What Lay Beneath the Larsen B Ice Shelf: Results of the First Survey of a Large Modern Sub Ice Shelf Deposystem

Extraordinary weather conditions this past Austral field season in the NW Weddell Sea allowed a marine geologic expedition to enter the formerly ice shelf bound coastal regions of the Oscar II Coast located along the eastern northern side of the Antarctic Peninsula. In February to March (2005) the US Antarctic Program Research Vessel L. M Gould conducted high resolution seismic reflection (3.5 kHz Chirp), single channel echo sounding, bottom imaging and sampling in conjunction with physical and biological measurements of the ocean water column. The region has only been an open system for about 3 years following the dramatic collapse of the Larsen B Ice Shelf in 2002 and hence we were provided with an opportunity to examine the depositional character of both ice shelf and post ice shelf sediments. Several new insights were provided by our data set including: the observation of a significantly (~25 m) thick syn- to post-glacial section of unconsolidated sediment in a rift-like basin, mapping of two distinct glacial troughs that reach 850-900 m depth and lead to the Hektoria/Evans and Crane Glacial systems, observations of a chemotrophic (cold seep) ecosystem, documentation of post-break up sedimentation patterns including rapid flux of ice rafted detritus, abundant and diverse sediment laden icebergs, biogenic sediment flux to the seafloor, and a spatially varying regime of modern organic (phytoplankton) carbon and lithogenic detritus across the seafloor. In addition, a circuitous grounding line zone was mapped based upon a unique, stratal geometry of dipping and terraced reflectors. Some of these features appear to relate to the historical grounding line of the Larsen B ice shelf system and an earlier and deeper positioned system that may be related to a pause in de-glacial recession of the ice shelf/ice sheet system from the continental shelf. Large diameter jumbo piston cores and gravity cores were collected from the different depositional regimes and the results of these coring efforts will be related to the history of the Larsen Ice Shelf system.