Expansion and rapid retreat of the West Antarctic Ice Sheet in eastern Ross Sea: possible consequence of over-extended ice streams?

Abstract

Marine geological data from the eastern Ross Sea provide compelling evidence that the West Antarctic Ice Sheet extended to the continental shelf break during the last glacial maximum. Swath bathymetry records show mega-scale glacial lineations within each of the four large glacial troughs on the shelf. These features extend virtually uninterrupted across the shelf and have sharp lateral boundaries. Sediment cores that sampled the lineations recovered diamicton that is interpreted as deformation till. The combined data indicate streaming ice. The great extent of the paleo-ice streams (up to 1000 km in length) is attributed to the fact that they flowed across unconsolidated sedimentary strata. Geomorphic features that are known to form at the retreating grounding line, such as recessional moraines and ridges, are restricted to the southeastern portion of the shelf. Iceberg furrows that occur at depths of up to 650 m overprint mega-scale lineations. These deep furrows are interpreted as having been formed by icebergs calved from the grounding line of the ice sheet, which indicates the absence of a fringing ice shelf. Sediment cores sampled a thin layer of glaciomarine sediments resting sharply on till. Sub-ice shelf deposits, similar to those sampled in the western Ross Sea, were not recovered in the eastern Ross Sea. These combined results imply rapid retreat of the ice sheet from the shelf. Crosscutting lineations with different orientations indicate changes in flow direction that probably occurred as the ice profile was lowered and the physiography of the sea floor across which the ice sheet was retreating changed. Grounding zone wedges vary in number and location within individual troughs, which indicates diachronous retreat of ice streams. Due to a lack of carbonate material for radiocarbon dating, all radiocarbon ages were derived from acid insoluble organic (AIO) fractions. These dates indicate early retreat of the ice sheet relative to the western Ross Sea and other areas of west Antarctica. However, the older dates must be viewed as suspect, due to a paucity of diatomaceous material and contamination from old carbon. While we were not successful at constraining the timing of ice sheet retreat from the continental shelf, the data do indicate that the ice sheet was grounded on the shelf during the LGM.