PALEOCEANOGRAPHY, VOL. 20, PA4002, doi:10.1029/2003PA000979, 2005
Intensification of the oxygen minimum zone in the northeast Pacific
off Vancouver Island during the last deglaciation:
Ventilation and/or export production?
J. L. McKay
Centre GEOTOP, Université du Québec à Montréal, Montréal, Québec, Canada
T. F. Pedersen
School of Earth and Ocean Sciences, University of Victoria, Victoria, British Columbia, Canada
J. Southon
Earth System Science Department, University of California, Irvine, California, USA
Received 6 November 2003; revised 27 April 2005; accepted 12 May 2005; published 4 October 2005.
[1] The oxygen minimum zone (OMZ) off Vancouver Island was more oxygen depleted relative to modern
conditions during the Allerød (13.5 to 12.6 calendar kyr) and again from 11 to 10 kyr. The timing of OMZ
intensification is similar to that seen throughout the North Pacific, although the onset appears to have been
delayed by 1500 years off Vancouver Island. Radiocarbon dating of coeval benthic and planktonic
foraminifera shows that between 16.0 and 12.6 kyr the age contrast between surface and intermediate waters
(920 m depth) off Vancouver Island was similar to, or slightly less than, that today. There is no evidence of an
increased age difference (i.e., decreased ventilation) during the deglaciation, particularly during the Allerød.
However, sedimentary marine organic carbon concentration and mass accumulation rate increased substantially
in the Allerød, suggesting that increased organic matter export was the principal cause of late Pleistocene OMZ
intensification off Vancouver Island.
Citation: McKay, J. L., T. F. Pedersen, and J. Southon (2005), Intensification of the oxygen minimum zone in the northeast Pacific off
Vancouver Island during the last deglaciation: Ventilation and/or export production?, Paleoceanography, 20, PA4002,
doi:10.1029/2003PA000979.
1. Introduction
[2] Extensive evidence indicates that the intensity (i.e.,
degree of oxygen depletion) of the oxygen minimum zone
(OMZ) along the eastern margin of the North Pacific Ocean
has fluctuated significantly over time. Laminated sediments,
preserved when bottoms waters contain <0.1 mL/L oxygen,
are observed in cores from the Gulf of California [Keigwin
and Jones, 1990], partially isolated basins within the
California Borderlands region (e.g., Santa Barbara Basin
[Behl and Kennett, 1996]) and from the open continental
margin off California and Mexico [Gardner and HemphillHaley, 1986; Anderson et al., 1987; Dean et al., 1994;
Ganeshram, 1996; van Geen et al., 1996; Gardner et al.,
1997; Lyle et al., 1997; van Geen et al., 2003]. These
laminated deposits are interbedded with partly homogenized
to well-homogenized sediments that were bioturbated when
the bottom water was more oxygenated. The assemblage of
benthic foraminifera has also fluctuated in response to
changes in OMZ intensity [Cannariato et al., 1999;
Cannariato and Kennett, 1999; Ohkushi et al., 2003]. In
the Santa Barbara Basin for example, the appearance of a
foraminiferal assemblage tolerant of low oxygen conditions
Copyright 2005 by the American Geophysical Union.
0883-8305/05/2003PA000979$12.00
was synchronous with the change to laminated sediments
[Cannariato et al., 1999]. The sedimentary concentrations
of redox-sensitive trace metals (e.g., Mo) have varied on
glacial to interglacial and shorter timescales, further suggesting that bottom water oxygen levels have fluctuated
[Nameroff, 1996; Dean et al., 1997; Zheng et al., 2000;
Ivanochko and Pedersen, 2004]. Collectively, the evidence
indicates that the OMZ along the eastern margin of the
North Pacific was less intense (i.e., relatively oxygen rich)
during colder intervals such as the Younger Dryas, marine
isotope stage (MIS) 2, and stadials within MIS 3 and 4, and
more intense during warm periods [Keigwin and Jones,
1990; Kennett and Ingram, 1995; Behl and Kennett, 1996;
Cannariato and Kennett, 1999].
[3] Intensity of the OMZ is a function of two primary
variables: (1) ocean circulation, henceforth referred to as
ventilation and (2) oxygen consumption [Wyrtki, 1962].
Ventilation refers to any process that transfers surface
conditions, in this instance high oxygen concentration, to
subsurface waters [Van Scoy and Druffel, 1993]. At present,
ventilation of the OMZ in the northeast Pacific reflects a
balance between the input of relatively oxygen-rich North
Pacific Intermediate Water (NPIW) from the northwestern
Pacific and oxygen-poor Subtropical Subsurface Water
(SSW) from the eastern tropical Pacific. Any change in
the relative input and/or oxygen concentration of either
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water mass could have influenced OMZ intensity. Oxygen
consumption refers to the utilization of oxygen during
degradation of organic matter, both as it settles through
the water column and after deposition on the seafloor. The
higher the export rate of labile organic matter from surface
waters, the greater the oxygen depletion of underlying
intermediate waters. In the northeast Pacific large changes
in primary production have occurred in the past [e.g., Lyle et
al., 1992; Ortiz et al., 1997; Mix et al., 1999; McKay et al.,
2004] and may have had a substantial impact on OMZ
intensity.
[4] There is considerable debate as to whether changes in
ventilation and/or production were responsible for fluctuations in OMZ intensity in the northeast Pacific during the
last deglaciation. Previous research has focused on the
regions off southern Oregon, California and Mexico, but
little is known about the history of the OMZ north of 42N.
This study is a step toward correcting this omission, with
the goal of better understanding why the intensity of the
OMZ has fluctuated.
[5] The study area is located off the west coast of
Vancouver Island, Canada, where the eastward flowing
Subarctic and North Pacific currents split into the northward
flowing Alaska Stream and southern flowing California
Current (Figure 1). At present, the region is characterized
by high primary productivity caused by seasonal (late spring
to early fall), wind-driven upwelling [Huyer, 1983]. However, primary productivity and organic carbon flux to the
sediment have varied substantially over the past 16 kyr
[McKay et al., 2004] and may have influenced the intensity
of the OMZ locally. The study area is also proximal to
regions of NPIW formation (Sea of Okhotsk) and ventilation (Alaska Gyre), and lies at the northernmost end of the
California Current system where the input of SSW is lowest.
