G 3. AN ELECTRONIC JOURNAL OF THE EARTH SCIENCES Published by AGU and the Geochemical Society

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1 Geosystems G 3 AN ELECTRONIC JOURNAL OF THE EARTH SCIENCES Published by AGU and the Geochemical Society Article Volume 3, Number XX Month 22.29/2GC283 ISSN: Ocean ventilation and sedimentation since the glacial maximum at 3 km in the western North Atlantic L. D. Keigwin Woods Hole Oceanographic Institution, McLean Laboratory, 36 Woods Hold Road, Woods Hole, Massachusetts 2543, USA (Lkeigwin@whoi.edu) M. A. Schlegel Department of Earth Sciences, Millersville University, P.O. Box 2, Millersville, Pennsylvania 755, USA [] Stable isotope, sedimentological, and radiocarbon data from cores at 3 km water depth on the Blake Ridge, western subtropical North Atlantic, reveal the history of deep water ventilation since the last glacial maximum (LGM). Bulk sediment accumulation rates varied locally by a factor of 2 under the influence of bottom currents in this sediment drift environment, but the sand flux, mostly foraminifera, was nearly identical at a given time. This suggests that the rain rate of foraminifera (mostly planktonic) was constant, that transport of foraminifera was negligible, and that current-controlled differences in clay and silt transport drive bulk accumulation. In two of the cores, flux peaks in the benthic foraminifera Cibicidoides and Uvigerina peregrina occurred during the Younger Dryas (YD) cold event, and at 8.2, 9.6, 2., 25., and 28. ka. Radiocarbon measurements on those benthic foraminifera show the ventilation age of bottom waters was years during the YD, and for older events it was as great as 2 years. These results contrast with Holocene ventilation, which was 5 years and 7 years at 5 years and 7 years before present, respectively. Components: 7248 words, 7 figures, table. Keywords: Sediment drifts; radiocarbon; ocean ventilation; Younger Dryas; last glacial maximum. Index Terms: 322 Marine Geology and : Marine sediments processes and transport; 4267 Oceanography: General: Paleoceanography; 4 Geochemistry: Isotopic composition/chemistry. Received 27 November 2; Revised 3 March 22; Accepted 2 March 22; Published XX Month 22. Keigwin, L. D., and M. A. Schlegel, Ocean ventilation and sedimentation since the glacial maximum at 3 km in the western North Atlantic, Geochem. Geophys. Geosyst., 3(),.29/2GC283, 22.. Introduction [2] Two decades of geochemical research into ocean history have established clearly that the export of North Atlantic Deep Water (NADW) and the compensatory northward flow of warm surface waters were interrupted during the LGM and numerous other times in the past [Duplessy et al., 98; Boyle and Keigwin, 982; Curry and Lohmann, 982; Clark et al., 999]. Most of this research was based on geochemical tracers with distributions similar to nutrients, where higher concentrations in deep waters would indicate a deep southern source to the North Atlantic. Although these tracers document the presence or absence of a water mass, they do not provide information on overall rates of water movement [LeGrand and Wunsch, 995]. Broecker et al. Copyright 22 by the American Geophysical Union of 4

2 Geosystems G 3 KEIGWIN AND SCHLEGEL: OCEAN VENTILATION AND SEDIMENTATION.29/2GC283 NW GGC4 JPC37 ODP 58 ODP 59 GGC39 SE 295 m 3 m 35 m 2 km Figure. KNR 4/2 3.5 khz line across seaward edge of Blake Ridge at 3 m water depth [after Keigwin et al., 998]. The unusually high deposition rates at the site of cores JPC37 and GGC39 allow 4 C dating of discrete peaks in benthic foraminiferal abundance, and flux calculations show that the foram mass accumulation rates (fluxes) are the same from the lowest deposition rate location (GGC4) to the highest (GGC39). Thus 4 C results on foraminifera at these sites should be largely unaffected by the current-controlled focusing of clay and silt. [988] pioneered 4 C in benthic foraminifera as a direct measure of Atlantic ventilation changes, but their results were subject to the uncertainties introduced by sediment mixing on the seafloor [Broecker et al., 999]. Recently, 4 C measurements on deepsea coral showed that large ocean ventilation changes occurred very rapidly (order decades) during deglaciation [Adkins et al., 998], but there is still no accurate estimate of North Atlantic ventilation during the LGM. [3] Here we investigate the history of ocean ventilation based on cores from 3 m water depth on the large sediment drift known as Blake Ridge (Figure ), where the high deposition rate and abundance peaks of foraminifera should minimize the bioturbation effects noted by Broecker et al. [999]. This paper has two purposes. First we show that the sediment drift environment is suitable for precisely