Change in geomorphological expression of palaeo-ice stream grounding zone retreat associated with change in bed slope along Belgica Trough, Bellingshausen Sea

Previous studies showed that during the Last Glacial Maximum one of the largest palaeo-ice streams around West Antarctica flowed along Belgica Trough in the Bellingshausen Sea. Based on radiocarbon dates on acid insoluble organic matter in shelf sediment core samples, grounding zone retreat has been interpreted as having started before the global glacial maximum and as having reached Eltanin Bay on the inner shelf before the start of the Holocene. A contributing factor to an early start to retreat could have been the fact that the continental shelf break in the trough is unusually deep (>650 m). Previous sparse bathymetry data showed that, unusually among palaeo-ice stream troughs on the continental shelves around West Antarctica, the shallowest part of the trough is on the middle shelf. The outer shelf part of the trough slopes down at a very gentle gradient towards the shelf edge, whereas inshore from the middle shelf “saddle” the trough is inclined more steeply towards a >1000 m deep basin in Eltanin Bay 

New multibeam bathymetry, acoustic sub-bottom profiler and multichannel seismic data were collected along the axis of Belgica Trough during RV Polarstern expedition PS134 in January and February 2023. These new data reveal a set of six grounding zone wedges (GZWs) on the gentle seaward-inclined slope from the middle to the outer shelf, with along-trough extents between 15 and 45 km and frontal heights between 20 and 40 m. A multichannel seismic profile shows the thickness of GZW deposits is mostly between 20 and 60 ms two-way time (~15–55 m) above angularly truncated older strata. The maximum thickness observed is 90 ms two-way time (70–80 m), at a location where deposits of one GZW extend over the backslope of an earlier one.  In contrast to the slope seaward of the mid-shelf saddle, we identify only three possible GZWs on the retrograde slope inshore from it, which are thinner and more widely spaced. The contrasting geomorphological character and GZW sediment volume either side of the mid-shelf saddle are consistent with what would be expected to result from a faster retreat with fewer pauses once the grounding zone moved onto the retrograde slope. The regularity of GZW formation on the seaward-inclined slope outboard of the mid-shelf saddle suggests the possibility of autocyclic ice stream behaviour during this phase of grounding zone retreat.