Warm Saline Deep Water Production
in the middle Eocene-early Oligocene
PI: Ellen E. Martin, University of Florida
Funded by: American Chemical Society, Petroleum Research Fund, 2005
1
Abstract
The goal of this proposal is to use Nd isotopes preserved in fossil fish teeth and
authigenic Fe-Mn oxide coatings on sediment to test the idea that warm saline deep
waters (WSDW) produced at low latitudes filled much of the deep ocean during intervals
in the middle Eocene through early Oligocene. The existence of WSDW has dramatic
implications for poleward heat transport, nutrient distributions and the preservation of
organic carbon; however, previous studies based on other proxies, such as stable isotopes
and sedimentary parameters, yield equivocal results. Nd isotopes preserved in deep
marine archives are considered “quasi-conservative” tracers of water mass because the
primary signature imparted in the source region of a particular water mass is
predominantly altered by mixing of water mass, but can also be affected by local
continental inputs. This study will apply a multi-proxy approach; the Nd isotopic records
will be compared to: 1) records of Pb isotopes from the coatings also collected during this
study, 2) published stable isotope records from benthic foraminifera, 3) published clay
mineralogy records, and 4) published sedimentary parameters, in order to isolate the
component of the Nd isotopic record that correlates to circulation. Records of these
proxies from 3 Ocean Drilling Program (ODP) sites in the Southern Ocean (sites 689,
690, and 699) will be used to test the theory that WSDW was an important component of
deep ocean circulation during the middle Eocene through early Oligocene and that it
contributed to the warm, equitable climate.
Project Description
Introduction to the Problem
The Eocene to Oligocene interval marks a critical time in the evolution of deep ocean
circulation patterns and the transition from a greenhouse to an icehouse world. For much
of the Cenozoic deep and bottom waters appear to have formed at high latitudes, as they
do today, resulting in deep ocean waters that are relatively cold and moderately salty.
Chamberlin (1906) suggested that these cold, dense bottom waters could have been
replaced by warm, salty deep waters formed at low latitudes when the polar regions were
relatively warm and ice-free, and the pole to equator temperature gradient was low, as in
the Eocene. Kennett and Stott (1990) named the ocean typified by this reversed form of
meridional overturn the “Proteus Ocean,” (Fig. 1) and argued that it represented the
dominant circulation pattern for most of the Eocene based on evidence from ODP sites
689 and 690. The goal of this proposal is to test the theory that warm saline deep water
(WSDW) formed during the middle Eocene through early Oligocene (~28-44 Ma) using
Nd isotopes, a relatively new tracer for water mass.
2
Fig. 1
The Proto-oceanus and Proteus
ocean circulation patterns for the
Atlantic proposed by Kennett and
Stott (1990). For Oligocene and
Eocene times warm, salty water
from high latitude sources is
common in the Southern Ocean.
AAIW = Antarctic Intermediate
Water; AABW = Antarctic
Bottom Water. From Kennett and
Stott (1990).
Determining the mode of ocean circulation in the past is critical for understanding the
role circulation plays in driving global climate change. Formation of WSDW has major
implications for poleward heat transport, nutrient distributions and preservation of
organic matter. Although the Proteus Ocean model represents a fundamentally different
mode of ocean circulation than the system that prevailed for most of the Cenozoic; 1)
general circulation models (GCMs) (Barron, 1987; Rind and Chandler, 1991; Barron and
Peterson, 1992; Sloan et al., 1995, Bice et al., 1997 and 1998; Bice and Marotzke, 2001),
2) stable isotopic studies (Kennett and Stott, 1990 and 1991; Mackenson and Ehrmann,
1992; Mead and Hodell, 1992; Pak and Miller, 1992; Zachos et al., 1992; Wright and
Miller, 1993), and 3) studies of a variety of sedimentary parameters, such as carbonate
preservation (Diester-Haass et al., 1993 and 1996; Diester-Haass, 1995; Diester-Haass
and Zahn, 1996), sediment composition (Oberhänsli and Hsü, 1986; Ehrmann and
Mackensen, 1992), and CCD fluctuations (Oberhänsli and Hsü, 1986), have not been able
to resolve whether or not there were significant intervals of WSDW production. This is
because of the limitations imposed on GCMs by our knowledge of boundary conditions
and because stable isotopes, carbonate preservation, and sediment composition are not
conservative tracers of water mass.