Therefore, if variations in the intensity of the OMZ along
the northeastern margin of the Pacific Ocean were related to
changes in the supply of NPIW and/or SSW, we would
expect to see evidence of this off Vancouver Island.
[6] Analyses were conducted on Core JT96-09 which was
raised from a water depth of 920 m (i.e., from within the
core of the present-day OMZ). The objectives were to
determine if the intensity of the OMZ has changed over
the last 16 kyr and, if it has, determine whether the changes
were the result of fluctuations in ventilation and/or export
production. Changes in OMZ intensity are inferred from
trace metal data and the abundance of certain oxygensensitive species of benthic forminifera. Radiocarbon dating
of coeval benthic and planktonic foraminifera and the
calculation of benthic-planktonic age differences are used
to assess if there have been substantial changes in ventilation over the past 16 kyr, while changes in paleoproductivity are inferred from the organic carbon record of McKay et
al. [2004].
2. Materials and Age Model
[7] Sediment cores were collected from the continental
slope west of Vancouver Island, British Columbia, Canada
(4854.760N, 12653.440W; Figure 1) during a 1996 Canadian Joint Global Ocean Flux Study cruise. Data are
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Figure 1. The study area is located off the west coast of
Vancouver Island, British Columbia, Canada (inset). Sediment cores were collected from Site JT96-09 which is
located on the continental slope at a water depth of 920 m.
presented here for a 374 cm long piston core (JT96-09pc)
and a corresponding 40 cm long multicore (JT96-09mc)
taken from a water depth of 920 m. The multicore and upper
51 cm of the piston core are composed of a homogeneous,
organic carbon-rich (1.2 to 3.1 wt %) olive green mud. In
the piston core this mud is underlain by 85 cm of grayish
green clay and at the base of this clay is a 16 cm thick sandy
turbidite. The remainder of the core (222 cm) is a dense,
organic carbon-poor (<0.6 wt %) gray clay. It is important to
note that no laminated sediments are observed in either core.
[8] The sediment-water interface was captured in the multicore and a comparison of geochemical data from the multicore and piston core suggest that 12 cm were lost off the top
of the piston core during its collection. Piston core depths
have been corrected for this loss (+12 cm) and for the presence
of the turbidite (16 cm), and the records for both cores were
merged to yield a composite record for Site JT96-09.
[9] The age model for Core JT96-09 is based on nine
accelerator mass spectrometry (AMS) radiocarbon dates
measured at the Lawrence Livermore National Laboratory
on a mixed assemblage of planktonic foraminifera (Neogloboquadrina pachyderma right- and left-coiling and
Globigerina bulloides). These data are provided in Table 1.
The results were converted from radiocarbon to calendar
ages using CALIB 4.3 [Stuiver et al., 1998]. A reservoir
age of 800 years was applied to radiocarbon ages younger
than 12,000 14C years and a reservoir age of 1100 years
for older samples as per Kienast and McKay [2001].
Sedimentation rates, which were calculated by assuming
linearity between calendar age dates, range from 5 cm/kyr
during the Holocene up to 169 cm/kyr during the early
deglaciation (Figure 2a).
[10] Alkenone paleothermometry measurements previously conducted on Core JT96-09 yielded evidence of rapid
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Table 1. Radiocarbon Ages of Planktonic and Benthic Foraminifera in Core JT96-09
Sample
Depth, cm
1
2
3
4
5
6
7
8
9
10
47.5
57.5
77.5
87.5
102.5
112.5
142.5
261.5
286.5
346.5
Calendar Age,
kyra
10.03
12.24
12.73
12.84
13.17
13.43
14.14
14.30
15.57
14
C Age Benthics
14
C Age Mixed
Planktonics
9760 ± 70
9360 ± 240
11210 ± 120
11500 ± 110
11600 ± 80
12460 ± 120
12640 ± 90
13410 ± 80
13520 ± 70
14140 ± 70
Uvigerina
Bolivina
Benthic-Planktonic
Age Difference, years
10710 ± 60
10650 ± 90
10910 ± 70
950 ± 92, 890 ± 114
1550 ± 250
12110 ± 80
610 ± 136
13170 ± 100
13500 ± 80
710
860
880
830
680
14290 ± 110
14350 ± 120
14830 ± 280
±
±
±
±
±
156
120
136
139
289
a
These calendar ages were used in the creation of the age model for Core JT96-09. A reservoir age of 800 years was applied
to samples 1, 3, 4, and 5 and 1100 years to ages 6 through 10. Further details about the age model are given by Kienast and
McKay [2001].
sea surface temperature (SST) fluctuations during the
deglaciation (Figure 2b) [Kienast and McKay, 2001]. The
pattern of SST fluctuations is remarkably similar to
temperature fluctuations in the GISP-2 ice core record
suggesting that the Bølling, Allerød and Younger Dryas
events are recorded in Core JT96-09. More importantly
these events appear to be nearly synchronous with those in
GISP-2, although the match is not perfect [Kienast and
McKay, 2001]. Offsets between the two records, in some
instances on the order of hundreds of years, most probably
reflect errors in the age model of Core JT96-09 resulting
from: (1) problems inherent to radiocarbon dating (e.g.,
14
C plateaus), (2) a poorly known reservoir age (e.g., using
a reservoir age of 800 years rather than 1100 years for
samples older than 12,000 14C years increases most ages
by 300 years), (3) bioturbation, although the effects of
this are limited by the high sedimentation rates during the
deglaciation, and (4) large errors associated with the
radiocarbon dating of small samples. In this paper we
assume that the GISP-2 and JT96-09 records are essentially synchronous, but we have not tuned the age model
for Core JT96-09 to make the records match exactly.
Therefore we refer to the warm period from 14.3 to
13.5 kyr as the Bølling, and that from 13.5 to 12.6 kyr
as the Allerød, and assume that the cool interval from
12.6 to 11.0 kyr is the Younger Dryas (Figure 2b).
3. Methods
[11] Manganese and Al concentrations were measured by
X-ray fluorescence spectrometry. Trace metal concentrations (i.e., Mo, Cd, Re and U) were measured by isotope-
Figure 2. (a) Plot of 14C ages of planktonic and benthic foraminifera versus corrected core depth for
Core JT96-09. Circled numbers correspond to the sample numbers in Table 1. (b) Sea surface temperature
(SST) record for Core JT06-09 derived from alkenone paleothermometry [Kienast and McKay, 2001].
