Marine Geological Survey off the Larsen-B Ice Shelf: Cruise NBP-01-07 (December 2001 to January 2002)

Domack, E.¹, Gilbert, R.², Berger, G.³, Tewksbury, D.¹, McCallum, S.⁴, Backman, E.¹, McMullen, K.¹, Duran, D.¹, McCloskey, A.¹, Rubin, A⁵, Amblas, D.⁶

¹Dept. of Geology, Hamilton College, Clinton, N Y, USA(edomack@hamilton.edu)
²Dept. of Geography, Queen's University, Kingston, Ontario, CANADA
³Desert Research Inst., University of Nevada, Reno, Nevada USA
⁴Dept. of Geology, Southern Illinois University, Carbondale Illinois, USA
⁵Dept. of Geology, Colgate University, Hamilton, New York, USA
⁶Dept. of Stratigraphy, Paleontology, and Marine Geosciences; University of Barcelona, Campus de Pedralbes, E-08028 Barcelona, Spain

Cruise 01-07 of the USAP research vessel N. B. Palmer was part of a multi-year investigation of the sediment processes and paleohistory of the Larsen Ice Shelf begun in May 2000 (cruise NBP 00-03). The project was first proposed to the US NSF in May of 1998 and approved for funding in December 1998. Our data set includes: 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. Our data 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 reveal 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 melt-water enhanced productivity. Over 600 km of multibeam mapping reveal a streamlined seafloor that clearly delineates the former flow path of glacial ice that at one time filled the embayment. Ice flow 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 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 peripheral ice rises. The 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 the silty clay lacks distinctive diatomaceous intervals suggestive of open marine conditions prior to the most recent ice shelf retreat. Rather, it appears from our preliminary stratigraphy that the Larsen-B ice shelf has not experienced a history of recession and reformation since the Last Glacial Maximum. Instead the ice shelf 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 angular 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.