Glacigenic debris-flows and down-slope gullies: evidence of a grounded ice margin during past glacials, South Shetland Trench, Antarctica

Abstract

The South Shetland Trench (SST) is located near the Antarctic Peninsula, around 100 km northwest, and parallel to, the South Shetland Islands. Although a number of studies examining glacial history have been undertaken in the Bransfield Strait located between the South Shetland Islands and the Antarctic Peninsula to the south, the authors believe this is the first study of the seafloor glacial geomorphology, and recent glacial history of the SST. This paper presents initial results from a EUROFLEETS Expedition to the SST that took place in December 2015. The Expedition collected three gravity cores, 3148 square kilometres of multibeam echosounder data and around 600 line kilometres of Topas sub-bottom data covering part of the southern flank and trench floor of the study area. Additional bathymetry data derived from the Global Multi-Resolution Topography (GMRT) Synthesis (Marine Geoscience Data System www.marine-geo.org) comprising a multi-resolution global Digital Elevation Model (DEM) that includes processed multibeam bathymetry data (100 m resolution) where available and gridded seafloor depths (30 arc-second resolution) derived from the General Bathymetric Chart of the Oceans (GEBCO www.gebco.net). These combined data reveal the study area, north of King George Island, on the southern flank of the SST to host a system of linear downslope gullies, glacigenic debris flows and glacigenic deposits. Within the study area glacigenic debris flows are found to extend from the continental shelf break to the lower continental slope. Sub-bottom profiler data penetrated up to 150 ms below seafloor in places and reveal a stacked sequence of debris flows suggestive of a fluctuating ice front that was grounded to, and retreated from, the shelf break on several occasions. More than eleven individual gullies (and their tributaries) were imaged between 450 m and 3600 m water depth. The gullies are incised up to 250 m below the surrounding sea bed with internal slope angles locally exceeding 45° and were influenced by shallow transform faulting related to subduction processes. Downslope gullies have been observed on other glaciated margins such as the Scotian slope offshore Canada, Ross Sea Antarctica, north-western Barents Sea and West Shetland Margin offshore north-western UK. The gullies are inferred as being eroded by turbidity currents comprising cold, dense, sediment-rich meltwater released from an ice front located at or near the continental shelf break.