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dilution inductively coupled plasma mass spectrometry.
Sample preparation for trace metal analysis involved adding
known amounts of isotopically enriched spike solutions to
20 mg of powdered sediment. Samples were then microwave digested in a mixture of concentrated HNO3, HCl and
HF. The digests were evaporated on a hotplate overnight
and then redigested in 5N HCl. Aliquots were taken for Mo
and U analysis and the remaining sample was run through
an anion exchange column (Dowex 1-X8 resin) to concentrate Re and Cd. Further details of the sample preparation
are given by Ivanochko [2001]. To check the precision, a
sediment standard (SNB) from the University of British
Columbia was analyzed with each batch of samples. The
resulting relative standard deviation (RSD, 1s) for Mo, Cd,
Re and U are 7%, 10%, 11% and 9%, respectively.
Accuracy, assessed by measuring the concentrations of
these metals in the National Research Council of Canada
sediment standard MESS-1, is 8% or better for Mo, Re and
U, and 14% for Cd.
[12] Approximately 5 cm3 of sediment were sieved to
obtain the >106 mm size fraction. The abundance of benthic
foraminifera in this size fraction, specifically those species
known to be sensitive to the bottom water oxygen concentration (i.e., Uvigerina and Epistominella species, Bolivina
argentea, Bolivina pacifica, and Buliminella tenuata), was
determined. When benthic foraminifera were particularly
abundant (>800 individuals) the sample was split once
before counting.
[13] Possible changes in intermediate water ventilation
were determined by calculating the age difference (using
uncorrected 14C ages) between benthic and planktonic foraminifera separated from the same sample. Planktonic samples
comprised a mixture of N. pachyderma (right and left
coiling) and G. bulloides. Benthic samples consisted of
either a mixture of Uvigerina species or Bolivina argentea.
This method assumes that the effects of bioturbation can
be ignored, a reasonable assumption given the high
sedimentation rates that prevailed during the deglaciation
(10 cm/kyr). However, the age difference may be biased
toward slightly smaller values because Uvigerina and
Bolivina species are infaunal organisms and probably grew
at depth alongside the shells of previously deposited
planktonic foraminifera. Here again, the high sedimentation
rates typical of the deglaciation minimized the problem and
no attempt was made to correct for it. Sample size ranged
from <1 to 4.8 mg carbonate (usually 1.3 to 4.8 mg). Prior
to radiocarbon dating, samples were sonicated in methanol
and briefly etched with 0.0001N HCl.
[14] Changes in paleoproductivity over the last 16 kyr
were inferred from the marine organic carbon concentration
and accumulation rate records of McKay et al. [2004]. The
concentration of marine organic carbon was obtained by
measuring the total organic carbon concentration and then
subtracting the percentage of terrestrial organic carbon. This
correction was necessary because terrestrial organic matter
comprises up to 70% of the organic material in sediments
deposited during the late glacial and early deglacial [McKay
et al., 2004]. The terrestrial fraction was estimated by first
measuring the d13C value of total organic matter. Then,
by assuming samples contained a mixture of marine and
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terrestrial organic carbon and that each end-member had a
distinctive d13C signature, it was possible to calculate the
terrestrial fraction (i.e., two end-member mixing model).
Further details about how the marine organic carbon
record was obtained and validated are given by McKay
et al. [2004]. The organic carbon mass accumulation rates
were calculated as the product of marine organic carbon
concentration (wt %), linear sedimentation rate (cm/kyr)
and sediment dry bulk density (g/cm3). The latter was
determined using chlorinity data and a pore water salinity
of 35 to estimate porosity, and by assuming an average
grain density of 2.5g/cm3.
4. Results
[15] Minor and trace metal data for Core JT96-09 are
presented in Figure 3. Mn/Al weight ratios range from
0.006 in the Holocene up to 0.010 between 16.0 and
12.6 kyr. Molybdenum concentrations range from 0.3 to
3.7 mg/g, Cd from <0.1 to 1.4 mg/g, Re from <1 to 65 ng/g
and U from 1.1 to 5.8 mg/g. To account for possible
fluctuations in trace metal content due to changes in dilution
by biogenic components (i.e., carbonate and organic matter)
the data are also presented as metal/Al ratios (Figure 3).
Concentrations and metal/Al ratios are relatively low in
glacial and early deglacial sediments (16.0 to 13.5 kyr). At
13.5 kyr (i.e., the start of the Allerød) these ratios increase
substantially and remain high until the Younger Dryas at
12.6 kyr. Concentrations and ratios are low throughout the
Younger Dryas and then rise again at 11.0 kyr. Early to
mid Holocene sediments host the highest trace metal enrichments and have correspondingly high metal/Al ratios. In the
late Holocene all values decrease to near glacial levels.
[16] No foraminifera occur in the upper 30 cm of Core
JT6-09 (0 to 5 kyr). In sediments deposited between 5 and
10 kyr foraminifera are present but extensive dissolution is
observed and fragments are abundant. The paucity of shells
in the Holocene deposits can be attributed to the highly
corrosive nature of North Pacific waters [Zahn et al., 1991;
Karlin et al., 1992] and the adverse effect that degradation
of organic matter has on carbonate preservation [De Lange
et al., 1994; Jahnke et al., 1997]. In contrast, foraminifera
are generally abundant and well preserved in sediments
older than 10 kyr apparently reflecting less corrosive conditions in the past. The only exception is the Bølling (14.3
to 13.5 kyr) when the number of foraminifera, in particular
benthic foraminifera, decreases significantly (Figure 4b).
[17] Core JT96-09 is generally characterized by two
distinct assemblages of benthic foraminifera. Assemblage
one is dominated by Bolivina species (e.g., Bolivina argentea, Bolivina pacific and Bolivina subadvena) and Buliminella tenuata, but contains few, if any, Uvigerina species.
This assemblage occurs in two zones that correspond to the
Allerød and the period from 11.0 to 10.0 kyr (Figure 4b).
