sedimentation rate has varied by up to 20% between the cores studied.
Inspection of the elemental data (Table 11-1) reveals the
presence of two dispersed ash layers which were not detected during
visual descriptions of the cores in the upper three meters of sediment.
The presence of the ashes is apparent in profiles of the
Ti/Al ratio of the bulk sediment (Figure 11-4) as two spikes with
Ti/Al ratios much lower than the fairly constant background ratio of
the detrital sediment.
In this region of the Pacific, the Ti/Al
ratio is a sensitive indicator of the presence of dispersed ash,
because ashes in this region have low Ti/Al ratios relative to
normal inorganic detritus.
The areal distributions, chemical com-
positions and stratigraphic positions of ashes in the region
(Drexler et al., 1979; Bowles et al., 1973; Ledbetter, 1985; Lin and
Ledbetter, 1984) allow identification of the Site H ashes.
The
upper ash is the Los Chocoyos ash, also known as ash layer D.
lower ash is ash layer K (Table 11-2).
The
cARa0UATE ( 7. )
0
C-
_.
C?.
0
CARBONATE (
-
U.
.-
-
7. )
CARBONATE (
b_s
7.
)
-.
C
C
SH 0 84kyr
C
C
aCO3
-u
-4
l55kyr
x
aCO3 200kyr
o
. CC)
aCO3 265kyr
N
0
0
SH K 33Okyr
N
0UI
7OGC
_aCO3 400kyr
49GC
38GC
Figure 11-3.
Calcium carbonate abundances vs depth for the Site H
gravity cores. The age picks used for the time-scale are
indicated.
0'
64
Figure 11-4.
Variations over the past 400 kyr in the Ti/Al, calcium
carbonate, Al, organic carbon, opal, Mn and ash contents of the
Site H gravity cores.
Ti/Al v.s. Age
00;
38GC
49GC
0.07
---7OGC
0.06
-
,#
0.05
p
'I
0.03
0.02
50
100
150
200
250
300
350
400
AGE (kyr)
Figure 11-4.
U'
Calcium Carbonate v.s. Age
25
'I
'I
-38GC
A
A
I'
I
*
It
I$
$
I
\
I
700C
I
I
It
It
I
$
9-
$
I
S
I
$t
w
Iz
'
I
I
I
S
$
St
It
I'
I \
I
$
I
I
$
I
S
I
I
gb
*
SI
I
I
I
I
I
/1
1
0
I
/
I
/
S
I
t
I
g
IS
S
I
'S
'S
S
II
It
It.
I
I
'4
I S
S
Si
I
I
I!
/'I
ru
I
50
I
100
150
I
200
250
-
I
300
350
400
AGE (kyr)
Figure 11-4. . . continued
a'
0'
Al v.s. Age
8
7
I,
6
I.-
I
-J
<3
-38GC
4SGC
2
1
I
0,
iuu
iou
uo
AGE (kyr)
Figure 11-4.
continued
I
I
300
350
400
Organic Carbon v.s. Age
1.0
A
49GC
It
---7OGC
,I
%
s
t
z
S
'
t
t
I
.-'S
I
oO.6
/
I''
c
OV-S
S
:1
I,
II
0
/
/
/
'
0.4
'
I
\.,
I
z
-
S
I
'I
o O2
''.
I
4...
S
S
S
-
50
I
I
I
I
I
100
150
200
250
300
AGE (kyr)
Figure 11-4.. . continued
350
400
Opal v.s. Age
14
38GC
49GC
12
----7OGC
I
'S
'S
I
S
I
I
1
5
-J
S
I
I
cL
S
.5
II\,
0
S
S
g
g
I
S
S
S
S
S
S
I
I
I
2
S
S
S
1
%_#
I
S
'I
'I
100
150
200
AGE (kyr)
Figure 11-4. . . continued
250
300
I
350
400
Mn v.s. Age
8
*
7"
'I
--38GC
1*
I'
I,
6
---70Gc
''
I
I
63GC
I
,-..,5 p t
Il
'Ni
4
z
4
ft\
2
1
00
50
100
150
200
250
300
350
400
AGE (kyr)
Figure 11-4.. .COntlnued
0
Ash v.s. Age
IiI
38GC
49GC
1iI
----7OGC
I'
;40
I
SI
I'
:tt
Is
I'
It
SAl
30
I'
I
I
I
IIIt
'
li/I
I
I
S
ii
'Il
I
I
I
II
I
10
/
I'
I
*
I
I
II.''
I
I
I
I
S
''
'I
50
100
150
200
AGE (kyr)
Figure 11-4.. .cpntinued
250
I
300
350
(II,]
Table 11-2.
ELEMENT
Chemical Compositions of Ash Layers at Site 11 and those of Ash D
and Ash K in Other Cores.
Sample MP 20659 Los Chocoyos "D"
70CC 54-55cm
(1)
(2)
(3)
Sample MP 20660
49CC 215-217cm
ASH "K"
(2)
Na (%)
2.123
2.107
2.22
2.453
2.32
Mg
0.101
0.04
0.24
0.231
0.46
Al
5.800
6.43
6.76
6.519
6.79
I
Si
34.36
34.31
36.45
I
K
3.165
3.02
3.25
3.26
Ca
0.424
0.393
0.61
0.64
Fe
0.614
0.63
0.059
0.065
I
Mn
Ti (ppm)
Ni
774
879
0.51
0.79
0.065
1080
I
I
895
1
25
Zn
27
36.36
3.249
2.91
3.46
0.657
0.41
1.12
1.176
0.54
1.05
0.070
1428
21
Cu
35.44
(3)
0.064
500
1266
22
26
I
61
Rb
142
128
138
119
130
Sr
112
107
108
145
161
Ba
1039
1104
1123
976
1147
(1) Average composition from Drexler et al. (1980)
(2) Average composition from Ledbetter (1985)
(3) Average composition from Bowles et al. (1973)
-1
73
The age of ash layer D is well constrained by oxygen isotope
stratigraphy to be 84 kyr (Drexler et al., 1979).
The age of ash
layer K, identified by Ledbetter (1985) in DSDP 503, as estimated
using the new oxygen isotope-based timescale for the core developed
by Rea et al. (1986) is between 305 and 330 kyr.
The age of this
ash in DSDP 503 is somewhat uncertain because of the coarselysampled oxygen isotope profile.
In addition, the location of the
ash in 503B (where the timescale was developed) is estimated from
its depth in 503A.
At Site H, the age of ash K can be determined
from its depth in core 35P which has been dated to this level by
(Kadko and Heath, 1984).
At the measured sedimentation rate
of 0.66 cm/kyr, the 217 cm sub-bottom depth corresponds to an age of
330 kyr.
Because these estimates are substantially older than the
270 kyr estimate of Ledbetter (1985), we suggest that the age of
this ash be revised.
For the Site H timescale, we will use the 330
kyr age estimate.
The ash layers form the basic framework for the Site H timescale.
The carbonate profiles are used for additional age control by correlation to carbonate records in cores in the eastern equatorial
Pacific that have been dated by oxygen isotope stratigraphy (Murray
et al., 1986; N. Pisias, pers. comm., 1986).
The ash and carbonate
age horizons used in the time scale are shown in Figure 11-3
.
Ages
for samples are calculated by assuming constant sedimentation
between the horizons.
The derived timescale, shown as calcium car-
bonate vs age in Figure 11-4, indicates that the sedimentation rate
has been fairly constant at the site over the past 400 kyr.
Sedi-
74
mentation rates between the time intervals for a given core vary by
less than 20%.
The mean sedimentation rate for the gravity cores is
0.70 cm/kyr, in good agreement with the sedimentation rates deter-
mined by 230Th for the site (Kadko and Heath, 1984).
We estimate
that the relative error in ages between cores is less than 10 kyr
and that the error in absolute age is less than 30 kyr.
75
RESULTS
Variations in the abundance of the major sedimentary components
(calcium carbonate, opal, aluniinosilicates and Mn-Fe oxyhydroxides)
and organic carbon at Site H during the past 400 kyr are shown in
Calcium carbonate concentrations range from 0-25% and,
Figure 11-4.
as previously discussed, show the pattern of higher abundances
during glacial periods usually observed in Pacific cores recovered
from beneath the lysocline.
The profiles of aluminum, which is
characteristic of the aluminosilicate fraction, show minor variations of up to about 20% which are inversely related to the carbonate cycles.
Organic carbon abundances, which range from 0.2-0.9%,
follow similiar patterns in all cores.
periodic with a period of about 100 kyr.
The variations are somewhat
The peak organic carbon
concentrations are lower in sediments older than 150 kyr, but concentrations in the intervening minima are uniform over 400 kyr.
Opal abundances range from 0-12%.
The opal and organic carbon
records are correlable, but do not always correlate with calcium
carbonate.
High abundances of opal, organic carbon and carbonate
coincided during the last two glacial periods, but the organic car-
bon and opal records appear to be out of phase with carbonate prior
to that time.
The Fe/Al ratio in the sediments is fairly constant except in
zones with ash layers.
The lack of significant Fe/Al variations
suggests that Fe has not been diagenetically remobilized at Site H,
a conclusion consistent with porewater studies at the site (Klinkhammer, 1980).
Although some Fe is leachable by hydroxylamine
76
hydrochloride solutions and undoubtedly is in an oxyhydroxide phase,
most of it is fixed in the detrital fraction (Graybeal and Heath,
1984).
The absence of any correlation between the downcore abun-
dances of Fe and Mn indicates little, if any, diagenetic transport
of Fe during Mn remobilization.
The uniformity of downcore Fe/Al
and Ti/Al profiles indicates that the composition of the detrital
component, except for the ashes, has been fairly constant over the
past 400 kyr.
Manganese, which is the dominant element in the oxyhydroxide
fraction of the sediment, has several large downcore peaks in concentration which are not consistently related to maxima or minima in
the carbonate content profiles (Figure 11-4); some Mn peaks corre-
spond to carbonate minima, others to carbonate maxima.
In addition,
the apparent ages of the Mn peaks differ from core to core.
The
large near-surface peak results from on-going remobilization and
upward diffusion of Mn in the sediments (Lynn and Bonatti, 1965;
Klinkhammer, 1980).
Possible origins of the downcore Mn peaks are
discussed later in this paper.
The Ni and Cu concentration profiles (Figure 11-5) follow Mn
quite closely.
The distribution of these elements is strongly
influenced by Mn remobilization in the sediments (Klinkhainmer,
1980).
Thus, the origin of the downcore peaks of these elements is
likely controlled by the same process that causes the downcore Mn
peaks.
Zn concentrations (Figure 11-5), however, are fairly con-
stant downcore.
This is surprising because the near-surface Zn
distribution resembles that of Mn quite closely (Graybeal and Heath,
77
Ni. Cu. Zn v.s. Age - 49GC
1000
800
0
0
400
200
50
100
150
200
250
300
350
400
Mn v.s. Age - 49GC
8
7
6
'5
'.4
x
2
I
n
50
100
150
200
250
300
350
400
AGE (kyr)
Figure 11-5.
The inter-relations of Ni, Cu, Zn and Mn over the past
400 kyr in core 49CC.
78
1984).
Other factors apparently control the burial of Zn in these
sediments.
Although variations in the downcore profiles of Ni, Cu
and Zn reflect both diagenetic alteration and any variations in the
input of these elements to the site, the fairly constant mean down-
core abundances suggest that there have been no major long term
changes in the input of these elements to the site over the past 400
kyr.
The different Ni/Cu ratios in the downcore peaks could reflect
either differences in the composition of the source material or
variations in the processes which control the burial of these elements.
79
DISCUSSION
The hypothesis that the organic carbon record preserved in
deep-sea sediments holds the key to reconstructing paleo-redox conditions is reasonable because early redox diagenesis in surface
sediments is largely controlled by the breakdown of organic matter.
The oxidation of organic carbon in sediments occurs via a characteristic sequence of reactions (Froelich et al., 1979).
Labile
organic carbon that remains after all available oxygen is consumed
is further oxidized by the secondary oxidants NO3, Mn+4, Fe+3 and
SO4Th with sediments becoming more reducing as the successive
secondary oxidants are consumed.
The fraction of organic carbon
that survives in suboxic sediments is largely regulated by the
balance between carbon flux and oxidation by 02, NO; and Mn02
(Bender and Heggie, 1984).
Thus, changes with time in the burial of
organic carbon should record changes in the redox state at the time
at which the carbon was buried.
Factors controlling the redox condition
of surface sediments
In order to reconstruct past variations in redox conditions, we
must know the relation between the concentration of buried (surviving) organic carbon and redox state.
Furthermore, we need to
know how other changes which affect the burial of organic carbon
might also influence the redox system.
Several factors are important in controlling the amount of
organic carbon that is buried.
An increase in the flux of organic
80
carbon to the seafloor should result in an increase in the burial
content of carbon and more rapid consumption of 02 in sediments if
other factors remain fairly constant (Janke et al., 1982, Emerson et
al., 1985).
A decrease in bottom water 02 also should also result
in enhanced carbon preservation and a decrease in the depth to which
02 is present (Emerson et al., 1985).
A decrease in the degradation
rate constant of organic carbon or an increase in the bioturbation
rate in the sediments both result in greater carbon preservation and
a shallowing in the 02 zero depth in the model of Emerson et al.
(1985).
