Last glacial maximum paleochemistry and deepwater circulation

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1 PALEOCEANOGRAPHY, VOL. 12, NO. 6, PAGES , DECEMBER 1997 Last glacial maximum paleochemistry and deepwater circulation in the Southern Ocean: Evidence from foraminiferal cadmium Yair Rosenthal Institute of Marine and Coastal Sciences, Rutgers University, New Brunswick, New Jersy Edward A. Boyle Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, Cambridge Laurent Labeyrie Universite Paris-Sud Orsay, Dept des Sciences de la Terre et Centre des Faibles Radioactivites, Centre National de la Recherche Scientifique-Commissariat 'a l'energie Atomique, Gif-sur-Yvette, France Abstract. South Atlantic benthic foraminiferal Cd/Ca shows no glacial-interglacial variation, suggesting that the glacial contribution of North Atlantic Deep Wate r to th e SOUthern OCean was not much different than at present. In contrast, Cd/Ca in southeast Indian Ridge cores show lower glacial bottom water Cd, comparable to levels in intermediate depths of the North Atlantic and significantly lower than in the deep South Atlantic. Low glacial Cd/Ca was also recorded in planktonic foraminifera, suggesting a substantial decrease in the nutrient concentration of Subantarctic surface water during the glacial maximum which most likely was caused by increased biological productivity. The Cd data are inconsistent with low glacial benthic foraminiferal 813C which suggest higher nutrient concentration. We propose that the low Cd/Ca in the Southeast Indian Ridge records reflects a local source of nutrient-depletedeepwater, formed during the last glacial maximum by open-ocean convectio near the Antarctic Polar Front, downstream of the Kerguelene Plateau. If this source was limited to the southeast Indian basin then its impact on the overall chemistry of glacial Circumpolar Deepwater was rather small. However, if during glaciations open-ocean convection became the dominant mode of bottom water formation, it might have had a greater impact on CPDW chemistry. 1. Introduction Ice core records extending back to the penultimate glacial period reveal that the atmosphericarbon dioxide concentration was substantially reduced during glaciations [Barnola et al., 1987]. It is generally accepted that the ocean is the only carbon reservoir which is large enough yet temporarily responsive to control atmospheric pco2 on a glacial-interglacial timescale. Surface waters of polar regions may be critical for regulating atmospheric pco2 because it is there that the deep ocean communicates directly with the atmosphere [Sarmiento and Toggweiler, 1984]; a series of models demonstrated the sensitivity of atmospheric pco 2 to the nutrient distribution in the Antarctic Ocean thereby hypothesizing that increased.efficiency of the "biological pump", resulting in complete utilization of nutrients from the Antarctic surface water, might have i'- tributed to loweringlacial atmospheric pco 2 [ Knox and McElroy, 1984; Sarmiento and Toggweiler, 1984; Siegenthaler and Wenk, 1984]. The higher efficiency could have resulted from either increased biological productivity, a decrease in the rate of exchange between the CO2 and nutrient-rich Circumpolar Deep Water (CPDW) and polar surface water, or any combination of these processes [Knox and McElroy, 1984; Sarmiento and Toggweiler, 1984; Siegenthaler and Wenk, 1984]. Because the chemistry of CPDW is strongly af- Copyright 1997 by the American Geophysical Union. Paper number 97PA /97/97PA fected by the influx of the nutrient-depleted North Atlantic Deep Water (NADW), variations in the flux of NADW into the CPDW could have had a dominant effect on the chemistry of Southern Ocean surface water. Paleochemical studies typically infer past deep ocean circulation from the carbon isotopic and cadmium composition of benthic foraminifera from deep sea sediments. Such reconstructions rely on the close association among the oceanic distributions of nutrients 13C and Cd, which reflect the biological cycling of organic matter as modified by ocean circulation [Kroopnick, 1985; Boyle, 1988]. Indeed, the independent confirmation of reduced NADW formation from studies using both nutrient proxies in the deep Atlantic Ocean have lent great supporto glacial ocean reconstructions and climate models. However, as more data became available, it also became evident that the agreement between foraminiferal Cd and 513C breaks down in the Southern Ocean [Boyle, 1992]. Glacial reconstructions showed large negative shifts in benthic foraminiferal 513C from the Southern Ocean, apparently suggesting higher nutrient concentration in CPDW [Curry et al., 1988; Oppo et al., 1990], possibly caused by the cessation of the NADW flux into the CPDW [Charles and Fairbanks, 1992]. However, this increase is not supported by Southern Ocean Cd records showing glacial values similar to those of today [Boyle, 1992]. Similarly, glacial BadCa records suggesting a small increase in CPDW alkalinity are inconsistent with the 5 3C data [Lea, 1995]. To further investigate this apparent discordance we study two new records from the southeast 787

2 788 ROSENTHAL ET AL.: SOUTHERN OCEAN PALEOCEANOGRAPHY Indian Ocean, located within the core of the CPDW. While the new fil3c records are consistent with previous Southern Ocean records, the new Cd/Ca records show lower ratios during the last glacial maximum (LGM) than during the Holocene thereby implying a glacial decrease in the CPDW nutrient content. We discuss the possibility that Southern Ocean paleochemical records idizing solution, (3) hot basic reducing solution, and (4) multiple weak acid leachings. We discovered that performing the reductive step before the oxidation eliminated the contamination. A more detailed description of the geochemical processes causing the high authigenic Cd enrichments and of the modified cleaning protocol are given elsewhere [Rosenthal et al., 1995a; Boyle and (Cd and 513C) were compromised by artifacts and, alternatively, Rosenthal, 1996]. Note that older Southern Ocean records suggesthat the Cd records indicate the presence of nutrient-depleted deepwater which formed during the LGM in the eastern Indian Ocean. [Boyle, 1992] which are used in this manuscript were reexamined with the modified cleaning technique. Cadmium and manganese were determined by graphite furnace atomic absorption spectrometry (Perkin Elmer model 5000) and calcium was analyzed by flame atomic absorption spectrome- 2. Oceanographic setting try (AAS). Replicates of foraminifera were mn wherever possi- A conspicuous feature of the Southern Ocean hydrography is the tongue of warm, saline, and nutrient-depleted water delineating the flow of NADW into the circumpolar current [Bainbridge, 1980; Spencer et al., 1982]. South of 30øS latitude, the upper layers of NADW spread eastward into Cape Basin extending over the mid-atlantic Ridge as part of the midlatitude anticyclonic gyre [Reid, 1990]. The NADW retains its distinctive characteristics despite vigorous mixing with the underlying Antarctic Bottom Water (AABW) and is thus easily recognizable as a thick, low-po4 tongue between =2000 and 3500 m throughout the South Atlantic and Antarctic Circumpolar Current (ACC; Figure 1). This paper presents four records from the South Atlantic and southeast Indian oceans (Table 1). South Atlantic cores of RC on the southern flank of the Walvis Ridge (WR) and RC in the deep Cape Basin (CB) are located within the modem mixing zone between the NADW and AABW. At presenthe shallower site of RC is primarily under the influence of NADW [Broecker et al., 1991], while the deeper site of RC is overlain to a large extent (=80%) by AABW [Oppo and Fairbanks, 1987]. The two additional cores of MD and MD are from the southeast Indian Ridge (SIR) located downstream of the Kerguelen-Amsterdam passage in the distal edge of the NADW tongue as it propagates through the ACC. Note that the densest water of the lower circumpolar current, returning into the Atlantic Ocean through the Drake Passage, is the nutrient-rich AABW which fills the deep Cape Basin as it flows northward under the NADW [Embley and Morley, 1980; Mantyla and Reid, 1983]. Therefore the four cores are in a good position for monitoring variations in the flow of ble. The pooled analytical uncertainty of Cd/Ca data, as determined by replicate analyses of spiked gravimetric standards over a period of 3 years, is about +8% (20). Manganese levels were measured to monitor the presence of secondary MnCO 3 overgrowths [Boyle, 1983]. Except for a few samples, MnCO 3 levels in the cores studied here are fairly low (Mn/Ca < 150 gmol moll). Samples with higher ratios wer excluded from consideration. The Cd data represent averaged measurements of three benthic species ( Uvigerina spp., Cibicidoides kullenbergi, and Melonis barleeanum). Isotope analyses were carried out in France using a Finnigan MAT 251 with automated