The second assemblage is characterized by Uvigerina
species (predominantly Uvigerina perigrina) and Epistominella species, as well as minor numbers of Bolivina species
(predominantly Bolivina argentea). This assemblage is
found in sediments younger than 10 kyr, older than
13.5 kyr, and in sediments deposited during the Younger
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Figure 3. Redox-sensitive minor and trace metal concentrations (symbols) and metal/Al ratios (thick
lines) in Core JT96-09. (a) Mn/Al, (b) Mo, (c) Cd, (d) Re, and (e) U. Two periods of high trace metal
accumulation (i.e., enrichment relative to modern surface sediments) are indicated by the shading
(zones 1 and 2). Zone 1 corresponds to the Allerød, and the period of relatively low trace metal
accumulation between zones 1 and 2 is the Younger Dryas. Typical lithogenic concentrations are as
follows: 0.6 mg/g Mo and 0.2 mg/g Cd (based on concentrations in oxic surface sediments in the region
[McKay, 2003]), 0.05 ng/g Re [Koide et al., 1986], and 1 mg/g U [Morford et al., 2001]. The average
shale Mn/Al ratio of is 0.010 [Turekian and Wedepohl, 1961].
Dryas (Figure 4b). It is important to note that the benthic
foraminiferal assemblage in the Bølling is distinctly different from that in the Allerød. It is dominated by Epistominella species, with only minor numbers of Uvigerina
species and essentially no Bolivina species (Figure 4b),
and most closely resembles the benthic foraminiferal
assemblage of glacial sediments.
[18] No single species of benthic or planktonic foraminifera is found throughout the entire core and therefore it
was necessary to use a variety of taxa to determine benthicplanktonic age differences. Radiocarbon data were obtained
for nine benthic-planktonic foraminiferal pairs spanning the
period between 10.0 and 15.6 kyr (Table 1). Benthicplanktonic age differences range from 610 to 1550 years.
The errors for these data (i.e., the square root of a2 + b2;
where a and b are the individual errors) range from ±92 to
289 years, with errors greater than 200 years resulting from
the use of very small samples (<1 mg carbonate). The
benthic-planktonic age difference was determined twice
for sample 1 (Table 1), once using a mixture of Uvigerina
species and a second time using Bolivina argentea. The
results are identical within the error (950 ± 92 and 890 ±
114 years, respectively; Table 1) and thus changes in the
benthic-planktonic age difference observed in Core JT96-09
are most probably not the result of using two different
benthic foraminifera taxa.
[19] The down-core changes in the benthic-planktonic
(B-P) age difference are illustrated in Figure 5. The single
late glacial sample yields a B-P age difference of 680 ±
289 years. Benthic-planktonic age differences range from
830 ± 139 to 880 ± 136 years during the early Bølling and
then decrease slightly during the Allerød to a low of 610 ±
136 by 12.6 kyr. Owing to a paucity of foraminifera no
data are available for the period from 12.6 to 11.0 kyr
(i.e., the Younger Dryas). The highest B-P age difference
(1550 ± 250 years; sample 2 in Table 1) occurs just after
the Younger Dryas. This large value appears to be partly the
result of an anomalously young planktonic age (Figure 2a).
At 10 kyr the B-P age difference ranges from 890 ± 114 to
950 ± 92 years. With the exception of sample 2, there are
no large changes in the B-P age difference. There are,
however, some intriguing, abet subtle, variations such as
the slight decrease in the Allerød (Figure 5).
5. Discussion
5.1. Evidence of OMZ Intensification
[20] The OMZ, defined as that portion of the water
column where oxygen is 0.5 mL/L, extends from approximately 750 to 1300 m water depth off the west coast of
Vancouver Island. During the 1996 research cruise the
lowest oxygen concentration (0.3 mL/L) was measured at
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Figure 4. Various paleorecords for the period between 16 and 8 kyr for Core JT96-09. Two periods of
oxygen minimum zone (OMZ) intensification (zones 1 and 2) are identified. (a) Sea surface temperature
record derived from alkenone paleothermometry [from Kienast and McKay, 2001]. (b) The distribution of
oxygen-sensitive benthic foraminifera. Note the high abundance of species tolerant of low oxygen
concentrations (e.g., Buliminella tenuata) during the Allerød (13.5 to 12.6 kyr). Uvigerina abundances
have been multiplied by 10 so that they could be plotted on the same x axis as the other species.
(c) Molybdenum concentration data.
a water depth of 920 m (i.e., the site where Core JT96-09
was collected). Unlike cores from the California and Mexican continental margins, no laminated sediments have been
preserved in Core JT96-09. However, this does not necessarily imply that bottom water oxygen concentrations did
not fluctuate because laminated sediments are only preserved when bottom water oxygen levels drop below
0.1 mL/L [Behl and Kennett, 1996]. Significant variations
in bottom water oxygen concentration (i.e., OMZ intensity)
over the past 16 kyr are inferred from changes in the
accumulation of redox-sensitive trace metals and by
changes in the assemblage of benthic foraminifera.
[21] The concentration of certain redox-sensitive trace
metals in sediments is directly or indirectly controlled by
redox conditions through either a change in redox state
(e.g., Re and U) and/or speciation (e.g., Mo) which results
in their accumulation or loss. Other redox-sensitive metals
(e.g., Cd) have a single redox state, but readily react with
the reduced forms of other elements such as sulphur and this
results in their accumulation under reducing conditions. The
authigenic flux into the sediment (i.e., the diffusion of
metals from the overlying water column into the sediment)
is primarily controlled by the oxygen concentration in the
sediment and overlying bottom water. In general, when
suboxic and anoxic redox boundaries (defined here, respectively, as the locations where the oxygen content falls to
zero and sulphate reduction commences) are shallow, the
flux from the overlying water column into the sediment is
enhanced. Such conditions commonly occur when organic
matter flux to the seafloor is high and/or oxygen concentration in the bottom water is low.
[22] The reduction and subsequent precipitation of Re and
U begins once pore water O2 is depleted and this leads to
their enrichment in both suboxic and anoxic sediments
[Ravizza et al., 1991; Colodner et al., 1993; Crusius et
al., 1996]. Cadmium has a single redox state, but form
insoluble sulphides when trace amounts of H2S are available [Rosenthal et al., 1995]. This leads to minor Cd
accumulation in suboxic sediments and large accumulations
in anoxic sediments. In comparison, Mo enrichment is only
observed in anoxic sediments [Francois, 1988; Emerson
and Huested, 1991; Crusius et al., 1996; Morford et al.,
2001; Ivanochko and Pedersen, 2004]. In the presence of
>11 mM H2S, molybdate (MoO2
4 ) is converted to thiomolybdate (MoS2
4 ) which is scavenged by pyrite [Helz et al.,
1996; Erickson and Helz, 2000]. The presence of zero-
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Figure 5. Benthic-planktonic age differences are plotted
against calendar age. The two periods of OMZ intensification (zones 1 and 2) are indicated by the shading. The sea
surface temperature record of Kienast and McKay [2001] is
also shown, and the Bølling (B), Allerød (A), and Younger
Dryas (YD) events are labeled. Circled numbers correspond
to the sample numbers in Table 1.
valent sulphur speeds up this process, and also aids in the
reduction of Mo and formation of Mo-Fe-S complexes
which are also rapidly scavenged by pyrite [Vorlicek et
al., 2004].