Model results (Emerson, 1985) indicate that changes of a
factor of two at rates in the range of sedimentation rates typical
of deep-sea sediments will not affect organic carbon preservation
greatly.
In all cases changes in the factors that enhance organic
carbon preservation will result in more reducing conditions in the
surface sediments.
At suboxic sites, such as Site H, the degree to which sediments
are reducing can be determined by the distributions of dissolved and
solid Mn in near-surface sediments.
In an idealized situation the
first downward increase in the concentration of Mn dissolved in
porewaters occurs at the depth where solid-phase Mn shows a peak in
concentration.
We refer to this depth as the "Mn redox boundary"
and the zone above as the "Mn oxidation zone".
The Mn redox
boundary represents the horizon where the downward diffusive flux of
02 is balanced by the upward diffusive flux of Mn-Il (Froelich et
al., 1979; Klinkhannner, 1980).
Both gradients, and hence fluxes,
are largely controlled by the oxidation of organic carbon in the
81
sediments (other sedimentary properties are uniform enough to have
The upward flux of Mn is con-
little impact on diffusion rates).
trolled by the rate of organic carbon oxidation in the Mn reduction
zone, which in turn is controlled by the flux of labile carbon into
and through this zone.
The flux of 02 is controlled by the bottom
water 02 concentration and the distribution and rate of 02 conFor a given bottom water 02
sumption in the Mn oxidation zone.
content, this flux is directly related to the rate of oxidation of
organic carbon in the sediments.
Thus the downward flux of oxidiz-
able organic carbon drives both reactions.
Data from the three MANOP sites (Sites H, M and C) where suboxic diagenesis has been studied, span a range of redox conditions.
The Mn redox boundary lies within 5 cm of the seafloor at Site M,
These differences allow
5-15 cm at Site H, and 15-20 cm at Site C.
us to disaggregate the factors that influence the redox conditions.
The organic carbon contents, depths to the Mn redox boundary
and other sedimentary characteristics for the three sites are listed
in Table 11-3.
The organic carbon content (C) of the surface sedi-
ments is strongly correlated (R2 - .96) with the depth to the redox
boundary (Z) at these locations (Figure 11-6).
The trend of Figure
11-6 is consistent with variations in the factors, discussed above,
which control organic carbon burial.
The best-fit line to the data
gives the following relation:
Z - 5.69 - 12.12 ln C
(2)
82
Table 11-3.
Organic Carbon and Manganese Contents and Characteristics of the Suboxic MANOP Sites (H, M and C)
Thickness of Mn
CORE
% Organic Carbon
oxidized zone (ciii)
Site H (633'N, 92°48'W, water depth = 3600 in, C-org flux = 89-95
40-160 cm2!
pg/cm2/yr S
.3 g/cm3, Db
.65 cm/kyr, D
kyr, bottom water 02 - 110 zm/1 (1)
INDOMED 33BC
1.00
5.0
VULCAN
37BC (2)
0.80
8.0
PLUTO
12BC (3)
0.75
10.0
0.60
15.0
INDOMED 27BC
Site M (8°48'N, 104°00'W, water depth
3100 in, C-org flux = 88-138
150-500
ig/cm2/yr 5
1.0 cm/kyr, D
.3 g/cm3, Db
cm2/kyr, bottom water 02 - 110 jnn/2 (1)
VULCAN 25BC (2)
1.40
1.25
PLUTO
1.30
1.5
1.20
3.5
VULCAN
25BC (3)
9BC
Site C (1°03'N, 138°58'W, water depth - 4400 in, C-org flux - 185
pg/cm2/yr S - 1.0-1.5 cm/kyr, D - .7 g/cm3, Db - 160
167 pm/2 (1)
cm2/kyr, bottom water 02
W8402A 14GC (4)
21.5
0.24
(1) C-org fluxes measured by sediment traps (Dymond and Lyle, 1985;
Walsh, 1985; Fischer et al., 1986; Dymond, personal communication 1986).
Sedimentation rates (S) from Kadko (1980), Kadko
and Heath (1984) and Murray et al. (1986).
Bottom water oxygen
concentrations from Emerson et al. (1985).
D
density.
dry bulk
Bioturbation rates (Db) based on Pb-210 from Cochran,
(1985).
(2) Lyle et al. (1984).
(3) Kalhorn and Emerson (1984).
(4) Murray et al. (1986).
83
% ORGANtC CARBON
.1
.2
.4
.6
.8
2.0
1.0
10
0
E
C,
I
I-
0
30
Figure 11-6.
The depth to the Mn redox boundary as a function of
the organic carbon content in surface sediments.
R2 - .96.
84
The plotted organic carbon content is the value at the depth where
the carbon concentration first reaches a constant concentration.
For these sites, this level which marks the virtual disappearance of
dissolved oxygen, lies within a few cm of the seafloor (Emerson et
al., 1985).
We assume that this organic carbon value is charac-
teristic of the current redox conditions.
Because of the relatively
rapid sedimentation at these sites, the carbon value should certainly be indicative of conditions for the most recent few thousand
years.
The residence time of organic carbon with respect to degradation at these sites is less than 150 years (Emerson et al., 1985).
Because the Mn redox boundary is within about 15 cm of the sediment-
water interface, the bioturbation rate is more important than the
sedimentation rate in controlling the downward flux of reactive
organic carbon (Crundinanis and Murray, 1982).
For this reason, the
redox state at these sites is not directly related to the burial
flux of organic carbon.
The relation of equation (2) may hold only
because the bioturbation rates (Cochran, 1985) and sedimentation
rates are similiar at the three sites.
The redox conditions at the three MANOP sites are not simply
related to surface water productivity or the organic carbon flux to
the seafloor measured by sediment traps.
Thus, organic carbon
preservation apparently is not related solely to organic carbon
flux.
Differences (Emerson et al., 1985; Emerson, 1985) could
result in part because of the different sediment types (carbonate,
heinipelagic with abundant ferromanganese nodules and metalliferous)
85
arid different benthic communities of microbes and animals living in
the different substrates.
Factors which enhance the burial of
organic carbon seem to have the same net effect of increasing reducing conditions in the sediments, however.
Consideration of an even
broader spectrum of sites does show that depositional conditions
become more strongly reducing as surface-water productivity
increases (Lynn and Bonatti, 1965; Lyle, 1983; Sawlan and Murray,
1983).
Paleo-redox chances at Site H
The depth to the Mn redox boundary at a given time can be hindcast from the measured organic carbon content using equation (2).
The reconstructed paleo-redox profile for the stacked organic carbon
record for the Site H gravity cores (Figure 11-4) is shown in Figure
11-7
.
The inferred depth to the Mn redox boundary has ranged from
6 to 25 cm during four major cycles over the past 400 kyr.
Generally, the depth to the Mn redox boundary has been shallower for
the most recent 180 kyr than it was prior to this time.
If the
degradation of organic carbon continues below the top few cm, then
the depths predicted by equation (2) are too large.
Because the
organic carbon values for the last 4 my at nearby DSDP Site 503 do
not show any long-term decrease, however (Gardner, 1983), it appears
that most organic carbon degradation does occur within the top few
cm.
Even if the absolute magnitude of the fluctuations is not pre-
cisely correct, the trend with time shown in Figure 11-7 should be
correct.
5
1::
mean
20
25
0
100
200
300
400
AGE (kyr)
Figure 11-7.
Variations in the depth to the Mn redox boundary at
Site H over the past 400 kyr.
87
Variations in near-surface redox conditions may explain the
absence of manganese nodules from certain areas within Site H.
These areas, called "bare patches" (Figure 11-8), are small depressions in which the sediments contain more organic carbon than the
surrounding nodule-covered areas (Figure 11-9; Walsh et al., 1981;
Murphy, 1984).
The organic carbon data indicate that these areas
should be more reducing than the surrounding areas.
Our redox
modelling predicts that the Mn redox boundary is, and generally has
been, significantly shallower in the bare patches than in typical
nodule-bearing areas (Figure 11-10).
Porewater measurements show
that the bare patches currently have a surface-oxidized layer, but
they may well have exported Mn in the recent past.
If the primary
Mn flux to the bare patches has been the same as the flux to the
nodule-covered areas, the lower observed Mn inventory in the bare
patch sediments (Walsh et el., 1981) supports the idea that such
areas have lost Mn.
We infer that conditions in the bare patches
are too reducing for the formation and preservation of ferromanganese nodules.
Downcore manganese variations
Downcore solid-phase manganese distributions (Figure 11-4) can
be used to test the hindcast redox variations (i.e., Figure 11-7).
In areas where Mn remobilization occurs, some cores show solid-phase
Mn distributions, such as multiple peaks, which are not consistent
with the steady-state distribution predicted from porewater Mn pro-
files (Froelich et al., 1979; Sawlan, 1982; Thompson et al., 1984).
These non-steady-state distributions could reflect recent changes in
I.
.
't
'.i
'-
.. .
,....
b%t1
'(,.,b
.
.. :..
.d ,
.. !
b
i:.'
.fT.1jI.
4j'i
:it.1:;
'.4?'
.
.
4..;
I
.'.
;'.'
,-
iii
. ;...
j;' .
r
P1.
.:
'p..
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.,1I
1'
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*t,.,
:'.
;y.
':;l
tfi4!4T. ;;; ..
.t
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:.*
r':-
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U0.
iS
=
t'#.a
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:;':
.
F
V
....,I
rLJ'
....
L
S.
% C-org
0.5
1.0
1.5
2.0
2.5
10
0
z
I-
0
w
30
50
Figure 11-9.
Organic carbon contents as a function of depth below
the seafloor for a nodule-free bare patch (33BC) and a
nodule-bearing area (37BC).
10
\
S..
0
10
20
30
40
50
AGE (kyr)
Figure 11-10.
The predicted variation in depth to the Mn redox
boundary for the two organic carbon records in Figure 11-9 over
the past 50 kyr.
0
91
redox conditions caused by changing oceanographic conditions (Froelich et al. 1979).
Because the distribution of solid-phase Mn
responds much more slowly to changes in redox conditions than does
porewater Mn, the solid-phase distribution has the potential to
record past changes in redox conditions (Froelich et al., 1979).
If the downcore Mn peaks result from changing redox conditions,
then they must be of diagenetic origin, as is the near-surface peak
discussed previously.
Several other hypotheses could, however,
explain the origin of the downcore peaks: (1) episodically increased
Mn deposition from primary sources, such as hydrothermal systems or
heniipelagic "leakage"; (2) increased abundance of authigenic, Mn-
rich material due to a reduction in the rate of supply of diluent
particles (Krishnaswami, 1976; Kadko, 1985); (3) trapping of Mn
below ash layers (Lynn and Bonatti, 1965); or (4) episodic formation
of authigenic Mn phases that resist reductive dissolution.
Increased input from primary sources is not a likely explanation because the peaks are not time-synchronous from core to core
(Figure 11-4).
In addition, Fe increases do not accompany the Mn
increases as would be expected for hydrothermal inputs.
The sedi-
ment record indicates that sedimentation has been very uniform;
there is no indication that the sedimentation rate decreased
markedly where Mn enrichments occur, as would be required if they
were of authigenic origin.
Ash layers are very dispersed in the
sediment and cause only minor porosity decreases.
In addition, most
of the downcore Mn peaks are not located near ash layers, so such
layers are not likely to be responsible for Mn peak formation.
The
92
authigenic Mn mineral most likely to be formed downcore in these
sediments is a Mn-carbonate (Pederson and Price, 1982), although the
degree to which the sediments are saturated with respect to this
phase is not known (Klinkhammer, 1980).
Because, many of the peaks
are found in sediments with essentially zero carbonate (Figure II-
4), however, it is unlikely that this phase occurs in any abundance
in the top several meters of the sediment at Site H.
The non-steady-state Mn distributions described by most workers
are relatively young features found near core tops and thought to be
unstable.
The following model outlines a mechanism by which such Mn
peaks can be trapped and preserved in the zone where Mn-Il is
present in porewaters.
A simple cartoon depicting the proposed mechanism is shown in
Figure lI-li.
The values used for the variables in Figures lI-ha
and b span the range of those presently observed at Site H.
At
steady-state, Mn peaks are not buried below the redox boundary,
regardless of the intensity of reducing conditions.
The burial flux
of Mn is equal to the input flux, and the concentration of buried Mn
is constant (Figure II-lla,b).
Thus, if the buried peaks originated
front Mn remobilization, they could be preserved only through nonsteady-state processes.
Two possible changes in redox conditions are shown in Figure
II-llc,d.
For simplicity, bioturbation is ignored.
In the first
case (C), the redox boundary shallows smoothly over a 20 kyr period.
The rate of change of the carbon content is an index of the rate of
change of the reducing conditions because, as previously discussed,
93
Steady State
Input Flux
Input Flux
%Mn
5
5
0
10
E
C-)
IIa-
w
20
I
ii
1
0
A% Organic Carbon
ure Il-Il.
trapping.
1
B
Cartoon showing the proposed mechanism of Mn peak
A and B are two steady-state examples at different
redox intensities.
C and D represent non-steady-state changes
from condition B to A and A to B, respectively.
The mechanism is
described in the text (solid lines - Mn, dashed lines - organic
carbon, sedimentation rate - 0.65 cm/kyr).
94
%Mn
B a-- A
A
0
-- B
5
kyr
E
a
I
I-
0
w
3
0
1
C
0
% Organic Carbon
Figure 11-il. . continued
1
D
95
the carbon content of the sediment controls the depth to the redox
boundary.
