Minimal change in Antarctic Circumpolar Current flow speed between the last glacial and I.N. McCave, S.J. Crowhurst, G. Kuhn, C-D. Hillenbrand and M.P. Meredith Methods Cores Twelve cores forming the transect across the Scotia Sea were identified in the collections of the British Antarctic Survey (BAS) and the Alfred Wegner Institute (AWI) (Fig. 1, details in Supplementary Table 1). Sediment distribution in the Scotia Sea was mapped as patchy 25,31. Due to bottom current effects at the sea floor, recent sediment cover is missing in many areas. However, at some places there are sediment accumulations mapped as contourite drifts or sediment wave fields with moderate to high accumulation rates 25,31. The recovered sediments consist of terrigenous mud, and muds with variable amounts of diatoms but very little carbonate 12,25,27,31-34. Age Models. Magnetic susceptibility (MS) records of the cores were used to provide ice core equivalent ages via the procedure of Pugh et al. 27 who correlated MS to the EPICA Dome C ice core dust records and established consistency with chronological constraints from AMS 14 C dating and biostratigraphy 25,32-34. The age limits for the LGM sensu lato (i.e. lower Marine Isotope Stage 2; 18 ka to base of MIS 2 at 28 ka) were based on the uniform temperature implied by the deuterium record from the EPICA Dome C ice core 35 (Supplementary Fig. 1) that is relevant to the environmental conditions over the ACC. The LGM sensu stricto as defined by the EPILOG group is 19-23 ka based on assessment of maximum land ice volume 36, whose growth is mainly in the northern hemisphere. A minimum of ten samples from each of the (0-12 ka) and LGM (sensu lato) sections in each core were analysed to generate averages for the two climatic extremes, averaged over their 12 and 10 ka duration respectively. Samples are thus ~ 1000 years apart (Supplementary Table 3). The BAS core samples are 1 cm thick while those from NATURE GEOSCIENCE www.nature.com/naturegeoscience 1
AWI cores are 2 to 4 cm thick. Compounded with the wide range of sedimentation rates (2.3 to 71 cm ka -1 ), this yields durations of 30 to 430 years, average 160 years. Ages of all samples are given in Supplementary Table 3. Sediment processing Unlike earlier work 25, carbonate and opaline silica were removed from the <63 m grainsize (mud) fraction, but the carbonate content was generally negligible. Grainsize analysis of the resulting terrigenous fine fraction was by Coulter Counter (Multisizer- 3) 37. and LGM averages of the sortable silt size (, the size of the 10-63 m fraction) proxy for flow speed 13,38 were calculated for each core. Numerous studies have shown this parameter to record relative flow speed and to be tightly linked to climatic and oceanographic variations 13,38,39 (for example Supplementary Fig. 2). Significance of the difference between s was assessed by a 2-tailed t-test where greater than 99% (P<0.01) was considered significant. Sortable Silt; A primer The grain-size of the sortable silt fraction () in sediments has been used as a proxy for the rate of ocean circulation based on the principle that higher current velocities tend to suppress deposition of finer grains more than low velocity