How far did the ice melt? Deciphering the minimum extent of the West Antarctic Ice Sheet during the last interglacial from a subglacial carbonate record.

The West Antarctic Ice Sheet (WAIS) has been expanding and contracting throughout the Pleistocene. While the maximum extend and thus its maximum contribution to global sea level fall is relatively well known, the minimum extent or retreat history of ice sheets is less well constrained. Deciphering such records in Antarctica is further complicated, as most records are hidden beneath extensive ice shelves or altered and covered by several kilometers of ice. However determining the minimum extent of ice sheets during different glacial periods as well as the extent and timing of different stages of retreat is important for determining the contribution of individual ice sheets to global sea-level change events. Here we present a new method to decipher important information on the subglacial paleo-environment in West Antarctica using subglacial carbonates. We have collected isotope data from subglacial carbonate precipitates that indicate a widespread freeze-on event, which may have been associated with shutdown of ice streams at the last interglacial-glacial transition. Subglacial carbonates have recently been found in sediment from beneath West Antarctic ice streams (Bindschadler, Kamb and Whillans Ice Stream). The subglacial carbonates from Whillans and Kamb ice stream have 18O of 36.5° to 39.6° (VPDB). If precipitated in equilibrium this corresponds to a 18O of 42° to 46° (SMOW) of the source water. This is lighter than the lightest ice measured within the modern catchment area of these ice streams. A process able to produce such light source water in the subglacial environment is basal freeze-on. Modeling isotope depletion during freeze-on indicates reservoir depletion of more than 95%. Such reservoir depletion is likely to concentrate solutes in the source water to over-saturation, causing carbonate precipitation. A freeze-on event associated with such large reservoir depletion may have also dewatered the subglacial environment enough to cause ice stream stoppage. Carbonate precipitates from Kamb Ice Stream seem to be unconnected to the recent stoppage of this ice stream. Therefore we hypothesize that these carbonates have formed during a period of extensive basal freeze-on penetrating possibly several meters of subglacial till, along the edge of a possibly thinner ice sheet at the last interglacial/glacial transition. Such an event could have shutdown all Siple Coast ice streams allowing subsequent thickening and advance of the WAIS in the early stages of the last glacial period. This method may be applicable to current and past ice sheet and glacial settings and bears the potential to elucidate processes in the subglacial environment otherwise not detectable, opening up a new field of paleo-glaciology.