carbonate device. The benthic and planktonic foraminiferal 13C data are from Cibicidoides wuellerstorfi and Neogloboquadrina pachydermal., respectively. External reproducibility for duplicate samples is %o (1{ ), and all data are calibrated to PDB (Peedee belemnite) using a carbonate standard from the National Bureau of Standards following (NBS 19; Coplen, 1988]. The isotope and Cd/Ca data from cores MD and MD were electronically archived, and are avilable at the World Data Center-A for Paleoclimatology, 325 Broadway, Boulder, CO; paleo.html; paleo@ngdc.noaa. gov. Isotope and Cd/Ca data from cores RC and RC were published previously [Curry et al., 1988; Oppo et al., 1990; Oppo and Rosenthal, 1994; Boyle and Rosenthal, 1996]. The cores' stratigraphy and chronology are based on correlating their 80 records to Accelerator Mass Spectrometry 14C -dated records in benchmark cores from the Southern Ocean. The ages are expressed in calendar years; radiocarbon ages were corrected for secular variations by using a NADW into the Southern Ocean. dendrochronological and UFFh coral calibration after subtracting 480 years as a reservoir age correction [Bard et al., 1990; 3. Methods Sample preparation and elemental analysis are generally as de- Edwards et al., 1993]. Because of differences in the structure of the isotopic records, the chronologies have an average age uncertainty of =2 kyr. scribed by Boyle and Keigwin [1985]. Recently, however, we discovered a new source of Cd contamination which occasionally was not eliminated by this cleaning protocol: glacial age 4. Results foraminifera from Subantarctic sediments exhibited Cd/Ca ratios Core top calibrations show that Cd/Ca ratios in benthic which were higher than previously encountered in any modem or glacial samples from other oceanic regions [Rosenthal, 1994]. A further investigation showed that high foraminiferal Cd/Ca was associated with sediments containing high levels of authigenic Cd, typically of glacial age [Rosenthal et al., 1995a]. The contamination problem was eliminated by a simple modification of the original cleaning technique. In the original procedure [Boyle and Keigwin, 1985], foraminifera were cleaned by a succession of treatments including (1) ultrasonic agitation, (2) hot basic oxforaminifera are proportional to seawater Cd concentrations but also depend on the calcification depth at water depths between 1 and 3 km [Boyle, 1992]. Core top Cd/Ca ratios in benthic foraminifera from the Southern Ocean fall along the line of Dcd = 2.6 (Figure 2) which is slightly lower but within the uncertainty of the global calibration of cores below 3 km water depth (i.e., DCd=2.9) [Boyle, 1992]. Therefore Cd/Ca records from deep ocean cores (water depth >3 km) can directly be compared without calculating the seawater Cd w content. The SIR and South

3 ROSENTHAL ET AL.' SOUTHERN OCEAN PALEOCEANOGRAPHY 789 Ao &O O CHN82 4PC (.38) '% RC RC I MD [./.... RCI ;_ I)*...L / MD _ t/o _..., B (.37.36) 'ao o o $o loo ao B South Atlantic A looo ACC 2.0 ' SIR E ';, 40S 30S 20S 120E 100E 80E 60E 40E Figure 1. (a) Core location map. Also marked is the location of the cross section (dashed line) as well as the last glacial maximum Cd w values (in parentheses); (b) Phosphate distribution along a South Atlantic-circumpolar cross section. The labels are defined as follows: WR, Walvis Ridge; CB, Cape Basin; and SIR, southeast Indian Ridge). Atlantic foraminiferal Cd/Ca and isotope records (Figures 3 and 4, respectively) are compared with their counterparts from the North Atlantic (NA) and the eastern equatorial Pacific (EEP). Two features in the Cd/Ca records are especially noteworthy. 1. the South Atlantic Cd/Ca records show no discernible glacial-interglacial variation suggesting that the glacial nutrient gradient between the WR and CB sites was similar to the modern one. During the LGM, as today, South Atlantic Cd/Ca was intermediate between the NA and EEP. These observations imply a persistent presence of nutrient-depleted water at the shallower, WR, site. 2. Cd/Ca records from the SIR show glacial levels which are significantly lower than those during the Holocene, comparable to levels observed in the WR and NA sites. In contrast, a slightly

