Origin of the Mertz Drift, George V Basin, East Antarctica

During February-March 2000, a joint Italian/Australian marine geoscience expedition (Project WEGA) discovered and cored a large drift deposit, named the Mertz Drift. The drift is about 390 km\\(^2\\) in area, and lies in the western end of the George V Basin, East Antarctica. I document a study of the Mertz Drift as the deposit contains an important high-resolution record of palaeoenvironment changes during Holocene. X-radiographs of cores, visual core logs and multi-sensor core logger are used to distinguish facies in the cores. A comparison of facies from all cores, radiocarbon ages, thin section analysis and seafloor photographs of the Mertz Drift allowed interpretation of five different facies into four generalised phases of environmental history. A diamicton corresponds to sub-ice shelf, water-lain till deposited during the Last Glacial Maximum, and underlies the Mertz Drift. Unconformably overlying the diamicton is a massively bedded siliceous mud and diatom ooze (SMO) with a high proportion of ice-rafted debris. This unit represents a period of glacial retreat of the ice shelf and a transition to an open marine environment, commencing about 14,000 yr BP and lasting for about 9000 years. A thick succession of laminated and cross laminated SMO follows, deposited during a mid-Holocene climate optimum from 5000 to 3000 yr BP. Overlying the Mertz Drift drape is a massively bedded fine sand/SMO with a high proportion of ice-rafted debris, deposited from 3000 yr BP to present. This drape reflects modern oceanographic conditions commencing from a time of climatic deterioration. This study found seasonal changes in diatom assemblages within lamination couplets with mean deposition times of between 2.6 to 4.3 years. Two currents are believed responsible for the construction of the drift. Upwelled Circumpolar Deep Water transports fine grained sediments southwards towards the inner shelf, and deep, high salinity Shelf Water flows northwesterly along the George V Basin, focusing sinking sediments into a drift deposit. A relative reduction of sea ice cover during a climatic optimum is believed to be a key factor for longer periods of upwelled Circumpolar DeepWater over the outer shelf. This leads to a surge in bottom currents and the concentration of greater volumes of sediment into the drift. This study has important implications for oceanographers as the results suggest a non-steady-state of Antarctic Bottom Water production during the Holocene.