3
The research plan for this proposal is to analyze Nd and Pb isotopic compositions of FeMn oxide coatings and fish teeth from three sites in the Atlantic sector of the Southern
Ocean in order to evaluate the existence and extent of WSDW formation during the
middle Eocene through early Oligocene. The Nd isotopic signal records a combination of
deepwater circulation plus weathering inputs. This project will use a multiproxy
approach to deconvolve these aspects of the Nd record. The circulation component of
this signal will be isolated by comparison to stable isotopes and sedimentary properties
related to water mass, while the weathering component will be identified by correlation to
Pb isotopes plus pre-existing data on clay mineralogy. This study will focus on sites 689,
690 and 699 in the Southern Ocean because there should be a clear distinction between
younger, cooler, nutrient-rich southern component deep waters with Nd values between 6 and -9 and older, warmer, nutrient-poor WSDW with Nd values less than –9.5 (Nd
represents deviations in parts per 104 of the 143Nd/144Nd ratio relative to the chondritic
uniform reservoir; Jacobsen and Wasserburg, 1980).
Scientific Significance
WSDW production associated with the Proteus Ocean model may impact climate
through: 1) poleward heat transport, 2) nutrient and productivity distributions, and 3)
hydrocarbon generation via organic carbon preservation. Some GCM’s for the Paleogene
indicate it is impossible to generate the documented low pole-to-equator thermal gradient
and ice-free conditions without increased poleward heat transport, which could be
accomplished through WSDW circulation (Barron, 1987; Rind and Chandler, 1991;
Barron and Peterson, 1992; Sloan et al., 1995). However, newer models with more
realistic seafloor bathymetry and continental runoff calculations indicate that it may be
difficult to generate WSDW in the Tethys region (Bice et al., 1998; Bice et al., 1997;
Bice and Marotzke, 2001) and Sloan et al. (1995) argued that extreme quantities of
WSDW would be required to generate sufficient poleward heat transport to sustain
Eocene conditions. Ultimately, the test of WSDW production and the Proteus Ocean
model will depend on data that can identify changes in water mass and ocean circulation
patterns.
4
A low latitude source of deep water would affect the distribution of nutrients in the
ocean. In high-nutrient, low-chlorophyll regions of the high latitude oceans (Chisholm
and Morel, 1991) surface nutrients are not fully utilized by biological productivity due to
limitations imposed by temperature, light and possibly iron availability (Martin and
Fitzwater, 1988). As a result, significant concentrations of nutrients are carried into the
deep ocean as “preformed” nutrients, representing a key inefficiency in the conversion of
dissolved nutrients to organic matter. In contrast, phytoplankton tend to extract nutrients
more efficiently in low latitude surface waters; thus, convecting waters in low latitude
source regions carry a greater load of organic matter into the deep ocean. Herbert and
Sarmiento (1991) pointed out that this load, combined with the lower saturation of
oxygen in warmer downwelling waters (Brass et al., 1982), could lead to deep water
anoxia on a large scale. Although deep sea records of bioturbation argue against such
periods of extensive anoxia, the model by Herbert and Sarmiento (1991) illustrates that
changes in deep sea circulation patterns could effect organic carbon preservation.
Typically >99% of the organic carbon rain is recycled before deep burial. In contrast,
periods of WSDW formation could be associated with enhanced organic carbon
preservation and deposition of potential hydrocarbon source rocks.
Background
Evidence for WSDW
Much of the evidence for the production of WSDW comes from studies of stable
isotopes. Kennett and Stott (1990) interpreted the 18O data from sites 689 and 690 to
indicate a temperature inversion with warmer waters underlying cooler waters (Fig. 2).
Other authors have documented evidence for WSDW, but often for shorter intervals of
time. In some studies the evidence supports smaller-scale, intermittent plumes of
intermediate to upper deep water masses from a low latitude source (Mackensen and
Ehrmann, 1992; Pak and Miller, 1992; Zachos et al., 1992; Wright and Miller, 1993), or
mixing of a plume of WSDW with high latitude source waters (Mead et al., 1993), rather
than a true Proteus Ocean model with WSDW filling the deep ocean.
