Testing the Extent and Timing of Past Glaciations Offshore of the Sub-Antarctic Island of South Georgia

• Main Authors: Graham, Alastair G. C., Kuhn, Gerhard, Hillenbrand, Claus-Dieter, Hodgson, Dominic A., Ehrmann, Werner U, Meisel, Ove, Larter, Robert D., Cofaigh, Colm Ó
• Format: Conference Object
• Language: unknown
• Published: 2015
• Subjects: Antarctic; Southern Ocean; Cumberland Bay; Royal Bay; Antarc*; Ice cap; Ice Sheet; Tidewater
• Online Access: https://epic.awi.de/id/eprint/39065/; http://isaes2015goa.in/docs/XII%20ISAES%202015%20Abstract%20Volume.pdf; https://hdl.handle.net/10013/epic.46347

There has been a long history of debate over the extent and timing of past glaciations on the small but climatically-sensitive islands of the sub-Antarctic. The largest of these islands, South Georgia, has been the focus of contention with two conflicting models proposed for the peak of the last glaciation (the Last Glacial Maximum, c. 21 k yrs B.P.): one suggesting an ice cap of restricted extent that saw tidewater outlets limited to the coastal basins of radial fjords; the other proposing an extensive shelf-wide ice cap based on well-preserved though as yet un-dated or un-sampled sea-bed geomorphology. It has been suggested that these two models form comparative end-members of 'South American‘ (restricted) and 'Antarctic‘ (extensive) modes of glaciation, and thus establishing whether the pattern of glaciation on sub- Antarctic islands follows one of these modes, or is in itself entirely different, can provide potentially important insight into past climate forcing of an otherwise poorly-constrained Southern Ocean region. The extent of island glaciation can also provide valuable far-field constraint for Antarctic ice sheet models, and forms vital information for biologists seeking to understand benthic marine communities around South Georgia, whose evolution and structure are intimately linked to the long-term history of ice advance and retreat. Here we investigate the geological record offshore of South Georgia to improve understanding of the extent and timing of past glaciation. Marine geophysical data from new and previous research cruises have been compiled and analysed to advance knowledge of palaeo-ice cap dynamics on the island‘s submerged sea bed and subsurface. We have also sought an independent verification of the glacial history of the island by linking new geomorphological analyses with dated marine sedimentary records recovered by coring, for the first time, on the continental shelf. We present results of new landform mapping on the sea-bed of the South Georgia block. Mapped moraine belts demarcate a minimum of three shelf limits of former glaciation: (1) a shelf edge maximum, (2) an outer basin/fjord mouth limit, and (3) an inner basin moraine position. In addition, numerous spatially-discontinuous moraines must represent further intermediary ice-marginal extents. Geomorphological observations are consistent with the larger configurations of the palaeo-ice cap being highly dynamic, reflected in the pattern, geometry, and sedimentary sequences of cross-shelf glacial troughs, extensive depositional moraine systems, subglacially-generated bedforms, and meltwater-carved channels mapped from multibeam bathymetric survey and sub-bottom echo-sounder data. Reconstructed limits suggest a complex offshore glacial history, and provide hitherto unrivalled insight into the spatial configurations of past sub-Antarctic ice caps. A key question is whether the most prominent moraines were formed during successive Cenozoic glaciations or during retreat and readvance(s) since the last glaciation. We present preliminary chronological investigations of marine gravity cores from the Cumberland Bay and Royal Bay areas of the north-eastern shelf that provide tests of the restricted vs extensive ice-cap hypotheses. We show additional sedimentological and physical properties data from the suite of sediment cores that give insight into fjord and shelf depositional environments, patterns of Holocene glacier behaviour, and contribute more broadly towards a better characterisation of South Georgia‘s long-term marine environmental history.