The organic carbon profile records the beginning of the
change at the level where the carbon content first increases, and
the profile continues to increase until a new steady-state condition
is reached.
As the Mn redox boundary migrates upward, a relict Mn
peak will be left behind if it cannot be fully remobilized.
Because
the peak is moving relative to the Mn reduction zone, at the accelerated rate of redox zone migration, Mn reduction may be less intense
than at steady-state when the redox zone migrates only at the sedimentation rate, and a higher concentration of Mn may survive and be
buried.
Once the Mn peak reaches the Mn equilibrium zone (the zone
where Mn-Il concentrations in porewaters reach a constant level),
further degradation can oniy occur if Mn-Il precipitation exactly
balances Mn-Ill reduction.
served.
Even in this case the peak will be pre-
The distance of the trapped peak below the break in organic
carbon will be equal to the thickness of the Mn oxidation zone
before the conditions changed.
In the other case (Figure Il-lId), conditions change such that
the redox boundary migrates to a deeper position relative to the
sediment-water interface.
Again, the sediments record the change,
in this case by a decrease in the organic carbon content, with the
slope of the carbon profile related to the rate at which the redox
conditions changed.
If the redox zone migrates downward at a rate
equal to or greater than the sedimentation rate, then the preexisting Mn peak will be left in the oxidized zone until the redox
conditions stabilize.
Then the peak will travel into the Mn reduc-
tion zone at the sedimentation rate, as it does at steady-state.
If
the reduction zone migrates at a rate slower than the sedimentation
rate, the peak will move through the Mn reduction zone slower than
at steady-state (at a rate given by the difference between the sedimentation rate and the redox migration rate).
It seems unlikely
that Mn peaks would be preserved in the long-term sedimentary record
by redox changes of this type.
This kind of change could, however,
explain non-steady-state peaks found near or above the present redox
boundary.
From the preceding discussion, it is clear that the survival
and burial of Mn peaks depends on how efficiently redox precesses
within the sediments are able to erode the peaks before they reach
the equilibrium zone.
The efficiency of remobilization of Mn is
related to the time it resides in the reduction zone, and the rate
of reduction in the zone.
Estimates of these variables can be used
to establish whether our mechanism for trapping Mn peaks is
feasible.
The time a peak resides in the Mn reduction zone is
related to its rate of advection relative to the zone.
Under non-
steady-state conditions, the rate is related to both the sedimen-
tation rate and the rate of migration of the redox front.
The mean
rate of redox migration (W) can be calculated between samples at
two different levels, zi and z2, with known organic carbon contents
Cl and C2 from the relation in equation (2)
The difference in
oxidation zone thickness (dD) between the two samples is
dD
12.12 (ln Cl - ln C2)
(3)
97
The time interval is easily calculated knowing the sedimentation
rate (S), and the equation for the redox migration rate becomes
(ln Cl - ln C2) 12.12
r
(4)
(z2 - zl)/S
For the hypothetical example in Figure Ilil Wr is 0.38 cm/kyr; a
significant fraction of the average sedimentation rate of 0.70
cm/kyr at the site.
Very little is known about the factors which govern the rate of
Mn reduction in deep-sea sediments.
If the reaction were purely
inorganic, the rate could be modelled as a first-order process proportional to the content or surface area of Mn in the reduction zone
(Burdige and Gieskes, 1983).
It seems more likely, however, that
the rate is influenced by microbially mediated reactions involving
the breakdown of organic carbon (Froelich et al., 1979; Emerson et
al., 1980; Klinkhammer, 1980).
In this case, the Mn reduction rate
could be modelled as a first-order process proportional to the flux
of labile carbon into the Mn reduction zone.
For the present dis-
cussion, we assume that the rate of Mn reduction under various redox
conditions can be estimated from the present day range in remobilization fluxes observed at the site.
The concentration of Mn that is buried under various conditions
can be estimated from a simple box model.
The burial flux (Fb
g/cm2 kyr) of Mn below the Mn reduction zone is equal to the flux of
Mn into the top of the reduction zone (F) less the flux of Mn
remobilized in the zone (F).
FbF.in -F r
where
F.
in
(5)
(S+W ).D.Mn /100
r
(6)
p
(Wr
the relative migra-
tion rate of the Mn redox
boundary relative to the
sediment-water interface,
is 0 at steady-state)
Fb = (S.D.M%)/l00
(7)
(relative to the sedimentwater interface)
Fr is estimated from solid and dissolved Mn profiles, D is the
dry bulk density (g/cm3), S is the sedimentation rate (cm/kyr) and
Mn
is the concentration (wt % of dry sediment) of the Mn peak in
the oxidation zone.
Solving for Mnb, the peak burial concentration of Mn, yields:
- 100 [((S+W).D.Mn/l0O) - FJ/S.D
(8)
The results (Table 11-4) indicate that the concentration of buried
Mn is enhanced above the steady-state concentration during the
period when the conditions rapidly change from less to more reducing, as in Figure Il-il, even though more Mn is reduced.
Enhanced
burial concentrations of Mn only occur during the non-steady-state
event.
For a period after returning to steady-state, the burial of
Mn may be less than before because of a reduced surface Mn reserThe burial concentration of Mn decreases as conditions become
less reducing.
Within the limitations of the assumptions, the
Table 11-4.
Burial Concentrations of Manganese Under Changing Redox
Conditions Calculated from the Box Model
"CONDITION"
Mn
(%)
Wr(cm/kyr)
Fr (gMn/cm2/kyr)
M% (%)
Calculations for the example in Figure lI-li
(at S - 0.65 cm/kyr and D - 0.30 g/cm3)
Steady-state "A"
7.0
0
0.0117
1.0
Steady-state "B"
5.0
0
0.0078
1.0
Change from "B"
to "A" (ex. "C")
5.0
+ 0.38
0.0078-0.0117
3.9-1.9
Change from "A"
to "B" (ex. "D")
7.0
- 0.38
0.0117-0.0078
0
Ranges in the variables measured at Site H
4.0-7.5
-0.63-+.63
0.003-0.008 (1)
0.4-2.5
0.006-0.012 (2)
(1) Calculated from pore water profiles (Rlinkhanimer, 1980; Bender
and Heggie,
1984)
(2) Calculated from the near surface solid phase Mn distribution using the
equation, Fr - (Mn - Mn.0).SD
100
mechanism of Mn peak trapping seems reasonable, and the observed
ranges of the depositional and diagenetic variables measured at Site
H indicate that the process has likely occured at this location.
The sedimentary organic carbon and Mn data allow us to check
the proposed mechanism of Mn peak trapping.
Qualitatively, the
mechanism predicts that a Mn peak will be trapped when the con-
ditions become more reducing, that is, when the organic carbon content of the sediments increases.
The redox boundary must migrate
upward at a rate comparable to the sedimentation rate, and the conditions must continue until the peak is transported through the Mn
reduction zone.
Quantitatively, the migration rate is estimated
from equation (4), and the peak is located a distance, given by
equation (2), below the change in organic carbon content.
The sedi-
mentary Mn profiles match the predicted trends quite closely (Figure
11-12).
Generally, Mn peaks of about the expected magnitude (Table
11-4), are located about 10-20 cm below the base of zones where the
organic carbon content increases abruptly.
Thus the downcore Mn
distribution is compatible with and supports the predicted changes
in reducing conditions at the site.
Factors controlling the organic carbon variations
Although the modelled redox variations are independent of the
mechanism responsible for the influx of organic carbon, it is worth
considering what processes might control the observed organic carbon
variations at site H.
The factors, some of which may be inter-
related, that could govern sedimentary organic carbon variations
include: (1) degradation; (2) sedimentation rate; (3) productivity
101
Figure 11-12.
Manganese variations and the rate of change of
organic carbon in the sediments (dC/dT, in wt % organic carbon
per kyr multiplied by 100) vs depth in the Site H gravity cores.
0
HHIIII1
0 IItII
0'
mwmuiii-'
105
of surface waters or organic carbon flux to the sea floor; (4) bioturbation; and (5) bottom-water oxygen content (Heath et al., 1977;
Miller and Suess, 1979; Aller and Mackin, 1984; Emerson et al.,
1985, Emerson, 1985).
The modelling of Muller and Suess (1979)
and Emerson (1985) suggest that the small variations in sedimentation rate observed at the site (less than a factor of two) cannot
account for the organic carbon variations.
Emerson (1985) showed
that variations of less than a factor of two in the bioturbation
rate presently observed at the site (Cochran, 1985) would not significantly effect the organic carbon content.
Thus the factors that
are most likely to be responsible for the organic carbon variations
at Site H are changes in the flux of organic carbon to the sea floor
and changes in bottom-water oxygen (Emerson, 1985).
The flux of
carbon to the sea floor has been shown to be related to productivity
in the surface waters (Suess, 1980; Muller and Suess 1979; Honjo,
1982; Deuser, 1986).
The model of Emerson (1985) suggests that the organic carbon
flux to the sea floor must have varied by about a factor of two,
with all other variables constant and simillar to those now found at
Site H, to account for the observed variations in carbon content.
Such productivity variations do not seem unreasonable over these
time-scales, but the carbon variations could also be explained by
variations of about 50 jiM/i (i.e., about 50%) in the oxygen content
of bottom waters at the site (Emerson, 1985).
The use of other
paleo-.productivity indicators can help determine if the variations
are due primarily to productivity or to bottom water oxygen changes.
106
Opal has been used as another index of productivity (Pisias, 1976;
Schrannn, 1983).
Its preservation in sediments is not directly
affected by bottom water oxygen.
The strong downcore correspondence
between opal and organic carbon (Figure 11.4) implies that surface
productivity has controlled the preservation of these components,
unless bottom waters at the site became depleted in oxygen and less
corrosive to opal at the same time, a possibility that modern bottom
water characteristics cannot rule out.
The present-day distri-
butions of opal and organic carbon in Pacific sediments (Johnson,
1976; Leinen et al., 1986; Prezumic et al., 1982) correlate well
with the surface water productivity, suggesting that productivity
controls the first order variations in these components.
In
addition, other studies have shown that productivity variations have
occurred in this region over the the past few glacial cycles
(Pederson, 1983; Aclelseck and Anderson, 1978; Rea et al., 1986).
The surface water circulation near the site (Wyrtki, 1965) is
complex and has strong yearly cycles.
The site lies near the
boundary of the North Equatorial Counter-Current (NECC) and the
Costa Rica Dome.
The waters of the eastward flowing NECC are rela-
tively nutrient depleted, so support low primary productivity.
In
contrast, the Costa Rica Dome is a site of seasonal upwelling, so
the surface waters support much higher productivity.
Consequently,
the integrated annual primary productivity at any location in the
region is very dependent on its location relative to the components
of the circulation system (Owen and Zeitzchel, 1970).
Climate-
107
induced shifts in the currents could, therefore, produce significant
variations in the annual primary productivity at a particular site.
The organic carbon to opal ratio does not decrease systematically with age, as would be expected if organic carbon degradation
continued below the surficial sediments.
108
CONCLUS IONS
The depth profiles of both organic carbon and manganese in the
sediments at MANOP Site H suggest that redox conditions have varied
during the last 400 kyr.
Data from Site H and other locations in
the equatorial Pacific with similiar (within a factor of two) sedi-
mentation rates, bioturbation rates and productivities, form the
basis for an empirical relation to predict the thickness of the zone
that is oxidizing with respect to Mn from downcore organic carbon
values.
The relation suggests that the thickness of this zone over
the past 400 kyr has varied from 5 to 25 cm in typical Site H cores.
The redox variations at the site have a period of roughly 100 kyr
over this time interval.
Such variations could markedly change the
early diagenetic cycling of redox-sensitive metals over these timescales.
In addition, such redox variations may control the accre-
tion of the fast-growing Mn-rich nodules at Site H.
In the extreme
case, in slight depressions known as bare patches, the accumulation
of more organic carbon than over the rest of the site appears to
have made the sediment too reducing for Mn nodules to form.
Changes in redox conditions can result in downcore variations
in solid phase Mn.
A simple mechanism by which solid phase Mn is
trapped and buried, as conditions rapidly become more reducing, is
compatible with the organic carbon and Mn records at Site H, and
supports the reconstructed variations in redox conditions.
The
observed variations in organic carbon content could have resulted
from fluctuations in the oxygen content of bottom water, or from
changes in the flux of organic carbon to the sea floor.
Concurrent
109
variations of the organic carbon and opal contents of sediment cores
suggest that productivity variations in the surface waters exerted
the major control of the redox variations.
110
CHAPTER III
COMMON GROWTH HISTORIES IN MANGANESE NODULES FROM MANOP SITE H
(EASTERN EQUATORIAL PACIFIC): CONTROL OF NODULE ACCRETION
BY SEDIMENTARY PROCESSES
111
ABSTRACT
Chemical, mineralogical and textural transects through five
manganese nodules from MANOP Site H (6°30 N, 92°40 W) show similiar
property variations with depth.
The variations in chemical data are
best explained by changes in the accretionary processes which form
the nodules.
Growth rate determinations indicate that chemical
variations can be correlated between the nodules.
The nodules show
that a major change in growth and composition occurred 160 kyr ago.
Since that time the nodules have been growing at average rates in
excess of 150 inni/my and have compositions highly enriched in Mn
(indicating accretion by suboxic diagenesis).
