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 theLate Pleistocene and Holocene have been reconstructed fromchanges in sedimentation processes. The evolution of asediment drift deposit (Mertz Drift) located in the deep trough on the shelf has been assessed using a sedimentologicalapproach. Up to 5.5 mlong cores have been collected. Four Late Pleistocene to Holocene sediment units have beendistinguished on the basis of their physical properties, grain-size distributions, geochemical characteristics and ages. Thebasal Unit 1 is a grey, sandy clayey diamicton (27–37wt% sand) deposited before 14,000 14Cyr before present (14CyrBP). 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 (11–30wt% sand), rich in ice-rafted debris, deposited between about14,000 and 5000 14Cyr BP. It likely represents an open-marine setting, proximal to a retreating ice-front, where strongbottom currents were frequent. The overlying Unit 3 is a laminated siliceous mud and diatomaceous ooze, with lessterrigenous material (4–5wt% average sand), deposited between 5000 and 3000 14Cyr BP. The low wet bulk densityvalues (o2 g/cm3) and mass-specific magnetic susceptibility values (<25x10-8 m3/kg) of sediments of Unit 3 are aconsequence of the high content of biogenic material (Corg content >1wt%; biogenic opal averaging 55 wt%). Unit 3 isprobably related to a phase of increased sediment accumulation on the drift site and low bottom current activity on theshelf, with a stratified upper water column that could have lead to an enhanced primary productivity. Occurrence ofcrossed lamination indicates episodes of slightly enhanced bottom currents, probably with velocities up to 3–5 cm/s. Thetop Unit 4 is a bioturbated siliceous sandy mud, on average 50 cm thick, deposited under an energetic bottom currentregime after about 3000 14Cyr BP. This regime is probably related to the activation of a brine-rejection mechanism andis linked to the formation of high-salinity shelf water (HSSW) in a coastal polynya, which causes winnowing of the shelfdeposits. In contrast, the mid-Holocene setting was characterised by weaker bottom currents that, together with highinputs of organic material from the photic zone, allowed for the deposition of thick fine-grained sediments of Unit 3,resulting in an anoxic environment.