Benthic Foraminiferal Assemblages
Changes in the relative abundances (frequency abundance) of benthic foraminiferal species in the late Quaternary sequences are shown in Figures S1–S3, as are their relationships to TOC, CaCO3, and grain size. The relative abundances of benthic foraminifera in the two cores (Figure S1) show strong similarity in trends even though each sequence contains different relative abundances of taxa. Assemblages in MD02-2503 reflect changes in basin-floor conditions during the last 34 kyr, whereas assemblages in MD02-2504 record environments close to sill depth (currently ~475 m) over the last 24 kyr.
Benthic foraminiferal assemblages in both cores changed dynamically on decadal through orbital timescales, and both the general and detailed patterns of change are well correlated between the cores (Figures S2 and S3). Benthic assemblages are distinctly different between warm and cold intervals. Frequent faunal changes during the Holocene are unassociated with major climate change and not matched in the relatively stable patterns of δ18O exhibited in the SBB [Hendy and Kennett, 1999] and Greenland records [Dansgaard et al., 1993; Stuvier and Grootes, 2000] (Figures 4 and 5). Blackman-Tukey spectral analysis of both cores shows weak cyclicity in % dysoxic species with 1350–1380 year periodicity during the Holocene (Figure S4).
The numbers of counted specimens of benthic foraminifera are shown in Figure 2a and listed in Tables S1 and S2. Ideally, 300 specimens should be counted for each sample. However, several laminated levels contain few to no foraminifera, and consequently, 109 of the 504 total samples with <50 specimens (Tables A1 and A2) were included in the census. Many samples with low benthic abundances contain well-preserved planktic foraminifera, indicating that postdepositional benthic dissolution was not a significant factor and that the numbers reflect past seafloor environmental conditions rather than poor preservation (Figure S5). The overall pattern of changing benthic foraminiferal abundance (number of specimens) is similar between the two cores (Figure 2a) and also corresponds well to variation in the weight % CaCO3 (Figure 2b), reflecting the predominance of benthic foraminifera in the sedimentary carbonate fraction. There is strong similarity of major trends between the ODP Site 893 and MD02-2503 records.
Figure 2. (a) Numbers of counted specimens, (b) weight % calcium carbonate, (c) number of counted species of benthic foraminifera in samples from cores MD02-2503 and MD02-2504, plotted by age, (d) the Shannon index (H) of benthic foraminifera, and (e) the abundance ratio of planktic foraminifera/benthic foraminifera. Climatic intervals indicated as L. Holocene: Late Holocene; M. Holocene: Middle Holocene; E. Holocene: Early Holocene; PB: Pre-Boreal warming of earliest Holocene; B/A: Bølling-Allerød warming; preB: pre-Bølling warming; H1: Heinrich cold event 1; LGM: Last Glacial Maximum, and IS: Interstadial warm episodes 6 through 2.
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The number of species counted (simple diversity) and the species diversity (Shannon index) in each sample are shown in Figures 2c and 2d. The time series shows large fluctuations in diversity (H = 0 to 3). These are well matched between the two cores except during the early Holocene (9.5 to 8 ka; Figure 2c) and in parts of the LGM. Thus, it appears that these data reflect widespread changes in simple species diversity in SBB during the latest Quaternary. With the exception of a few intervals in the Holocene, higher diversity and greater species number are generally associated with cooler intervals and lower diversity and number with warmer intervals. The species diversity at the deeper site (MD02-2503) is low in the early Holocene, B/A, and IS3-IS6 but especially from 11 to 9 ka. However, in the shallower site close to sill depth (MD02-2504), diversity values are high (~2) in the same interval. This reflects strong vertical differences in diversity at this time with low species diversity at the more dysoxic deeper site. After 9 ka, species diversity gradually increased with similar trends exhibited in each of the cores in response to increased basin oxygenation.
Planktic/benthic foraminiferal ratios (P/B) peaked in the Pre-Boreal and B/A (Figure 2e), reflecting decreased production of benthic foraminifera. Intervals of high P/B ratio are most closely related to intervals of relative warmth and high carbonate content (Figure 2). However, several negative spikes of P/B ratio are exhibited at the onset of abrupt warming episodes IS2-6, B/A, and Pre-Boreal. These spikes may reflect major reduction in planktic foraminiferal production during the onset of abrupt warming and/or increase in sea surface salinity.
For better understanding of the history of basin oxygenation in MD02-2503 and MD02-2504, we grouped species into assemblages ranked according to how they reflect bottom water oxygenation (Table S4 and Figures 3 and 5) following the example of recent studies that have moved toward quantitative interpretations of foraminiferal assemblages [e.g., Jorissen et al., 2007]. These rankings were established using available hydrographic data showing oxygen ranges and tolerance for each species (Table S4 and references therein; Moffitt et al., in review). These groupings build on (and broadly agree with) rankings previously established by Cannariato and Kennett  and Cannariato et al.  and are further supported by Principle Components Analysis, discussed below.