Processes associated
with oxic diagenesis were more important in controlling nodule
growth before that time, resulting in average growthrates of less
than 50 mm/my and compositions enriched in Fe, Co, Ni, Cu, Zn, Si
and Al.
Data from cores taken at Site H show that significant
variations in the intensity of Mn-reducing conditions in surface
sediments have occurred during the past 400 kyr.
The change in the
mode of accretion of the nodules is consistent, within the uncertainties of the growth rate and sedimentation rate estimates, with
an intensification of reducing conditions at about 180 kyr ago.
The size of a nodule may play a role in its growth history at
the site.
Small nodules, which do not extend as far into the sedi-
ment and are disturbed more often by benthic organisms, are influenced less by suboxic processes.
The mineralogy of the nodules is dominated by two 10 A phases.
An unstable phase which collapses to 7 A upon dehydration forms
112
initially under suboxic accretion conditions.
This phase stabilizes
with time and may diagenetically transform into todorokite.
A
stable todorokite forms under oxic diagenetic conditions, and the
crystallinity of this phase increases with age.
Aging is a closed
system process, at least for the elements studied.
The accretionary
mode may control the initial mineral form, but cannot account for
the observed diagenesis of the minerals.
The large flux of Mn and Ni into nodule bottoms during suboxic
accretion can be accounted for by fluxes from the sediment porewaters.
Porewater profiles show that these metals are extensively
reniobilized at relatively shallow depths and can diffuse upward.
fluxes to the nodules are consistent with supply from both nearsurface reactions arid deeper release associated with the reduction
of Mn.
The fluxes of Fe and Co to both the tops and bottoms of
these nodules have been fairly constant during the last 500 kyr.
Cu
113
INTRODUCTION
The manganese nodules at MANOP Site H (6°33'N, 92°49'W) occupy
an extreme position in the spectrum of ferromanganese concretions
found in the deep ocean with regard to chemical compositions, growth
rates and accretionary mechanisms.
The Mn/Fe ratio of Site H
nodules ranges up to 85, much greater than the typical values of 1-5
found in most deep-sea nodules.
They are also strongly depleted in
minor elements such as Cu, Co, Ti and Th relative to typical
nodules.
In addition, the average growth rates range from 20-200
imn/my, much faster than the growth rate range of 1-10 mm/my found in
most deep-sea nodules.
Only in the Peru Basin of the South Pacific
have comparable nodules been described (Reyss et al., 1982, 1985).
The extreme chemical compositions and growth rates of Site H
nodules are a consequence of the mechanisms by which they accrete.
In most areas where nodules form, the underlying sediments are
oxidized to great depths.
Measurements of dissolved Mn and Ni in
porewaters at these sites show that the diffusive flux from porewaters cannot supply these elements at the rate in which they are
incorporated into nodules (Klinkhaxnmer et al., 1982).
The com-
position of these nodules results from contributions primarily from
two accretionary mechanisms, hydrogenous supply from seawater and
oxic diagenesis in sediments (Dymonci et al., 1984).
the sediments at Site H are much more reducing.
In contrast,
Porewater studies
have shown extensive remobilization of Mn and Ni near the sedimentwater interface (Klinkhammer, 1980).
This remobilization fraction-
ates Mn from most other elements and results in a large flux of Mn
114
into surface sediments and nodules.
This process, which is termed
suboxic diagenesis, results in the Mn enrichments and fast growth
rates found in Site H nodules (Dymond et al., 1984).
Recent work on Site H nodules has shown that their chemical
compositions and growth rates can best be explained by non-steadystate accretion (Dymond et al., 1984; Finney et al., 1984).
These
studies concluded that nodule accretion at the site is largely controlled by diagenetic processes within the sediment.
Therefore,
variations in the flux of biogenic material or metal-rich detritus
across the site or with time which would change the supply of essential metals and the relative contributions of the accretionary pro-
cesses, could explain non-steady-state growth and variations in
nodule chemistry.
More specifically, non-steady-state growth was
largely attributed to variations in the contribution from suboxic
diagenesis, implying that accretion by this process is intermittent.
Because suboxic diagenesis is driven by the flux of organic carbon
into the sediments (Froelich et al., 1979), variations in the flux
of organic carbon to the seafloor could produce the observed variations in nodule accretion.
This report is part of a study designed to examine the processes which control the history of accretion and composition of
Site H nodules and the relation of these processes to the associated
sedimentary environment, through coordinated studies of nodule
growth histories and the history of sedimentation at the site
(Finney et al., l986a).
Because of the fast growth rates of these
nodules, they can easily be sampled on time scales fine enough to
115
allow comparison to changes in sedimentation at the site and to
variations in climate and oceanic circulation during the Quaternary.
The basic premise of this work is that nodules respond in a
consistent way to the environment in which they form.
Therefore,
variations in the chemical composition of nodules with time that are
of primary depositional origin should be related to variations in
the sedimentary environment and should be recorded in the sediment
column.
Singular events, such as the burial or turnover of a
nodule could, however, significantly perturb the expected profiles
of chemical variations.
116
SETTING
Site H (Figure Ill-i) is centered at 6°33'N, 92°49'W in the
Guatemala Basin where the water depth averages 3600 m.
The sedi-
ments are Mn-rich, siliceous, calcareous clays which accumulate at
average rates of 0.66 cm/kyr (Kadko and Heath, 1984).
Sediments at
the site are suboxjc; reduced Mn is found at about 10 cm depth, but
reduced Fe is not detected in porewaters (Klinkhammer, 1980).
Site IS currently at or below the CCD;
less than 1% CaCO3.
The
surface sediments contain
Downcore variations in the abundance of CaCO3
reflect late Cenozoic climate changes.
Productivity in the surface
waters is high and has a strong seasonal cycle (Fischer, 1983).
Manganese nodules are abundant over most of the site except for a
few scattered nodule-fee patches which range in size from about 100
in to 1 km.
cm.
Nodule diameters average about 4 cm but range up to 20
Spheres and flat "hamburgers" are the most coimnom shapes.
40°N
20
S
C
160° N
Figure 111-i.
140
120
100
Locations of the Manganese Nodule Project (MANOP)
Sites.
-4
118
SAMPLING AND METHODS
The manganese nodules used in this study were collected during
the Vulcan I cruise aboard the R/V Melville in September 1980.
The
nodules were hand selected from box cores (Figure 111-2) and the
orientation of each at the time of collection was carefully noted.
After being described briefly, the nodules were stored under moist,
dark and refrigerated conditions.
Nodules were cut in half, normal
to the plane of the sediment-water interface at the time of
recovery, using a slow speed bandsaw.
Layers were sampled in tran-
sects from bottom to top using a stainless steel spatula.
In most
cases, layers on the order of 1 mm could be easily pried from the
nodule but some layers were sampled by scraping.
The thicknesses of
the sampled layers were measured by calipers and checked by measurements of the change in nodule diameter.
For some samples, the
thicknesses were estimated from the weights of samples scraped from
measured surface areas.
Because the thickness of some layers varies
over short distances, the estimated error of individual layer thickness can be as much as 20%.
Such errors are not systematic, how-
ever, because the sums of layer thicknesses in complete transects
agree well with measured nodule diameters.
The samples were ground to a fine powder in an agate mortar and
pestle and oven dried at 110°C to constant weight.
Atomic absorb-
tion spectrometry (AA) was used to measure Mn, Fe, Ni, Cu, Zn, Si,
Al and Co using the procedures described in Robbins et al.
(1984).
The accuracy of these determinations, determined by analysis of
U.S.G.S. manganese nodule standards A-i and P-1, is within 5%.
U
119
SITE H
6°36
(
37
6°34'
r
66
3575
150
6°32'
351
6°30'
.
Figure 111-2.
.J '.J
I L.
?
'
-r
Bathynietry of Site H and locations of the cores
discussed in this report.
120
and Th isotope determinations basically follow the procedure of Ku
(1966).
Briefly, 0.1 to 0.3 g of sample is dissolved in a mixture
of HF and HNO3 to which a U-232, Th-228 spike has been added.
The
sample is passed through anion exchange columns to separate the U
and Th isotopes which are then extracted using TTA and plated on
stainless steel discs.
The U and Th isotopes are counted using a
silicon surface barrier detector alpha spectrometer with a multichannel analyzer.
X-ray diffraction (XRD) analysis were conducted
on splits of samples to which 10% by weight of internal standard
boehmite was added.
One hour time averaging runs on a Scintag PAD V
diffractometer were used to determine peak areas.
The ratios of
peak areas of minerals to an internal standard is used as a quantitative measure of the abundance of the manganese minerals.
Dupli-
cate analysis indicates a precision of 10-15% for such determinations.
121
RESULTS AND DISCUSSION
Physical and Textural Proierties
Cross-sectional views of the Site H nodules under reflected
light (Figure 111-3) reveal several interesting features.
The
nodules consist chiefly of two textural types; smooth, hard layers
and rough, less dense, friable layers.
The rough layers have Mn/Fe
ratios that range from 4 to 50 and always have lower Mn/Fe ratios
than the intervening smooth layers that have Mn/Fe ratios from 40 to
85.
Thus the smooth layers record accretion controlled by suboxic
diagenesis, whereas the rough layers represent more oxic diagenetic
control (Dymond et al, 1984).
The smooth layers are concentrated on
the bottom sides of the nodules, as expected from their chemical
compositions and inferred mode of accretion (Dymond et al., 1984).
These textural patterns are opposite to those reported for most
pelagic nodules (Raab, 1972; Halbach et al., 1975).
Most layers are continuous across the bottom of a nodule, but
are not fully concentric.
As the layers are traced toward the top
of a nodule they thin and curve upward, and eventually are truncated
by growth from the top.
The curvature of the layers shows that the
bottom of each nodule is much thicker than its top; the bottom
layers account for about 75% of the volume of these nodules.
This
proportion reinforces the conclusion that growth from below depends
on a sedimentary source for the key metals and is much faster than
growth from above.
Thickness variations are greatest for layers in
nodules with a highly lobate structure (nodule 52BC, Figure 111-3)
and show that the variations produce the lobes.
In all cases, the
U
I..
U
'1
4OBC
Figure 111-3.
48BC
5OBC
52BC
Photographs of sections through the nodules studied.
t'.)
123
core of a nodule is dominantly layered oxyhydroxide material which
appears to be conformable with the surrounding layers.
The pattern of textures through all four nodules is similiar.
Alternating smooth and rough layers make up the lowest 25 mm or so
of the nodule bottom but grade inward to rough layers which make up
the remainder of the nodule bottom and top.
textured intervals in the nodule bottoms.
There are three smooth
The outermost layer is
smooth, as are layers at 5-10 mm and 15-25 mm from the nodule
bottoms.
These layers have distinctive chemical compositions which
are discussed in the next section.
Chemical Conmosition
The analytical data for the individual layers within the
nodules (Table 111-1 and Figure 111-4) show the interelement correlations discussed by Dymond et al. (1984) and Finney et al.
(1984).
The sampling of smaller discrete units in this study, however,
allowed more extreme end member samples to be identified.
Overall,
the nodules show stronger Mn enrichments and lower minor element
contents than typical slowly growing pelagic nodules.
The abun-
dances of most elements can be explained largely by varying contributions from two accretionary processes, which Dymond et al.
(1984) have termed oxic and suboxic diagenesis.
Oxic diagenesis
includes several processes that enrich transition metals in components of sediments that are oxidized.
These include: reaction of
opal and Fe-Mn oxides to form nontronite and Mn oxyhydroxides (Heath
and Dymond, 1977; Lyle et al., 1977; Rein et al., 1979), transfer of
sorbed metals from sedimentary particles to nodules through
U- -
Chemical Composition (AA data) for Site H nodules.
Table 111-1.
Vulcan 4OBC
SAMPLE$
DEPTHmm
Mr/.
Fe)
4001
0.0-
1.3
42.3
063
40. 02
2. 8
40. 04
40. 05
40. 06
3. 1 4. 4 6. 7 -
43.
42.
43.
43.
40.
0. 84
40.03
1. 3 2. 8 -
40.07
40.08
40.
40.
40.
40.
40.
40.
40.
40.
40,
09
10
11
12
13
14
15
16
17
3. 1
4. 4
6. 7
8. 0
9.0 - 8.8
9.8 - 11.2
11. 2 - 12.0
12.
13.
15.
15.
19.
20.
21.
22,
0
8
4
6
4
6
0
9
-
13. 8
15. 4
15. 6
19.4
20.
21.
22.
24.
6
0
9
8
4
6
9
8
1
41.6
46.3
41.
44.
44.
43.
46.
45.
43.
45.
36.
9
5
2
1
6
6
4
0
7
1.00
0. 65
0. 57
1. 38
1,33
O.
1. 22
0.
0.
0.
0.
0.
0.
0.
3.
68
65
76
56
68
98
72
95
40.18
24.8 - 26.9
42.5
40. 19
40. 20
28. 7
30. 5
40. 1
1. 71
34. 6
32.8
38.8
40. 22
40. 23
40. 24
26.
28.
30.
32.
35.
36.
35. 1
36. 8
40. 3
40.25
40.26
9
7
5
8
1
9
-
1.11
CPPPI
37
36
30
26
19
21
21
44
30
29
27
33
27
27
29
48
177
56
69
34. 2
36. 0
38. 9
4.
