Glacimarine Record of the Disintegration of the Larsen A Ice Shelf, Antarctic Peninsula.

Robert Gilbert, Queen's University
Department of Geography, Kingston ON K7L 3N6

Eugene W. Domack, Hamilton College
Department of Geology, Clinton NY 13323

About 6000 km² of the northern part of the Larsen Ice Shelf on the east side of the Antarctic Peninsula has disintegrated in the twentieth century, especially during the mid 1990s. This has allowed access for study of the sea floor in Prince Gustav Channel and north of the Seal Nunataks (Larsen A). The former ice shelves had been floating over much of this area, and probably reformed during the late Holocene (Domack et al. 2001: EOS 82: 13). Using five large-diameter gravity cores recovered from these regions during a winter cruise aboard USAP N.B. Palmer in 2000 an assessment of the sedimentary conditions during disintegration was made based on the physical properties of the upper 25 cm of the cores. The sediment was dated by ²¹⁰Pb activity series from 12 to 15 samples per core.

Three cores in a transect along the Greenpeace Trough (Larsen A region) were recovered in depths to 978 m. In the most southerly core, closest to the present front of the ice shelf the rates of sediment accumulation before about 1900 (at 7 cm depth) were from 100-300 g m^-2 a^-1 (0.5-1 mm a^-1), increasing subsequently to 1.6 g m^-2 a^-1 (1.9 mm a^-1) by the mid 1990s. Grain size of the sediment shows a similar change with mean values before the 1930s of 6-8 µm increasing to 23 µm by 1990 and decreasing to 20 µm by the top of the core (2000). Sand (63-2000 µm) was absent from the core before the 1930s, increasing to a maximum of 30% by 1990. The particle size distribution (PSD) of the sediment changed significantly, unimodal at 8 µm below 13 cm depth, with a second mode at 600 µm coming in above and progressively dominating the PSD. A decrease in ²¹⁰Pb activity from the peak of 0.2 Bq g^-1 in 1990 to 0.6 Bq g^-1 in 2000 occurred in response to this coarser sediment. This pattern we ascribe to sediment input during the disintegration of the ice shelf when melt on the surface concentrated sediment, including aeolian material, and it was washed through moulins to the sea below prior to and during breakup. Two other cores to the north show a different pattern. Accumulation at an intermediate site remains low during the twentieth century at 200-250 g m^-2 a-¹ (0.27-0.32 mm a^-1). Grain size also shows no trend with mean values of 5-6 µm and small amounts of sand (<1.6%) throughout the core except since 1923 when it is absent, although a second mode about 50 µm does occur in this period. The distal core is similar with a modest increase in accumulation from 250-300 g m^-2 a^-1 (0.3-0.5 mm^-1) in the nineteenth century with up to 600 g m^-2 a^-1 (0.7 mm ^-1) early and late in the twentieth century with somewhat lower values between. No sand occurs, although a secondary mode in the PSD occurs at 50 µm throughout the core.

Two cores from beneath the former ice shelf in Prince Gustav Channel show a similar pattern that documents breakup as the proximal core above. Rates of accumulation increase from 300 - 500 g m^-2 a^-1 (0.5 - 1.1 mm a^-1) from 1840s to 1960s, to 800-900 g m^-2 a^-1 (3.4-3.8 mm a^-1) in 1990 - 1993, decreasing to intermediate values since. The sand fraction also increased dramatically since the 1960s in both cores. Remarkable are the lower values of ²¹⁰Pb activity in 1993 and 1994 associated with an increased coarse fraction. These were unusually warm years according to Scambos et al. (2001, Journal of Glaciology 154: 516).