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Keywords:

  • oxygen minimum zone;
  • paleoceanography;
  • northeast Pacific

Abstract

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Materials and Age Model
  5. 3. Methods
  6. 4. Results
  7. 5. Discussion
  8. 6. Summary
  9. Acknowledgments
  10. References
  11. Supporting Information

[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.

1. Introduction

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Materials and Age Model
  5. 3. Methods
  6. 4. Results
  7. 5. Discussion
  8. 6. Summary
  9. Acknowledgments
  10. References
  11. Supporting Information

[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 Hemphill-Haley, 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 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 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 42°N. 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.

image

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.

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[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 oxygen-sensitive 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

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Materials and Age Model
  5. 3. Methods
  6. 4. Results
  7. 5. Discussion
  8. 6. Summary
  9. Acknowledgments
  10. References
  11. Supporting Information

[7] Sediment cores were collected from the continental slope west of Vancouver Island, British Columbia, Canada (48°54.76′N, 126°53.44′W; Figure 1) during a 1996 Canadian Joint Global Ocean Flux Study cruise. Data are 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).

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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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Table 1. Radiocarbon Ages of Planktonic and Benthic Foraminifera in Core JT96-09
SampleDepth, cmCalendar Age, kyra14C Age Mixed Planktonics14C Age BenthicsBenthic-Planktonic Age Difference, years
UvigerinaBolivina
  • 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].

147.510.039760 ± 7010710 ± 6010650 ± 90950 ± 92, 890 ± 114
257.5 9360 ± 240 10910 ± 701550 ± 250
377.512.2411210 ± 120   
487.512.7311500 ± 110 12110 ± 80610 ± 136
5102.512.8411600 ± 80   
6112.513.1712460 ± 120 13170 ± 100710 ± 156
7142.513.4312640 ± 90 13500 ± 80860 ± 120
8261.514.1413410 ± 8014290 ± 110 880 ± 136
9286.514.3013520 ± 7014350 ± 120 830 ± 139
10346.515.5714140 ± 7014830 ± 280 680 ± 289

[10] Alkenone paleothermometry measurements previously conducted on Core JT96-09 yielded evidence of rapid 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., 14C 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

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Materials and Age Model
  5. 3. Methods
  6. 4. Results
  7. 5. Discussion
  8. 6. Summary
  9. Acknowledgments
  10. References
  11. Supporting Information

[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-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, 1σ) 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 μm 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 δ13C value of total organic matter. Then, by assuming samples contained a mixture of marine and terrestrial organic carbon and that each end-member had a distinctive δ13C 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

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Materials and Age Model
  5. 3. Methods
  6. 4. Results
  7. 5. Discussion
  8. 6. Summary
  9. Acknowledgments
  10. References
  11. Supporting Information

[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 μg/g, Cd from <0.1 to 1.4 μg/g, Re from <1 to 65 ng/g and U from 1.1 to 5.8 μg/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.

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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 μg/g Mo and 0.2 μg/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 μg/g U [Morford et al., 2001]. The average shale Mn/Al ratio of is 0.010 [Turekian and Wedepohl, 1961].

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[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).

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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.

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[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 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 benthic-planktonic age differences. Radiocarbon data were obtained for nine benthic-planktonic foraminiferal pairs spanning the period between 10.0 and 15.6 kyr (Table 1). Benthic-planktonic 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).

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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.

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5. Discussion

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Materials and Age Model
  5. 3. Methods
  6. 4. Results
  7. 5. Discussion
  8. 6. Summary
  9. Acknowledgments
  10. References
  11. Supporting Information

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 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 μM H2S, molybdate (MoO4−2) is converted to thiomolybdate (MoS4−2) which is scavenged by pyrite [Helz et al., 1996; Erickson and Helz, 2000]. The presence of zero-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, near-surface sediments never become fully anoxic and thus a large Mo enrichment above the typical lithogenic concentration (i.e., ∼0.6 μg/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.

[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 sediment-water 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 Keller, 1980; Mullins et al., 1985; Sen Gupta and Machain-Castillo, 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.

image

Figure 6. Down-core profiles of Mo concentration: (a) Core JT96-09 from the Vancouver Island Margin (48°54.76′N, 126°53.44′W), (b) Core 1019A from off northern California (41°40.963′N, 124°55.979′W), and (c) Core 893A from the Santa Barbara Basin, California (34°17.25′N, 120°02.19′W). 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.

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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-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 δ15N 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 δ13C 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 Bølling-Allerød [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 (35°N), 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 (41°N) 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 benthic-planktonic age difference should be greater, reflecting the 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 Δ14C 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 responsible for the smaller B-P age differences during the Allerød. It is more likely that increased upwelling of 14C-depleted 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 ∼38°N to ∼28°N and is replaced by the Aleutian Low, winds switch direction and upwelling ceases north of ∼40°N [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.

image

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 JT96-09. Data are from McKay et al. [2004].

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[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 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].

Table 2. Oxygen Exposure Times for Sediments in Core JT96-09
Time PeriodSedimentation Rate, cm/kyrOxygen Penetration Depth,a cmOET,a,b years
  • 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 oxic-suboxic boundary.

Holocene50.2c40
Allerød1160.2 (0.1)1.7 (<1)
Bølling1500.2 (1.0)1.3 (7)
Late glacial480.2 (1.0)4.2 (20)

6. Summary

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Materials and Age Model
  5. 3. Methods
  6. 4. Results
  7. 5. Discussion
  8. 6. Summary
  9. Acknowledgments
  10. References
  11. Supporting Information

[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 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.

Acknowledgments

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Materials and Age Model
  5. 3. Methods
  6. 4. Results
  7. 5. Discussion
  8. 6. Summary
  9. Acknowledgments
  10. References
  11. Supporting Information

[42] 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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  2. Abstract
  3. 1. Introduction
  4. 2. Materials and Age Model
  5. 3. Methods
  6. 4. Results
  7. 5. Discussion
  8. 6. Summary
  9. Acknowledgments
  10. References
  11. Supporting Information
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Supporting Information

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Materials and Age Model
  5. 3. Methods
  6. 4. Results
  7. 5. Discussion
  8. 6. Summary
  9. Acknowledgments
  10. References
  11. Supporting Information
FilenameFormatSizeDescription
palo1117-sup-0001-t01.txtplain text document1KTab-delimited Table 1.
palo1117-sup-0002-t02.txtplain text document1KTab-delimited Table 2.

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