Subglacial topography of Coats Land records the geological evolution and past ice behaviour of the eastern Weddell Sea, East Antarctica

The East Antarctic Ice Sheet (EAIS) formed circa 34 million years ago and is now the largest reservoir of freshwater on Earth, containing an ice volume equivalent to ~52 metres of global sea-level rise. Although the EAIS is approximately in balance today, there is substantial uncertainty as to the sensitivity of certain sectors, particularly those underlain by widespread low-lying bed topography. The Wilkes and Aurora Subglacial Basin catchments have notably been the focus of recent observation- and modelling-based work, but comparatively little is known about the long-term history of Coats Land and the eastern margin of the Weddell Sea, which is the third major marine-based catchment of the EAIS. In total, the eastern Weddell Sea catchments contain ~9 metres of sea-level equivalent, which is more than the entire West Antarctic Ice Sheet.

However, offshore and onshore geological records of past ice-sheet change are particularly sparse in this region, and the subglacial landscape has been little studied. Here, we describe the use of radio-echo sounding and ice-surface morphology data to characterise distinct physiographic regions of the ice-sheet bed in Coats Land. Our mapping reveals a widespread low-relief, seaward-dipping topographic surface immediately inland of the grounding line, which resembles similar features documented around the East Antarctic margin that are inferred to be remnants of once-contiguous coastal plains formed by fluvial erosion after the separation of East Antarctica from Gondwana (ca. 180 Ma) and prior to glaciation. The preservation of these landforms indicates a lack of intense, selective erosion of the surfaces throughout Antarctica’s glacial history.

We also identify deep subglacial troughs that crosscut (i.e., post-date) these pre-glacial erosion surfaces. The morphology of these troughs resembles that of typical half-graben basins associated with continental rifting; the overlying ice is largely stagnant, indicating that these features did not form beneath the modern EAIS. Based on these observations, geophysical measurements, and geomorphological and geochronological constraints from local nunataks, we infer that these troughs originally formed as ‘failed rift branches’ during Gondwana breakup and were subsequently overdeepened by ice in the Oligocene–Miocene (ca. 34–14 Ma), when ice first expanded to continental-scale but in a different configuration to the modern EAIS. Together, our observations provide new insights into the Mesozoic–Cenozoic tectonic and geological evolution of this sector of East Antarctica, as well as the long-term behaviour of the ice sheet that initially modified this landscape but now acts to preserve signatures of pre- and early-glacial processes.