2.
2.
2.
2.
84
88
98
51
76
252
138
90
40.3 - 44.3
44.3 - 30. I
35.4
31.0
5.06
6.04
40. 27
40. 28
0, I - 32. 7
32. 7 - 55. 2
36. 7
31. 7
3. 9â
6. 58
40.29
35.2
29.5
741
316
400
250
392
425
40.21
57.1
77
126
NiPPM
4500
2900
3000
3000
2200
2800
4800
3600
2900
3000
2200
2200
2000
2100
2800
2800
5700
4000
4700
8500
6700
7800
7000
8500
8100
8900
6300
9900
10540
CPPM
1100
800
500
700
900
1000
900
900
600
700
800
400
500
900
900
1400
2000
2300
3500
3800
4300
5100
4000
5100
6000
6700
6800
6400
5600
ZnPPM
1400
700
600
800
BOO
900
700
1200
900
1000
1000
700
800
1000
900
1000
1300
1600
1700
1800
2100
SiX
1.64
2. 13
A1%
0.81
1. 68
1. 86
3. 13
1.
1.
0.
1,
1.
2.61
1.24
0.65
2.39
2. 30
1.
1.
1.
1.
49
13
44
11
12
15
90
08
80
1.28
1. 16
0. 77
0.75
2. 98
0.
0.
0.
1.
0.
1.
1.41
0.85
2. 30
1. 50
1. 89
1.30
1. 83
1. 09
3.70
2.94
68
59
75
03
69
50
1. 61
2300
2400
2500
2100
3. 86
4. 19
2. 59
3.86
1.83
1800
1700
5.31
2.70
3. 25
4. 24
1. 70
2, 10
4.73
2.31
2000
2000
2. 96
2. 25
1. 83
I-.'
Table 111-1. . . corttinued
Vulcan 48BC
CoPPM
48.01
48.02
0.00.5 -
1.0
44.3
FeZ
0.59
0.57
48. 03
1. 8 -
2. 0
41. 4
1. 13
62
48.04
48.05
48.06
2.0 2.6 4.2 -
2.8
4.2
6.0
44.7
40.8
43.8
0.62
0.95
0.67
56
47
52
48. 07
6. 0 8. 1 -
9.
40. 2
46. 2
1. 30
36
49
25
36
36
30
35
DEPTHmm
SAHPLE
48.08
48.09
48.10
48. 11
48.
48.
48.
48.
48.
48.
12
13
14
15
16
17
9.8
11.0
12.4
13.6
14.8
14. 8 - 15.?
9.8
11.0
12.4
13.6
14.
15.
18.
19.
8
9
1
9
0. 56
43.6
42.0
42.4
43.8
0.68
15. 9
18. 1
43. 8
42. 5
45. 4
0. 71
0. 81
0. 66
19.9
44.4
0.70
20. 8
40. 4
0, 94
-
tln%
43.
110
1. 13
0. 74
35
44
42
25
25
27
32
39
48.18
20.8 - 21.3
40.7
1.07
48. 1?
48. 20
48. 21
21. 3 - 23. 9
23. 9 - 25.9
25. 9 - 27. 9
40. 7
35. 2
35. 1
0. 97
4. 13
3. 47
151
2.48
131
253
206
48.22
27.9-30.0
38.2
48. 23
30. 0 - 32. 2
33. 8
4. 81
48.24
32.2 - 35.3
35.8
3.23
153
NPP(1
4000
b000
5200
5100
4200
4600
5600
5800
3500
4200
4700
4200
3800
5300
3500
3200
3400
3900
3700
7000
8400
6500
8800
10200
CuPPM
1100
1100
800
1100
1100
1400
1400
1000
1100
1100
1000
1100
1000
1400
1000
1200
1400
1200
2100
2900
4900
5300
5900
7500
ZnPPM
2200
1400
1000
1400
1100
1400
1500
1800
1300
1300
1200
1300
1300
1500
1500
1500
1600
1400
1700
1600
2200
2300
2300
2300
3.70
1.60
AiX
0.72
0.75
2. 65
1. 26
1.81
0.92
1.43
3.65
2.45
1.21
1. 45
1. 69
0. 71
1.39
2.34
0.60
1.20
2. 14
1.01
1.62
I. 56
1. 73
1. 30
0.
0.
0.
0.
1.63
0.87
2. 04
1. 16
2.46
1.33
1. 72
1. 09
2. 17
2. 19
3. 65
4.33
4. 38
90
81
94
76
2.57
1.44
3. 81
2. 00
2.88
1.95
J1
UUUUUUUUUIIUI,U
JJUIUjUpj3
OOOOOOOø
<
H
PP-P9?pppppppp000000,
000O0OOOO.
r
J J M '
.- '-.-
U'
U
U0UOuo,.00
4 .
o
0 0
.
O
U PJ 0
P..)
IIIIItI(II$Il),r
IlIIuIlIIIIIiI
I
'-4 U 0
.-' U
U 0 U
*0
P4 00
uUU0-.oao'U*pJ-0
UU.***UUPJPJPJ---.U P4
UQsJ 0' *
U U 0- IU '100' U P4 U * 0'
UUUUUWUU
P.1 0 0 0- U P4 0 0
0 '0
W b .-.
0 U 0 U '-4 - 0 -' * - U UI P4
0
-
0-U *
-O
0 U U '4'-4 * P4 0
.
**
'-JUO.
0Q00r00rpp
I'
I
.D
.00- '
Pp999pop
0-
'4
0 .D '4 * P.1 . '4
- CI 0' 0' O P4 '1 U '-4 CI '0 CI
.0 * U 114 '4 .0 * 0 0' '4 U0 0' U ." 0' -0 U CI 0'
127
Figure 111-4.
Al and Si.
The strong negative correlations of Mn to Fe, Ni, Cu,
Box - Nodule 4OBC, triangle - Nodule 48BC, star Nodule 5OBC and cross - Nodule S2BC.
*
*
11*
0
t
çe(°
*
129
Mn v.s. Ni
48
A
46
£
£
DA
rn
44
0
0
42
ADA
a
;40
A
0 £AAAA
A
0
A
0A
0
A
'38
A
00
36
0
0
0
A
A
34
A
32
0
0
30
I
,.
'J'J'J
'*'Juu
b000
Nt (ppm)
Figure 111-4. . .continued
8000
0
10000
130
Mn v.s. Cu
48
46
44
42
A
W
A
A
;40
0
A
''38
C
0
0
36
0
A
A
34
A
32
0
0
30
I
I
4000
Cu (ppm)
11-4. . .con.tlnued
I
0
6000
131
Mn v.s. Al
48
46
AA
44
AIW
42
AA
;40
LA
0
A
38
00
A
0
0
34
A
32
0
0
30
I
1.0
I
I
1.5
2.0
AI(%)
[1-4.. .continued
0
2.5
3.0
132
Mn v.s. Si
48
A
46
t;
44
0A
0
0
A0
D0A
42
A
;40
A
A
00
0
A
A
A
C
00
36
A
0
0
0;
34
32
0
30
0
0
1
3
2
si(%)
Figure 111-4. . .coatinued
4
133
bioturbation (Lyle et al., 1984), and local micro-reduction in oxic
sediments (Kaihorn and Emerson, 1984).
Suboxic diagenesis is
thought to supply Mn to nodules through reduction of Mn-IV to Mn-Il
by oxidation of organic carbon in sediments and subsequent diffusion
of the dissolved reduced Mn to nodules (Froelich et al., 1977;
Klinkhammer, 1980).
Because the reduced Mn is oxidatively pre-
cipitated from solution soon after it reaches the oxidized zone,
this process can only be important where sediments become reducing
just below the growing nodules.
Because of the fast growth rates of
Site H nodules (Huh and Ku, 1984; Finney et al., 1984), the slow
supply of hydrogenous material directly from seawater is not an
important source of most elements to these nodules.
Because suboxic diagenesis enriches nodules only in Mn, U, Na
and Sb relative to oxic diagenesis, variable mixtures of oxic and
suboxic components yield negative correlations between Mn and most
other elements such as Cu and Ni (Figure 111-4).
In contrast, mix-
tures of oxic and hydrogenous components result in the positive
correlations between Mn and Cu and Ni observed in typical pelagic
Chemical variations with depth
The cross-sectional compositional profiles of the nodules
studied are similar.
The gross pattern is characterized by very
high Mn/Fe ratios (30-85) over the outermost 21-29 mm of the nodule
bottoms, followed by a sharp decrease to relatively low Mn/Fe ratios
(4-15) with smaller overall compositional variations through the
remainder of the bottom, core and top (Figure 111-5).
Mn/Fe ratios
Bottom
+
8C
4'
5OBC
8(
52BC
41
Mn/Fe
8
4
48BC
8
4OBC
4
0
20
40
60
Distance above nodule bottom (mm)
Figure 111-5.
(n/Fe ratio vs distance above nodule bottom for the
four Site FL nodules studied.
135
vary significantly within the high Mn/Fe zone.
These fine scale
variations appear similiar from nodule to nodule.
Four intervals of
Mn/Fe enrichment, separated by significantly lower Mn/Fe ratios, can
be identified in the bottom zone (Figure 111-5).
Sharp Mn/Fe maxima
are located within the bottom 1-2 mm and at depths of about 5 and 10
mm.
A broad region with high Mn/Fe ratios is centered at 15-20 nun
from the bottom.
The only major deviation from this pattern is in
nodule Vulcan 5OBC which contains only three maxima.
The distributions of the other elements follow the Mn/Fe variations closely. The Mn/Fe rich intervals are depleted in Fe, Cu, Ni,
Zn, Si, Al and Co relative to the rest of the nodules (Figure III6).
Generally, these elements are least abundant in the zones with
the highest Mn/Fe ratios.
Uranium Series Distribution and Growth Rates
U and Th isotopes were measured in the bottom layers of two
nodules to constrain their growth rates and determine whether the
chemical variations are time syncronous.
Previous U-series work on
Site H nodules (Huh and Ku, 1984, Finney et al., 1984) has shown
unusual nuclide behavior due to suboxic accretion and fast growth.
U is one of the few measured elements whose abundance is positively
correlated with Mn.
lated with Mn.
In contrast, Th is strongly negatively corre-
The suboxic Mn-U rich component grows at rates much
faster than the oxic component which is relatively enriched in Th
(Finney et al., 1984).
Thus the Th isotope variations, which are
used to determine growth rates, reflect not only radioactive growth
and decay but also the relative abundances of the U- and Th-bearing
136
-9
Mn/Cu 6
3
2
Mn/Ni1
'J.
8
Mn/Fe 4
0
Figure 111-6.
Nodule 4OBC.
20
Distance above nodule bottom
40
Mn/Ni, Mn/Fe and Mn/Cu vs distance from the
bottom of
137
components.
Therefore, the raw U arid Th data must be modeled in
conjunction with the stable-element abundances in order to assess
the rates of input from the two sources.
The U-series data (Table 111-2) show the same general pattern
of isotope abundance with depth for both nodules.
To convert these
data to growth rates, we used the model of Finriey et al. (1984)
which was designed for such nodules.
This model can treat varia-
tions in U and Th abundances due to both chemical compositions and
radioactive growth and decay.
Samples for U-series determinations
were picked from the two nodules on the basis of similiar chemical
compositions and "stratigraphic" positions.
Because the samples are
very similiar chemically (the concentrations of most elements vary
by less than 10% betweem samples, see Table 111-1), a correction for
variable dilution of Th is not required.
This simplifies inter-
pretation of the raw data and eliminates one source of error in
correcting the U-series data.
Because of the fast growth rates
expected for these nodules, the determination of 230The
dent on the age of the sample.
ex
230Th
sup
- 230Th
- 234U
is depen-
This can be expressed as:
tot
(x)
-
23°Th
sup
(1 - exp (-Xx/g))
(1)
(2)
where x is depth in the nodule, g is the growth rate, and A is the
decay constant of 230Th.
Assuming that the U and Th isotopes are
Immobile and the ratio of initial 230Th to its supplying phase is
138
Table
111-2.
Sample
U-series Data for Site H Nodules
depth
(mm)
U A.R.
234U
23°Th
(dpm/g)
(dpni/g)
Nodule Vulcan 40 BC
Bi #1
40 Bi #2
40
0-1.30
mean
B5 #1
40 B5 #2
40
4.4-6.6
mean *
40 B8 #1
40 B8 #3
mean *
40 B13 #1
12.0-13.75
15.6-19.4
40 B13 #2
mean *
40 B16
.86
.33
.40
.51
.34
.81
1.22±.09
1.16±.04
1.18±.12
1.26±.07
8.75-11.2
40 B8 #2
40 BlO
16.46±
15.90±
15.98±
7.69±
8.84±
8.35±
21.0-22.9
8.90±.52
8.10±.20
8.22±.56
7.25±.25
8.40±.82
5.99± .17
6.03± .16
6.01± .03
3.08± .11
7.50± .80
8.29± .66
7.90± .56
7.65± .30
1.26±.07
1.37±.23
.99±.05
7.54±.18
7.65±.61
5.61±.20
7.69±.19
7.08±.22
7.38±.43
7.64±.32
13.89±1.32
6.15± .12
7.29± .57
6.20±1.0
1.69±.26
1.13±.03
1.15±.13
1.16±.30
7.61±.19
6.18±.22
6.79±.34
6.96±.20
1.16±.05
1.22±.05
l.17±.14
Nodule Vulcan 48 BC
48 B2
48 B6
48 B8
48 B15
0.5-1.8
4.2-6.0
8.1-9.8
15.9-18.0
1 S.D. counting error unless noted
* error based on duplicate analysis
139
constant
'with time, the growth rate can be determined iteratively by
obtaining the best fit to the data (Reyss et al., 1982; Huh and Ku,
1984).
