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Domack, E.1, R. Gilbert2, G. Berger3, D. Tewksbury4, S. McCallum1, E. Backman1, K. McMullen, D. Duran, A. McCloskey1, and A. Rubin5.
1 Department of Geology, Hamilton College, Clinton, NY, USA. edomack@hamilton.edu
2 Department of Geography, Queen's University, Kingston, Ontario, CANADA
3 Desert Research Institute, University of Nevada, Reno, NV, USA
4 Department of Geology, Southern Illinois University, Carbondale, IL, USA
5 Department of Geology, Colgate University, Hamilton, NY, USA
Abstract
A marine geological investigation was conducted in front of the Larsen-B Ice Shelf during cruise 01-07 of the USAP research vessel N. B. Palmer. This study was part of a multi-year investigation of the sediment processes and paleohistory of the Larsen Ice Shelf (NW Weddell Sea) begun in May 2000, first proposed to the US NSF in May of 1998, and approved for funding in December 1998. We collected a diverse data set including: real-time satellite derived (SEAWIFS) surface productivity estimates, surface pC02, salinity and temperature measurements, surface to bottom CTD measurements, bottom photographs, swath bathymetry, and surface sediment grabs, kasten cores and multi-cores. We present the basic observations from this data set that document the characteristics of the oceanographic and seafloor setting prior to the most recent collapse of the Larsen B system (late February-March of 2002) but following the penultimate retreat (in 1999). Oceanographic observations revealed a low salinity melt layer emanating from the front of the Larsen-B ice shelf. This surface layer was associated with pronounced gradients in surface chlorophyll-a concentrations and significant draw down in pC02 (to 112 ppm), thus indicating a melt-water enhanced productivity. Approximately 600 km of multibeam (SEABEAM) mapping revealed a glacially streamlined erosional/depositional seafloor that clearly delineates the former flow path of glacial ice that at one time filled the embayment. Iceflow paths veered from SE, in the inner embayment, toward the NE in the outer embayment. Iceberg scours interrupt this lineated seascape at depths above ~400 m, notably around shoals near Robertson Island, Jason Peninsula and a mid-embayment high near the 1999 ice shelf front. A bathymetric deep or broad trough extends to depths in excess of 700 m in the center of the embayment. This deep extends beneath the edge of the Larsen-B ice shelf as it stood in December-January 2001-2002. A pavement of angular pebbles and cobbles covers the seafloor within the embayment (in places to 100% of the surface). The stone pavement is notably lacking in significant encrustation by epifaunal organisms near the center of the trough but contains more encrustation near the periphery, near Jason Peninsula. This surface veneer of ice rafted clasts is underlain by 20 to 60 cm of silty clay, followed by a poorly sorted granulated sediment and, finally, a gray diamicton (interpreted as a till). This stratigraphy is unlike that previously documented from the Larsen-A embayment in that it lacks distinctive biosiliceous intervals suggestive of open marine conditions prior to the most recent ice shelf retreat. Rather it appears from our preliminary stratigraphic studies that the Larsen-B ice shelf has not experienced a history of full recession and reformation since the Last Glacial Maximum but instead appears to have been in place for some time while embayements to the north were experiencing open marine conditions (Domack et al., 2001; Pudsey et al., 2001). The surface veneer of ice rafted stones may mark the break-up event as recorded in 1999 as it was associated with innumerable icebergs and complete shelf disintegration compatible with a massive flux of englacial IRD.
(To be presented at "Antarctic Peninsula Climate Variability: an Historical and Paleoenvironmental Perspective," Hamilton College, New York, 3-6 April 2002.)