currents, leading to a coarser size. It is important that the particles should be deposited in a similar form (i.e. single grains) as that in which they are analysed (i.e. mechanically disaggregated). The size of the re-deposited silt (within the 10 63 m fraction which mainly behaves as single particles thereby fulfilling the above criterion) is found to be proportional to the current velocity 13,38,40. A test for whether the sortable silt data are recording flow (rather than input) is to plot versus % the percentage of the <63 µm fraction that lies in the 10-63 µm range. A current-sorted population should display a clear relationship in which, for relatively fast flows, a deposition bias towards coarser sortable silt components (higher ) is expected to be accompanied by an overall decrease in the deposition of material <10 µm (giving higher %) 13. Such data can be obtained only from the SediGraph that measures the whole sample 37. Such data were measured by Pugh 41 for two BAS cores TPC288 and TPC 290 used in this study (Figs. 3, 4). Fine fraction sizes show that, for most samples 2 NATURE GEOSCIENCE www.nature.com/naturegeoscience
of TPC288 and TPC290, % and plot in the expected manner (Supplementary Fig. 3). The relationship may be compared with an example from a deposit under the Iceland- Scotland overflow on Gardar Drift in the North Atlantic (Supplementary Fig. 4) 39. From these comparisons we conclude that the relationship here in the Scotia Sea is not biassed by input of ice-rafted detritus (IRD). Although there is clearly input of IRD in this region 34, the current resuspends and deposits the fine material thereby imposing a current-sorted size distribution on the deposit. Flow speed Few attempts have been made to relate fine particle size to changes in current strength, the only one of significance being Ledbetter 40 who shows a positive relation with r = 0.90 for a slightly different size range than that used here. His calibration yields flow speed differences such that a 1 µm size change equates to a flow speed differences of ~2 to 3 cm/s at 22 to 14 µm. Today the current penetrates the whole water column and the near-bottom flow reflects that 42 (Supplementary Fig. 5). However it is not possible to extrapolate a change in the near-bottom flow to the ACC s transport which has a significant baroclinic element and is much faster near the surface (Fig. 5). Summer Sea Ice (I) Summer sea-ice positions have been mapped in this area by Collins et al. 12 and more recently by Allen and Peck 43. Both use the abundance of the sea ice proxy diatom Fragilariopsis obliquecostata 44. The age models used by these authors have evolved such that Collins et al 12 indicate the period 29-23.5 ka for the ~56 S northern limit of I, whereas the more recent assessment by Allen and Peck 44 employing the same core puts it at 30.6-22.2 ka with the I limit north of 55 S. The effect of sea ice on the flow depends on how tightly packed are the ice floes. Fast ice attached to the shore causes the greatest reduction in air-sea wind stress coupling which limits movement of tight pack ice and so drag applied to the water is low. In contrast highly mobile pack ice couples well and is a strong intermediary between the