4 790 ROSENTHAL ET AL.: SOUTHERN OCEAN PALEOCEANOGRAPHY Table 1. Core Locations Core Latitude Longitude Depth, m Location CHN 82 4PC 41ø72'N 32ø85'W 3427 North Atlantic (NA) RC ø20 ' S 11ø12 ' E 3204 Walvis Ridge (WR) RC ø30 ' S 11ø20' E 4191 Cave Basin (CB) RC ø52' S 79ø87' E 3135 southeast Indian Ridge (SIR) MD ø45 ' S 82ø56' E 3330 southeast Indian Ridge (SIR) MD ø04 ' S 90ø07' E 3420 southeast Indian Ridge (SIR) TR163-31B 03ø10 ' S 83ø97'W 3210 East Ecluatorial Pacific (EEP) shallower core from the SIR (RC11-120) does not show the low glacial Cd levels; glacial Cd levels in this core are similar to Holocene levels [Boyle, 1992] and comparable with glacialevels in the CB (RC13-229). Note that this pattern was confirmed by the new cleaning meth0d'(table 2) o.oo I o.o i i i I 1.0 Bottom water Cd, nmol kg -1 North Atlantic [3 East Equatorial Pacific!9 South Atlantic Southeast Indian Ridge DCd = DCd = 2.6 Figure 2. Southern Ocean foraminiferal Cd calibration curve. Modem Cd concentrations were estimated from PO4 profiles at the nearest Geachemical Ocean Sections Study (GEOSECS) stations using the global oceanic deepwater Cd-PO4 relationship [Boyle, 1988]. The line Dcd = 2.9 was estimated from the global calibration [Boyle, 1992] whereas Dcd = 2.6 was estimatedfrom the Southern Ocean data. The glacial-interglacial behavior of 513C in CPDW was discussed in detail elsewhere [Curry et al., 1988; Duplessy et al., 1988; Oppo et al., 1990; Boyle, 1992]. The new il3c record is consistent with the previous data: glacial 513C levels in all four Southern Ocean records are substantially lower than during the Holocene and are lower than in the EEP (Figures 3 and 4). Among these cores, MD shows the largest drift with an average LGM 5 3C value of-0.7%0, which is = %0 lower than glacial 5 3C in the EEP. This pattern is consistent with the -0.7%0 glacial drift observed in the nearby core of MD [Labeyriet al., 1996]. Interestingly, glacial 5 3C values in RC are significantly more positive than in other SIR cores (MD and MD88-769) and comparable with the deep Cape Basin [Curry et al., 1988]. Evidently, both Cd/Ca and 5 3C records of RC are very different than the records of MD and MD even though the three sites are from the same region. Although the sampling resolution in RC is lower than in the other two cores, it seems unlikely that two different studies both missed the true LGM level. After correct- ing for a whole ocean change of-0.4%0 [Curry et al., 1988; Duplessy et al., 1988] we obtain glacial-interglacial il3c shifts of =-0.4 to -0.5%0 in the South Atlantic and =-0.7%0 in the Southeast Indian Ocean. If strictly related to nutrient re-distribution, these negative shifts translate to glacial PO 4 concentrations of gmol kg -1 in the WR and CB sites, respectively (relative to modem concentrations of 1.6 to 2.2 gmol kg-, respectively), and =3.0 gmol kg -1 in the SIR (relative to 2.1 gmol kg -1 at present). These levels are higher than estimated for the deep equatorial Pacific and therefore cannot be explained by a simple decrease of the NADW flux [Curry et al., 1988]. Low Cd/Ca ratios were obtained also in glacial planktonic foraminifera from the SIR (Figure 3), suggesting lower surface water nutrient concentrations during the glacial period. Glacial Cd/Ca ratios in Globigerina bulloides from this core are about half the Holocene ratios (0.025 and 0.06 gmol mol-1 during the glacial and late Holocene periods, respectively). Likewise, Cd/Ca ratios in N. pachyderma I. from the same core show a factor of 2 increase from =0.04 to 0.09 gmol mol - during the transition from the last glaciation to the Holocene. Similar behavior was observed in the Pacific, Subantarctic core E11-2 (56ø03'S 115ø04'W) [Mashiotta and Lea, 1995]. The planktonic foraminiferal Cd/Ca records provide a strong evidence that the increased glacial productivity in the Subantarctic zone[charles et al., 1991; Francois et al., 1993; Rosenthal et al., 1995a; Bareille et al., 1997] resulted in a substantial reduction of the nutrient concentration in Subantarctic surface water. The new Cd/Ca and