dating foraminifera to measure paleoocean ventilation. We conclude that although bulk sediment accumulation differs considerably among nearby cores, the sand (foraminiferal) accumulation rate is the same at these sites, and they differ only in clay and silt flux. Thus the paleoceanographic signal in foraminifera is not compromised by lateral sediment transport. Second we develop high resolution proxy data at two sites in order to evaluate the history of ocean and climate change during and since the LGM. By measuring 4 C in pairs of benthic and planktonic foraminifera from the same samples, we find that the ventilation age of the western North Atlantic at 3 km ranged from to 2 years for several time slices during the interval 27 2 ka. 2. Methods [4] In November 993 R/V Knorr (KNR) cruise 4/2 surveyed and cored Blake Ridge in preparation for Ocean Drilling Program Leg 72 [Keigwin et al., 998]. One surprising result was the discovery of a location at 3 km water depth with an anomalously thick accumulation of sediment. The KNR4 3.5 khz profile indicates that this location is effectively a small sediment drift superimposed on the larger Blake Ridge drift (Figure ). Along that 3.5 khz line we recovered several cores, three of which are discussed here (see map of region at www-odp.tamu.edu/publications/72_sr/synopsis/ cs_f.htm#384). Piston core KNR4 37JPC is at the same location as ODP Site 58, gravity core 39GGC is 3 km to the southeast (and near ODP Site 59), and 4GGC was recovered 7 km to the 2of4

3 Geosystems G 3 KEIGWIN AND SCHLEGEL: OCEAN VENTILATION AND SEDIMENTATION.29/2GC283 Calendar Age, years Average δ 8 O G. ruber Average δ 8 O G. ruber -2 - * * * * * * * * * * Depth cm -2 * b Depth cm c Depth cm d * a northwest of 37JPC. Together, these sites form a transect from a relatively low deposition rate environment (4GGC) at 2924 m, to a very high rate environment (39GGC) at 2975 m. Each core was split, described, and sampled at spacing of 4 cm using a thin wall tube from which sample volume could be measured. Samples were dried, weighed, and washed over a 63 mm screen. The sand fraction was dried and weighed, and foraminifera >5 mm were picked. Fluxes (mass accumulation rates) were determined by multiplying dry bulk density, rate of sedimentation, and the mass fraction of sediment. Stable isotopes of oxygen and carbon were measured on single to several individuals of the benthic foraminifera Uvigerina peregrina and Cibicidoides wuellerstorfi and on 5 specimens of Globigerinoides ruber (white variety) from the 5 to 25 mm fraction. Analytical procedures are described elsewhere [Keigwin, 998]. Radiocarbon age determinations were made on ultrasonically cleaned foraminifera at the National Ocean Sciences Accelerator Mass Spectrometer (AMS) facility at Woods Hole, Massachusetts, and calendar ages were calculated from AMS dates using Stuiver et al. [998] and the equation of Bard et al. [998]. 3. Results 3.. Stable Isotopes and Chronology [5] (All results are archived at the World Data Center A for Paleoclimatology, NOAA/NGDC, Boulder, Colorado; paleo.) Oxygen isotope results on G. ruber from core 39GGC exceed a 2% range from a minimum of about.5% in the upper cm (the Holocene) to maximum d 8 Oof>.5% deeper than 25 cm at the end of the LGM (Figure 2a). Calendar Age, years Depth cm Figure 2. (opposite) (a, b) Oxygen isotope stratigraphy and (c, d) chronology at a high (39GGC) and a low deposition rate site (4GGC), respectively, from 3 km on Blake Ridge. AMS 4 C dated levels indicated by asterisks in Figures 2a and 2b. In Figures 2c and 2d the calendar age-depth relationship is fitted with a polynomial age model; for 39GGC the age model usually falls within the error envelope (dashed blue lines) of the AMS dates (Figure 2c). The age-depth relationship for 4GGC (Figure 2d) is based on the sedimentological data and correlations in Figure 3. 3of4

4 Geosystems G 3 KEIGWIN AND SCHLEGEL: OCEAN VENTILATION AND SEDIMENTATION.29/2GC283 Between those end-members, there are at least three oscillations, with d 8 O maxima at 27, 75, and 263 cm, and minima between 35 and 67 cm and at 87 and 235 cm. These are probably the same changes in surface salinity and temperature of the western North Atlantic that were discussed previously [Keigwin et al., 99]. Core 4GGC generally shows the same pattern but in lower resolution because of the lower rate of sedimentation (Figure 2b). A detailed chronology was developed for 39GGC based on AMS dates on three samples of mixed planktonic foraminifera from the upper cm and on nine samples of G. ruber from deeper in the core (Table, Figure 2a). Rates of sedimentation increase from cm kyr near the top of the core to cm kyr near 3 cm. A calendar age model was derived by fitting