Figure 3. Changes in relative percent abundances of benthic foraminifera as three oxygenated-ranked groups: Weakly hypoxic-oxic, Suboxic, and Dysoxic in late Quaternary cores (a) MD02-2504 and (b) MD02-2503, compared with benthic foraminiferal assemblages in (c) ODP 893A in the SBB [Cannariato et al., 1999] and (d) ODP 1017E off Point Conception [Cannariato and Kennett, 1999], and (e) Greenland (GISP2 δ18O) climatic record. Stratigraphic distribution of individual species in both MD cores is shown at high resolution in Figures S2 and S3. Abbreviations as in Figure 2.
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The Dysoxic assemblage contains Nonionella stella, Bolivina tumida, Buliminella tenuata, Bolivina pacifica, Bolivina pseudobeyrichi, Cassidulina delicata, and Cassidulina limbata. This assemblage is considered to be associated with severely dysoxic conditions (O2 = <0.1 mL/L), even more so than in the modern basin (O2 = 0.1–<2 mL/L), in which Bolivina tumida is not observed and N. stella is the dominant foraminifera [Bernhard and Bowser, 1999]. This assemblage grouping is based upon a combination of lowest reported oxygen concentration where the individual species were observed (0.1 to 0.02 ml/L, depending upon species [Harman, 1964; Douglas and Heitman, 1979; Mackensen and Douglas, 1989; Jannink et al., 1998; Bernhard et al., 2001; Bernhard et al., 2010]) and associations with methane-rich environments, preserved laminations, and symbionts or sequestered plastids [Bernhard and Bowser, 1999; Cannariato et al., 1999; Bernhard et al., 2000; Bernhard et al., 2001; Stott et al., 2002; Hill et al., 2003]. This grouping replaces the Dysoxic I and II assemblages of Cannariato and Kennett .
The suboxic assemblage (O2 = 0.1 to 0.5 mL/L) contains Globobulimina spp., Bolivina argentea, Bolivina spissa, Cancris sp., Cassidulina carinata, Epistominella pacifica, Epistominella smithi, Uvigerina peregrina curticosta, and Valvulineria araucana. This grouping is based upon the lowest reported oxygen concentration where individual species were observed (from 0.2 to 0.5 mL/L for most species [Smith, 1964; Blake, 1976; Douglas and Heitman, 1979; Quinterno and Gardner, 1987; Mackensen and Douglas, 1989; Mullins et al., 1985; Maas, 2000]), in addition to documented associations with the open slope OMZ, methane-rich environments, laminated sediments, and/or presence of symbionts or sequestered plastids [Ingle and Keller, 1980; Douglas, 1981; Quinterno and Gardner, 1987; Bernhard et al., 1997; Cannariato et al., 1999; Rathburn et al., 2000; Bernhard et al., 2001; Hill et al., 2003]. This grouping replaces Suboxic I and II groups found in Cannariato et al. .
The Weakly Hypoxic-Oxic assemblage contains Nonionella labradorica, Pyrgo spp., and Quinqueloculina spp. We consider this assemblage to represent O2 concentrations >1.5 mL/L, based upon lowest reported oxygen concentrations for these three species [Alve, 1990; Cedhagen, 1991; Kaiho, 1994] and the observation that these are thick-walled, predation-resistant, motile fauna more likely to occur outside of the OMZ [Linke and Lutze, 1993; Kaiho, 1994].
To assist in differentiating the taxonomic groupings making up these communities, we conducted a principal component analysis using 87 species with abundances of more than 1% in at least one sample. Principal component analysis indicates that 56% of the total variance can be expressed by three principal components (PC) (supporting information appendix). PC1 explains 22% of the variance with Bolivina tumida displaying the highest loadings, supported by other high loadings in Bolivina argentea, Buliminella tenuata, and Uvigerina peregrina curticosta (Table S5). Thus, PC1 reflects variations in the Dysoxic and Suboxic assemblages. Trends in PC1 are similar between the two cores with especially high values in MD02-2503 during the early Holocene, B/A, and IS3 to 6, reflecting the most extreme, poorly oxygenated conditions in this sequence (Figures 4 and 5).
Figure 4. (a) Time series changes in score values of three PC in principal component analysis of benthic foraminifera in cores MD02-2503 and MD02-2504 compared with foraminiferal δ18O records of (b) MD02-2503 [Hill et al., 2006a], (c) planktic foraminiferal δ13C records of ODP 893A and MD02-2503 [Hill et al., 2006b; Hendy and Kennett, 2003], (d) benthic foraminiferal δ13C records of ODP 893A [Kennett et al., 2000], (e) Behl dissolved oxygen (DO) index in cores MD02-2503 and MD02-2504, (f) Schmiedl DO index in cores MD02-2503 and MD02-2504, (g) benthic foraminiferal δ13C records in core MR01-K03 PC4 in northeastern Pacific [Hoshiba et al., 2006], and (h) Δ14C intermediate waters off Baja California [Marchitto et al., 2007]. Abbreviations as in Figure 2.