wind and the ocean (ice-ocean drag coefficient is 5x10-3, air-sea coefficient is 1 to 2x10-3 and air-ice coefficient is ~1x10-3 ) resulting in a strong current 45,46. Although during part of the LGM sensu stricto (ss) (23-19 ka) the summer sea-ice limit may have lain farther poleward 11,12, winter sea ice at this time filled the whole Scotia Sea, and, if sufficiently tightly packed, would have been a partial shield to reduce flow speeds. NATURE GEOSCIENCE www.nature.com/naturegeoscience 3
The LGM sensu lato (sl) averages given here include the whole 28-18 ka period. However, the separate averages of the grain-size data from cores south of 56 S calculated for both the LGM(ss) and the period 28-23 ka show the same spatial trends of slower flow in the south relative to the, so the flow may have been reduced in winter at this time. Alternatively, wind forcing throughout the LGM(sl) may have been similar to present and winter sea ice may have been mobile, but this would not account for the reduced LGM flow south of 56 S. North of 56 S, where no flow reduction relative to the is seen, the area might have experienced strong summer winds to compensate for winter ice cover. Supplementary references 31. Maldonado, A. et al., Contourite deposits in the central Scotia Sea: the importance of the Antarctic Circumpolar Current and the Weddell Gyre flows, Palaeogeog., Palaeoclim., Palaeoecol. 198, 187-221 (2003). 32. Allen, C.S., Pike, J. & Pudsey, C.J., Last glacial interglacial sea-ice cover in the SW Atlantic and its potential role in global deglaciation. Quat. Sci. Rev. 30, 2446-2458 (2011). 33. Brathauer, U., Abelmann, A., Gersonde, R., Niebler, H.S. & Fütterer, D.K., Calibration of Cycladophora davisiana events versus oxygen isotope stratigraphy in the subantarctic Atlantic Ocean a stratigraphic tool for carbonate-poor Quaternary sediments. Mar. Geol. 175, 167 181 (2001). 34. Diekmann, B., et al., Terrigenous sediment supply in the Scotia Sea (Southern Ocean): response to Late Quaternary ice dynamics in Patagonia and on the Antarctic Peninsula. Palaeogeog., Palaeoclimatol., Palaeoecol. 162, 357-387 (2000). 35. Jouzel, J. & Masson-Delmotte, V., EPICA Dome C Ice Core 800 kyr deuterium data and temperature estimates. doi:10.1594/pangaea.683655 (2007). 36. Mix, A., Bard, E. & Schneider R., Environmental processes of the ice age: land, oceans, glaciers (EPILOG). Quat. Sci. Rev. 20, 627 657. 37. Bianchi, G.G., Hall, I.R., McCave, I.N. & Joseph, L., Measurement of the sortable silt current speed proxy using the Sedigraph 5100 and Coulter Multisizer IIe: Precision and accuracy. Sedimentology 46, 1001-1014 (1999). 38. McCave, I.N., Manighetti, B. & Robinson, S.G., Sortable silt and fine sediment size/ composition slicing: parameters for palaeocurrent speed and palaeoceanography. Paleoceanography 10, 593-610 (1995). 39. Kleiven, H.F., Hall, I.R., McCave, I.N., Knorr, G. & Jansen, E., Coupled deep-water flow and climate variability in the Mid-Pleistocene North Atlantic. Geology 39, 343-346 (2011). 40. Ledbetter, M. T., A late Pleistocene time-series of bottom-current speed in the Vema Channel, Palaeogeogr. Palaeoclimatol. Palaeoecol., 53, 97 105 (1986). 41. Pugh, R.S., Late Quaternary Changes in the Antarctic Circumpolar Current in the Scotia Sea. Ph.D. thesis University of Cambridge, 192 pp. (2009). 42. Renault, A., Provost, C., Sennéchael, N., Barré, N. & Kartavtseff, A., Two full-depth velocity sections in the Drake Passage in 2006 Transport estimates. Deep-Sea Res. II. 58, 2572-2591 (2011). 4 NATURE GEOSCIENCE www.nature.com/naturegeoscience