5 ROSENTHAL ET AL.' SOUTHERN OCEAN PALEOCEANOGRAPHY Table 2. Cd/Ca Ratios in Uvigerina spp. From RC11-120, (1) and Modified (2) Cleaning as Obtained by the Original Technique. a. o N. pachydermal. Age Depth, Cd/Ca, gmol mol -] cm 1 a 1 a 2 b 2 b Holocene C. wuellerstorfi ' I ' I ', LGM c a From Boyle [1992]. b From this study. c LGM is the last glacial maximum ,.i..., '... '' significance C. wuellerstorfi 1.5, ' i ', 513C records from the SIR are apparently out of phase. The increase in benthic foraminiferal Cd/Ca ratios leads the northern hemisphere deglaciation, starting at about ka and reaching maximum levels at ka. The increase planktonic foraminiferal Cd/Ca leads the benthic foraminiferal record and is in phase with the N. pachyderma 1. 8]80 record. However, because of the relatively large uncertainty in the planktonic foraminiferal Cd/Ca record (--_+0.01), it is difficult to assess the of these relationships. In contrast, the benthic foraminiferal 813C and 8]80 records are in phase Discussion Possible Mechanisms for Decoupling fi13c and Cd in Seawater d, ' I ' I ' I The glacial distribution of ]3C and Cd in CPDW appears to be irreconcilable by current hypotheses of glacial deepwater circulation. The large glacial nutrient enrichment, inferred from foraminiferal ]3C (using a simple nutrient analogy), is not supported by the Southern Ocean Cd record. Further, while the 813C data may be taken to indicate substantial changes in circulation [Charles and Fairbanks, 1992], the Cd data, showinglacial Cd levels intermediate between the NA and EEP, suggest a persistent flux of NADW into the Southern Ocean. The latter conclusion is supported by a recent study of the oceanic budget of 231pa which suggests that the export flux of this radioisotope from the Atlantic to the Southern Ocean was not very different during the LGM MD CHN82 G. bulloides ' I ' I ' ' Age, ka B P 4PC... <>... MD TR163-31b Figure 3. Foraminiferal Cd/Ca and isotope records from the SIR: (a) ]80 in benthic and palnktonic foraminifera (chronologies are based on correlating these records with their counterparts in the Accelerator Mass Spectrometry (AMS)]4C-dated core of MD [Labeyriet al., 1996]; (b) ]3C in Cibicidoides wuellerstorfi;/(c) average, mixed-species Cd/Ca; and (d) planktonicfisraminiferal Cd/Ca. The Southern Ocean records are compared with their counterparts from the North Atlantic (NA)(CHN82 sta. 24 core 4PC) [Boyle and Keigwin, 1985] and the eastern equatorial Pacific (EEP)(TR16 3lb) [Shackleton al., 1988; Boyle, 1992]. Note that the ]3C records were reversed and scaled so they can be visually compared with the Cd/Ca records; that is, in both cases an upward shift signifies higher nutrients.

6 792 ROSENTHAL ET AL.' SOUTHERN OCEAN PALEOCEANOGRAPHY o bo... : i :.,d C. wuellerstorfi 0.00 ß.,., Age, ka BP = RC RC CHN82 4PC... TR163-31b Figure 4. Foraminiferal Cd/Ca and isotope records from Cape Basin: (a) benthic foraminiferal 8180 (chronologies are based on the correlation with the AMS ]4C dated South Atlantic record of RCll-83 [Charles and Fairbanks, 1992]; (b) fi13c in C. wuellerstorfi; and (c) average, Cd/Ca. Isotopes data are from Curry et al. [1988] andoppo et al. [1990], and the Cd/Ca data are from Oppo and Rosenthal [1994] and Boyle and Rosenthal [1996]. For other details see Figure 3 caption. than it is today [Yu et al., 1996]. In the past few years, several mechanisms have been offered to explain the apparent paleoceanographic conundrum Postdepositional dissolution. Core top calibrationshow that Cd/Ca ratios in benthic foraminifera are proportional to g, seawater Cd concentration, but also depend on the depth of calcification [Boyle, 1992]. More recently, it was suggested that the low apparent distribution coefficient of Cd (Dcd) in shells of C. wuellerstorfi from the equatorial Pacific may be an artifact of dissolution which preferentially removes Cd from calcitic shells [McCorkle et al, 1995]. This hypothesis relies primarily on the observed correlation between Dcd and the bottom water calcite saturation state (ACO3): a compiled data set of all documented core tops [Boyle, 1988, 1992; McCorkle et al., 1995] shows a substantial decrease in the apparent distribution coefficients of Cd, Ba, and Sr as ACO 3 changes from =+10 to -10 gmol kg -] [McCorkle et al., 1995]. However, on the basis of available data one cannot exclude the possibility that saturation-related effects during calcification rather than postburial dissolution cause the observed depth-dependent changes [Lloyd-Kindstrand et al, 1994; Elderfield et al., 1996]. The correlation between foraminiferal Cd (as well as Ba) and their seawater concentration in much of the modem ocean implies that this effect is a secondorder phenomenon. However, if dissolution is an important factor, then it is possible that increased dissolution of sediments in the glacial Southern Ocean may account for some of the discrepancies between the different proxies (i.e., Cd, Ba, and fi 3C) [McCorkle et al., 1995]. If dissolution had a significant impact on the chemical composition of glacial foraminifera, how