[23] Modern sediments deposited within the OMZ off
Vancouver Island become suboxic within millimeters of
the sediment-water interface and are thus characterized by
relatively low Mn/Al ratios (i.e., similar to, or less than, the
average shale ratio of 0.010; Figure 3a). However, nearsurface sediments never become fully anoxic and thus a
large Mo enrichment above the typical lithogenic concentration (i.e., 0.6 mg/g [Morford et al., 2001]) is not
observed in the upper 35 cm of Core JT96-09 (Figure 3b).
In comparison, two periods of high Mo accumulation,
relative to modern sediments, are observed in older sediments (zones 1 and 2, Figure 3b). These two zones are also
enriched in Cd, Re and U (Figures 3c – 3e). Molybdenum
enrichment suggests that in the past sediments were more
reducing (i.e., anoxic conditions existed in the near-surface
sediments), although it does not imply that the overlying
bottom water was anoxic. The fact that these sediments are
not laminated indicates that the bottom water was somewhat
oxygenated and that bioturbation was occurring.
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[24] The first episode of marked trace metal enrichment
is observed in the Allerød (zone 1, Figure 3). There is no
evidence that this enrichment is result of metal remobilization due to oxygen influx (i.e., burn down) and subsequent reprecipitation in underlying reduced sediments
because the distribution of all redox-sensitive metals is
similar. Burn down commonly redistributes elements at
different depths because of their different chemical behaviors [Colodner et al., 1992; Thomson et al., 1993, 1995;
Crusius and Thomson, 2000]. Therefore metal enrichment
in zone 1 must reflect the development of anoxic conditions
within the sediment shortly after deposition. Such conditions
may have developed as a result of (1) increased sedimentation rate and corresponding decreased oxygen influx,
(2) decreased ventilation of the bottom water, and/or
(3) increased carbon flux to the sediment. Increased sedimentation is ruled out as the principle cause of trace metal
enrichment because the sedimentation rate was substantially
higher during deposition of the trace metal-poor sediments in
the Bølling (average 114 versus 169 cm/kyr; Figure 2).
Whether oxygen depletion in near-surface sediments was
the result of decreased ventilation of bottom waters and/or
increased organic carbon flux to the sediment, which also
would have caused water column oxygen content to decline,
is discussed in sections 5.2 and 5.3. In either case, these trace
metal data imply that oxygen depletion of the OMZ was
more pronounced during the Allerød.
[ 25 ] The second interval of trace metal enrichment
begins at 11.0 kyr (i.e., the end of the Younger Dryas)
and continues into the Holocene (zone 2, Figure 3). The
increase in Re at 4 kyr (i.e., 20 cm below the sedimentwater interface) reflects the approximate depth where Re
reduction and accumulation are occurring at present [McKay,
2003]. The relatively low Holocene sedimentation rate
(5 cm/kyr) coupled with sufficient oxidant demand has
allowed authigenic metal enrichment to occur well below the
sediment-water interface, possibly overprinting the paleosignal. Although this observation constrains detailed interpretation of OMZ history during the early Holocene, these data
clearly indicated that near-surface sediments at Site JT96-09
have been continuously reducing since the Younger Dryas,
and imply relatively low bottom water oxygen concentrations during the Holocene. This conclusion is consistent with
deductions made off California [Anderson et al., 1987;
Behl and Kennett, 1996; Cannariato and Kennett, 1999;
Ivanochko and Pedersen, 2004] and Mexico [Ganeshram
et al., 1995], all of which indicate that a relatively intense
OMZ has been a permanent fixture during the Holocene.
[26] Molybdenum enrichment in the Allerød is contemporaneous with a dramatic increase in the numbers of
Bolivina species (predominantly B. argentea, B. pacifica
and B. subadvena) and Bulliminella tenuata (Figure 4b). A
similar, although more subtle, increase occurs between
11.0 and 10.0 kyr (Figure 4b), which is also a period of
enhanced Mo accumulation. Bottom water oxygen concentration has a direct influence on the species of benthic
foraminifera that occur in sediments [Kaiho, 1994].
Bolivina-dominated assemblages are typical of the most
intense portions of the OMZ in the eastern Pacific and
thrive at dissolved oxygen levels of <0.3 mL/L [Ingle and
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Figure 6. Down-core profiles of Mo concentration: (a) Core JT96-09 from the Vancouver Island Margin
(4854.760N, 12653.440W), (b) Core 1019A from off northern California (4140.9630N, 12455.9790W),
and (c) Core 893A from the Santa Barbara Basin, California (3417.250N, 12002.190W). Molybdenum
data for the California cores are from Ivanochko and Pedersen [2004]. The age model for Core 1019A is
from Mix et al. [1999], and the age model for Core 893A is from Ingram and Kennett [1995]. The
locations of calibrated 14C dates are indicated by the stars.
Keller, 1980; Mullins et al., 1985; Sen Gupta and MachainCastillo, 1993]. In contrast, Uvigerina species prefer a
more oxygenated environment (0.3 to 1.5 mL/L oxygen
[Cannariato and Kennett, 1999]). This relationship has
been documented at many locations along the northeastern
Pacific margin [e.g., Mullins et al., 1985; Quinterno and
Gardner, 1987] and it appears to hold true in the past [Behl
and Kennett, 1996; Cannariato et al., 1999; Cannariato and
Kennett, 1999]. The presence of Uvigerina species also may
be related to a high organic carbon supply to the sediment
[Quinterno and Gardner, 1987], but this association is not
observed if bottom water oxygen is too low [Kaiho, 1994].
