Ancient pre-glacial erosion surfaces preserved beneath the West Antarctic Ice Sheet

We present ice-penetrating radar evidence for ~150 km wide planation surfaces beneath the upstream Institute and Möller Ice Streams, West Antarctica. Accounting for isostatic rebound under ice-free conditions, the surfaces would be around sea level. We, thus, interpreted the surfaces as ancient, marine erosion (wave-cut) platforms. The scale and geometry of the platforms are comparable to erosion surfaces identified in the Ross Sea embayment, on the opposite side of West Antarctica. Their formation is likely to have begun after the development of the deep ocean basin of the Weddell Sea (~160 Myr ago). In order to form wave-cut platforms, the sea must be relatively free of sea ice for a sustained period to allow wave erosion at wave base. As a consequence, the most recent period of sustained marine erosion is likely to be the Mid-Miocene Climatic Optimum (17–15 Ma), when warm atmospheric and oceanic temperatures would have prevented ice from blanketing the coast during periods of ice-sheet retreat. The erosion surfaces are preserved in this location due to the collective action of the Pirrit and Martin–Nash Hills on ice-sheet flow, which results in a region of slow flowing, cold-based ice downstream of this major topographic barrier. This investigation shows that smooth, flat subglacial topography does not always correspond with regions of either present or former fast ice flow, as has previously been assumed.