Carbon isotopes are potential tracers of water mass age (Kroopnick, 1985) and, therefore,
circulation patterns. However, interpretations of 13C data from the Eocene to Oligocene
5
Fig. 2
18O data from Kennett and
Stott (1990) vs. age for ODP
sites 689 and 690. The ages
have been converted to
Cande and Kent (1995) and
corrected using Sr
chemostratigraphy. Intervals
with lower 18O values in
site 690 relative to site 689
indicate times when waters
bathing the deeper site (690)
may have been warmer than
those bathing the shallower
site (689).
are complicated by a very small 13C gradient throughout the ocean basins, which may
indicate one dominant source region with rapid ventilation (Wright and Miller, 1993),
oligotrophic surface conditions, or multiple deep water sources (Mead et al., 1993).
Based on 13C studies Wright and Miller (1993) argued that the Southern Ocean was the
dominant source of deep water from the late Eocene to Miocene. In contrast, Zachos et
al., (1992) found evidence for a Tethyan-Indian Saline Water mass at intermediate to
deep water sites in the Indian Ocean, and Pak and Miller (1992) cite evidence for possible
WSDW in the North Atlantic and Pacific.
Interpretations of stable isotope records are complicated by the fact that 18O is not a pure
tracer of temperature and 13C is not a true tracer for water mass. Although studies of
18O from several sites support the presence of a warmer deep water mass (Fig. 2), 18O
is influenced by both the isotopic composition of seawater (reflecting ice volume and
salinity) and the temperature. Ice volume is more difficult to estimate for the Oligocene,
but the Eocene is generally assumed to be ice-free. The two remaining variables,
temperature and salinity, are exactly the two water mass properties that change during
WSDW formation. 13C interpretations are limited by the small gradient in the
Eocene/Oligocene. In additional, 13C is affected by the rain rate of organic carbon,
6
mixing between water masses with different compositions, and variations in the
carbonate ion concentration (Spero et al., 1997).
Diester-Haass and co-workers (Diester-Haass, 1995; Diester-Haass et al., 1996) identified
intervals of WSDW production using estimates of surface productivity and carbonate
preservation. They argued that intervals of high productivity coupled with high
carbonate preservation indicated the presence of WSDW that would chemically favor
carbonate preservation. In contrast, intervals with low productivity and high carbonate
dissolution were interpreted to represent corrosive proto-AABW (Antarctic Bottom
Water).
Nd Isotopes as Water Mass Tracers
Nd isotopes are viewed as quasi-conservative tracers of water masses. The residence
time of Nd in seawater (~1000 yrs; Elderfield and Greaves, 1982; Piepgras and
Wasserburg, 1985; Jeandel et al., 1995; Tachikawa et al., 1999) is shorter than the mixing
time of the oceans (~1500 yrs; Broecker and Peng, 1982), and Nd is predominantly
sourced from the continents (see Frank, 2002). As a result, the cores of individual water
masses have distinct Nd values that reflect continental weathering in their source regions
(Piepgras and Wasserburg, 1982 and 1987; Piepgras and Jacobsen, 1988; Bertram and
Elderfield, 1993; Jeandel, 1993; Shimizu et al., 1994; Jeandel et al., 1998). The “quasiconservative” terminology reflects the fact that the initial signal imparted in the source
region can be modified by weathering inputs during circulation; however, the isotopic
distinction between many water masses is large enough that it is still possible to detect
mixing and changes in circulation despite regional weathering effects (Piepgras and
Jacobsen, 1988).
Pb Isotopes as tracers of Weathering Inputs
Pb has a much shorter residence time in the ocean (~80-100 yrs), thus Pb isotopic studies
illustrate regional variations that are attributed to changes in weathering inputs (e.g.
Abouchami et al., 1997; Frank and O’Nions, 1998; Reyolds et al., 1999; Claude-Ivanaj et
al., 2001; von Blanckenburg and Nägler, 2001; Abouchami et al., 2003; Frank et al.,
7
2003). von Blanckenburg and Nägler (2001) warned that Pb isotopic ratios in seawater
may not exactly reflect source materials due to incongruent weathering; however,
numerous studies have identified reasonable source components using Pb-Pb isotope
crossplots.