We conclude that the occurrence of a benthic foraminiferal
assemblage composed almost exclusively of Bolivina species and Buliminella tenuata, in sediments characterized by
relatively high Mo concentrations, is further evidence that
the bottom water oxygen concentration was lower than at
present during the Allerød (13.5 to 12.6 kyr) and between
11.0 and 10.0 kyr. There is however no evidence that
bottom water oxygen concentration was low during the
Bølling. In fact, it could be argued that bottom water oxygen
concentration was relatively high given that the most
abundant species of benthic foraminifera in Bølling sediments is Epistominella which prefer an oxic environment
(i.e., 0.3 to 1.5 mL/L oxygen [Cannariato and Kennett,
1999]).
[27] OMZ intensification off Vancouver Island brackets
the Younger Dryas, and in this respect the timing is similar
to that observed throughout the North Pacific, with one
notable difference. Intensification of the OMZ off Vancouver Island began in the Allerød while at many other
locations it began much earlier [Cannariato and Kennett,
1999; Zheng et al., 2000; Crusius et al., 2004; Ivanochko
and Pedersen, 2004]. On the California margin, for example, OMZ intensification based on high Mo concentrations,
commenced at 15 kyr while off Vancouver Island it was
delayed until 13.5 kyr, a lag of 1500 years (Figure 6).
There is no prerequisite that the timing of OMZ intensification be the same throughout the North Pacific. In fact,
because of regional differences in surface water productivity
and proximity to sources of NPIW and SSW variability is to
be expected. Even cores that are located relatively close to
one and other can exhibit nonsynchronous behavior [van
Geen et al., 2003]. It remains to be determined whether the
delay observed off Vancouver Island is regional, reflecting
some large-scale oceanographic difference, or local.
5.2. Ventilation Changes?
[28] At present, ventilation of the OMZ along the eastern
margin of the North Pacific reflects a balance between the
input of relatively oxygen-rich North Pacific Intermediate
Water (NPIW) from the northwestern Pacific and oxygen-
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poor Subtropical Subsurface Water (SSW) from the eastern
tropical Pacific. Any change in the supply and/or oxygen
concentration of either the NPIW or SSW would directly
impact the OMZ.
[29] Oxygen-depleted SSW is transported northward
along the west coast of North America as far north as
Vancouver Island by the California Undercurrent [Reed and
Halpern, 1976; Mackas et al., 1987]. Mixing with adjacent
water masses modifies the physical and chemical properties
of SSW as it moves northward (e.g., decreasing temperature
[Halpern et al., 1978] and decreasing d15N values [Kienast
et al., 2002]). Off Vancouver Island the highest percentage
of SSW is observed at a depth of 100 to 300 m where the
California Undercurrent is strongest [Reed and Halpern,
1976]. Weaker flow below 300 m allows increased mixing
of SSW with other water masses; however, SSW is still
recognizable as deep as 1300 m [Reed and Halpern, 1976].
[30] Oxygen-rich NPIW, which forms just east of the Sea
of Okhotsk, occurs throughout the North Pacific Subtropical
Gyre and extends as far north as the Gulf of Alaska. A
number of studies have suggested that enhanced formation
of NPIW can explain the weakening of the OMZ during
cold climatic periods [Duplessy et al., 1988; Keigwin and
Jones, 1990; Kennett and Ingram, 1995; van Geen et al.,
1996; Behl and Kennett, 1996; Keigwin, 1998; Zheng et al.,
2000]. Better ventilation of NPIW during the last glacial has
been inferred from relatively high d13C values of benthic
foraminifera which suggest the presence of a younger,
relatively nutrient-poor intermediate water mass in the
northwest Pacific [Duplessy et al., 1988; Keigwin, 1998].
Radiocarbon data for coeval benthic and planktonic foraminifera (i.e., benthic-planktonic age differences) for cores
from the northwestern Pacific also suggest increased ventilation between 17 and 13 kyr and during the Younger
Dryas, as well as decreased ventilation during the BøllingAllerød [Duplessy et al., 1989; Ahagon et al., 2003].
Radiocarbon data from the Santa Barbara Basin (Core
893A, 588 m), on the eastern side of the Pacific, also
indicate increased ventilation during the Last Glacial Maximum and Younger Dryas [Ingram and Kennett, 1995;
Kennett and Ingram, 1995]. However, radiocarbon data
from cores collected on the open California margin are
more ambiguous. Core F2-92-P3, taken from 800 m (35N),
yields evidence of decreased ventilation between 11 and
9 kyr [van Geen et al., 1996]. In contrast, 14C measurements
made on ODP Core 1019 collected from the continental
slope (980 m water depth) off northern California (41N)
suggest increased, not decreased, ventilation during the
early Holocene and Bølling-Allerød at the same time as
OMZ intensification occurred [Mix et al., 1999].
[31] The age of intermediate water off Vancouver Island is
recorded by benthic foraminifera in Core JT96-09 while the
age of surface water is recorded by planktonic foraminifera.
The difference in the ages of benthic and planktonic
foraminifera obtained from the same sample therefore
establishes the age difference between the intermediate
and surface waters. If intensification of the OMZ inferred
from trace metal and benthic foraminifera data was the
result of a substantial decrease in ventilation, the benthicplanktonic age difference should be greater, reflecting the
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reduced influence of relatively young NPIW and the
increased influence of older, oxygen-depleted SSW.
[32] Direct determination of the modern benthic-planktonic (B-P) age difference is not possible because of the lack
of foraminifera in surface sediments. However, using D14C
water column data of Östlund and Stuiver [1980] for the
North Pacific off the coast of California we estimate that
benthic foraminifera from a depth of 900 m off Vancouver
Island should be 1780 years old or perhaps slightly
younger given that our study area lies closer to regions of
NPIW ventilation. The age of planktonic foraminifera
presently growing within surface waters (i.e., reservoir age
of surface water) is 800 years [Robinson and Thompson,
1981; Southon et al., 1990; Southon and Fedje, 2003;
Hutchinson et al., 2004]. Thus the modern B-P age difference should be 1000 years.
