ALBERT

Description: The main purpose of cruise JR298 was to collect marine geological and geophysical samples and data to support International Ocean Discovery Program (IODP) proposal 732- Full2, “Sediment drifts off the Antarctic Peninsula and West Antarctica” (Channell, Larter, Hillenbrand et al.). The ship time was allocated for this purpose on the basis of a Site Survey Investigation grant from the NERC UK-IODP Programme (NE/J006548/1: Depositional patterns and records in sediment drifts off the Antarctic Peninsula and West Antarctica) to R.D. Larter, C.-D. Hillenbrand (both BAS), D.A. Hodell (University of Cambridge) and A.G.C. Graham (University of Exeter). The data and samples collected will also be used in two Collaborative Gearing Scheme projects, an Antarctic Science Bursary project, a University of Cambridge PhD studentship, and within the National Capability remit of the BAS Science Teams in “Geology and Geophysics” and “Palaeoenvironments and Climate Change”. These projects are: • Tracing and reconstructing the neodymium and carbon isotopic composition of circum-Antarctic waters (CGS-100, PI: A.M. Piotrowski, Department of Earth Sciences, University of Cambridge; cruise participants: A.M. Piotrowski and T.J. Williams). • Structural characterisation of Late Quaternary sediments from West Antarctic contourite drifts using three dimensional X-ray imaging (CT-scanning) (CGS-98, PI: C. Ó Cofaigh, Department of Geography, Durham University; cruise participant: J. Horrocks) • Tracing the Quaternary evolution of the Antarctic Peninsula and West Antarctic Ice Sheets using lead isotopes in ice-rafted feldspar mineral grains (Antarctic Science Bursary awarded to C. Cook). • Seismic imaging of oceanographic structures and processes in the Southern Ocean south of the Polar Front (component of University of Cambridge/BP Institute PhD studentship; primary supervisor: N.J. White, Department of Earth Sciences, University of Cambridge; PhD student and cruise participant: K.L. Gunn). • Modelling crustal structure across the Bellingshausen Gravity Anomaly and oceanic fracture zones formed at the Antarctic-Phoenix Ridge through integration of marine potential field and seismic data (Collaboration between two BAS Science Teams; cruise participant: T.A.R.M. Jordan). 2 The cruise also provided support for physical oceanography projects by deploying six Argo floats and rescuing a malfunctioning sea glider. Towards the end of the cruise, RRS James Clark Ross was diverted to Rothera to uplift 16 personnel who had been flown across from Halley in two ALCI Basler aircraft because the sea ice situation in the Weddell Sea was considered to pose a significant risk to the scheduled last call of the season at Halley by RRS Ernest Shackleton. This uplift resulted in a two-day delay to arrival at Punta Arenas at the end of the cruise, which was in addition to a two-day extension already agreed as a result of departure from Punta Arenas having been delayed by slow refuelling. Adverse weather conditions, particularly during the first half of the cruise, resulted in more downtime than the amount of contingency time that had been allowed in the proposal. As a result, one less piston core and about 20% fewer line-km of seismic data were collected than had been planned. Nevertheless, the key objectives were achieved and the cores and data that were collected are of very good quality. The data and cores collected on cruise JR298, combined with existing data and cores, should satisfy all of the requirements of the Site Characterisation Panel and the Environmental Protection and Safety Panel of IODP. They will also provide a good basis for addressing the science objectives set out in the UK-IODP Site Survey Investigation proposal and those of the ancillary projects listed above.

Description: Subglacial meltwater facilitates rapid ice flow beneath concurrent ice sheets, and there is widespread evidence for a dynamic subglacial water system beneath the Antarctic ice sheet. It steers and affects the pattern of ice flow and is a direct result from boundary processes acting at the base of the ice sheet, i.e. pressure-induced basal melting. Consequently, the occurrence of subglacial meltwater plays an important role in bedrock erosion, subsequent resedimentation, and in shaping the topography of ice-sheet beds. Here we present new geological and geochemical data from sediments recovered on the West Antarctic continental shelf in Pine Island Bay that we interpret as reliable indicators for deposition in a palaeo-subglacial lake beneath the formerly expanded West Antarctic ice sheet, presumably during or following the Last Glacial Maximum. Characteristic changes of sedimentary facies and geochemical profiles within these cores taken on RV Polarstern expeditions ANTXXIII/ 4 (2006) and ANT-XXVI/3 (2010) support the presence of an active subglacial lake system during the late stages of the last glacial period. These findings have important implications for palaeo ice-sheet dynamics, suggesting there was considerable water available to lubricate the bedrock–ice interface and deposit water-saturated subglacial sediments (soft tills). Based on our investigations performed so far, we suggest that the transition from subglacial lake to contact with the ocean took place in the early Holocene. During this time we speculate that the ice sheet thinned and successively transformed into an ice shelf with sub-ice cavities flushed by tidal currents. Based on bathymetric maps and relative sealevel curves we will aim to estimate ice thickness as the grounding line retreated across the subglacial lake threshold further inland. Our findings may also have implications for ice-sheet models, which have to consider the predominantly non-linear effects related to subglacial hydrology.