these results with a third-order polynomial, and the model age is almost always within the s errors of the 4 C date (Figure 2c). Our age model for 4 GGC (Figure 2d) was developed by correlation of various sedimentary parameters between 39GGC and 4GGC and one 4 C date [Schlegel, 998]. In the upper cm of 4 GGC we correlated two maxima in percent carbonate between the cores (Figures 3a and 3b), and three maxima in magnetic susceptibility deeper than cm (Figures 3c and 3d). These five tie points and the planktonic 4 C date (Table ) give a near perfect linear fit of 4GGC to 39GGC (r 2 =.98). Ages for the tie points were taken from 39GGC, and a polynomial age model was developed (Figure 3d). [6] At 37JPC the planktonic foraminifera Globorotalia inflata was dated in four samples between 35 and 65 cm (Table and Figure 4f ). Continually deceasing d 8 OofG. ruber in the upper 5 cm is evidence that the late Holocene was not recovered, but d 8 O maxima at 5 cm, 95 cm, and below 22 cm, and the minimum at 2 cm probably correlate with similar events in 39GGC (Figure 3b). Table also presents AMS results on abundance maxima in benthic foraminifera from both 37JPC and 39GGC and planktonic foraminifera from the same samples. [7] The benthic stable isotope stratigraphy of 39GGC is limited to the upper 2 cm because Cibicidoides are rare deeper in the core and specimens are not usually C. wuellerstorfi (Figure 4c). Analysis of one individual at 39 cm produced LGM d 8 O (4.43%); results were lower in the 2 2 cm interval, and there is a noteworthy decrease to minimum d 8 O above 2 cm. Carbon isotope ratios generally increase from the LGM sample (.32%) to the core top (.2%), with the exception of one sample at 7 cm, where d 3 C of a single large C. wuellerstorfi was.39%, and three smaller specimens were.4%. For this paper, we have resampled 37JPC extensively and replicated analyses to develop a more reliable benthic stratigraphy [Hagen and Keigwin, 22] also present Isotope Stage 2 results at JPC37, but they emphasized Stage 3 variability). As at 39GGC, there are no U. peregrina in the upper cm of 37JPC, and its absence from the 2 3 cm interval (Figure 4f) corresponds to the 2 4 cm interval at GGC39 (Figure 4e). Analysis of 2 individuals per sample gives a reliable stratigraphy, except for two or three samples between 23 and 243 cm where d 8 O appears anomalously low (Figure 4d). C. wuellerstorfi d 8 O results at 37JPC are similar to those from 39GGC, with several samples defining maximum d 8 O between 3 4 cm (Figure 4c). However, d 3 C of this species is more erratic at 37JPC, with several excursions of anomalously low d 3 C in the upper 2 cm Sediment Fluxes [8] Before discussing the 4 C evidence for ventilation changes on Blake Ridge, it is important to verify that the sediment focusing in this sediment drift environment has not transported sand-sized foraminifera. We do this by comparing the sediment mass accumulation rates (fluxes) at JPCs 39 and 4. Bulk mass accumulation rates are about the same for the past, years at 39 and 4GGC, but by about 8 ka, 39GGC has more than twice the mass accumulation rate (Figure 5b). In contrast, the two core locations have nearly identical sand (mostly foraminifera) fluxes, including peaks at 5. and 8 ka (Figure 5c). The only significant difference in sand accumulation rates occurred 7.5 ka, when a pteropod layer was deposited at 4GGC but not at 39GGC. 4of4

5 Geosystems G 3 KEIGWIN AND SCHLEGEL: OCEAN VENTILATION AND SEDIMENTATION.29/2GC283 Table. AMS Radiocarbon Results on Foraminifera From 3 m on Blake Ridge Depth Interval, cm Midpoint Depth, cm Species NOSAMS Number Convent. 4 C Error Age, Years a ±s D conv. Age, Years Calendar Age, years b Cal Age, Years s Cal Age, Years s KNR 4 39GGC mixed planktonic 7,39, ,527,563,53 " 7 mixed benthic,889 2, mixed planktonic 7,29 6, ,58 7,23 7, mixed benthic,89 7, G. ruber 7,4, ,96 3,7 2,676 " 27 Cibicidoides 7,38 2,3 65 " 27 mixed benthic,89 2, G. ruber 6,368 3,25 7 5,393 5,574 4, G. ruber 26,45 3,9 8 6,99 6,432 5, G. ruber 26,46 4,5 6,789 7,55 6, G. ruber 26,47 5,5 3 7,537 7,838 7, G. ruber 26,48 5, 8 7,365 7,636 7, G. ruber 26,49 5,55 8 7,998 8,287 7, G. ruber 26,4 5,7 9 8,7 8,469 7, G. ruber 26,432 6,9 75,25 9,55 9,875 9,24 " 429 mixed benthic,96 8,5 KNR 4 4 GGC 9 mixed planktonic,888 2, ,293 2,39 2,22 KNR 4 37JPC C. wuellerstorfi 29,284 2, U. peregrina 29,283 2, G. inflata 27,342 5, ,228 8,524 7, U. peregrina 29,28 6, G. inflata 27,343 8,26 95,4 2,6 2,464 2, U. peregrina 29,282 9, G. inflata 27,34 2,25 2,5 25,39 c U. peregrina 29,28 22, G. inflata 27,34 23,9 25, 28,96 c U. peregrina 29,279 24,9 8 a Including a correction based on a background of.6 fraction modern. b Calendar ages were calculated using Calib v. 4.3 [Stuiver et al., 998], assuming a 4 yr reservoir correction and no error. c Calculated from polynomial by Bard et al. [998]. 5of4

