Minimal change in Antarctic Circumpolar Current flow speed between the last glacial and Holocene

Transcription

1 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, 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, 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 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 1

2 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 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 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 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

3 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 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 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 3

4 The LGM sensu lato (sl) averages given here include the whole 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 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, (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, (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, (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, (2000). 35. Jouzel, J. & Masson-Delmotte, V., EPICA Dome C Ice Core 800 kyr deuterium data and temperature estimates. doi: /pangaea (2007). 36. Mix, A., Bard, E. & Schneider R., Environmental processes of the ice age: land, oceans, glaciers (EPILOG). Quat. Sci. Rev. 20, 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, (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, (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, (2011). 40. Ledbetter, M. T., A late Pleistocene time-series of bottom-current speed in the Vema Channel, Palaeogeogr. Palaeoclimatol. Palaeoecol., 53, (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, (2011). 4 NATURE GEOSCIENCE

5 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, (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 5

6 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

7 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 7

8 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 TPC BAS TPC PS PS67/ BAS TC290/ PC078 6 PS67/ PS67/ PS67/ BAS TPC PS67/ PS BAS TPC 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

9 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 TPC BAS TPC PS PS67/ BAS TC290/PC PS67/ < PS67/ < PS67/ < BAS TPC < PS67/ PS > BAS TPC significantly greater non-significantly greater All data 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 9

10 Table 3. Sample data. Mean grainsizes in µm measured by Coulter Counter. PS67/205-2 PS67/224-1 depth age ka depth age ka LGM LGM * *omitted ~10 s.d. > PS PS67/186-1 depth age ka depth age ka NATURE GEOSCIENCE

11 PS67/197-1 PS depth age ka depth age ka LGM age ka LGM age ka LGM LGM NATURE GEOSCIENCE 11

12 PS67/219-1 BAS TPC 063 TC063, depth age ka depth age ka LGM LGM PC BAS TPC077 BAS TPC287, TC077, TC287 depth age ka depth age ka NATURE GEOSCIENCE

13 LGM TPC077 LGM PC BAS TPC288 BAS TC290/PC078 depth age ka depth age ka TC PC LGM - TPC288 LGM - PC078 depth age ka depth age ka NATURE GEOSCIENCE 13

14 NATURE GEOSCIENCE