A Submarine Landform Continuum across Marine Ice-Stream Margins

The interface between grounded ice and the open ocean is one of the most critical for resolving processes of change in Earth‘s ice sheets. However, this environment remains one of the least explored on the planet and is poorly understood as a result. While the sedimentological facies succession of the transition from grounded ice to (seasonal) open marine conditions is relatively well known from sediment cores recovered from the continental shelf around Antarctica, the corresponding geomorphological signature has been less well studied. Here we analyse high-resolution marine geophysical surveys from palaeo-ice stream troughs on Antarctica‘s continental shelf, including bathymetry from sub-ice shelf settings, that allow for the geological imprint of the transition between grounded and floating ice to be fully explored. We document six styles of landform assemblage from Antarctic palaeo-ice stream beds, which we further interpret as diagnostic of particular stages in the transition of a marine ice-stream margin from a fully grounded state to an open water setting. Descriptively, the six types comprise of (i) Mega-scale glacial lineations, evolving into (ii) Elongate lineations with corrugated ridges, (iii) Sub-parallel elongate keel-marks, (iv) Broadly-spaced elongate furrows, (v) Elongate furrows with sinuous or ridged termini, and (vi) Curvi-linear, randomly-oriented ploughmarks. Whilst several of these landform types have been described in isolation in previous bathymetric studies, rarely have they been considered in the context of such assemblage groupings and have not been viewed together as a continuum. In drawing together observations, these landform types, exposed by past ice-margin fluctuations across the sea bed, demonstrate a close similarity to one another but significant differences in their extremes. We thus argue that they form a recognisable continuum (evolution) of forms within fully grounded, through ice-plain, sub-ice shelf, pro-marginal, distal marginal, and finally, polar open-marine zones. These observations, including recognition of a geological signature for ‗lightly-grounded‘ ice plains, provide mechanistic explanations for the creation of some key landforms (e.g. corrugated ridges) on circum-Antarctic sea floors, and offer a more detailed framework for interpreting ice-sheet change from the buried and exposed sea-bed geomorphological records of glaciated marine basins worldwide