6 Geosystems G 3 KEIGWIN AND SCHLEGEL: OCEAN VENTILATION AND SEDIMENTATION.29/2GC GGC % CaCO % sand chi a (Pteropod layer) 5 4GGC c % CaCO % sand chi 4 b Depth, cm 5 2 d Depth, cm Figure 3. Correlations between high deposition rate site (a, c) 39GGC and (b, d) low deposition rate site 4GGC. In Figures 3a and 3b, 2% carbonate (small black squares) and 2% sand maxima (large red diamonds) are correltated as indicated tie lines. In Figures 3c and 3d, three maxima in magnetic susceptibility are also correlated. The linear fit of carbonate and magnetic susceptibility data, and one 4 C date, are used to assign 39GGC ages to the 4GGC events. Percent sand data (Figures 3a and 3b) are not used in the correlation to avoid circularity in discussion of fluxes. [9] Our flux measurements on Blake Ridge support earlier observations to the east (on the Bermuda Rise) that fine-grained sediments are selectively advected and focused, whereas sand-sized particles generally settle directly to the sea floor [Suman and Bacon, 989]. This is to be expected because the sand fraction in Blake Ridge sediments is mostly planktonic foraminifera and because these cores all underlie the same surface waters. In contrast, the large differences in bulk accumulation rate must reflect the greater deep advection of clay- and silt-sized particles at 39GGC (Figure 5b). This most likely reflects the influence of fine particle transport by the Deep Western Boundary Current (DWBC) at the location where the 3.5 khz data indicate the thicker sediment accumulation. Here some component of the DWBC probably spilled over the crest of Blake Ridge and deposited its sediment load around LGM time. 6of4

7 Geosystems G 3 KEIGWIN AND SCHLEGEL: OCEAN VENTILATION AND SEDIMENTATION.29/2GC283 δ 8 O G. ruber δ 8 O G. ruber YD YD Depth, cm a b δ 8 O C. wuellerstorfi Benthic δ 8 O YD KNR4 39GGC d8-o KNR4 37JPC C.w. d3c C.w. d8-o d3c 5 YD U. peregrina d8-o Depth, cm c d δ 3 C C. wuellerstorfi δ 3 C C. wuellerstorfi YD Cibicidoides U. peregrina YD U. peregrina Cibicidoides Depth, cm Figure 4. Stable isotope stratigraphy and benthic foraminiferal abundance at cores 39GGC and 37JPC. Correlation of the Younger Dryas (YD) d 8 O maximum in G. ruber and the 8.2 ka event between (a) 39GGC and (b) 37JPC, shown by dotted lines, indicates that the deglacial oscillations between 8.2 ka and the YD are better-defined in 39GGC because the sedimentation rate is >5% higher at that site. However, 37JPC recovered a much longer section with maximum d 8 O. Cibicidoides d 8 O (blue data) and d 3 C (black data) are shown for (c) 39GGC, and (d) 37JPC. Uvigerina peregrina data (red) are only available for 37JPC. Abundance trends in Cibicidoides (black) and U. peregrina (red) are distinctive and correlatable in each core. The YD peak at (e) 39GGC is Cibicidoides spp., whereas it is C. wuellerstorfi at (f) 37JPC. Planktonic dates (in calendar ka) are shown for the benthic abundance peaks that were also dated to determine ventilation ages. 25. e f Number/g 5 Number/g [] Although the history of clay and silt accumulation at these two cores is consistent with expectation at sediment drifts, one should nevertheless exercise caution when considering the sand flux data. Just because the sand fluxes are so similar at 39GGC and 4GGC, we should not assume all foraminiferalsized particles are in situ. For example, if a downslope transport event affected both sites at the same time, we would not necessarily know it. Obviously, this did not happen with the pteropod layer in 4GGC, but the occasional presence of G. menardii > ka in 39GGC and other nearby sites (L.D. Keigwin, unpublished results, 998) and unusually low d 8 OinU. peregrina at 243 cm in 37JPC (Figure 4d) indicate that not all sand grains are in situ. The isotopically light U. peregrina were probably transported from shallower water because this species is absent from the Holocene at this location, yet their d 8 O is low enough to be interglacial. [] The episodes of increased sand flux common to both cores 39 and 4 probably occurred during times of cooling in the surface waters over the Blake Ridge because the flux maxima are coincident with maxima in the d 8 O of G. ruber (Figure 6). Higher sand fluxes could result from increased production of foraminifera, but they cannot result from increased winnowing by deep current activity unless our dating failed to resolve brief condensed intervals. Furthermore, increased winnowing is inconsistent with the higher bulk accumulation rate (dominated by clay and silt flux) during the LGM at 39GGC (Figure 5b). Nevertheless, Haskell et al. [99] reported grain size evidence for increased current speed at a core from 27 m on Blake Ridge at 4,5 4 C years B.P. (= 7 ka calendar), at 2,5 4 C years B.P. (= 5.3 ka calendar), and at the end of the Younger Dryas (, 4 C years B.P.). Although we would not necessarily expect the same timing of DWBC events at our cores 3 m deeper, these events are close enough in age to our sand flux maxima (Figure 5c) that several processes may have operated in concert. How- 7of4