Archives of Nd Isotopes
Published studies of Nd and Pb isotopes as proxies for ocean circulation and weathering
rely on Fe-Mn crusts as archives (e.g. Abouchami et al., 1997; Burton et al., 1997 and
1999; Ling et al., 1997; O’Nions et al., 1998; Frank et al., 1999; von Blanckenburg and
O’Nions, 1999; Frank et al., 2002); however, these crusts have limited spatial and
temporal distributions, are difficult to date, and accumulate extremely slowly, on the
order of 1-10 mm/myr. This slow accumulation rate is problematic because it means the
crusts tend to integrate the bottom water isotopic composition over long periods of time.
Therefore, the isotopic records from crusts lack the resolution necessary for detailed
studies of paleocean circulation.
Fossil fish teeth and authigenic Fe-Mn oxide coatings on sediment represent more ideal
archives for Nd isotopes. Martin and Haley (2000) and Martin and Scher (2004)
documented that fish teeth incorporate deep water Nd isotopes during early diagenesis
and preserve that value during burial. As a result, fossil teeth can be used to construct
high-resolution records of secular variations in Nd isotopes. The one disadvantage of fish
teeth as an archive is that they are time consuming to pick and clean, and they are not
always available in sufficient quantities.
Dispersed authigenic Fe-Mn oxide coatings on sediment particles represent an alternative
archive for Nd isotopes. These coatings represent the same material that accumulates in
Fe-Mn crusts, but in this case the oxides precipitate on the surface of sedimentary
particles. The coatings are ubiquitous in marine sediments and they incorporate high
concentrations of Nd along with Pb and other trace metals. Published records
demonstrate that coatings on biogenic and detrital particles record high-resolution
variations in NADW over Pleistocene glacial/interglacial cycles (Rutberg et al., 2000;
8
Bayon et al., 2002), and even millennial time scales (Piowtrowski et al., 2004). Palmer
and Elderfield (1986) presented a low-resolution record of Nd isotopic data from coatings
on uncleaned foraminifera for samples spanning the Cenozoic. Recent work at the
University of Florida (UF) illustrates that coatings leached from the >63 m fraction of
the sediment from two of the sites proposed for this study and site 1090 yield Nd isotopic
values comparable to values preserved in fish teeth (Fig. 3) (Blair et al., 2004).
Fig. 3
Nd(0) vs. age for a) site
1090, b) site 689, and
c) site 690. Blue
symbols illustrate the
values for fish teeth,
while red symbols
represent values from
Fe-Mn oxide coatings.
For almost all sample
pairs the values for the
two different archives
agree within error.
Note different scales
in each plot.
a
b
689
c
690
9
Work Plan
The Nd isotopic record for site 689 has already been generated and submitted for
publication (Scher and Martin, submitted). The plan for this proposal is to have a
graduate student compile Nd and Pb isotope records for the middle Eocene through early
Oligocene from ODP sites 690 and 699 and complete a Pb isotopic record for site 689
(Table 1, Fig.4). Derrick Newkirk is currently completing is first year of a MS program
with a teaching assistantship. This proposal would support his research, stipend and
tuition for the additional 1.5 years projected until his completion (summer ’05 through
Fall ’06), as well as partial support for travel to one meeting (Fall AGU) each year for the
PI and Derrick.
Fig. 4
Location map for sites
to be studied for this
project.
Site 699
Site 690
Site 689
Table 1. Location and information on the ODP sites proposed for this study.
Site/Leg
Location
Site 689/1131
Site 690/1131
Site 699/1142
64°31’S, 3°06’E
65°09S, 1°12’E
51°32’S, 30°40’W
Modern water
depth (m)
2080
2914
3705
Paleo depth
(~40 Ma)
1438 3
2240 3
3431 3
Average sed.
Rate (m/m.y.)
5 4,5
35
14 3
1
Site 689 and 690 location and water depth from Barker, Kennett et al. (1988a and b).
Site 690 location and water depth from Ciesielski, Kristoffersen et al. (1988).