To provide an initial estimate for g in the above model, the
range of possible values of 230Thex was calculated for each sample
and plotted vs depth.
sup
The maximum
230
Th
values are obtained when
0 (i.e., the measured value) and the minimum values are
obtained when 230Th
(i.e.,
sup
23O..
ex
-
230Th
23411).
The
tot
lines with the minimum and maximum slopes that encompass the ranges
of all the data points then define the limits of possible growth
rates.
The range in possible growth rates is similiar for both
nodules (100-280 mm/my for 4OBC and 100-220 mm/my for 48BC) and is
consistent with previous results for nodules with similiar chemical
compositions (Finney et al., 1984; Reyss et al., 1982, 1985).
The best-fit growth rates using the model are 168±25 mm/my and
150±28 mm/my for the bottoms of 4OBC and 48BC, respectively (Figure
111-7).
These values assume constant growth rates, even though
previous studies and the variations in composition over the intervals dated indicate that such constancy is unlikely.
The sampling
scheme does not support growth rate determinations on a finer scale,
however.
Crowth rates for intervals without radiometric data and with
Mn/Fe ratios less than 15 were estimated using equation 3 (Lyle,
1982).
G
-
16 (Mn/Fe2)
.448
(3)
140
Figure 111-7.
230
Th
vs depth into the nodule bottom.
cx
used is described in the text.
is 168±25 mm/my.
mm/my.
The method
(a) Nodule 48BC (the growth rate
(b) Nodule 48ZC (the growth rate is 150±28
6
6
0
C
K
K
0
4
0
C,)
C,"
I-
I
2
a
2
I
I
10
DEPTH (mm)
Figure 111.7
20
U
10
20
DEPTH (mm)
I-,
142
This relation, which predicts growth rates from the Mn and Fe contents, has been shown to yield results that are consistent with
U-series determinations for growth rates up to about 50 mm/my,
including rates for Site H nodules (Finney et al., 1984; and, using
a similiar equation, Huh and Ku, 1984).
The predicted growth rates
for most of the layers where the model was applied to the Site H
nodules is less than 50 mm/my, suggesting that the estimates are
reasonable.
The sharp break in the Mn/Fe ratio in the bottom of nodules
5OBC and 52BC was correlated to the equivalent break in the Mn/Fe
profiles of the radiometrically dated bottoms of 4OBC and 48BC
(Figure 111-5) to estimate the average growth rates of the high
Mn/Fe zones of the bottoms of the undated nodules.
Fluxes of elements into nodules
The growth rate and chemical data allow us to calculate fluxes
of elements into the nodules.
Fluxes of Mn, Fe, Co, Cu, Ni, Si and
into the tops and bottoms at various time intervals are shown
in Table 111-3.
The fluxes are consistent with the proposed modes
of accretion and the pore water fluxes of elements at the site
(Klinkhanuner, 1980).
The fluxes of Ni and Mn to nodule bottoms
during the last 160 kyr, have been much higher than at other times,
consistent with accretion controlled by suboxic diagenesis.
The
fluxes calculated from the present day porewater gradients suggest
that these elements follow each other closely and could be supplied
from this source (Klinkhammer, 1980).
The present day porewater
143
Table 111-3.
Source
Fluxes of Mn, Fe, Ni, Cu, Co, Si and 230Th
in
ex
nodules and sediments at Site H
Location
Time
Interval
Mn
Fe
Ni
Cu
Co
Si
230
Th
(kyr)
Nodules
4OBC
Bottom
0-160
10,0
.20
6.7
1.8
7.8
.42
.18
48BC
Bottom
0-160
8.7
.16
8.9
2.4
7.8
.41
.14
5OBC
Bottom
0-160
8.8
.17
52BC
0-160
8,7
.18
37BC
Bottom
Bottoma
12.1
.20
13.4
3.1
16.2
.44
.27
4OBC
Bottom
160-545
2.0
.19
4.3
2.6
9.3
.20
48BC
Bottom
160-430
2.0
.21
4.7
3.0
10.2
.21
4OBC
Top
0-200
0.3
.09
1.3
0.7
5.2
.06
5OBC
Top
0-140
1.0
.14
37BC
TOPa
0-250
2.7
.23
5.8
3.6
10.0
.28
.30
Site H Pore water datab
3-8
0
Guatemala Basin hemipelagic
sediment porewater fluxesc
2-33
0
Site H ave. Burial Fluxd
3.5
60
9
0-40
Sediments
8
11
10-57 17-46
12
12
12
10
Mn, Fe and Si in mg/cm2/kyr, Ni and Cu x 100, Co x 10000
230
Th in dpm/cm2/kyr (9.5 is the theoretical production flux of
.
.
from the overlying water at Site H)
aF
et al. (1984)
bKlinkhanlmer (1980), Bender and Heggie (1984)
Sawlan and Murray (1983)
dFi
et al. (1986a), Kadko and Heath (1984)
144
fluxes are on the same order as the suboxic fluxes to nodules, and
they may have been greater in the recent past when reducing con
ditions were more intense (Finney et al., 1986a).
The flux calculations for Cu show that the fluxes to nodule
tops are comparable with those to the suboxic bottoms.
This is
consistent with the porewater profiles of dissolved Cu, which differ
from those of Mn and Ni.
The flux of Cu is not only related to the
reduction of Mn at depth, but Cu is also released in near surface
sediments, presumably from reactions involving the breakdown of
organic matter (Klinkhammer, 1980; Fischer et al., 1986).
The near-
surface flux is sufficient to account for Cu fluxes to the nodule
tops.
The remobilization of Cu at depth in the sediments is much
weaker than the remobilization of Ni.
This difference results in
the fractionation of Ni from Cu in nodule bottoms.
Ni to Cu ratios
in suboxic samples range from 2 to 8, whereas this ratio in oxic
samples ranges from 1 to 2 (Dymond et al., 1984).
In contrast, Fe and Co fluxes vary little between tops and
bottoms or between layers of different ages.
The flux data suggest
that Fe, Co and 230Th are not supplied through suboxic diagenesis.
This is consistent with the porewater measurements which show no
detectable Fe in solution at Site H (Klinkhammer, 1980).
Because Fe
is fractionated from Mn through both oxic and suboxic processes and
because Fe and Co are strongly correlated, both may be supplied to
nodules at a constant rate (Halbach et al., 1983) by hydrogenous
processes.
Our data suggest that this source has supplied these
145
elements to both tops and bottoms at a fairly constant rate over the
past 500 kyr.
Mineralogy
Characteristics and identification of minerals
Previous X-ray diffraction (XRD) studies of the mineralogy of
Site H nodules (Dymond et al., 1984) have shown the dominance of two
separate 10 A phases with different properties.
One form is more
abundant in the relatively Ni- and Cu-rich nodule tops where oxic
diagenesis is the dominant accretionary mechanism.
This is a stable
10 A phase which does not collapse from 10 A to 7 A upon
dehydration.
The other phase, found in the Mn-rich nodule bottoms
that accreted under conditions of suboxic diagenesis, is unstable
when dried.
Its main basal spacing collapses from 10 A to 7 A
when samples are heated and dried in the laboratory (Figure 111-8).
The amount of crystalline material in both phases, judged by relative peak areas, is greater than that of the more Ni- and Cu-rich
todorokites of the equatorial Pacific (Piper et al., 1984; Dyniond et
al., 1984).
The variations in chemical composition found in Site H nodules
support the XRD findings, and also indicate that the nodules are
comprised primarily of two end member phases.
The abundance of most
elements measured in the nodules is well explained by the mixing of
oxic and suboxic end members using the chemical partitioning linear
programming model of Dymond et al. (1984).
Delta Mn02 is undetect-
able by XRD in the Site H nodule samples, as predicted by the
146
'C
cPs
Be
90
81
86
72
70
63
60
54
50
45
40
36
C)
z
0
C)
w
38
27
20
18.
18
9.
C,)
B
w
aC')
'C
cps
00
iee.
96
98.
F-
z
0
0
80
80.
76
70.
66
60.
58
50.
40
40.
30
30.
20
28.
10
l0.
0
ire 111-8.
X-ray diffractograms for the outermost bottom sample
)f Nodule 4OBC (sample 40.1).
The upper diagram is for the
;ample before dehydration, and the lower diagram shows the effect
)f drying the sample at 105°C.
147
minimal amount of hydrogenous component estimated by the linear
programming model, even in the samples with the lowest Mn/Fe ratios.
If other phases are important in controlling the chemical
composition on millimeter and larger scales, their compositions must
either be the same as or intermediate between end members, or covary
uniformly with another phase such that they can be modelled as part
of a single end member.
The XRJJ data indicate that if such phases
exist their abundances are very low, they are X-ray amorphous, or
they have diffraction patterns very similiar to those of the end
member phases.
The identification of the dominant 10 A mineral present in
the Site H nodule samples with chemical compositions representative
of accretion under oxic conditions is aided by XRD, chemical and
high-resolution transmission electron microscopy (HRTEM) studies.
The non-collapsing behavior and XRD patterns are consistent with the
properties of todorokite.
The chemical composition of these samples
is also consistent with that of todorokite.
The molar ratio of I'm
to charge balancing cations (expressed in +2 equivalents) in the
most oxic samples at H is about 3.
This is close to the ratio of
Mn-IV to charge balancing cations found in todorokites (Burns and
Burns, 1977; Golden et al., 1986), and may indicate the predominance
of 3 x 3 tunnel structures.
HRTEM studies by Turner and Buseck
(1982) revealed todorokite-like tunnel structures in Site H nodules.
The chemical composition in regions where these structures are found
(Turner and Buseck, 1982) is compatible with that of the oxic component.
As this evidence indicates that the mineral characteristic
148
of the oxic end member belongs to the todorokite family, we will
refer to it as todorokite in the following discussion.
In contrast, the XRD studies of samples from the outer few mm
of nodule bottoms that are dominated by suboxic deposition indicate
that todorokite is not present.
The basal spacing in these samples
collapses totally to 7 A upon dehydration; a behavior not con-
sistent with the fixed tunnel structure of todorokite (Arrhenius and
Tsai, 1981; Siegel and Turner, 1983; Siegel, 1981).
Therefore, a
layer or sheet mineral phase without a fixed tunnel structure seems
to dominate these samples.
HRTEM studies (Turner and Buseck, 1982)
have shown that a layer mineral is present in Site H nodules.
Regions where this material is dominant are depleted in most ele-
ments except Mn and Ni (Turner and Buseck, 1982), consistent with
the chemical composition of our suboxic-rich samples with the collapsing 10 A phase.
For purposes of discussion, we will refer to
this mineral as "collapsing 10 A phase".
To better understand the mineralogy of the collapsing 10 A
phase, stability tests were conducted on end member samples.
The
relative peak area centered at about 7 A does not vary with the
length of time the sample is heated over periods of several days,
once it has been dried to constant weight at 100-110°C.
The
reaction leading to the collapse of the structure is not reversible
by exposing samples to air or water for periods of several weeks.
Thus the relative area of the 7 A peak in heat-dried samples is a
true measure of the degree of instability of the mineral structure.
The sharpness of the 10 A peak increases with increasing abundance
149
of todorokite and decreasing abundance of collapsing 10 A phase,
which is consistent with the indication of mineral stability based
on the 7 A peak area in dried samples.
The 10 A peak position
does shift slightly upon dehydration, even for samples dominated by
todorokite.
lJndried refrigerated samples have sharp peaks at about
9.8 A (see Figure 111-8), whereas dried samples show a peak shift
to 9.6-9.4 A.
Mineralogical variability within nodules
Mineralogical profiles for nodules 4OBC and 48BC reveal that
the area of the 7 A reflection decreases inward from both top and
bottom surfaces (Figure 111-9).
The trend is more obvious in the
nodule bottoms because they have more intense initial 7 A reflections.
The oldest samples in both nodules (40.26 and 48.24) have
essentially no collapsing 10 A phase as defined by our method.
The nodule bottoms, which lack initial todorokite, show an exponential decrease in the abundance of the 7 A reflection toward the
interior even though the outer 25 mm or so is compositionally fairly
uniform and indicative of dominance by suboxic diagenesis (Figure
111-10, Table 111-4).
The area of the 10 A reflection increases
from both top and bottom surfaces towards the core.
The outermost
top layer has about half the amount found in the core.
Thus the
decreases with depth into the nodule of the unstable 10 A phase
are mirrored by increases in the area of the todorokite 10 A peak
(Figure 111-10).
150
Figure 111-9.
A representative suite of diffractograms illustrating
the mineralogical profile through Nodule 4OBC.
Samples 1 through
16 are for the nodule bottom, sample 26 is the core, and sample
29 is the top.
boebmite.
The peak at about 6 A is the internal standard,
151
71
C')
aC)
Figure 111-9.
152
(I)
aC-)
Figure 111-9. . . continued
153
A
10
4
I..
'..
I
£
0
0
m
I-
4--0
'IA
26
/
29
154
Table 111-4.
