Extensive fluvial surfaces at the East Antarctic margin have modulated ice-sheet evolution.

Antarctic bed topography influences how the overlying ice sheet responds to climate change and provides a record of long-term glacial history. However, knowledge of the processes that governed the development of the landscape before glacial inception and how this modulated subsequent ice-sheet evolution remains limited. Here we use radio-echo sounding to reveal extensive flat surfaces beneath the ice margin between Princess Elizabeth Land and George V Land, East Antarctica. When their elevations are isostatically adjusted for unloading of the present-day ice load, these surfaces cluster at 200-450 metres above sea level and dip gently in an offshore direction. We show that the surfaces are fragments of a once-contiguous coastal plain formed by fluvial erosion, which dates from between the separation of East Antarctica from Australia (~100-80 Ma) and the onset of Southern Hemisphere ice-sheet glaciation (~34 Ma). The preservation of these landforms indicates a lack of intense, selective erosion of the surfaces throughout Antarctica's glacial history. Fast-flowing ice has instead been directed through inherited tectonic structures and fluvial valleys, leading to the incision of overdeepened subglacial troughs between the flat surfaces and thus modulating the responsiveness of the ice sheet to climate change.