3
Mead et al.(1993)
4
Spiess (1990)
5
Bohaty and Zachos (2003)
2
10
Site Information
Sites 689 and 690 located on Maud Rise were chosen because intervals of WSDW have
been identified at these locations, particularly the deeper site 690 (Fig. 2). There are no
sediments from the deep Tethys preserved in the ocean today; therefore, site 690 will also
provide the framework for determining whether a unique water mass signature is
associated with intervals of purported WSDW, and it will be used to define the Nd value
of that water mass. Site 699 is included because it is a deeper site and will test whether
the WDSW occurred as a true deep water mass or a plume at intermediate depths. Age
models for all 3 sites will be established by adjusting biostratigraphic datums (Table 2) to
the time scale by Berggren et al. (1995), and magnetostratigraphic data (Table 2) to the
time scales of Cande and Kent (1995). Sr chemostratigraphy for the late Eocene to early
Oligocene will be applied as needed; however, the Sr curve is too flat in the middle
Eocene to allow accurate age determinations.
Table 2. Supplementary data for sites for this study.
Site
Stable
Clay
Additional
Isotope
Mineralogy
Sedimentary
Data
Parameters
689
1, 2, 3, 4
2, 6, 7, 8
2, 3, 8 9
690
1, 2
2, 6
2, 10
699
5
11
1
2
3
4
Kennett and Stott (1990 and 1991)
Diester-Haass and Zahn (1996)
5
Mead et al. (1993)
7
Robert and Kennett (1997)
9
Barker and Kennett et al (1988a)
11
Ciesielski and Kristoffersen et al. (1988)
13
Wei (1992)
15
Madile and Monechi (1991)
17
Speiss (1990)
Biostrat
Datums
Magnetostrat
Datums
12, 13, 14
12, 13, 14
13, 15, 16
17
17
18
Mackensen and Ehrmann (1992)
Bohaty and Zachos (2003)
6
Ehrmann and Mackensen, 1992
8
Robert et al. (2002)
10
Barker and Kennett et al. (1988b)
12
Stott and Kennett (1990)
14
Wei and Wise (1990)
16
Wei (1991)
18
Hailwood and Clement (1991)
Basic information about the locations, water depths, age coverage and sedimentation
rates is available in Table 1 and Fig. 4 and 5. All of the sites for this study were selected
because they have been extensively studied and literature exists that documents
subsidence histories, stable isotopic records and records of sedimentary parameters, such
as clay mineralogy, biogenic carbonate and opal content and accumulation rates,
11
carbonate preservation, and estimates of export production (see Tables 1 and 2 for
information and relevant references).
Fig. 5
Sediment coverage and
distribution of hiatuses for ODP
sites 689, 690 and 699 corrected
to the time scale by Cande and
Kent (1995). Samples for this
study will range from 46 to 28
Ma.
Water Masses and Nd
To reconstruct water circulation patterns in the Southern Ocean from Nd isotopic records
we need the isotopic values for different potential end-member water masses and those
values must be analytically distinct. Values for modern and paleo- water masses are
listed in Table 3. There are only five published records of Nd isotopes from the
Eocene/Oligocene, and these are low-resolution records (Fig. 6). In contrast, Scher and
Martin (submitted) have compiled high-resolution records from fish teeth for two sites in
the Southern Ocean: site 689 (Fig. 7) (Scher and Martin, submitted) and site 1090 in the
Cape Basin (Martin and Scher, 2004). Based on these seven records we can make some
estimates of the end-member isotopic composition of several water masses, but work
from this proposal is required to refine these estimates. It is apparent from Table 3 that
deep waters produced in the Mediterranean/Tethys region have been less radiogenic than
Southern Ocean waters by at least 0.5 Nd units today and in the past. Our external error
12
on Nd analyses is +/-0.23; thus, we are able to analytically distinguish potential WSDW
from SCW.
Table 3. Nd values for modern and Eocene water masses.