8 Geosystems G 3 KEIGWIN AND SCHLEGEL: OCEAN VENTILATION AND SEDIMENTATION.29/2GC283 δ 8 O G. ruber Oxygen isotopes 39GGC 4GGC Calendar Age, years Bulk Mass Accumulation Rate Sand Flux a b c Bulk mass accumulaton rate (gcm-2kyr) GGC 4GGC Calendar Age, years (pteropod layer) 39GGC 4GGC Calendar Age, years Figure 5. Accumulation rate differences between low accumulation rate site 4GGC (dotted lines and open circles) and high accumulation rate site 39GGC (solid lines and squares) at 3 km on Blake Ridge. (a) The age model in these sites is based on data in Figures 2 and 3. (b) Bulk mass accumulation at 39GGC begins to diverge from 4GGC around ka and is more than twice as great at about 8 ka. (c) Despite the bulk accumulate rate differences, the sand flux (mostly foraminifera, except in the pteropod layer) is nearly identical at these two sites. Sand flux (gcm-2kyr) ever, it is notable that peaks in abundance of benthic foraminifera at 39GGC do not always coincide with the peaks in sand flux from the same samples, so they may reflect a different process (Figure 6). In the 97s, benthic foraminiferal assemblage changes were thought to reflect changing water mass and circulation patterns [Streeter, 973; Schnitker, 974], but later research suggested a greater role for changes in surface ocean fertility [Loubere, 99]. In particular, the abundance of U. peregrina is thought to correlate with high organic carbon flux and low dissolved oxygen in bottom waters [Miller and Lohmann, 982; Zahn et al., 986] Ocean Ventilation and Stable Isotopes [2] Most studies of deep ocean ventilation are based on passive tracers such as Cd/Ca and d 3 Cin the shells of benthic foraminifera. Here we have no trace metal data, and the preferred benthic foram species for d 3 C, C. wuellerstorfi, is extremely rare in deglacial and LGM sediments (Figures 4e and 4f). Nevertheless, where maximum d 8 OinC. wuellerstorfi clearly identifies at least some part of the LGM in cores 39GGC and 37JPC (Figures 4c and 4d), the low d 3 C (about.3%) is consistent with isotope and trace metal results from other western North Atlantic cores at this depth that indicate reduced ventilation by NADW [Boyle and Keigwin, 985/ 986; Keigwin and Lehman, 994]. Deep Pacific d 3 C is much lower during the LGM, suggesting that despite reduced NADW production, the deep Atlantic remained nutrient depleted (better ventilated) than the Pacific at the same depth [Boyle and Keigwin, 985/986]. [3] The fact that Holocene d 3 C is sometimes lower than LGM at 3 km on Blake Ridge is troubling. The d 8 O of those Holocene samples is typical, yet the d 3 C is occasionally much lower than can be accounted for by downslope transport of Holocene specimens. If we assume the d 3 CofC. wuellerstorfi faithfully records the d 3 C of SCO 2 in today s oxygen minimum zone (.7%), then there should be no C. wuellerstorfi living in shallower water with d 3 C as low as.5%. These anomalously light data are similar to Holocene observations on the Bermuda Rise, where transport can be ruled out [Keigwin et al., 99]. Either these data reflect previously unreported changes in the Holocene flux of NADW (nutrient depletion), or there may be some vital effects associated with organic carbon rain rate [e.g., Mackensen et al., 993] Radiocarbon Evidence for Ocean Ventilation [4] Directly measuring paleoocean ventilation by AMS 4 C dating pairs of benthic and planktonic foraminifera is a promising technique, but one that 8of4

9 Geosystems G 3 KEIGWIN AND SCHLEGEL: OCEAN VENTILATION AND SEDIMENTATION.29/2GC Sand flux (gcm-2kyr) 2 δ 8 O G. ruber δ 8 O G. ruber sand flux U. peregrina flux 75 Cibicidoides flux 5 25 Benthic foram flux (number cm-2kyr) Calendar age (yrs) Figure 6. Comparison of oxygen isotope results, sand fluxes and benthic foraminiferal abundances at 39GGC. Peaks in sand flux (g cm-2kyr; black line) occurred during events of cooler climate, as indicated by increased d 8 O (red squares and line; top). These sand flux peaks are highlighted across the two panels by the stippled pattern. Peaks in flux of U. peregrina (blue) and Cibicidoides (red) (bottom) do not necessarily coincide with peaks in sand flux, which are mostly planktonic foraminifera. has its own pitfalls. Because a large number of specimens is required for analysis (typically 5 mg of CaCO 3 or 