43. Allen, C. & Peck, V., (2013). 44. Gersonde, R. & Zielinski, U., The reconstruction of late Quaternary Antarctic sea-ice distribution - the use of diatoms as a proxy for sea-ice. Palaeogeog. Palaeoclimatol. Palaeoecol., 162, 263-286 (2000). 45. Leppäranta, M., The Drift Of Sea Ice, 266p, (2 nd ed., Heidelberg, Springer-Praxis, 2011). 46. Leppäranta, M., Sea-ice dynamics, In: Encyclopedia of Ocean Science, 2009). NATURE GEOSCIENCE www.nature.com/naturegeoscience 5
Supplementary figures. Fig 1. Deuterium record (temperature proxy) from the EDC ice-core showing the limits of and LGM time intervals chosen. The temperature is at a minimum and shows low variability in the period 28 - ~18 ka. (data from PANGAEA database 35 ). Fig. 2. Time series from Gardar Drift 38 showing close correspondence between climate (δ 18 O of benthic (Cibicides wüllerstorfi) and planktonic (Neogloboquadrina pachyderma) foraminifera) and flow speed proxies and %. Data in Fig. S4 are drawn from this record. 6 NATURE GEOSCIENCE www.nature.com/naturegeoscience
Fig 3. Cross plot of and % measured by Sedigraph for cores TPC288 and TPC290 showing current-sorted relationship 41. Fig. 4. For comparison with Fig. S3, - a similar plot from Gardar sediment Drift (N Atlantic) under a deep western boundary current 38. NATURE GEOSCIENCE www.nature.com/naturegeoscience 7
Fig. 5. Flow speed across the Drake Passage transect by LADCP (from Renault et al. 2011) 42. Note overall speed increase to the north, high speed at fronts, and penetration of high flow speed down to the sea bed. Fronts are labelled as in Fig. 1. Supplementary Tables. Table 1. Positions of cores used in the study Core # Core ID Lat. decimal, S Minutes Long. decimal, W Minutes Depth, m 1 BAS TPC063-53.9333-56.00-48.0433-2.60 3956 2 BAS TPC077-53.9167-55.00-45.4667-28.00 3774 3 PS2514-1 -53.6667-40.00-43.7667-46.00 2537 4 PS67/197-1 -55.13733-8.24-44.10467-6.28 3837 5 BAS TC290/ -55.55-33.00-45.015-0.90 3826 PC078 6 PS67/205-2 -56.70183-42.11-43.3575-21.45 3790 7 PS67/219-1 -57.22033-13.22-42.467-28.02 3619 8 PS67/224-1 -57.94283-56.57-44.1965-11.79 2868 9 BAS TPC288-59.1420-8.52-37.96467-57.88 2864 10 PS67/186-1 -59.49883-29.93-41.32267-19.36 3671 11 PS2319-1 -59.7883-47.3-42.6833-41.00 4323 12 BAS TPC287-60.3060-18.36-36.65117-39.07 1998 TPC cores are from British Antarctic Survey, Cambridge; PS are piston and gravity cores from Alfred-Wegener-Institute, Bremerhaven. (TPC signifies trigger core (TC) and piston core (PC) spliced to give a continuous record) 8 NATURE GEOSCIENCE www.nature.com/naturegeoscience