Modern
Water Mass
PDW
NADW
MOW
Nd
-4 1
-13.5 2
-9.4 3
AABW
AAIW
CPW
-8 to -9 4,5
-7 to -9 4,5
-6 to -9 5
Eocene (~40 Ma)
Water Mass
PDW
NCW
Tethys/WSDW
SCW
Nd
-4.5 6
-10.5 7,8
-9.2 to -9.7 9
>-9.5 10
-6 to -9 10,11
Modern Values
1
Pacific Deep Water, Piepgras and Jacobsen, 1988
2
North Atlantic Deep Water, Piepgras and Wasserburg, 1987
3
Mediterranean Outflow Water, Spivack and Wasserburg, 1988
4
Antarctic Bottom Water, Jeandel, 1993; Albarede et al., 1997
5
Antarctic Intermediate Water, Jeandel, 1993; Albarede et al., 1997
Eocene Values (~40 Ma)
6
Pacific Deep Water, Ling et al., 1997
7
Northern Component Water, Burton et al., 1997
8
Northern Component Water, Burton et al., 1999
9
Tethys Water, Stille and Fisher, 1990
10
Warm Saline Deep Water, Southern Component Water, Scher and Martin, submitted
11
Southern Component Water, Martin and Scher, 2004
The Tethyan Nd values reported in Stille and Fischer (1990) (Table 3) come from
authigenic glauconites in the Helvetic belt of the Alps, which are believed to have been
deposited on the northern continental shelf of the extinct Tethys Ocean. Nd isotopes
through time are generally stratified with depth, with the most radiogenic values near the
surface (O’Nions et al., 1997; Frank, 2002). Thus, deep water in the Tethys during the
Eocene should have been less radiogenic than –9.2 to –9.7 (Fischer and Stille, 1990), and,
therefore, analytically distinct.
Multi-proxy Approach
The quasi-conservative nature of Nd isotopes implies that they contain a signature of both
large-scale circulation and more local weathering. Isolating these two effects is critical
13
Fig. 6
PublishedNd(T) data from Fe-Mn oxide
crusts and coatings. The shaded
rectangle highlights the interval of
interest for this study. The five records
with coverage during this interval are
from Ling et al (1998) for the Pacific,
Palmer and Elderfield (1986) for DSDP
site 357 in the South Atlantic, and
Burton et al (1997 and 1999) for the
North Atlantic. Figure modified from
Frank (2002)
357
689 eNd(T)
-7
Fig. 7
Nd isotopic data
vs. age based on
fossil fish teeth
from site 689.
This relatively
high resolution
record will be part
of the data set used
for this study.
Modified from
Scher and Martin
(submitted).
-7.5
Nd
 (T)
-8
-8.5
-9
-9.5
-10
15
20
25
30
35
40
45
50
Age (Ma)
14
for the application of Nd isotopes as water mass tracers. We are proposing a multi-proxy
approach combining our Nd records with Pb isotopic records with pre-existing datasets of
stable isotopes and sediment properties. None of these proxies stands alone. 18O, 13C,
carbonate and opal preservation, and estimates of export production contain records of
the chemical and physical properties of the water mass, while clay mineralogy and Pb
isotopes provide information about the style and sources of continental weathering. This
combination of proxies can help us deconvolve the water mass and weathering signals.
Scher and Martin (submitted) applied a similar approach at site 689 to argue that most of
the Nd signal reflected water mass mixing. This conclusion is supported by a pilot study
at site 689 (Blair, unpublished data) that documented variations in Nd isotopes
unaccompanied by variations in Pb isotopes.
Sampling Strategy
Most of the isotopic records will be produced from leachates of Fe-Mn oxide coatings
from the >63m fraction of the sediment with a resolution of one sample every 200 kyr
from 44 to 28 Ma. The coatings are a relatively new archive for Nd isotopes on
Eocene/Oligocene time scales, but they are readily available, require minimal preparation
time, and incorporate high concentrations of Pb as well. As a continued test of the
integrity of the oxide coatings we will also analyze fossil fish teeth every 2 myr. If we
encounter problems with the Fe-Mn coatings we can always resort to the more laborintensive fossil fish teeth for Nd isotopes.
Disregarding hiatuses, this sampling strategy translates to ~160 Nd isotopic analyses of
coatings, ~18 Nd analyses of fish teeth, and ~240 Pb isotopic analyses. Although there
are hiatuses, we will need to run replicates and the fish teeth samples, therefore these
numbers represent a minimum estimate; actual analyses are estimated at 200 Nd and 275
Pb samples.
Analytical Methods
Fossil fish teeth are hand picked from the >125 m fraction of dried, disaggregrated and
sieved sediment. The teeth are cleaned using the oxidative/reductive cleaning technique
15
from Boyle (1981), Boyle and Keigwin (1985) and Boyle (pers. comm., 1993) to
chemically remove Fe-Mn oxide coatings, thereby yielding isotopic ratios derived purely
from the teeth. Nd concentrations are ~200 ppm. Therefore, we need ~40 g of cleaned
teeth (~1-2 teeth).