5 large U. peregrina), the role of bioturbation [Broecker et al., 999] or sample contamination cannot be as easily evaluated as it can with stable isotope analysis of individual foraminifera. This problem is especially acute where sedimentation rates are low and may lead to underestimates of the ventilation age [Broecker et al., 988]. Another problem, especially in polar and subpolar waters, is the largely unknown effect of changing surface ocean reservoir effects [Bard et al., 994]. In extreme cases, it can lead to age reversals, with the deep ocean apparently younger than the surface ocean [Voelker et al., 998]. [5] Blake Ridge sediments allow us to circumvent some of these problems. The sedimentation rates are high and where possible we have dated peaks in flux of benthic foraminifera. Furthermore, we have ground-truthed the method by dating Holocene benthic and planktonic pairs, and we apply the projection age method of Adkins and Boyle [997]. 9of4

10 Geosystems G 3 KEIGWIN AND SCHLEGEL: OCEAN VENTILATION AND SEDIMENTATION.29/2GC283 Delta 4 C late Holocene KNR4 GGC-39 3km on Blake Outer Ridge early Holocene Younger Dryas glacial maximum Age, yrs B.P. Figure 7. Projection age method [Adkins and Boyle, 997] of determining ocean ventilation for four time slices at 3 km on Blake Ridge. The solid curve is the radiocarbon activity of the atmosphere versus calendar age according to Stuiver et al. [998]. For each time slice (late Holocene, etc.) the length of the short vertical line reflects the D 4 Cof benthic foraminifera (y axis) at the sample calendar age (x axis) from planktonic D 4 C. The dashed curves for each time slice reflect the exponential decay of the bottom water (benthic D 4 C) from its initial composition set by exchange with the atmosphere. The intercept of this curve with the solid curve is the age and the initial D 4 Cofthe surface ocean from which the deep water mass originated. For each time slice, the ventilation age of the water at 3 km is the difference between the long vertical line (the time of water mass formation) and the short vertical line (sample calendar age). One sigma errors (vertical dashed lines) on the time of water mass formation become visible in a plot of this scale for the YD and the LGM. Results for the YD and for LGM time slices show that deep ocean ventilation age in the western North Atlantic was considerably greater than it is today, despite the errors. [6] Results of the projection method of determining ventilation ages for four time slices in core 39GGC are shown in Figure 7. The first sample pair, at the core top, has a calendar age on mixed planktonic foraminfera of 5 years (Table ). Mixed benthic foraminifera from the core top have a conventional 4 C age only 5 years older than the conventional planktonic age (Table ) and the ventilation age by projection is only 5 years (Figure 7). Although this result might be affected more than those deeper in the core by upward bioturbation of older foraminfera [e.g., Manighetti et al., 995], we assume the effect is about the same for both the benthic and planktonic foraminifera. Our core top ventilation age is essentially the same as the 4 year age for deep waters in the western North Atlantic today [Broecker et al., 988], and at 76 yrs B.P. the ventilation age is only slightly greater. Thus, where the sedimentation rates and the benthic accumulation rates are relatively low on Blake Ridge and where we analyzed mixed species, we nevertheless get results that are reasonable. of 4

11 Geosystems G 3 KEIGWIN AND SCHLEGEL: OCEAN VENTILATION AND SEDIMENTATION.29/2GC283 [7] The next oldest sample, with a calendar age of 2,9 yrs B.P., is at the peak of d 8 OinG. ruber that has been correlated with Younger Dryas cooling in the western North Atlantic region [Keigwin and Jones, 989]. For that sample we dated G. ruber, the flux peak of Cibicidoides spp. (7% C. wuellerstorfi) (Figure 6) and mixed benthic foraminifera. The two benthic samples gave nearly identical results (2,24 and 2,3 conventional 4 C years), 95 years older than the conventional result on G. ruber (Table ). In order to test the validity of analyzing mixed benthic species, we also determined the conventional 4 C age of C. wuellerstorfi and U. peregrina to be 2,5 and 2,25 years, respectively, at the Younger Dryas abundance peak in 37JPC (Figure 3f). Although the projection age for these benthic results is less certain than in the Holocene because of 4 C plateau effects, we estimate that the ventilation age of western North Atlantic deep waters was years during the Younger Dryas (Figure 7). This estimate is in fundamental agreement with the conventional 4 C age difference of 65 ± 36 reported earlier from the Bermuda Rise [Keigwin et al., 99]. [8] For