Ripple cross-laminated sediments on the East Antarctic Shelf: evidence for episodic bottom water production during the Holocene? P.T. Harris * Antarctic CRC and Australian Geological Survey Organisation, GPO Box 252-80, Hobart, Tasmania 7001, Australia Received 21 September 1999; accepted 19 July 2000 Abstract Biosiliceous sediments sampled from a submarine valley system on the continental shelf of East Antarctica contain intervals of ripple cross-lamination interspersed with massively bedded units. Based on radiocarbon dates from one core collected on the Mac.Robertson Shelf, the most intensely cross-laminated sediments were deposited between 6 and 3.5 kyr BP, with isolated cross-laminae deposited at other times in the Holocene. The cross-laminated sediments are interpreted here as a signal of episodic density currents ¯owing across the outer shelf, which result from the formation of high salinity shelf water (HSSW). This HSSW is formed in winter by brine rejection during sea ice formation and by the exchange and cooling of upwelled saline slope water, and it contributes to the bottom water produced along the continental margin of Antarctica. If this interpretation of the cross-laminae is correct, then bottom water formation and export from the East Antarctic shelf has exhibited temporal, and probably also spatial, variability throughout the Holocene. Such variability would have implications for oceanographers attempting to quantify Antarctic bottom water production rates based only on present day observations. Crown Copyright q 2000 Published by Elsevier Science B.V. All rights reserved. Keywords: Antarctica; Palaeoceanography; Cross-bedding; Biosiliceous sediments 1. Introduction The continental shelf of East Antarctica is charac- terised by numerous valleys and basins that were formed by glacial erosion processes during Cenozoic glaciations when the Antarctic ice sheet expanded across the continental shelf. The Nielsen Shelf Valley, located on the Mac.Robertson Shelf is one such incised glacial trough (Fig. 1). It is arcuate in plan form and about 200 km long. The inner shelf portion of the valley includes a deep basin (the Nielsen Basin) that is locally up to 1200 m deep (Harris and O'Brien, 1996; 1998; see Fig. 1). The Holocene sediments deposited in the deeper (.500 m depth) sections of the Nielsen Shelf Valley are mainly biosiliceous mud and diatom ooze (SMO; Harris and O'Brien, 1998). SMO was ®rst described by Domack (1988) from the Mertz Glacier trough in George V Land, and is probably common to East Antarctic shelf basins; it contains no measurable calcium carbonate, an average of about 1.4% TOC, 20±50% biogenic silica derived principally from diatoms (Fragiliapsis curta and F. kerguelensis), about 10% angular, ®ne sand to coarse-silt quartz with the remainder being terrigenous mud and clay Marine Geology 170 (2000) 317±330 0025-3227/00/$ - see front matter. Crown Copyright q 2000 Published by Elsevier Science B.V. All rights reserved. PII: S0025-3227(00)00096-7 www.elsevier.nl/locate/margeo * Tel.: 161-3-6226-2506; fax: 161-3-6226-2973. E-mail address: p.harris@utas.edu.au (P.T. Harris).