The Fe-Mn oxide coatings are leached from the >63m fraction of ODP sediment (Blair
et al., 2004) using a procedure modified from Chester and Hughes (1967), Rutberg et al.
(2000) and Bayon et al. (2002). Carbonate is removed by repeated dissolution in
buffered acetic acid. These coatings are reduced using 0.02M hydroxylamine
hydrochloride in 25% glacial acetic acid. The sample is placed on a wrist action shaker
for 1 hour, then centrifuged and the supernate is decanted and split into two fractions for
Nd and Pb isotopic analyses, which are dried down for column chemistry.
Nd is separated from both types of samples using two sequential cation exchange
columns. The first column separates the bulk rare earth elements (REE) from other
cations using Mitsubishi resin and HCl as an eluent. The REE cut is dried down and
loaded onto a second Mitsubishi resin column and Nd is isolated using methylactic acid.
The total procedural blank is 10 pg Nd.
The smaller Nd samples for the teeth are analyzed in dynamic mode on a Micromass
Sector 54 thermal ionization mass spectrometer (TIMS) at the UF at a recharge rate of
$35/sample. These small samples are analyzed as NdO, which significantly increases the
efficiency. The samples are loaded on zone-refined Re filaments with silica gel. All
ratios are normalized to 146Nd16O/144Nd/16O = 0.722254. Using 142Nd/16O as a monitor
peak, beams of 0.5V are measured for 200 ratios. The 143Nd/144Nd value for repeat
analyses of the international Nd standard JNdi-1 is 0.512102 (+/-0.000012, 2 external
reproducibility, n=65). This uncertainty translates to 0.24 Nd units.
The larger oxide coating samples are analyzed on a Nu Plasma MC-ICP-MS at UF.
Sample and standard solutions are aspirated into the plasma source via a Micromist
desolvating nebulizer with a GE spray chamber (wet plasma). Preamplified gains are run
16
before each analytical session. All ratios are normalized to 146Nd/144Nd = .7129 to correct
for instrumental mass bias. Baselines are measured by ESA (electrostatic analyzer)
deflection of the beam. Typical beam sizes for leachate runs are 5-10 V of total Nd. Our
143
Nd/144Nd value for repeat analyses of the JNdi-1 standard is 0.512110 +/- 0.000014
(2).
Pb is separated from the Fe-Mn oxides using Teflon microcolumns and HBr as an eluent
(modified from Manhes,1978). The Pb cut is dried, diluted with 2% HNO3 and
introduced into the Nu as for Nd. The isotope ratios are collected in static mode and
normalized using 205Tl/203Tl = 2.38750 (Belshaw, 1998). The SRM 981 standard is run
between every other sample. Values vary slightly with the tuning of the instrument, but
42 analyses in April-June, 2003 yielded 206Pb/204Pb = 16.9369 (+0.0039, 2), 207Pb/204Pb
= 15.4904 (+0.0034, 2),208Pb/204Pb = 36.6949 (+0.0087, 2). We can run ~3
unknowns/hour on the Nu at a recharge rate of $50/hour.
Summary
The goal of this proposal is to generate high resolution Nd and Pb isotope records for the
middle Eocene through early Oligocene at three sites in the Atlantic sector of the
Southern Ocean and compare these data to existing data including stable isotopes and
sediment composition in order to determine:

Whether there was a water mass with distinct Nd isotopes on Maud Rise during
times of purported WDSW.

The Nd value of WSDW, if it exists, compared to SCW.

The changes in weathering associated with dramatic changes in climate at the
Eocene/Oligocene boundary.

The vertical extent of WSDW. In particular, whether it extends into the deep
ocean at site 699 or is composed of a tongue of intermediate water that is only
observed at site 690 and possibly 689.
Broader questions to be addressed include:
17

The nature of the connection between circulation and climate. Do changes in
ocean circulation drive changes in climate or do changes in climate drive changes
in ocean circulation?

The evolution of the Antarctic Circumpolar Current (ACC). Data from site 689
suggest the introduction of water into the South Atlantic with a Pacific Nd
isotopic signature at least as early as the earliest estimates for the opening of the
Drake Passage Lawver and Gahagan, 2004). The convergence of Nd isotopic
values from various depths might indicate the onset of the ACC circulation.
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23