would this affect our results? The modem hydrographic lysocline is located at a depth of =4200 m in Cape Basin. The estimated bottom water ACO3 is =20-25 gmol kg -1 in the site of RC (3204 m) and 0-5 gmol kg -1 in RC (4191 m) [Howard and Prell, 1994]. In the southeast Indian Ocean the lysocline depth is at =3400 m with ACO3 concentration of =0-5 gmol kg -1 in water overlying the sites of MD and MD Although the estimated Dca from Southern Ocean core tops is slightly lower than the Dca calculated from the global calibration (Figure 2), it provides no convincing evidence for postdepositional dissolution in Southern Ocean surface sediments. During the LGM the lysocline was shallower in both sites by m, corresponding to --12 mol kg -1 lower CO3 ion content [Howard and Prell, 1994]. This implies that during the LGM, cores RC13-229, MD and MD were well below the lysocline (ACO3=-I 0 gmol klg- 1), while RC was above it (ACO3= +10 gmol kg-s). Consequently, it seems unlikely that the discrepancy between fi 3C and Cd in RC can be entirely due to dissolution. On the other hand, if dissolution affected the foraminiferal Cd record of RC one, expects an =35-45% decrease in the apparent Dca [McCorkle et al., 1995] implying "undissolved" ratios above 0.2 gmol mol- which are similar to Cd/Ca ratios in the glacial EEP. In this case the enhanced nutrient gradient in the South Atlantic, implied by the Cd/Ca data, is not matched by the fi 3C data Changes in the Cd oceanic inventory. A diagenetic mechanism for decoupling Cd from nutrients was discussed elsewhere [Rosenthal et al., 1995a; Van Geen et al., 1995]. Briefly, the decoupling occurs because of the precipitation of Cd (most likely as CdS) [Rosenthal et al., 1995b] in suboxic sediments underlying highly productive areas. In the modem ocean, suboxic sediments are the major sink for dissolved Cd. Consequently, the possibility that climatically induced variations in the area of suboxic sediments might have changed the modem relationship between Cd and 5 3C was explored by Rosenthal et

7 ROSENTHAL ET AL.' SOUTHERN OCEAN PALEOCEANOGRAPHY ' 1000 A. 110E B. Atlantic Ocean 45S \ 30s 0 30N 60N. ACC 30, E////[ [ IGlacial,, Cd, w (nmol, kg,,,, 70E,,..21 ß -1) 24/-': :;;.:4,/-'.."5 SIR '551 C '"'ø'5'"'x ß 45 ' ".34'.37_ if'.-. "5.36'" l 21 '.52 / '? '".37 N. 51 ) ] ' I! I I /,.'-":.' ' ',.' (RCl Figure 5. Last glacial maximum distribution of (a) Cdw and (b) 13C in the Atlantic and Southern Oceans, along a similar cross section as the one presented in Figure 1. The data are from previous studies [Curry et al., 1988' Duplessy et al., 1988; Boyle, 1992] as well as this study. Note that in three South Atlantic cores (V25-59, RC12-294, and RC17-69) the Cd data were revised from the original paper [Boyle, 1992] because of suspected CdS contamination. al. [1995a], who concluded that on a glacial-interglacial timescale such changes had only a minor effect (<5%) on the oceanic inventory of Cd Accuracy of foraminiferal fil3c. C. wuellerstorfi has been shown to record seawater fil3c with a typical accuracy of _+0.2%o[Duplessy et al., 1984]. However, more recently, it was suggested thathe fil3c of live foraminifera may be up to -0.5%0 lower than expected from the isotopic composition of the water in sediments underlying highly productive surface waters [Sarnthein et al., 1988; Mackensen et al., 1993]. Paleoproductivity reconstructions suggest that during the LGM the four cores discussed here were directly under zones of high productivity [Charles et al., 1991; Rosenthal et al., 1995a; Bareille et al., 1997], and therefore it is plausible that productivity-driven effects might more deepwater formed by open-ocean convection rather than underneath the ice (as suggested below), the thermodynamic equilibration should have been enhanced, as is seen today in Southern Ocean polynyas [Mackensen et al., 1996] Glacial Circumpolar Deepwater Circulation The distributions of glacial Cd w and 13C in the Atlantic Ocean and CPDW are shown along a similar cross section with the one presented for the modem ocean (Figure 5). The data are from previous studies [Curry et al., 1988; Duplessy et al., 1988; Boyle, 1992; Lynch-Stieglitz and Fairbanks, 1994] as well as this study. The Cd- and 13C-based reconstructions of the nutrient distribution in the glacial Atlantic Ocean were discussed elsewhere [Boyle and Keigwin, 1987; Oppo and Fairbanks, 1987; have compromised their foraminiferal 513C records. Interestingly, Duplessy