SEDIMENTARY EVIDENCE OF THE RECENT BREAK-UP OF THE NORTHERN LARSEN ICE SHELF: IMPLICATIONS FOR ASSESSING THE HISTORY OF ANTARCTIC ICE SHELVES

Study of rates of accumulation based on 210Pb dating, and of the particle size of sediments deposited beneath the former Larsen-A, Larsen-B, and Prince Gustav ice shelves on the northern Antarctic Peninsula provides evidence of the processes associated with the disintegration of the ice shelves. Glacimarine deposition both under the ice shelves and in open water beyond is characterized by fine-grained sediments (silt and clay size) and occurs at rates of about 0.2 to 0.5 mm/a. This indicates that advection of particulates beneath the ice shelf occcurred prior to the early stages of disintegration of the ice shelf. At a number of sites, sediment texture coarsens upward with up to 40% sand at the surface and rates of accumulation increase to 4 mm/a. Radiometric 210Pb dating shows this to have occurred between 1985 and 1993, several years before break-up of the ice shelves. These events are related to the release of eolian sediment in periodic rapid draining of small lakes and crevasses on the ice shelf before break-up. Spacing of these lakes and thus the points of release of this sediment is 0.5 – 2 km; in addition the timing and thus the location of release is irregular as the ice shelf advances about 0.5 km/a. When considered with the pattern of spreading associated with settling through the water column of sediment released from point sources, this suggests that the footprint of these deposits covers somewhat less than one half of the sea floor. X-ray radiographs of sediment cores also document the recent influx of coarse particles (gravel) related to ice rafting during ice shelf disintegration. In this case, the catastrophically rapid break-up of the shelf into a mix dominated by brash ice and small bergs allows a significant part of the dominantly englacial basal and lateral morainal debris to be released. Because the content of this material varies spatially through the ice, again, only a part of the sea floor receives these materials. These coarser sediments associated with break-up which are deposited at much greater rates provide a signal that may be used to assess the history of ice shelves in Antarctica, although that record is probably not evident everywhere beneath the shelves because they occur in a spatially irregular pattern.