Continental slope records indicate a grounded ice sheet margin during past glacials, South Shetland Trench, Antarctica

The South Shetland Trench (SST) is located around 100 km northwest, and parallel to, the South Shetland Islands, located between the Antarctic Peninsula and South America. 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 the results from a Eurofleets2 expedition to the SST that took place in December 2015. There is evidence from other sub-Antarctic islands such as the South Orkney Islands and Elephant Island for glaciations that extended well onto their continental shelves although a lack of age constraint from marine cores means it cannot be tied to a specific glaciation. Multibeam echosounder data reveal the study area on the southern flank of the South Shetland Trench to host a system of linear downslope gullies and glacigenic debris flows. 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. Glacigenic debris flows are found to extend from the continental shelf break to the lower continental slope. Sub-bottom profiler data penetrated up to 150ms 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. The trench floor appears to be devoid of major geomorphological features as revealed by the multibeam echosounder data and comprise a relatively well layered sequence imaged by the sub-bottom profiler. In addition 3 gravity cores up to 2.79 m in length and one core catcher sample were recovered from the study area. The cores will be analysed on their return to the UK and combined with the acoustic data to produce a shallow geological model specifically looking at the fluctuating ice margin located north of the South Shetland Islands

Submarine landforms and shallow acoustic stratigraphy of a 400 km-long fjord-shelf-slope transect, Kangerlussuaq margin, East Greenland

Kangerlussuaq Fjord is a relatively uniform, steep-walled basin, whose floor has an almost smooth surface. Debris is supplied mainly from icebergs from the fast-flowing Kangerlussuaq Glacier. Sedimentation after iceberg release from multi-year sea ice is mainly by rain-out of fine-grained englacial debris. Streamlined glacial lineations and drumlins were produced at the sedimentary bed of an ice sheet that expanded into Kangerlussuaq Trough at the Last Glacial Maximum (LGM). Bedrock channels and crescentic overdeepenings indicate warm-based ice and free water beneath parts of the former ice sheet. Cross-cutting iceberg scour marks, which characterise outer Kangerlussuaq shelf, were produced not only during deglaciation, but also occasionally through the Holocene by deep-keeled icebergs from further north in East Greenland. The outward-convex contours of the shelf edge and slope beyond Kangerlussuaq Trough, and debris flows on the slope, suggest a glacier-influenced high-latitude fan. The distribution of streamlined subglacial landforms demonstrates that the Greenland Ice Sheet extended throughout Kangerlussuaq Fjord and reached at least 200 km across the shelf in Kangerlussuaq Trough at the LGM. Streamlined landform orientation indicates ice flow from the interior of Greenland down the axis of Kangerlussuaq Trough. There is little evidence for discrete sedimentary depocentres in the trough, implying that ice probably retreated rapidly from the outer and mid shelf during deglaciation

Submarine evidence of ice-streaming and ice-sheet re-advance of the British Ice Sheet

A number of major ice streams have been identified within the Pleistocene British–Irish Ice Sheet (BIIS), primarily based on onshore mapping and single beam bathymetric datasets offshore (e.g. Bradwell et al., 2008). The identification of these ice streams is crucial to better understand the flow pattern and demise of the last BIIS, although evidence is currently lacking for key marine sectors such as the North Sea Basin. An improved understanding of the past cycles of glaciation, timing and extent, has far reaching implications for better understanding global climate change and ice-sheet behaviour. This work focuses on the offshore geomorphological signature of the ice sheet sector referred to as the North Sea Lobe. New interpretations derived from high-resolution multibeam echosounder data and single-beam echosounder bathymetry data have been viewed in conjunction with onshore topographic digital surface models (NEXTMap). Derived layers of rugosity, slope, bathymetric positioning index and aspect were utilised during geomorphological mapping. Where available, multibeam backscatter intensity data have also been utilised as part of a semi-automated approach to identify areas of glacigenic deposits. Mega-scale glacial lineations and drumlin fields indicate the North Sea Lobe was fed by an ice stream emanating from the Forth–Tay catchment in central Scotland. Large arcuate moraine ridges, smaller-scale moraine ridges and grounding-zone wedges indicate a complex advance, recession and re-advance of the North Sea Lobe during the Last Glacial Maximum

A Late Pleistocene channelized subglacial meltwater system on the Atlantic continental shelf south of Ireland