Table 2. Summary data of averages, all grainsizes in µm Core # Core ID Holo 2σ/ n LGM 2σ/ n Difference Holo-LGM Significance 1 BAS TPC063 21.37 0.604 19.78 0.620 1.59 0.002 2 BAS TPC077 19.60 0.811 20.29 0.597-0.69 0.2 3 PS2514-1 18.97 0.463 19.57 0.615-0.60 0.2 4 PS67/197-1 17.35 0.625 17.72 0.499-0.37 0.4 5 BAS TC290/PC078 16.11 0.334 16.86 0.283-0.74 0.002 6 PS67/205-2 18.06 0.561 16.20 0.261 1.86 <0.001 7 PS67/219-1 17.84 0.238 16.58 0.540 1.26 <0.001 8 PS67/224-1 15.97 0.261 15.47 0.315 0.50 <0.04 9 BAS TPC288 17.53 0.160 16.70 0.247 0.83 <0.001 10 PS67/186-1 15.19 0.326 14.90 0.227 0.29 0.2 11 PS2319-1 15.76 0.459 15.55 0.498 0.21 >0.5 12 BAS TPC287 15.60 0.225 15.96 0.293-0.36 0.1 significantly greater non-significantly greater 17.45 17.13 0.31 All data 17.383 0.322 17.108 0.318 0.28 P>0.2 i.e. not significant Note that the analytical error of ± 0.5 µm has not been additionally propagated into the standard error of the, 2σ/ n. NATURE GEOSCIENCE www.nature.com/naturegeoscience 9
Table 3. Sample data. Mean grainsizes in µm measured by Coulter Counter. PS67/205-2 PS67/224-1 depth age ka depth age ka 0-1 0.085 18.71 14-17 4.285 15.72 16-19 0.329 20.32 21-24 5.406 15.85 64-67 1.172 17.82 24-26 5.806 15.70 96-99 1.728 18.16 31-34 7.007 15.82 127-130 2.267 17.61 34-37 7.488 16.21 158-161 2.806 17.07 41-44 8.448 16.51 296-299 5.852 18.33 44-47 9.089 16.27 316-319 7.196 17.50 51-54 10.210 16.28 354-357 9.750 17.26 54-57 10.530 15.04 384-387 11.766 17.80 61-64 11.811 16.34 LGM LGM 473-476 19.579 15.64 105-8 18.344 15.85 486-489 20.866 15.78 114-7 18.938 15.84 504-507 21.858 16.35 124-7 19.597 15.16 513-516 22.331 15.73 131-4 20.059 15.28 523-526 22.856 16.51 141-4 20.719 16.03 536-539 23.612 16.07 151-4 21.418 15.00 553-556 24.629 16.39 161-4 22.844 14.92 564-567 25.287 20.37* 171-4 24.268 14.63 572-575 25.766 16.56 *omitted 181-4 25.693 15.85 584-587 26.484 16.81 ~10 s.d. > 191-4 25.719 16.09 PS2319-1 PS67/186-1 depth age ka depth age ka 11-13 1.458 17.29 20-23 0.155 15.07 26-28 2.707 16.89 61-64 0.450 15.50 36-38 3.539 16.05 130-133 0.946 15.05 52-4 4.871 15.08 191-4 1.385 16.14 62-4 5.704 15.45 320-3 2.313 15.85 84-6 7.535 15.62 450-3 3.241 15.33 98-100.5 8.722 15.57 570-3 4.112 15.29 108-110 9.533 15.44 770-3 7.790 14.62 122-4 10.699 15.24 831-4 9.998 14.48 132-4 11.531 15.00 899-901 12.472 14.61 10 NATURE GEOSCIENCE www.nature.com/naturegeoscience
PS67/197-1 PS2514-1 depth age ka depth age ka 12-16 0.704 17.51 0.5-1.5 10.9153 19.96 20-24 1.070 17.93 2-4.5 11.074 20.02 32-36 1.618 16.44 4.5-6 11.16 18.37 62-66 2.990 16.60 4-6 11.171 19.39 92-96 4.362 16.77 8-9 11.39 18.62 128-132 6.008 16.99 9-10 11.42 18.44 160-164 7.471 18.31 13-14 11.65 18.69 192-196 8.934 17.05 14-15 1.778 19.70 226-230 10.489 19.67 16-17 11.84 18.87 256-260 11.861 16.27 19-20 12.03 17.66 LGM age ka LGM age ka 232-34 18.926 16.79 1059-62 19.074 14.57 248-50 19.890 15.16 1079-81 20.222 14.16 257-59 20.532 15.65 1099-1102 21.447 14.57 267-9 21.305 16.66 1109-12 22.055 14.82 277-9 22.594 15.68 1129-32 23.268 15.25 287-9 24.013 15.05 1139-42 23.865 15.12 291-3 24.528 14.73 1159-62 25.059 15.41 298-300 25.292 15.08 1169-72 25.656 14.94 307-9 25.829 16.33 1189-92 26.393 15.01 316-8 26.366 14.33 1204-12 26.799 15.17 LGM LGM 464-8 18.773 16.59 188.5-89.5 21.997 20.78 492-6 19.494 17.42 200-202 22.402 19.88 528-32 20.599 16.71 213-214 22.790 19.46 560-4 21.627 17.34 224-225 23.161 18.67 592-6 22.654 17.01 237-238 23.633 19.04 620-624 23.562 18.95 248.5-9.5 24.004 18.07 652-656 24.609 18.17 260-262 24.426 21.57 688-92 25.370 17.89 273-274 24.847 19.97 722-6 26.088 18.50 296-297 25.623 19.00 752-56 26.722 18.64 384-385 30.371 19.30 NATURE GEOSCIENCE www.nature.com/naturegeoscience 11
