Glaciomarine sedimentation and its paleoceanographic implications along the fjord margins in the South Shetland Islands, Antarctica during the last 6000 years

Abstract

Two 3 m long cores were collected from Maxwell and Admiralty Bays on King George Island, one of the South Shetland Islands, West Antarctica. These cores were examined for their benthic foraminifer (Globocassidulina biora), delta(18)O and delta(13)C records, diatom abundance counts and biogenic constituents (carbon and silica). These high-resolution data were used to reconstruct the paleoceanography of the bays and to link the marine record with deglaciation of the South Shetland Islands Ice Cap. In Maxwell Bay, extremely low diatom abundance, the depletion of total organic carbon (TOC) and biogenic silica (bioSi), and enriched delta(18)O between 6200 and 4000 BP indicate that this period was characterized by cold conditions with deposition of waterlain till (unit 1)just seaward of the grounding line, low sedimentation, limitation of primary production on surface water, and lack of meltwater supply. Deglaciation along the Maxwell Bay margin was dated from about 4000 BP until at least 2700 BP, with increasing TOC, bioSi and diatom abundance, and decreasing the C/N ratio up core. At this time, subglacial meltwater streams began to emanate from the glacier front. Sediment-laden meltwater plumes from these streams deposited interlaminated sand and mud (unit 2) in the ice-proximal zone. Deglaciation was followed by marked climatic warming at around 2700 BP, with evidence of TOC and bioSi maxima, depleted-delta(18)O, and markedly increased diatom valves. With further glacier recession, diatomaceous pebbly mud (unit 3) was deposited in open marine conditions. A large influx of organic materials by enhanced production during this period caused both rapid depletion of CaCO(3) in sediment and resulted in marked increase in sedimentation since 2700 BP at 200 m water depth along the Maxwell Bay margin (Core S-19). In contrast, the shallower platform along the Admiralty Bay margin (Core S-2) was deglaciated later about 1900 BP, that is, 2000 years after the Maxwell Bay margin. This is possible, as Core S-19 is situated (>200 m water depth) more distal to the fjord head glacier, while Core S-2 in Admiralty Bay lies on a shallower platform (<45 m water depth). In spite of certain correlation problems, we tentatively correlate our results to other studies of Holocene environments in Antarctica, and from that it can be suggested that the climatic optimum at 4000 to 2700 BP and preceding cold event at 6200 to 4000 BP were of circumpolar significance. Since 2700 BP, the contrast between the maritime position of the South Shetland Islands and the continental climate of Antarctic Peninsula, where the Neoglacial appeals to have begun ca. 2700 BP, however, implies that the westerly stormtracks transporting humid, warm air from northwest would not be weakened and displaced northwards from the South Shetland Islands. (C) 2000 Elsevier Science B.V. All rights reserved.