The study of palaeo-glacial landforms and sediments can give insights into the nature and dynamics of ice sheets. This is particularly the case with regards to the subglacial record, which is challenging to observe in contemporary glaciated settings and hence remains only partially understood. The subglacial hydrological system is an essential component of ice dynamics, where increased water pressure enhances ice motion and sediment deformation, thus reducing ice-bed contact. Tunnel valleys are large, sinuous, steep-sided incisions that, together with smaller scale meltwater channels, indicate subglacial meltwater discharge beneath large ice sheets. Through the use of high-resolution marine geophysical data, a system of buried and exposed tunnel valleys, possible subglacial or proglacial meltwater channels and palaeo-fluvial valleys have been identified across the shelf of the Celtic Sea between Ireland and Britain. The presence of steep-sided and overdeepened tunnel valleys is indicative of a large channelized meltwater drainage system beneath the former Irish Sea Ice Stream, the most extensive ice stream to drain the last British–Irish Ice Sheet. After the rapid ice expansion across the Celtic Sea shelf around 28–26 ka, the tunnel valleys were carved into both bedrock and glacigenic sediments and are associated with rapid ice stream retreat northwards into the Irish Sea Basin between 25.6 and 24.3 ka. The presence of a major subglacial meltwater system on the relatively shallow shelf suggests that significant erosive meltwater discharge occurred during the last deglaciation and highlights the important contribution of meltwater to the retreat of the British–Irish Ice Sheet on the continental shelf

Geomorphic and shallow-acoustic investigation of an Antarctic Peninsula fjord system using high-resolution ROV and shipboard geophysical observations: Ice dynamics and behaviour since the Last Glacial Maximum

© 2016 Detailed bathymetric and sub-bottom acoustic observations in Bourgeois Fjord (Marguerite Bay, Antarctic Peninsula) provide evidence on sedimentary processes and glacier dynamics during the last glacial cycle. Submarine landforms observed in the 50 km-long fjord, from the margins of modern tidewater glaciers to the now ice-distal Marguerite Bay, are described and interpreted. The landforms are grouped into four morpho-sedimentary systems: (i) glacial advance and full-glacial; (ii) subglacial and ice-marginal meltwater; (iii) glacial retreat and neoglaciation; and (iv) Holocene mass-wasting. These morpho-sedimentary systems have been integrated with morphological studies of the Marguerite Bay continental shelf and analysed in terms of the specific sedimentary processes and/or stages of the glacial cycle. They demonstrate the action of an ice-sheet outlet glacier that produced drumlins and crag-and-tail features in the main and outer fjord. Meltwater processes eroded bedrock channels and ponds infilled by fine-grained sediments. Following the last deglaciation of the fjord at about 9000 yr BP, subsequent Holocene neoglacial activity involved minor readvances of a tidewater glacier terminus in Blind Bay. Recent stillstands and/or minor readvances are inferred from the presence of a major transverse moraine that indicates grounded ice stabilization, probably during the Little Ice Age, and a series of smaller landforms that reveal intermittent minor readvances. Mass-wasting processes also affected the walls of the fjord and produced scars and fan-shaped deposits during the Holocene. Glacier-terminus changes during the last six decades, derived from satellite images and aerial photographs, reveal variable behaviour of adjacent tidewater glaciers. The smaller glaciers show the most marked recent retreat, influenced by regional physiography and catchment-area size

A stratigraphic investigation of the Celtic Sea megaridges based on seismic and core data from the Irish-UK sectors

The Celtic Sea contains the world's largest continental shelf sediment ridges. These megaridges were initially interpreted as tidal features formed during post-glacial marine transgression, but glacigenic sediments have been recovered from their flanks. We examine the stratigraphy of the megaridges using new decimetric-resolution geophysical data correlated to sediment cores to test hypothetical tidal vs glacial modes of formation. The megaridges comprise three main units, 1) a superficial fining-upward drape that extends across the shelf above an unconformity. Underlying this drape is 2), the Melville Formation (MFm) which comprises the upper bulk of the megaridges, sometimes displaying dipping internal acoustic reflections and consisting of medium to coarse sand and shell fragments; characteristics consistent with either a tidal or glacifluvial origin. The MFm unconformably overlies 3), the Upper Little Sole Formation (ULSFm), previously interpreted to be of late Pliocene to early Pleistocene age, but here shown to correlate to Late Pleistocene glacigenic sediments forming a precursor topography. The superficial drape is interpreted as a product of prolonged wave energy as tidal currents diminished during the final stages of post-glacial marine transgression. We argue that the stratigraphy constrains the age of the MFm to between 24.3 and 14 ka BP, based on published dates, coeval with deglaciation and a modelled period of megatidal conditions during post-glacial marine transgression. Stratigraphically and sedimentologically, the megaridges could represent preserved glacifluvial features, but we suggest that they comprise post-glacial tidal deposits (MFm) mantling a partially-eroded glacial topography (ULSFm). The observed stratigraphy suggests that ice extended to the continental shelf-edge