[33] The paucity of foraminifera in Core JT96-09 places
some limitations on our use of B-P age differences to infer
changes in ventilation. Most notably, we can say nothing
about changes in ventilation during the Younger Dryas and
Holocene. However, we do have data for those times when
intensification of the OMZ occurred. With the exception of
one data point (sample 2), most of the B-P age differences
are similar to, or slightly less than, the estimated modern
value of 1000 years (Figure 5). There is no evidence of
decreased ventilation (i.e., larger B-P age differences)
during the first period of OMZ intensification (i.e., Allerød;
Figure 5). In fact, the B-P age difference decreases slightly
from 860 to 610 years throughout this interval. This
decrease implies slightly better ventilation of the OMZ,
which is inconsistent with trace metal and foraminiferal data
that indicate the exact opposite. The explanation for this
apparent contradiction is discussed below. The only substantial decrease in ventilation appears to occur just after the
Younger Dryas and is coincident with the second period of
OMZ intensification. This interpretation is based on a single
data point however and there is evidence that the planktonic
age may be anomalously young (Figure 2), leading to the
larger B-P age difference.
[34] When using B-P age differences to infer changes in
ventilation we assume that the reservoir age of the surface
water, and thus the planktonic age, is constant. However,
this is not always true. Fluctuations in atmospheric 14C
concentration and atmosphere-ocean exchange, as well as
changes in oceanic circulation (e.g., upwelling) can all
influence the reservoir age of surface waters. The modern
reservoir age in the northeast Pacific is 800 years and has
not changed significantly since the Younger Dryas [Southon
et al., 1990; Southon and Fedje, 2003]. There is however
evidence of larger reservoir ages (900 to 1300 years)
during the late glacial and early deglacial [Kovanen and
Easterbrook, 2002; Hutchinson et al., 2004]. These authors
suggest that melting of Cordilleran ice might have supplied
relatively old (i.e., 14C depleted) CO2 to surface waters,
thereby increasing the reservoir age. If this were the case,
B-P age differences off the west coast of Vancouver Island
should have decreased when rapid retreat of the Cordilleran
ice sheet commenced between 15,000 and 14,000 14C years
[Clague and James, 2002]. Since most of the meltwater
influx occurred prior to the Allerød it was probably not
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responsible for the smaller B-P age differences during the
Allerød. It is more likely that increased upwelling of 14Cdepleted subsurface waters caused the planktonic ages to
increase, thus decreasing the B-P age difference. Upwelling was greatly reduced along the northern and central
portions of the California Current system during the Last
Glacial Maximum, and was reestablished by the Allerød
(13.0 calendar kyr [Sabin and Pisias, 1996]). The lower
B-P age difference observed for the Allerød in Core JT96-09
could reflect the return of upwelling conditions off Vancouver Island. If this hypothesis is correct primary production
should have increased at the same time. We present evidence
in support of this scenario in section 5.3.
5.3. Changes in Productivity?
[35] Modern primary production off the west coast of
Vancouver Island is influenced by large-scale atmospheric
circulation. In late spring to early fall the North Pacific High
drives northerly winds, offshore Ekman transport and
upwelling of nutrient-rich water [Huyer, 1983]. The strength
of these winds and thus upwelling intensity is affected by
the strength of the pressure gradient between the North
Pacific High and the continental thermal low, such that the
larger the gradient the more intense the upwelling [Bakun,
1990]. In winter when the North Pacific High shifts southward from 38N to 28N and is replaced by the Aleutian
Low, winds switch direction and upwelling ceases north of
40N [Huyer, 1983].
[36] Climate modeling and paleoevidence suggest that
during the Last Glacial Maximum the North Pacific High
was positioned further south in summer [COHMAP
Members, 1988; Thunell and Mortyn, 1995; Mortyn et al.,
1996; Sabin and Pisias, 1996; Doose et al., 1997], a situation
analogous to modern winters. As a result, upwelling, and thus
primary production, along the central and northern portions of
the California Current system were greatly diminished during
the Last Glacial Maximum [Dymond et al., 1992; Lyle et al.,
1992; Sancetta et al., 1992; Ortiz et al., 1997; Dean and
Gardner, 1998; Mix et al., 1999]. Off Vancouver Island the
accumulation rate of marine organic matter was also relatively
low during the late glacial (Figure 7) [McKay et al., 2004].
However, during the Bølling-Allerød (14.3 to 12.6 kyr)
marine organic carbon accumulation increased substantially
(Figure 7). The most dramatic increase (i.e., an apparent
sixfold increase relative to late the glacial) occurred in the
Allerød, coincident with the first period of OMZ intensification. A small increase in the marine organic carbon accumulation rate is also evident during the second period of OMZ
intensification (11.0 to 10.0 kyr; Figure 7). On the basis of the
marine organic carbon record and the B-P age differences we
conclude that OMZ intensification during the Allerød was the
result of increased primary productivity, rather than decreased
ventilation. Enhanced productivity was most likely caused by
the onset of upwelling off Vancouver Island as atmospheric
circulation switched from a glacial mode (i.e., influenced by
the Aleutian Low throughout the year) to an interglacial mode
(i.e., influenced by the North Pacific High from late spring to
early fall). Upwelling of relatively old waters also may
explain why B-P age differences decrease slightly during
the Allerød.
Figure 7. Marine organic matter concentrations (wt %)
(open squares) and mass accumulation rates (MAR) (solid
squares) for the period between 16 and 8 kyr for Core JT9609. Data are from McKay et al. [2004].
[37] Despite apparently high organic carbon mass accumulation rates during the Bølling there is no evidence of
OMZ intensification at this time. It is possible that the high
organic carbon accumulation rates are an artifact of how
mass accumulation rates are calculated. During the Bølling
the sedimentation rate was exceptionally high (169 cm/kyr)
leading to high organic carbon mass accumulation rates even
though the organic carbon concentration did not increase. In
comparison, during the Allerød both the concentration and
accumulation of marine organic matter increased (Figure 7).
It is also possible that the OMZ was slightly better ventilated
during the Bølling and that this counterbalanced the increase
in organic carbon flux to the sediment. A small change in the
ventilation would not be detected when using B-P age
differences because of their large associated errors.