et al., 1988; Boyle, 1992; Oppo and Lehman, 1993; RC which is located about 2 ø north of MD and Bertram et al., 1995]. Briefly, both tracers suggest a downward MD [Labeyrie et al., 1996] does not show the very low 513C, possibly because of the fact that this site was out of the maximum productivity belt during the LGM Gas exchange. The 5 3C composition of deepwater reflects, to a large extent, the biological cycling of organic matter as modified by deep ocean circulation. The decomposition of isotopically light organic matter in the deep ocean modifies the relatively heavy preformed 3C composition of surface water at the sites where deepwater forms. It was postulated that reduced gas exchange between the ocean and the atmosphere, due to greater ice cover in the glacial polar oceans, might have altered the distribution of 13C independently of the distribution of nutrients [Charles and Fairbanks, 1990; Boyle, 1992; Broecker and Maier-Reimer, 1993]. However, if during the glacial interval shift of the nutrient maximum in the Atlantic water column, most likely due to a northward expansion of nutrient-rich, southern source bottom water. Likewise, both proxies suggest low nutrient content in intermediate depths of the North Atlantic, suggesting a shoaling of the glacial NADW [Boyle and Keigwin, 1987; Duplessy et al., 1988; Lehmann and Keigwin, 1992]. Apparently, the glacial Atlantic Ocean was characterized by weaker latitudinal gradients but greater stratification of the water column than the modem ocean. The similarity between Cd and fi 3C breaks down in the Southern Ocean where low fi 3C values are unmatched by high Cd w. The SIR Cd w data from cores MD and MD are among the lowest in the glacial deep ocean, comparable with Cd w levels in intermediate depths in the Atlantic Ocean and high

8 794 ROSENTHAL ET AL.: SOUTHERN OCEAN PALEOCEANOGRAPHY northern latitude deepwater (Figure l a and 5). These low levels contrast with the very low i13c values observed in these cores. Note that glacial SIR Cd w levels are substantially lower than in the deep CB site, in contrast with modem hydrography. Unlike the uniformity of modem CPDW, the Cd distribution in the glacial Southern Ocean was significantly heterogeneous. The heterogeneity of glacial CPDW is also expressed in the distribu- A newly formed deepwater in the southeast Indian basin will initially flow northward along the Kerguelen Plateau but thereafter will turn eastward along the mid-ocean ridge (at around 45øS) while mixing with the ACC. In the modem ocean, estimates of open ocean deep convection vary from between a few Sverdrop in the Southern Ocean to =10 Sv in the Labrador Sea (where 1 Sv = 106 m 3 s -1)[Gordon, 1982; Dickson et al., 1990]. For compartion of Southern Ocean glacial surface water i 13C [Ninnemann ison the modern flow rate of ACC is estimated at =130 Sv and Charles, 1997]. To explain the glacial heterogeneity, we propose that a local source of relatively nutrient-depletedeepwater might have formed in the southeast Indian Ocean (downstream of the Kerguelen Plateau) during the last glaciation. The glacial mode of deepwater formation was probably different than at present. In the modem ocean the principal mode of bottom water formation around the Antarctic perimeter is by convection near ocean boundaries, primarily in the Weddell and, to a lesser extent, in the Ross Seas [Gordon, 1971; Killworth, 1983; Foldvik and Gamrnelsrod, 1988]. In the process a dense water [Foldvik and Garnrnelsrod, 1988]. Therefore a local deepwater formation should have been recorded only in cores located close to the formation site (e.g., MD and MD88-769) before its distinctive properties are completely eroded because of mixing with the ACC. In this case it is not unlikely that deepwater flowing along a trajectory such as proposed above might explain the Cd differences between the westernmost core (RC11-120) and the eastern cores (MD and MD88-769). A local source of nutrient-depletedeepwater in the southeast Indian basin may also explain the fact that the benthic foraminiferal Cd records are mass which forms on the continental shelf because of brine re- out of phase with the northern hemisphere deglaciation. It is jection during wintertime ice formation descends down the continental slope while mixing with surrounding water (primarily of CPDW origin) thereby forming a very cold and dense bottom water. The lowering of the sea level during glaciations resulted in