Contrasting sources for glacial and interglacial shelf sediments used to interpret changing ice flow directions in the Larsen Basin, Northern Antarctic Peninsula

Marine sediment cores from the former Larsen A Ice Shelf area reveal three lithological units deposited during ice sheet advance and retreat from the continental shelf. The uppermost Unit 1 consists of a diatom-bearing silty clay with sparse ice-rafted debris (IRD). Clasts include rock types which can all be matched to onshore outcrops along the eastern side of the Antarctic Peninsula. Most clasts are very angular, angular or subangular. The clasts in this unit are interpreted as supraglacial or englacial debris deposited below an ice shelf distal from the grounding line and/or by icebergs in an open marine setting during the Holocene. In the underlying Units 2 and 3 clast lithologies are quite different and mainly comprise Cretaceous and Jurassic sedimentary rocks. These lithologies were derived locally from the continental shelf (the Larsen Basin) rather than the limited onshore outcrops. Of the 2750 clasts studied in Units 2 and 3, most are subrounded or subangular, and some are striated. Unit 2 is interpreted as being deposited beneath an ice shelf proximal to the grounding line. Unit 3 is interpreted as subglacial diamict deposited by grounded ice flowing across the shelf during the last glacial period. The availability of sedimentary rocks at the seabed may have facilitated the development of a deforming till layer with low shear strength (i.e. soft or deformation till) within Unit 3. There are no consistent differences in clast composition or roundness between the deformation till, which was probably deposited at the base of an ice stream, and an underlying till with high shear strength (i.e. stiff till). On the inner shelf where the bedrock lies very close to the seabed, clast composition in the subglacial diamicts affords a way of identifying the presence of Mesozoic stratigraphic units. The contrasting provenance of the coarse fraction between Unit 1 and Units 2 and 3 is used to interpret ice sheet basal thermal regime and produce a conceptual model of changing palaeo-flow directions between glacial and interglacial periods. Grain roundness and petrology are shown to be a potent method for interpreting different glacial/interglacial processes and environments.