The origin of massive diamicton in Marian and Potter coves, King George Island, West Antarctica

Abstract

Marine sediment cores were obtained from in front of the tidewater glaciers in Marian and Potter coves in the South Shetland Islands in the austral summer of 1998–1999. Sedimentological and geochemical data from these cores document an advance of ice tongue for the deposition of clast-supported, massive diamicton, interpreted as having been produced by ice rafting in front of glacier margin and/or releasing of clasts from basal debris zones in the sub-ice tongue setting. A C-14 chronology for a core indicates that glacial advance took place ca. 1450–1700 yrs B.P., coincident with warm, humid phase in the study area. During this period, the glacier margin was likely to advance and release diamicton clasts, inferred from a reduction in the total organic carbon content, and an increase in sand and clasts within the diamicton facies. The glacial advance probably caused enhanced ice-edge blooms near the core sites, resulting in increased abundance of sea-ice related diatoms i.e.,Fragilariopsis curta andFragilariopsis cylindrus in the diamicton. The warm and humid conditions between 1450–1700 yrs B.P. might allow the intrusion of warm circumpolar deep water within the fjords, bringing about increased abundance of warm water form, i.e.,Fragilariposis kerguelensis. On the other hands, this warming condition probably prohibited the intrusion of Weddell Ice shelf water from the fjord, as evidenced by lack of cold water form,Thalassiosira antarctica, in the diamicton. Clearly, the response of the outlet glacier system along the periphery of the South Shetland Islands Ice Sheet during the late Holocene warm, humid period (1450–1700 yrs B.P.) was expansion. Thus the process of clast-supported massive diamicton formation is likely to be applicable to a number of areas of the modern and Quaternary Antarctic Peninsula.