Late pleistocene and holocene sedimentation on the George V Continental Shelf, East Antarctica

Climatically controlled glaciological and oceanographic environmental changes off the George V Coast during the Late Pleistocene and Holocene have been reconstructed from changes in sedimentation processes. The evolution of a sediment drift deposit (Mertz Drift) located in the deep trough on the shelf has been assessed using a sedimentological approach. Up to 5.5 m long cores have been collected. Four Late Pleistocene to Holocene sediment units have been distinguished on the basis of their physical properties, grain-size distributions, geochemical characteristics and ages. The basal Unit 1 is a grey, sandy clayey diamicton (27-37 wt% sand) deposited before 14,000 C-14 yr before present (C-14 yr BP). Unit 1 represents a Late Pleistocene, sub-ice shelf water-lain till, overlain by a thin current-winnowed sandy subunit. Unit 2 is a massive siliceous sandy mud (11-30 wt% sand), rich in ice-rafted debris, deposited between about 14,000 and 5000 C-14 yr BP. It likely represents an open-marine setting, proximal to a retreating ice-front, where strong bottom currents were frequent. The overlying Unit 3 is a laminated siliceous mud and diatomaceous ooze, with less terrigenous material (4-5 wt% average sand), deposited between 5000 and 3000 C-14 yr BP. The low wet bulk density values (<2g/cm(3)) and mass-specific magnetic susceptibility values (<25 x 10(-8) m(3)/kg) of sediments of Unit 3 are a consequence of the high content of biogenic material (C-org content > 1 wt%; biogenic opal averaging 55 wt%). Unit 3 is probably related to a phase of increased sediment accumulation on the drift site and low bottom current activity on the shelf, with a stratified upper water column that could have lead to an enhanced primary productivity. Occurrence of crossed lamination indicates episodes of slightly enhanced bottom currents, probably with velocities up to 3-5 cm/s. The top Unit 4 is a bioturbated siliceous sandy mud, on average 50 cm thick, deposited under an energetic bottom current regime after about 3000 C-14 yr BP. This regime is probably related to the activation of a brine-rejection mechanism and is linked to the formation of high-salinity shelf water (HSSW) in a coastal polynya, which causes winnowing of the shelf deposits. In contrast, the mid-Holocene setting was characterised by weaker bottom currents that, together with high inputs of organic material from the photic zone, allowed for the deposition of thick fine-grained sediments of Unit 3, resulting in an anoxic environment. (C) 2003 Elsevier Science Ltd. All rights reserved.