suggested that the warming of Southern Ocean SST and freshening due to extensive ice melting during deglaciation resulted in the inhibition of open-ocean deep convection and the returning of the system to its interglacial mode of bottom water formation. the grounding of the Antarctic ice sheets on the continental shelves and, in some cases, seaward expansion toward the shelf break which led to a great reduction of the supply of dense, saline bottom water (see review by Broecker and Denton, [ 1989]. A further reduction in brine production was probably due to the fact that the summer-fallimit of ice meltback was greatly reduced, so refreezing near the continental shelves probably did not occur. Therefore, under glacial conditions, deep open-ocean convection in polynyas within the sea-ice zone became a more important mode for deepwater formation than it is today [Mackensen et al., 1994, 1996]. Observations and model results suggesthat enhanced ice growth associated with stronger catabatic winds can destabilize the water column thereby leading to deepwater convection, as was observed in the Weddell Polynya [Gordon and Huber, 1990; Martinson, 1990]. Indeed, the records of ice-rafted debris and floral abundance provide evidence for extensive ice growth as far north as 45øS in the southeast Indian Ocean [Burckle and Burak, 1988; Howard and Prell, 1992]. Increased whole ocean salinity due to the lowering of glacial sea level and lower surface temperatures, coupled with relative freshening of glacial Antarctic subsurface and intermediate waters due to the 6. Conclusions South Atlantic Cd/Ca records show no glacial-interglacial variation, suggesting that the glacial contribution of NADW to the Southern Ocean was not much different than at present. In contrast, benthic foraminiferal Cd/Ca data from southeast Indian Ocean cores suggest lower glacial bottom water Cd. Planktonic foraminifera from these cores also show low glacial Cd/Ca values. Unlike the uniformity of modem CPDW, the Cd distribution in the glacial Southern Ocean was significantly heterogeneous. We propose that the lower Cd/Ca in the SIR records reflects a local source of nutrient-depletedeepwater formation during the LGM by open-ocean convection near the Antarctic Polar Front, downstream of the Kerguelene Plateau. This mode of bottom water production, which is limited in the modem Southern Ocean, might have been more important during glaciations because of the grounding of Antarctic ice shelves and the reduced limit of the summer-fall ice meltbacks. If the extent of open-ocean deepwater formation was limited to the southeast Indian basin, as may be concluded from the available data, its impact on the overlower contribution of the North Indian Ocean intermediate water all chemistry of the glacial AABW entering the Atlantic and [Lynch-Stieglitz and Fairbanks, 1994], might have resulted in a further destabilization of the glacial water column. Taken together, the data are consistent with a weaker vertical density gradient during the LGM than at present near the Antarctic Polar Front in the eastern Indian Sector which, in principle, increased deepwater formation was limited to the southeast Indian basin, as may be concluded from the available data, its impact on the overall chemistry of the glacial AABW entering the Atlantic and Pacific Oceans was very small. However, if open-ocean bottom water formation became the dominant mode during glaciations, it the likelihood of wintertime open-ocean deep convection [Gordon and Huber, 1990; Martinson, 1990]. At present, deepwater masses originating in the Southern Ocean are enriched in nutrients, reflecting the high nutrient concentrations of Antarctic surface water. The planktonic foraminiferal Cd/Ca record (Figure 3) suggests a significant depletion of surface water nutrients north of the Antarctic Polar Front during the LGM which is consistent with our proposal, calling for nutrient-depletedeepwater formation in this region. may have had a greater impact on the CPDW chemistry. To test this hypothesis we need to map the Cd/Ca distribution in the glacial Southern Ocean. Acknowledgments. We thank D. Oppo, B. Curry, and M. Bender for fruitful discussions. Critical reviews by C. Charles, P. Froelich and D. Hodell greatly improved the manuscript. Core curation of the RC and MD cores is by LDEO (USA) and CNRS (France), respectively. This work was supported by grant # OCE

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