PS67/219-1 BAS TPC 063 TC063, depth age ka depth age ka 5-9 1.648 18.24 0-1 3.233 23.29 11-15 2.011 18.14 2-3 4.165 22.52 21-5 2.617 18.55 5-6 5.563 21.03 31-5 3.222 18.09 6-7 6.029 22.29 81-5 5.674 17.72 8-9 6.961 20.62 95-9 6.138 17.43 10-11 7.893 20.6 111-5 6.668 17.66 12-13 8.825 21.3 125-9 7.132 17.68 14-15 9.757 20.18 161-5 8.326 17.29 16-17 10.689 20.63 195-9 9.452 17.61 18-19 11.621 21.22 LGM LGM PC063 491-5 19.738 15.72 51-52 19.19 17.26 524-9 21.056 16.23 76-77 19.99 20.45 535-9 21.557 16.20 105-106 20.91 19.29 555-9 22.467 16.01 133-134 21.57 19.68 571-5 23.159 16.71 160-161 22.16 19.98 591-5 23.704 18.41 189-190 22.79 20.95 616-20 24.387 15.95 219-220 23.73 19.87 638-42 25.140 15.69 245-246 24.83 20.7 655-9 25.865 17.40 274-275 26.07 19.4 684-8 26.622 17.43 301-302 27.24 20.18 BAS TPC077 BAS TPC287, TC077, TC287 depth age ka depth age ka 0-2 8.215 19.45 0-2 0 16.16 4-5 8.909 22.80 5-6 0.028 15.76 8-9 9.702 20.34 9-10 1.174 15.47 12-13 10.496 18.70 13-14 2.319 15.84 16-17 11.289 19.56 17-18 3.465 15.65 20-21 12.083 19.64 21-22 4.610 15.01 24-25 12.876 18.64 25-26 5.756 15.64 28-29 13.670 19.90 29-30 6.901 15.67 32-33 14.463 19.30 33-34 8.047 14.95 36-37 15.271 17.67 37-38 9.193 15.87 12 NATURE GEOSCIENCE www.nature.com/naturegeoscience
LGM TPC077 LGM PC287 65-66 19.055 20.39 49-50 21.409 16.18 70-71 19.495 21.25 53-54 21.970 15.94 77-78 20.116 21.73 57-58 22.532 15.81 83-84 20.648 19.55 61-62 23.094 15.56 89-90 21.180 20.71 65-66 23.655 14.98 95-96 21.711 21.03 69-70 24.217 15.90 101-102 22.281 19.52 73-74 24.779 16.16 109-110 23.615 19.83 77-78 25.340 16.11 114-115 24.585 18.35 81-82 25.902 16.10 125-126 26.239 20.53 85-86 26.463 16.89 BAS TPC288 BAS TC290/PC078 depth age ka depth age ka 0-1 0.000 18.34 TC290-2-3 0.404 17.1 4-5 0.870 17.52 6-7 1.213 16.56 8-9 1.383 17.36 10-11 2.022 15.65 12-13 1.896 17.73 14-15 2.831 15.66 16-17 2.409 17.39 18-19 3.640 16.19 20-21 2.922 18.06 22-23 4.449 16.55 24-25 3.435 17.17 26-27 5.258 15.32 28-29 3.948 18.35 34-35 6.876 15.24 48-49 6.513 17.68 42-43 8.494 16.71 56-57 7.539 17.39 46-47 9.303 15.91 60-61 8.052 16.76 54-55 10.921 15.05 64-65 8.565 17.76 58-59 11.730 14.93 68-69 9.078 17.13 PC078-44- 45 8.899 16.47 76-77 10.104 17.34 48-49 9.708 16.2 80-81 10.617 17.42 52-53 10.517 16.46 84-85 11.130 17.04 56-57 11.326 17.32 88-89 11.643 17.71 60-61 11.730 16.6 LGM - TPC288 LGM - PC078 depth age ka depth age ka 168-9 19.190 16.24 96-7 18.883 16.58 172-3 19.665 15.85 100-1 19.387 15.99 176-7 20.360 16.15 104-5 19.892 17.61 180-1 21.056 16.30 108-9 20.396 16.38 188-9 22.230 16.82 112-3 21.033 15.93 192-3 22.493 16.33 116-7 21.713 16.8 196-7 22.755 17.11 120-1 22.394 16.97 NATURE GEOSCIENCE www.nature.com/naturegeoscience 13
208-9 23.824 16.48 124-5 23.075 16.81 212-3 24.273 17.06 128-9 23.756 16.7 216-7 24.723 17.31 132-3 24.436 17.72 220-1 25.173 16.22 136-7 25.117 16.5 224-5 25.622 16.71 140-1 25.798 16.73 228-9 25.982 17.51 144-5 26.479 17.18 232-3 26.251 16.95 148-9 27.160 17.81 236-7 26.520 17.52 152-3 27.917 17.13 240-1 26.789 16.66 14 NATURE GEOSCIENCE www.nature.com/naturegeoscience