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Figure 5. Changes in relative percent abundances of benthic foraminiferal assemblages grouped into oxygenated-ranked groups for cores (a) MD02-2504 and (d) MD02-2503. Patterns and abbreviations as in Figure 3. Percentages total organic carbon (TOC), calcium carbonate (CaCO3), silt (grain size) for cores (e) MD02-2503 and (b) MD02-2504 and bioturbation index for (f) MD02-2503 and (c) MD02-2504: 1 = well-laminated sediments, 2 = indistinctly laminated sediments, 3 = sediments with trace laminations, 4 = bioturbated sediments [Behl, 1995].
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PC2 explains 19% of the variance, with the highest loadings exhibited by Nonionellina labradorica, supported by high loadings in Epistominella pacifica and U. peregrina curticosta. These associations indicate that PC2 reflects suboxic, weakly hypoxic, and oxic environments, with highest values exhibited during cold episodes including the LGM, H1, and YD (Figure 4). PC2 exhibits similar values in the LGM, H1, YD, and the late early to early middle Holocene between the two cores. However, PC2 in MD02-2504 reveals higher values during the B/A, PB, and late Holocene compared with MD02-2503, implying the presence of higher oxygen levels at the shallower sill-depth site.
PC3 explains 15% of the variance, with the highest loadings in Bolivina argentea, supported by high loadings in U. peregrina curticosta, and low loadings in Nonionellina labradorica. Thus, PC3 reflects a transitional assemblage between dysoxic and suboxic but is lacking the extreme dysoxic indicator species, B. tumida. This assemblage likely reflects conditions similar to the modern OMZ in SBB. PC3 also reveals intermittent especially high Holocene values in MD02-2503, reflecting millennial and possibly centennial-scale dysoxic episodes in the deep basin during this interval (Figure 4).
TOC, CaCO3, Silt Content, and Bioturbation Index
Sedimentary geochemistry and texture also vary through the record, with similarities between the two cores (Figure 5). TOC (1.1 to 4.5 wt.% for MD02-2503, 1.1 to 3.7 wt.% for MD02-2504) displays a very similar pattern in both cores, gradually increasing from the last glacial episode though the late Holocene. No distinct fluctuations in TOC are evident between stadial and interstadial intervals, or at glacial terminations; a slight and gradual increase in TOC is observed toward the late Holocene (Figure 5). CaCO3, likewise, is quite similar in both cores (0 to 7.6 wt.% for MD02-2503, 0.4 to 9.6 wt.% for MD02-2504) but displays more variation in relation to climatic change (Figures 2 and 5). CaCO3 is more abundant through the entire glacial interval to about 17 ka, followed by a decrease to a minimum between 15 and 11 ka (from Termination 1A to 1B), increasing again to values greater than the glacial episode by 10 ka and persisting at higher levels through the early Holocene. The two cores differ somewhat through the remainder of the Holocene, with carbonate remaining high in MD02-2503, but decreasing to the present in core MD02-2504. As with TOC, CaCO3 does not evidently vary with stadial-interstadial oscillations during the last glacial interval (Figures 2 and 5).
Grain size of the siliciclastic portion of the sediment is quite different between the two core locations (Figure 5). In MD02-2503, grain size of the <63 µm fraction (37% to 57% silt) is relatively constant for the past 34 kyr but displays short-lived peaks (<few hundred years), generally during cold intervals at approximately 24, 16, 13, 12, 10, and 1 ka. In contrast, at shallower site MD02-2504, the sediment is slightly coarser (41% to 61% silt) and displays a distinct pattern related to climatic changes. The sediment is consistently coarser during the colder intervals associated with the entire glacial and YD intervals from 24 to 14.5 ka and 12.9 to ~11 ka (Figure 5). Sediment becomes distinctly finer during the B/A from 14.7 to 12.9 ka and throughout the Holocene from ~11 ka to present. In some cases, individual benthic foraminiferal species become more abundant in association with increased grain size during colder intervals that likely reflected lower sea level in the shallower and more proximal core MD02-2504 (Tables S2 and S3 and Figure 5).
The presence and relative strength of lamination/bioturbation was semiquantitatively described according to the bioturbation index of Behl . Bioturbation index 1 represents well-laminated sediments, index 2 represents indistinctly laminated sediments, index 3 represents sediments with trace laminae, and index 4 represents massive or strongly bioturbated sediments. Bioturbation index 1 represents deposition and preservation of fine laminations under 0–0.1 mL/L O2 and index 2 between ~0.1 and 0.2 mL/L O2 [Savrda and Bottjer, 1991; Diego and Douglas, 1999]. In the basin-center core MD02-2503, well-laminated sediments are associated with warm intervals including the Holocene, B/A, and IS2 to 6. Strongly bioturbated sediments are associated with cold episodes including the LGM, H1, and YD. At the shallower core site MD02-2504, distinct laminations are only preserved during the B/A and IS2. The remainder of the core is massive to distinctly burrowed.