The presence of polynyas in the Weddell Sea during the Last Glacial Period with implications for the reconstruction of sea-ice limits and ice sheet history

Abstract

Cores from a contourite drift on the continental slope in the NW Weddell Sea, Antarctica, contain up to four sandy foraminifer-rich layers (WA1–4), with dispersed ice-rafted debris (IRD), interbedded with silty–clayey sands and sandy muds. Accelerator Mass Spectrometry (AMS) 14C dating on Neogloboquadrina pachyderma from the foraminifer-rich layers WA2 and WA3 yielded ages spanning the Last Glacial Maximum (LGM) and Marine Isotope Stage (MIS) 2 (20,319–28,543 cal yr BP) and the middle part of MIS 3 (41,349–43,242 cal yr BP), respectively. We attribute the high abundances of planktonic and benthic foraminifera to deposition in a seasonally open-marine environment. Given that the NW Weddell Sea shelf was covered by grounded ice at the LGM (and probably during MIS 2 and MIS 3) and that diatom-based reconstructions place the northern limit of winter and summer sea-ice coverage to the north of the core sites during the Last Glacial Period, our results document the presence of a seasonal or perennial polynya in the NW Weddell Sea. The polynya may have been formed by katabatic winds blowing off the Antarctic Peninsula Ice Sheet (APIS) when it was grounded at the continental shelf break, although polynya formation through upwelling of deep water cannot be ruled out. In addition we argue that previously published data from the southern, southeastern and southwestern Weddell Sea as well as the Ross Sea may indicate the widespread occurrence of polynyas along the Antarctic continental margin during the Last Glacial Period. The widespread occurrence of polynyas could help explain how (i) AABW formation (through brine rejection) continued throughout the LGM, and (ii) Antarctic shelf benthos survived glacial periods. The presence of polynyas through glacial periods also has implications for sea-ice reconstructions from ice core records, possibly biasing the signal towards a reduced sea-ice cover. Finally we propose that the fringes of the APIS, which probably influence the presence or absence of polynyas in the NW Weddell Sea, are sensitive to changes in Antarctic temperature that may be related to a reduction in oceanic heat transport from the North Atlantic.