Sea ice expansion and northward shift of the Weddell-Scotia frontal system in South Orkney Plateau at 4,900 cal yrB.P.

Abstract

A 533cm-long gravity core sediment retrieved from the northwestern end of the South Orkney Plateau, west Antarctica, was analyzed for magnetic susceptibility, grain size distribution, total organic carbon content (TOC), carbonate content (CaCO3), content of biogenic opal (BSi), and diatom species composition to reconstruct climatic and oceanographic changes during the last 8,700 years. Light grayish olive diatomaceous silt and mud are predominant throughout the core with several diatom-rich laminae below 498cm. Age of sediment was determined by AMS radiocarbon dating for organic matter in six sediment samples including one from box-core top, which shows a very old reservoir age of ~2900 radiocarbon years. Sedimentation rate ranges from 46 and 84 cm/kyr with a higher rate in the upper part. The core is divided into two parts at 351 cm (4,900 cal yr B.P.) based on TOC, CaCO3 and BSi. Lower part is characterized by variable TOC, high CaCO3, and BSi that varies similar to TOC. Upper part shows higher TOC and lower CaCO3 than lower part, and BSi that does not correlate well with TOC. These characteristics imply that study area was under the influence of Scotia Sea Water prior to 4.9 ka and of Weddell Sea Water later. High concentration of open water diatom species and subpolar form of Eucampia antarctica in the lower part also suggests less winter sea ice during early Holocene. Cooling at 4.9 ka marks the onset of Neoglacial condition in the South Orkney Plateau and it accompanied northward shift of Weddell-Scotia frontal system.