The Late Quarternary Drainage System of the Southern Bellingshausen Sea, West Antarctica
Prof. Dr. Werner Ehrmann
Dr. Claus-Dieter Hillenbrand
British Antarctic Survey, Cambridge, United Kingdom
Dr. H. Grobe
Alfred-Wegener-Institut für Polar- und Meeresforschung, Bremerhaven
Dr. R. Larter, Dr. A.. Graham
British Antarctic Survey, Cambridge, United Kingdom
British Antarctic Survey, Cambridge
The West Antarctic Ice Sheet probably is the most vulnerable part of the Antarctic ice sheet, because it is largely grounded below sea level. Its disintegration would result in a global sea level rise of about 5 m. The knowledge of the past and current mass balance of the West Antarctic Ice Sheet therefore is crucial for an accurate estimation of future sea-level rise. In the last few years, some parts of the West Antarctic Ice Sheet have shown dramatic signs for a negative mass balance, including thinning, flow acceleration and grounding-line retreat of glaciers. It is unclear, however, if the ice sheet is still responding to climatic changes following the Last Glacial Maximum or to the anthropogenic climate warming. Opinions exist that in response to the present global warming the West Antarctic Ice Sheet could disintegrate during the next 500-700 years.
Fig. 1Map of the southern Bellingshausen Sea with ship track of cruise JR104 of the British research vessel James Clark Ross and core locations of cruises JR104 and ANT-XI/3 with the German research vessel Polarstern (Hillenbrand et al., 2007).
Swath bathymetric data collected during cruise JR104 of the Research Vessel "James Clark Ross” in 2004 in Bellingshausen Sea show that a huge ice stream, which was fed by ice draining through Eltanin Bay and Ronne Entrance, formed a major trough (Belgica Trough) and an associated trough mouth fan. The reconstructed ice flow lines indicate an only minor contribution by ice draining through Ronne Entrance (Ó Cofaigh et al., 2005). Clay mineralogical investigations on sediment cores recovered with RV "Polarstern” in 1994 showed that (1) Eltanin Bay was the source for glaciogenic debris deposited as subglacial till on the outer shelf, and (2) the clay mineralogical fingerprint of debris flow deposits forming a fan on the slope in front of the trough point to Ronne Entrance as the main source of terrigenous detritus (Hillenbrand et al., 2005). However, because this detritus is likely to have been transported by the ice stream to the shelf edge and to have been redeposited down the slope by mass wasting, the clay mineral assemblage of the debris flows should correspond to that of the subglacial till recovered from the shelf. Therefore, the investigation of clay mineral assemblages in the sediment cores recovered during JR104 is important to solving the obvious contradiction between swath bathymetry and core data.
Fig. 2Lithology, physical properties (magnetic susceptibility, water content, shear strength, WBD = wet bulk density), grain-size distribution and clay mineral composition of core GC374 (Hillenbrand et al., 2007).
The purpose of our collaborative project, which is integrated in the Antarctic Funding Initiative Project "Glacial-Interglacial Changes in the Lost Drainage Basin of the West Antarctic Ice Sheet" hosted at the British Antarctic Survey and the Scott Polar Research Institute (Cambridge, UK), is to reconstruct the glacial drainage system in the southern Bellingshausen Sea during the last glacial period and the depositional processes acting during the phase of deglaciation. This goal is achieved by the mapping of seabed features formed by grounded ice masses, ice shelves, meltwater flows and/or icebergs using swath bathymetry and subbottom profiler data, and by the investigation of sediment cores, which will enable the identification of the particular depositional processes forming these seabed features and the reconstruction of the temporal sequence of ice advance and retreat. Our collaborative project focuses on deciphering the source areas and transport processes for the fine-grained sedimentary components using clay mineralogical analysis.
The clay mineral assemblages in the glacial sediments (deformation tills, sub-ice shelf diamictons, iceberg-rafted diamictons, the glaciogenic debris flows) are remarkably homogenous with contents of smectite, illite, chlorite and kaolinite exhibiting hardly any variations at a particular core site (Fig. 2). As in the transitional and the post-glacial sediments, illite and smectite form the dominant clay mineralogical components in all cores. However, the clay mineral contents in the sub-glacial sediments on the shelf vary significantly between core sites with no simple spatial pattern recognizable. This finding is surprising because the clay mineral distribution in the lithogenic fraction of the surface sediments shows a clear relation to both source rocks in the continental hinterland and modern transport pathways of the detritus. The geographical heterogeneity of the clay mineral composition may indicate that the subglacial diamictons on the shelf have different ages. However, we do not exclude the possibility that during the LGM the ice stream remobilised and reworked older sediments on its way across the shelf.
Hillenbrand, C.-D, Benetti, S., Ehrmann, W., Larter, R., Ó Cofaigh, C., Dowdeswell, J., Grobe, H., Graham, A. (2007): Glacial dynamics of the West Antarctic Ice Sheet in the southern Bellingshausen Sea during the last glacial cycle. (ISAES, Santa Barbara).
Hillenbrand, C.-D., Ó Cofaigh, C., Larter, R., Dowdeswell, J., Pudsey, C., Ehrmann, W. & Grobe, H. (2005): Glacial and glaciomarine environments in the southern Bellingshausen Sea since the last glacial maximum a reconstruction based on the sedimentary record. (Glacial Sedimentary Processes and Products, Aberystwyth).