the LGM we dated several levels in cores 37JPC and 39GGC (Figures 4e and 4f). The youngest datable peak of LGM benthic foraminifera (U. peregrina) in each core has a planktonic calendar age of 8.2 ka, based on G. ruber in 39GGC and G. inflata in 37JPC. The next oldest peak, at the bottom of 39GGC, has a G. ruber age of 9.6 ka. This event is probably equivalent to the undated peak at 35 cm in 37JPC and is preceeded by additional G. inflata dates on U. peregrina peaks at 2., 25., and 28. ka (Figure 4f). Although the cause of these U. peregrina peaks is uncertain (current winnowing, increasing production, bottom water changes), as suggested above, they are probably climatically driven and seem to have the 3 year recurrence interval noted for the cold events in deglacial and Holocene records of the North Atlantic [Keigwin and Jones, 989; Bond et al., 997]. Here they extend through the LGM and into isotope stage 3. [9] Ventilation ages for LGM and older events are highly uncertain because the INTCAL 98 calibration curve extends only to 5.6 ka (Figure 7) [Stuiver et al., 998]. Beyond that there are only a few 4 C and U-series pairs of dates on early deglacial and LGM coral [Bard et al., 998]. As an example, consider the 9.6 ka event at 39GGC (Figure 4e). Because of the large one sigma error envelope around the calibration line >5 ka, the ocean ventilation age could have ranged from 8 to 25 years (Figure 7). It would only confuse Figure 7 to have plotted all four of the LGM sample pairs <25 ka, but the ventilation age by projection would be about the same for each. The conventional 4 C benthic-planktonic difference for the 9.6 ka event in 39GGC (Table ) is 25 years. For the 37JPC events at 8.2, 2., 25., and 28. ka, the differences are 8, 4, 5, and years, respectively. These are probably minimum estimates because G. inflata may live deep, or in the winter mixed layer [Deuser, 987], in waters slightly older than the surface water habitat of G. ruber. We conclude that there were several times during the LGM and late stage 3 when the projected ventilation age for the deep North Atlantic increased to an average of 2 years. Because of the large uncertainty in the 4 C calibration curve >5.6 ka and YD plateau effects that can only increase the projection age, the ventilation suppression at 3 km could have been about the same for both the Younger Dryas and the LGM/stage 3 events. [2] At least for the LGM, a deep Atlantic ventilation age of 2 years is consistent with stable isotope and trace metal evidence for a substantial replacement of NADW by deep water of southern and/or Pacific source. Measurements on a solitary coral from 8 m in the North Atlantic show that these changes can happen within a century [Adkins et al., 998]. More recent data from a late glacial/deglacial (6.6 ka) coral in the Drake Passage show that projection ventilation ages were 9 years, an increase of 2 4% over today [Goldstein et al., 2]. Because this coral was collected within Circumpolar Deep Water (at 25 m), it may represent a Southern Ocean endmember for comparison to North Atlantic results. As noted by Goldstein et al. [2], this southern component water may have reached the North of 4

12 Geosystems G 3 KEIGWIN AND SCHLEGEL: OCEAN VENTILATION AND SEDIMENTATION.29/2GC283 Atlantic with no dilution by younger waters early in deglaciation. However, because our LGM results are for time slices at least 5 years older than theirs, they may not be directly comparable. If the Circumpolar Deep Water was the source of bottom water at 3 km on Blake Ridge at 8.2 ka, then it may have been even years older at that time than it was at 6.6 ka. [2] There are relatively few pairs of 4 C dated foraminifera in the deep Pacific [Duplessy et al., 989; Broecker et al., 99; Shackleton et al., 988] for comparison to our North Atlantic results. Those published prior to 997 were reevaluated by Adkins and Boyle [997]. Although these LGM data are scattered spatially and in time, Adkins and Boyle [997] concluded that deep Pacific projection ages were greater than today by 6 years (making them 22 years). In contrast, Sikes et al. [2] provided new foraminiferal data from the LGM of the southwest Pacific Ocean that indicate much larger surface reservoir ages and deep ventilation ages than reported from anywhere else. Other controversial data come from Yu et al. [996], who used 23 Pa/ 23 Th ratios in the South Atlantic to argue that there was no reduced export of North Atlantic water during the LGM. In summary, however, we argue that the weight of the 4 C evidence suggests that a circumpolar water mass as old as 2 years may have filled much of the deep Atlantic and deep Pacific during the LGM. [22] Our estimates of Atlantic Ocean ventilation for the LGM are 