Defining past volume of grounded ice in the Ross Sea.

A large portion of global sea-level increase over the last glacial cycle is believed to originate from reduction in the volume of West Antarctic ice including ice grounded across the Ross Sea. The evidence for this comes from observation of glacigene sediments in the Ross Embayment, both onshore and offshore, indicating a much greater ice extent in the past. However, much of this evidence is not dated directly. Marine geophysical surveys show that the Antarctic Ice Sheet did in many locations ground out to the continental shelf edge, but it is not clear whether this happened at the Last Glacial Maximum. On land, our understanding of former ice extent in the Ross Sea region comes primarily from a glacial sedimentary deposit known as the Ross Sea Drift. The limit of these sediments has been used to define the volume of grounded ice in the Ross Sea and by further inference in the adjacent West Antarctic, representing an ice volume of some 9 million km3, or 14 m sea level equivalent. However, the Ross Sea Drift is defined by common processes and facies rather than as a time restricted unit and conflicting interpretations from different locations suggest different timing and different ice configurations. The geographic area of the southern McMurdo Ice Shelf (between Black Island, Minna Bluff and Mount Discovery) is identified as a location where the reconstructions are in greatest conflict. We present new constraints on the age of formation of these moraines from the application of cosmogenic exposure dating on 16 sandstone erratics collected from moraines on the flanks of Minna Bluff and Mount Discovery. These ice shelf marginal moraines have traditionally been included in the Ross Sea Drift. The sandstone erratics themselves are ideal targets for exposure dating as they include large blocks (> 1 m) that sit proud of the surrounding drift, they are quartz rich and are from an Eocene formation for which there is no known outcrop. While the origin of the erratics is not confirmed, each of the available reconstructions infers them to have been exhumed by advancing ice grounded in the Ross Sea. Thus we can assume that their exposure age relates to glacial erosion and deposition