Mineralogical and Chemical Data for Site H Nodules
Peak Area Ratios
Sample 1OA/Boehmite
7A/Boehmite
1OA/7A
Mn/Fe
Mn/(Ni+Cu)
Nodule 4OBC
1 (Bottom)
5
0
7.4
0
67
75
.71
4.0
.20
76
142
.62
84
102
8
1.6
2.6
13
1.9
1.4
1.3
84
182
16
2.2
1.2
1.8
62
106
18
2.0
1.2
1.7
38
68
21
1.9
1.4
1.4
14
36
24
1.4
.75
1.9
14
29
26 (core)
2.3
0
5.1
20
28
1.8
.24
7.3
4.8
20
29 (top)
1.1
.31
3.6
4.0
18
Nodule 48BC
2 (Bottom)
0
9.3
0
78
62
6
.17
7.0
.02
66
74
8
.62
3.9
.16
83
68
15
1.3
1.6
.82
69
102
19
1.6
1.6
.95
47
70
20
1.3
1.1
1.2
8.5
35
24
2.9
10.7
11
20
.27
155
The changes in the relative areas of the 7 A and 10 A peaks
seem to be a clear indicator of diagenetic inineralogic changes
within Site H nodules.
These mineralogical changes occur over
intervals of very similiar chemical composition in both nodule tops
and bottoms (Table 111-4).
Thus variations in the mode of accretion
or post-depositional diffusion of elements cannot explain the changing mineralogy.
Samples 40.29 (the top), 28 and 26 (the core) have
almost identical chemical compositions (Tables Ill-i, 111-4) with
very little indication of suboxic influence.
The increase in the
abundance of the 10 A peak toward the core (Figure 111-10)
reflects a significant increase in the crystallinity of todorokite
over about 500 kyr in these nodules.
Improved crystallinity of
todorokite inside nodules has been reported for other locations
(Piper and Williamson, 1981; Siegel and Turner, 1983; Sung et al.,
1975) suggesting that this aging process may be widespread.
Samples from the outer 20-25 mm in both nodules have similiar
compositions indicative of dominance by the suboxic component
(Tables Ill-i, 111-4).
The exponential decrease with time in abun-
dance of the collapsing 10 A phase in this zone is shown in Figure
111-11.
If the amount of collapsing phase is reliably estimated by
our heating method, then this phase has a half-life of about 45 kyr
for the first 120-150 kyr after accretion, at least in suboxic-rich
samples.
The rate of decrease in abundance of this phase appears to
decrease after this initial period.
The collapsing 10 A phase
completely stabilizes in about 500 kyr within the Site H nodules.
156
IC
r
E
C,
0
0<
2
0
60
120
180
AGE (kyr)
Figure Ill-il.
The exponential decrease with time in the ratio of
the 7 A peak to boebniite in nodule bottoms 4OBC and 48BC.
decrease suggests that the diagenetic transformation has a
half-life of about 45 kyr.
The
157
Collapsing 10 A phases, with decreasing abundances towards the
interior of nodules, have been reported in other locations (Siegel
and Turner, 1983; Piper and Williamson, 1981; Sorem and Fewkes,
1979) indicating that the stabilization can occur in a variety of
seafloor environments.
The initial ratio of todorokite to collapsing 10 A phase
appears to be controlled by chemical composition and, thus, mode of
accretion.
Samples dominated by the oxic component initially have
more todorokite.
Although todorokite is absent in the surface layer
of Mn-rich Ni- and Cu-poor suboxic bottoms, with time, the stability
and relative area of the 10 A peak approach those of the Ni- and
Cu-rich samples dominated by oxic diagenesis.
It is not clear
whether the collapsing 10 A phase simply stabilizes with time or
is diagenetically transformed into todorokite.
Although the todoro-
kite structure prefers cations such as Cu, Ni and Zn, they are not
essential for its formation (Ostwald, 1982, 1986).
Thus the low
abundances of these cations in suboxic-dontinated samples do not rule
out the possibility of transformation to todorokite.
The structural models of todorokite proposed by Turner and
Buseck (1981) and Arrhenius and Tsai (1981) suggest that the fixed
tunnel structure is closely related to a sheet structure.
There-
fore, the diagenetic transformation of a layer structure to todorokite may take place without major ionic rearrangement.
Arrhenius
and Tsai (1981) suggest that todorokite may be derived directly from
a collapsible/expandable 10 A "manganate" by oxygen bridging
through tunnel walls.
Recent laboratory experiments (Golden et al.,
158
1986) in which todorokite (confirmed by HRTEM, XRD and infrared
measurements) was synthesized from buserite, a collapsable synthetic
layer manganate, support this idea.
If the collapsing 10 A phase
is diagenetically transformed to todorokite within Site H nodules,
then this reaction seems to occur under deep-sea conditions over
timescales of tens of thousands of years.
The difference in chemical composition between samples rich in
todorokite and suboxic samples with the stabilized collapsing 10 A
phase has implications for the structures of these phases.
Although
variable substitution among a number of suitable charge-balancing
cations may occur, the ratio of Mn to charge-balancing cations must
satisfy the structural requirements of todorokite.
This ratio in
oxic dominated todorokite samples at H is about 3 (Dymond et al.,
1984).
As mentioned previously, this is consistent with the todoro-
kite structure, and may represent the predominance of 3 x 3 tunnels
with two substitution sites per unit cell.
The Mn to cation ratio
in extreme suboxic samples is greater than 5, however.
Several
hypotheses may explain this greater ratio:
(1) Some of the Mn in the suboxic samples could be in lower
valence states.
Dymond et al. (1984) suggested that if the sample
with the highest Mn to cation ratio contained about 10% Mn-lI, then
the ratio of Mn-IV to charge-balancing cations would be the same in
oxic and suboxic samples.
Studies on the oxidation state of Mn in
Site H nodules has not detected 10% Mn-lI in suboxic samples (Murray
et al., 1984; Piper et al., 1984).
The Mn-Il may, however, have
oxidized after incorporation in the structure or after recovery from
159
the sea-floor.
state.
Alternatively, some Mn may be in the Mn-Ill valence
Mn-Ill has been shown to exist in todorokite; indeed its
smaller size may allow it to be more readily accomodated than Mn-Il
in the structure (Post et al., 1982; Giovanelli, 1985; Burns et al.,
1985).
If Mn-Ill accounts for the difference in charge balance,
then about 5% Mn-Ill is present in the Site H sample with the
highest Mn-to-cation ratio.
The resultant 0/Mn ratio would be
1.975, a value within the uncertainty of the measurements for Site H
nodules (l.95±.05; Murray et al., 1984).
If Mn in lower valence
states can account for the different ratios, then the chemical data
alone cannot eliminate the possibility that the stabilized collapsing 10 A phase may have the same structure as the todorokite
in oxic samples.
(2) The different Mn to cation ratios could indicate that the
stabilized 10 A phase in suboxic samples and the todorokite in
oxic samples are structural variants within the todorokite family.
If Mn is fully oxidized in H nodules, increased Mn to cation ratios
may simply indicate a greater ratio of Mn to exehangable sites in
the todorokite.
As the most common substitution site in todorokite
is thought to be on the "wall" positions (Burns et al., 1983;
Arrhenius and Tsai, 1981; Crane, 1981), an increase in the ratio of
"floors and ceilings" to "walls" could increase the ratio of Mn to
substitution sites.
If suboxic samples do transform to todorokite,
and the Mn to cation ratio is controlled by the tunnel dimensions,
then the tunnels in the suboxic samples with the highest Mn to
cation ratio may average 3 x 10.
Alternatively, the higher Mn to
160
cation ratio in suboxic samples could indicate the presence of a
different mineral phase, a conclusion not supported by the chemical
or XRD data, however.
(3) The Mn to cation ratio could be greater in suboxic samples
because todorokite is diluted by an amorphous phase with a high Mn
to cation ratio (Murray et al., 1984).
If such an amorphous phase
were free of cations, samples with the highest observed Mn to cation
ratio would contain 60% todorokite and 40% amorphous material.
Our
quantitatitive XRD data do not show a 40% reduction in 10 A peak
abundance in suboxic samples with stabilized 10 A peaks and high
Mn to cation ratios, nor is there a trend to decreasing peak area
with increasing Mn content.
From the preceeding discussion, it is clear that the mineralogy
of Site H nodules cannot be fully resolved by XRD and chemical data
alone.
HRTEM profiles through the nodules we have studied to
identify and quantify the mineral structures could better define the
initial phases and the diagenetic transformations that occur in the
nodules at the site.
Our results do show the lack of a clear dis-
tinction between classic 10 A "todorokite and 7 A "birnessite",
and suggest that the latter phase is not present in situ at Site H.
The change from collapsing 10 A phase to todorokite occurs over
several centimeters in the fast growing Site H nodules, where the
change may be observed much more easily than in typical slowly growing pelagic nodules.
161
Variations in the Chemical Conmosition
of Site H Nodules with Time
The growth rate data and chemical profiles can be combined to
show how the composition of the nodules has varied with time.
These
data show that the similiar-shaped chemical profiles through the
nodules are time correlative (Figure 111-12).
The Mn/Fe ratio is a
good index of the nature of the accretionary processes for both
typical pelagic nodules and hemipelagic nodules (Halbach et al.,
1980, l981a, 1981b; Calvert and Price, 1977; Piper and Williamson,
1977; Dymond et al., 1984).
The most distinctive feature of the
Mn/Fe ratio profiles through the bottoms of the Site H nodules is
the large increase which occurred about 160 kyr ago.
In the Mn/Fe-
rich outer zone, intervals of increased Mn/Fe ratios are correlative
between nodules, except perhaps, for 5OBC, to within the uncertainty
of the rate estimates.
The similarity of the high and low Mn/Fe
values in the outer zones of all five nodules supports the contention that they have all responded to the same sequence of
growth-controlling events.
Two processes other than radioactive decay can produce compositional variations within nodules: (1) post-depositional migration or diagenesis of elements, and (2) changes in primary accretionary processes.
Diagenetic processes
The clearest indication of post-depositional diagenesis within
Site H nodules is the increase in mineral stability with time.
ever, this process appears to be an aging phenomenon that is not
How-
162
Figure 111-12.
Variations over the past 500 kyr in the Mn/Fe ratio
through five Site H nodules.
by 230Th
al., 1984).
(this study).
Nodule bottoms 4OBC and 48RC dated
Nodule 37BC dated by 230Thex (Finney et
Nodule bottoms 5OBC and S2BC dated by correlation
(horizontal dashed line) to the other nodules.
using equation of Lyle (1982).
nodule tops - dashed line.
Other areas dated
Nodule bottoms - solid line,
4
5OBC
0
si,u,i ,IU
u,i.,,,,,a,.uuIIuh,uhIflhIItII1mIIflIUhIlIfllUIIIIIIaIUham1IIIIIII
UI I4
80
4ofhiJ_iin1j
0
52BC
I
80
Mn/Fe
I.'
U,',
0
40.ii.uIulI.II1.
801h
,-
ft
r'-
37BC
,,U,UU;l,lu,,lU,U,U,,
I
4
48BC
40
4OBC
0
III
100
III
200
III
300
III
400
III
500
AGE (kyr)
Figure 111-12.
C'
164
related to any elemental migration, because intervals of essentially
uniform composition show major changes in mineral stability.
Piper
and Williamson (1981) report a similiar trend in a nodule from the
equatorial Pacific.
Because they found that the mineralogical
changes were not related to compositional variations, they concluded
that the mineralogy resulted from post-depositional recrystal-
lization, whereas the chemical variations reflected changes in the
sources of metals to the nodule.
The fast growth rates and youth of Site H nodules also argue
against diagenesis after deposition as the process responsible for
the patterns we observe.
Because of their fast growth rates, the 6
cm nodules we have studied are only about 500 kyr old.
Typical
pelagic nodules of the same size would be 3 to 30 million years old.
The fast growth rates allow us to sample fairly thick slices (1-2 mm
on average) while still maintaining good resolution relative to the
time scales we are studying.
Such a sampling strategy should
minimize the influence of post-depositional compositional changes,
if they are present, as the thick samples integrate out the smallscale diagenetic effects usually observed in profiles through
nodules (see, for example, the niicroprobe studies of Sorem and
Fewkes, 1979).
Features such as opal dissolution and the growth of
accessory secondary minerals have been observed in the course of
high powered microscope and electron microprobe studies of nodules
(Burns and Burns, 1978), but the influence of such features on the
gross composition at the scale we are concerned with has not been
assessed.
Because Site 1-1 nodules are very pure oxides with minimal
165
amounts of phillipsite, opal and smectite (based on chemical partitioning and XRD data), however, these trace components cannot
account for the variation in the abundance of transition elements
observed in these nodules.
The post-depositional metal enrichment
processes suggested by Burns and Burns (1978) for the Ni and Cuenriched Clarion/Clipperton nodules could lead to the positive NiCu-Mn relations they observe.
These processes cannot be very
effective at Site H, however, because we do not observe the same
correlated variations in Ni, Cu and Mn abundances (Figure 111-4).
The observed Mn/Fe variations cannot be explained by postdeopositional migration of Mn within the nodules because dilution by
15% Mn (the maximum difference in concentration between intervals
with high Mn/Fe and low Mn/Fe ratios) cannot produce the more than
five-fold decrease in Fe between these intervals.