[38] It could be argued that rather than causing intensification of the OMZ, increased organic carbon burial during
the Allerød was the result of better preservation of recalcitrant organic matter because of lower oxygen concentrations
in the bottom water [e.g., Dean et al., 1994; Zheng et al.,
2000]. In the geological record, laminated sediments commonly have high organic carbon contents and this led to the
hypothesis that organic matter preservation is enhanced in
anoxic environments [Emerson, 1985]. The presumption
being that anaerobic bacteria are less efficient at degrading
complex organic molecules. However, the sediments in
Core JT96-09 are bioturbated, even during periods of
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inferred OMZ intensification, and thus bottom waters never
became anoxic. Therefore enhanced preservation of organic
matter resulting from anoxic bottom waters cannot explain
increased organic carbon burial during the Allerød. More
recently, it has been suggested that high sedimentation rate
and low oxygen concentration work together to enhance
organic matter preservation by controlling the length of time
that refractory organic compounds are exposed to oxygen
[Hedges and Keil, 1995; Gélinas et al., 2001]. We have
estimated oxygen exposure times (OETs) for the Holocene,
Allerød, Bølling and the Late Glacial at Site JT96-09
(Table 2). If the oxygen penetration depth remained constant,
then OETs for the Allerød and Bølling are similar (i.e., 1.7
and 1.3 years, respectively). If the oxygen penetration depth
decreased in the Allerød, as the trace metal data imply, but
remained the same for the Bølling, computed OETs remain
similar (i.e., <1 and 1.3 years). To establish a substantial
difference in OETs would required a deeper oxygen penetration depth during the Bølling, which is unlikely given the
higher sedimentation rate at the time. There is also no
geochemical evidence to suggest that oxygen penetrated
more deeply during the Bølling. Sedimentary Mn/Al
ratios, for example, are relatively low during both periods
(Figure 3a) implying that near-surface sediments were continuously suboxic throughout the Bølling-Allerød. We cannot rule out the possibility that a combination of high
sedimentation rate and low bottom water oxygen concentration played a role in enhancing organic matter accumulation
during the Allerød. However, the large increase in organic
matter accumulation during the Allerød relative to the
Bølling, given that OETs were probably similar, suggests
that high export productivity was the dominant factor leading
to high organic carbon accumulation. This conclusion is
supported by data for other paleoproductivity proxies (e.g.,
biogenic barium and opal) [McKay et al., 2004].
6. Summary
[39] Trace metal and benthic foraminifera species data
indicate that the OMZ in the northeastern Pacific off
Vancouver Island, Canada was more intense (i.e., more
oxygen depleted) relative to modern conditions between
13.5 and 12.6 kyr (i.e., the Allerød) and again between
11.0 and 10.0 kyr. The timing of OMZ intensification is
similar to that throughout the North Pacific (i.e., bracketing
the Younger Dryas), with one notable difference. For
reasons that are not presently known intensification appears
to have been delayed by 1500 years off Vancouver Island.
[40] Radiocarbon dating of benthic-planktonic foraminiferal pairs indicate that between 16.0 and 12.6 kyr ventilation of the intermediate water mass (920 m water depth)
off Vancouver Island was similar to that at present. There is
no evidence of decreased ventilation during the Allerød
when the first period of OMZ intensification occurred.
There is however ample evidence that primary productivity
increased dramatically in the Allerød, most probably related
to the onset of seasonal upwelling as atmospheric and
oceanic circulation switched from a glacial to an interglacial
mode. In summary, it appears that marine productivity,
rather than ventilation, was the dominant control on OMZ
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Table 2. Oxygen Exposure Times for Sediments in Core JT96-09
Time Period
Sedimentation Rate,
cm/kyr
Oxygen Penetration
Depth,a cm
OET,a,b
years
Holocene
Allerød
Bølling
Late glacial
5
116
150
48
0.2c
0.2 (0.1)
0.2 (1.0)
0.2 (1.0)
40
1.7 (<1)
1.3 (7)
4.2 (20)
a
A second set of oxygen penetration depths and resulting OET values are
given in parentheses.
b
Oxygen exposure time (OET) is oxygen penetration depth divided by
sedimentation rate [Hedges and Keil, 1995].
c
The oxygen penetration depth of 0.2 cm is based on the thickness of the
brown ‘‘fluff’’ layer observed in Multicore JT96-09. The transition from
brown to greenish sediments is commonly assumed to represent the oxicsuboxic boundary.
intensity off Vancouver Island during the Allerød. During
the second period of OMZ intensification (11.0 to 10.0 kyr)
decreased ventilation, in combination with increased productivity, may have played a role; however, more data are
required to verify this conclusion.
[41] Whether or not increased marine productivity played
a major role in OMZ intensification throughout the North
Pacific is still debatable. The period from 13 to 8 kyr
(excluding the Younger Dryas) was generally a time of high
marine productivity along the eastern margin of the North
Pacific [Lyle et al., 1992; Ganeshram, 1996; Gardner et al.,
1997; Dean and Gardner, 1998; Mix et al., 1999; Hendy et
al., 2004; Ivanochko and Pedersen, 2004; Ortiz et al., 2004;
this study], as well as in the Northwest Pacific [Keigwin et
al., 1992; Crusius et al., 2004] and Gulf of Alaska [de
Vernal and Pedersen, 1997]. However, in the Santa Barbara
Basin (California Borderlands region) there is no conclusive
evidence that productivity increased during periods of OMZ
intensification [Behl and Kennett, 1996], except in the
Holocene [Ivanochko and Pedersen, 2004]. This is in
contrast to Site 1017E just 50 km north of Santa Barbara
Basin [Hendy et al., 2004; Ivanochko and Pedersen, 2004].
Upwelling within the California Borderlands region is almost absent in summer when it is strongest along the
northern and central portion of the California Current System
[Hickey, 1998]. Instead, upwelling occurs in winter and is
driven by local offshore winds. As a result primary productivity within the Borderlands region is generally lower than
primary productivity off northern California [Thomas et al.,
1994]. This many explain why paleoproductivity records
from the Santa Barbara Basin do not correlate with similar
records from elsewhere within the California Current System, most notably Site 1017E. It also may make Santa
Barbara Basin an ideal location for detecting subtle changes
in ventilation that are lacking or obscured elsewhere.
[42] Acknowledgments. This work was supported by NSERC
through the Canadian JGOFS program and the Climate System History
and Dynamics program. The authors would like to thank the officers and
crew of the Canadian Coast Guard Ship the John Tully for their assistance
in collecting cores. We are grateful to K. Gordon, B. Mueller, B. Nielsen,
and M. Soon (University of British Columbia) for their assistance in the
laboratory. Samples for benthic foraminiferal counting were provided by
A. de Vernal and C. Hillaire-Marcel (Centre GEOTOP, UQAM). We would
also like to thank K. Cannariato for his assistance in the identification of
the benthic foraminifera. Last, but not least, we would like to thank the
reviewers. Their thorough and thoughtful reviews helped to greatly improve
this paper.
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