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 14C yr before present (14C yr BP). Unit 1 represents a Late Pleistocene, sub-ice shelf water-lain till, overlain by a thin current-winnowed sandy sub-unit. Unit 2 is a massive siliceous sandy mud (11–30 wt% sand), rich in ice-rafted debris, deposited between about 14,000 and 5000 14C 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 14C yr BP. The low wet bulk density values (<2 g/cm3) and mass-specific magnetic susceptibility values (<25×10−8 m3/kg) of sediments of Unit 3 are a consequence of the high content of biogenic material (Corg 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 14C 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.