3 times those made by Broecker et al. [99]. Some of this difference reflects the greater ventilation ages provided by the projection method, but the average difference between our conventional ages ( years) is still 5% greater than 675 year average reported earlier [Broecker et al., 99]. We suspect this is attributable to the higher rate of accumulation in our cores and our dating of the distinct benthic abundance peaks. If that is so, then it supports a conjecture of Broecker et al. [99] that Atlantic ventilation changes may have occurred repeatedly during the LGM, on Dansgaard-Oeschger (D-O) timescales. Those authors speculated that an age of only 675 years for equatorial Atlantic deep waters during the LGM may have resulted from mixing of benthic foraminifera that lived during brief events of reduced ventilation and those that lived when NADW production was more like today. We cannot test this hypothesis with the present data because our benthic 4 C determinations come only from peaks in abundance. However, it could be tested with chemical measurements such as Cd/Ca on individual benthic foraminifera that should give bimodal results on the Blake Ridge if Broecker et al. [99] are correct. Although we do not trust d 3 CofUvigerina as a proxy for d 3 C of SCO 2 [Zahn et al., 986; Keigwin, 998], there is no relationship in our data between the abundance of U. peregrina during the LGM and the d 3 Cofthis species. [23] Because there is much recent discussion of a possible relationship between a minimum threshold for ice volume and thermohaline circulation changes [McManus et al., 999; Chapman and Shackleton, 999], it would be very interesting to know if ocean ventilation continued to cycle on and off during the LGM, when ice volume was maximum and approximately constant. If the accumulation rate of benthic foraminifera is indeed driven by climate changes and if it is related to D-O events, then the high LGM variability is surprising. As noted by Shulz et al. [999], the amplitude of the 47 year D-O signal in ice cores is low and insensitive during ice volume maxima. This observation is also borne out by modeling studies that suggest the colder the climate, the more difficult it becomes to switch to a warm mode of thermohaline circulation [Ganopolski and Rahmstorf, 2]. Although they may not be typical of the entire LGM, our results for a few time slices between 8 and 2 ka show that the age of deep waters was 2 or 3 times as great as previously thought, and 5 times as great as modern NADW. 4. Conclusions [24] The enhanced accumulation of sediment at 3 m on Blake Ridge forms what amounts to a small sediment drift superimposed on a larger one. Because sedimentation rates vary by about a factor of 2 within just a few kilometers at this location, we are able to use this location to show 2 of 4

13 Geosystems G 3 KEIGWIN AND SCHLEGEL: OCEAN VENTILATION AND SEDIMENTATION.29/2GC283 that rates of foram (sand) accumulation are constant and that mass accumulation variability is mostly a function of clay and silt flux. Thus sediment focusing does not appear to involve the foramsized fraction studied by most paleoceanographers. To our knowledge, this assumption has been tested previously only on the Bermuda Rise sediment drift [Suman and Bacon, 989]. [25] Peaks in accumulation of benthic foraminifera on Blake Ridge occur during the Younger Dryas (2 ka) and at 8.2, 9.6, 2., 25., and 28. ka. These events may be associated with extremes in climate cooling because the four youngest are associated with maximum in d 8 O of planktonic foraminifera. Although they are also sometimes associated with maxima in sand accumulation (during the YD and at 8.2 ka, for example), this is not always the case (9.6 ka, for example). We conclude they are not a result of winnowing because winnowing cannot produce flux maxima, but their origin remains uncertain. Radiocarbon measurements on these benthic foraminifera and planktonic foraminifera in the same samples show that during each event ocean ventilation ages at 3 kminthe western North Atlantic were at least years, 5% greater than previous estimates. This result is qualitatively similar to trace metal and carbon isotope evidence for glacial NADW reduction and for reduced Gulf Stream transport [Lynch-Stieglitz et al., 999]. Acknowledgments [26] We thank Jess Adkins and Jerry McManus for many helpful comments, Chris Charles and Tom Guilderson for their thoughtful reviews, Eben Franks and Ellen Roosen for laboratory assistance, and the staff at the National Ocean Sciences Accelerator Mass Spectrometer facility for providing radiocarbon results. This work was funded by NSF grants OCE and ATM References Adkins, J. F., and E. A. 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