Variability in Glaciomarine Sedimentary Processes on a West Antarctic Trough Mouth Fan

The sedimentology of 12 gravity cores from the slope of the Belgica Trough Mouth Fan (TMF) located in the southern Bellingshausen Sea (Western Antarctica) revealed a variety of depositional processes acting during the last glacial cycle. Hemipelagic sedimentation of foraminifera-bearing muds prevailed in the Holocene throughout the TMF area. The high concentration of manganese encrusted ice-rafted debris in the core tops may result from very slow sedimentation rates and/or current winnowing in the last few thousand years. The fact that foraminifera-bearing muds are thicker on the western and deeper side of the Belgica TMF suggests a westward current. One core recovered east of the TMF contains Holocene debris flows, which may be favoured by the steeper slope gradient beyond the TMF, where the sedimentation rate during the last glacial period was probably lower than on the TMF. Glacial sedimentation may have resulted from (1) gravity flows including turbidity currents and (2) vertical settling from proglacial plumes coupled with ice rafting. The resulting deposits are found in different parts of the TMF. This suggests that, although TMFs are traditionally associated with debris flow deposition due to high supply of glacigenic debris in front of ice streams, a plethora of factors affect downslope deposition in these areas. Local topography, the rapid pinch-out of debris flows compared to turbidites resulting in localized rather than widespread deposition, and the amount of meltwater at the ice front may be some such factors. At least one core penetrated the last interglacial and includes 2 m of hemipelagic glacial sediments, which represents the background sedimentation for the last glacial period. This study shows that deposition from turbidity currents and hemipelagic settling are important processes on TMFs and calls into question the usually assumed dominance of glacigenic debris flow deposition on TMFs during glacial periods. This observation highlights a need for additional coring on Antarctic TMFs and a comparison with TMFs on other glaciated margins in order to better understand the sedimentary processes in these areas, and the factors affecting their occurrence.