The consistant
shapes and similar concentration ranges of the profiles of most
elements in the Site H nodules also argue for control by primary
aecretionary processes rather than diagenesis.
Variability in primary accretionary processes
The observed variations in Mn/Fe ratio and minor element concentrations, particularly the abrupt changes at 160 kyr (Figure
111-12), appear to represent major changes in the mode of accretion
of the nodules.
Over the past 160 kyr, the nodule bottoms have
grown at average rates of 150-170 mm/my, with average Mn/Fe ratios
of about 60 and with most minor elements strongly depleted.
Prior
to this time, however, the nodules grew at rates averaging less than
50 mm/my, had average Mn/Fe ratios of about 10, and were much more
1W
enriched in the minor elements.
The growth and composition of the
outer section represent accretion dominated by suboxic processes;
the layers with the highest Mn/Fe ratios are very pure examples of
the suboxic end member of Dyinond et al. (1984), whereas the layers
with lower Mn/Fe values also contain a minor oxic component.
In the
deeper layers, oxic accretion was dominant, although the compositions of most of the layers indicate some suboxic influence.
The marked contrast in composition and growth is best explained
by a change in the process or processes that control suboxic diagenesis, or the supply of its products to nodules.
As suboxic
accretion is believed to result from the reduction of Mn in the
sediment and its subsequent diffusion to nodules, changes in this
process through time may be recorded in the sediment column.
Changes in the intensity of redox conditions
Variations in the Mn/Fe ratio in a nodule may reflect timedependent variations in the degree of reduction of surface sediments.
Any increase in the diffusional supply of Mn-Il from pore-
waters to growing nodules will result in faster nodule growth and
enhanced accretion of the suboxic component (marked by an increase
in the Mn/Fe ratio).
Thus, variations with time in the intensity of
reducing conditions in the surface sediment could explain the
observed variations in the chemical composition of Site H nodules.
Because the reduction of Mn in sediments is driven by the oxidation
of organic carbon (Froelich et al., 1979), variations in the
processes that control carbon sedimentation or utilization in sediments will effect the intensity of reducing conditions.
An increase
167
in the carbon content of surface sediment leads to the shoaling of
the Mn redox boundary and a resultant increase in the upward flux of
Mn in porewaters (Sawlan and Murray, 1983; Finney et al., 1986a).
As the redox boundary approaches the sediment-water interface and
the nodules residing there, the influence of suboxic diagenesis in
controlling nodule accretion should increase.
Thus, the Mn flux to
nodules should respond to changes in the organic carbon content of
the sediment.
The range of sediment, porewater and nodule compositions across
Site Fl at the present time supports this idea.
The thickness of the
Mn oxidation zone is inversely related to the content of organic
carbon in surface sediments (Finney et al., 1986a; Figure 111-13).
Furthermore, the thickness of the Mn oxidation zone is inversely
related to the Mn/Fe ratio, or proportion of suboxic component, in
the outer few mm of nodule bottoms.
Thus, the bottoms of nodules
from cores 66BC and 69BC, which have thick oxidation zones (18-20
cm), contain very little suboxic component (Mn/Fe 18-26).
In con-
trast, the compositions of the bottoms of the nodules reported here,
which came from cores with thinner oxidized zones (7-12 cm), are
dominated by suboxic accretion (Mn/Fe ratios greater than 60).
At
the extreme, core 33BC has the thinnest oxidized zone (5 cm) and the
highest organic carbon content of all.
It contains no nodules.
model of the variations in reducing conditions with time suggest
that such an area has exported Mn to the ocean, so has been too
reducing to support nodule growth (Finney et al., l986a).
A
168
% ORGAffiC CARBON
.1
.2
.4
.6
.8
1.0
2.0
IF]
E
0
I
I.-
a-
0
w.
30
Figure 111-13.
The depth to the Mn redox boundary as a function of
the organic carbon content in surface sediments (from Finney et
al., 1986a).
169
As part of our study of the sedimentary history of Site H, we
modeled variations in the intensity of Mn reduction during the past
400 kyr (Finney et al., 1986a), an interval comparable to the time
scale recorded by the manganese nodules studied here.
The relation
between modern surface organic carbon contents and Mn reduction
depths at the same sites was applied to the downcore organic carbon
content of the sediment to hjndcase variations in the thickness of
the Mn oxidation zone with time at the site (Figure 111-14).
The
reconstructed variations in the thickness of the oxidation zone were
attributed to climatically induced changes in sedimentation.
If this hypothesis is correct, times of relatively shallow Mn
reduction in the sediment (Figure 111-14) should correspond to times
of increased suboxic growth in the bottoms of the manganese nodules
(Figure 111-12).
For example, the strong increase in suboxic accre-
tion at 160 kyr corresponds to a time when the Mn redox boundary was
relatively shallow.
The modeled redox changes in the sediment would be expected to
produce variations in suboxic accretion to the nodules.
The lack of
correspondence between the sediment data and nodule composition for
younger intervals may be due to errors in the ages of nodule layers
resulting from the use of an average growth rate through this
interval.
The hindcast variations in suboxic accretion imply great
changes in growth rates.
The rates for oxic and suboxic accretion
given by Dyniond et al. (1984), suggest that if suboxic accretion
took place for only about 25% of the last 160 kyr it would produce
about 80% of the nodule material.
Thus, the layers with low Mn/Fe
E
10
0
w
0
mean
20
25
V
1UU
200
300
400
AGE (kyr)
Figure 111-14.
Variations in the depth to the Mn redox boundary at
Site H over the past 400 kyr (modelled by Finney et al., 1986a).
0
171
ratios almost certainly represent longer time intervals than calculated from average growth rates, leading to uncertainties in the
mass balance calculations.
Unfortunately, the geochemical behavior
of U-series isotopes in these nodules and the relatively high frequency of climate change makes the use of radiometric dating techniques less than ideal for determining fine-scale growth rates.
An increase in reducing conditions that lasted less than a few
thousand years could generate a Mn/Fe rich layer in a nodules. The
record of such an event would be mixed by bioturbation of the sedi-
ment and either "lost" from the sedimentary record or missed due to
the spacing of samples.
Sediment trap studies show variations by a
factor of 2 to 4 in the flux of organic carbon to the site over a
year (Fischer, 1983).
Occasional phytoplankton "blooms" or annual
variations in carbon flux may be important in controlling sedimentary redox conditions and nodule accretion.
These factors would
be superimposed on the long term redox changes at the site (Figure
111-14), and would be difficult to infer from the sedimentary
record.
The conclusion that suboxic accretion was not the dominant mode
prior to 160 kyr ago may imply that reducing conditions in the
sediments prior to that time were less intense.
The thickness of
the Mn oxidation zone, as modeled from sedimentary organic carbon
data at the site, was significantly deeper on average before about
180 kyr; the Mn redox boundary was always deeper than about 13 cm
prior to that time (Finney et al., 1986a; Figure 111-14).
The
magnitude of the peaks in organic carbon at about 10, 40 and 150 kyr
172
indicate that these were the most reducing periods over the past 400
kyr, corresponding to depths of 10 cm or less in the Mn redox
boundary.
The hindcast depth of the Mn redox boundary assumes no
long-term degradation of buried organic carbon in the sediment.
Support for this assumption comes from the organic carbon record at
nearby DSDP Site 503, which spans the past four million years, yet
shows no systematic decrease with age in organic carbon (Gardner,
1983).
In addition, the iodine conctents of downcore sediment
samples from intervals of low and high organic carbon abundance
indicate that all the carbon is present in a very degraded form,
(T.
Pederson, personal communication, 1986), suggesting that the
concentration variations reflect variable supply rather than variable decomposition of a constant supply.
Other changes in sedimentation have occurred at the site over
the same time-scale.
The sedimentation rate at the site has slowed
from an average of 1.0 cm/kyr for the period 200 kyr to 730 kyr, to
an average rate of 0.65 cm/kyr past 200 kyr.
The decrease corre-
sponds to a reduction in the average carbonate content of about 15%.
In general, the sedimentary evidence suggests that the factor or
factors responsible for the change in mode of nodule accretion are
subtle, as there are no abrupt changes in sedimentation rate or
character at the 160 kyr level.
The sedimentary conditions may have
been poised near a threshold, so that a minor change in sedimentation resulted in the domination of nodule accretion at the site by
suboxic diagenesis.
173
Still unclear is the mechanism by which the suboxically
remobilized transition metals are incorporated in the nodules.
Since the bases of the nodules always lie some distance above the
base of the Mn oxidized zone, some transport process other than
steady-state diffusion of dissolved metals is required.
Desorption
from bioturbated particles (Lyle et al., 1984) and intermittent
shoaling of the redox boundary (perhaps due to enhanced carbon
influx due to phytoplankton blooms; Dymond and Finney, 1983; Murphy,
1984) have been proposed as solutions to this enigma, but neither is
yet supported by hard evidence.
Nodule size
The possible influence of nodule size on its mode of accretion
introduces an additional complication to the Site H story.
Smaller
nodules are likely to be rotated or displaced more often than larger
nodules.
Models of bioturbation at Site H (Kadko and Heath, 1984),
imply an exponential decrease in mixing with depth.
Larger nodules
extend deeper into the sediment, so are subject to less benthic
activity.
Their larger size and mass will also tend to keep them
from being moved as often as small nodules.
Our earlier studies
(Finney et al., 1984) suggest that small nodules at Site H are more
likely to be buried than large nodules until they reach a diameter
of 2 cm.
Such small nodules appear to behave more like sedimentary
particles than their larger companions.
Large and small nodules also differ compositionally and texturally (Table 111-5).
Small nodules do not have the distinctive
174
Table Ill-S.
Chemical and Textural Properties as a Function of
Nodule Size at Site H
Diameter
Mn/Fe Bottom
Mn/Fe Bottom
Mn/Fe Top
(cm)
O-.2
% Without
Smooth Bottom
16.8
---
--
.5-2
37.0
2,5
50
2-4
48.3
5.0
43
4-6
50.8
5.5
39
6-8
56.6
8.1
29
8-10
- -- -
- --
0
(micronodules)
Chemical data from Dymond et al., 1984, Finney et al., 1984, and
this work.
Textural data, this work.
175
surface microtopography and top-bottom textural differentiation of
large nodules.
Small nodules also tend to have lower Mn/Fe ratios
in their bottoms and less of a contrast between the compositions of
their tops and bottoms.
This implies that small nodules turn over
more often and, therefore, do not experience the long periods of
stable accretion recorded by the large nodules.
Bottom photographs
show that nodules of all sizes at Site FL lie with their tops flush
with the sediment-water interface.
Consequently, smaller nodules,
which do not extend as deep into the sediment, and are further from
the source of suboxically mobilized elements.
This size-dependent behavior may explain why the bottom of
nodule 37BC, which is about 2 cm smaller in diameter than the other
nodules studied, was less influenced by suboxic diagenesis from 160
to 40 kyr ago.
The onset of intense reducing conditions 40 kyr ago
(Figure 111-13) combined, perhaps, with the nodule achieving the
critical size of about 3 cm, initiated dominant suboxic accretion by
this nodule.
Most of the nodule bottoms studied were not dominated
by suboxic accretion until the nodule had grown to a size of about 3
to 3.5 cm (Figure 111-5).
Thus, the abrupt change in accretionary
mode at 160 Kyr may be due partially to most of the studied nodules
attaining a critical size at that time.
Additional analyses of a
spectrum of nodule sizes should resolve this issue.
Turnover or change in orientation
Because suboxic accretion affects primarily the bottoms of
nodules, the overturn of a nodule should result in a sudden decrease
in suboxic supply to the former bottom surface.
Conversely, the
176
overturned top would be expected to be more strongly influenced by
suboxic accretion.
Because none of the present nodule tops display
any change in Mn/Fe ratio at a level corresponding to the 160 kyr
bottom change, overturn can be ruled out as a contributing factor.
Vertical displacement of a nodule without any change in orientation
could change the distance of the bottom from the suboxic source
without affecting the suboxic supply to the top.
The synchroneity
of the compositional changes we observe in several nodules would not
be expected to result from this process, which would be expected to
affect nodules randomly through time (Piper and Fowler, 1980).
Summary of accretionary control
Both variations in redox conditions within the sediments and
nodule size seem capable of influencing the way in which a nodule
accretes over time.
A combination of these factors can explain, the
genesis of nodules at Site H.
Small nodules which are disturbed
more often and do not extend as far into the sediment as large
nodules, are less likely to accrete metals of suboxic origin.
As
such nodules grow, predominantly through oxic accretion, they extend
closer to the suboxic source so are more likely to gain access to
metals supplied by suboxic diagenesis.
Thus, at Site H, nodules
which began to grow about 500 kyr ago were never large enough to be
be dominated by suboxic accretion until the onset of more reducing
conditions about 160 kyr ago.
Younger and therefore smaller
nodules, however, were less affected by this change in redox con-
ditions, and were not dominated by suboxic accretion until the even
more recent intensification of reducing conditions.
Because of
177
spatial and temporal variations in the depth to the Mn redox
boundary at Site H (Finney et al., 1986a), the critical size that a
nodule must exceed to acerete primarily by suboxic diagenesis
probably is not constant.
Chemical transects and growth rate deter-
minations for larger nodules could determine whether conditions in
the sediments were reducing enough for the dominance of suboxic
growth in nodule bottoms prior to 160 kyr ago.
178
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