Ice-sheet grounding-zone wedges (GZWs) on high-latitude continental margins

© 2015 Elsevier B.V. Grounding-zone wedges (GZWs) are asymmetric sedimentary depocentres which form through the rapid accumulation of glacigenic debris along a line-source at the grounding zone of marine-terminating ice sheets during still-stands in ice-sheet retreat. GZWs form largely through the delivery of deforming subglacial sediments. The presence of GZWs in the geological record indicates an episodic style of ice retreat punctuated by still-stands in grounding-zone position. Moraine ridges and ice-proximal fans may also build up at the grounding zone during still-stands of the ice margin, but these require either considerable vertical accommodation space or sediment derived from point-sourced subglacial meltwater streams. By contrast, GZWs form mainly where floating ice shelves constrain vertical accommodation space immediately beyond the grounding-zone. An inventory of GZWs is compiled from available studies of bathymetric and acoustic data from high-latitude continental margins. The locations and dimensions of GZWs from the Arctic and Antarctic, alongside a synthesis of their key architectural and geomorphic characteristics, are presented. GZWs are only observed within cross-shelf troughs and major fjord systems, which are the former locations of ice streams and fast-flowing outlet glaciers. Typical high-latitude GZWs are less than 15. km in along-flow direction and 15 to 100. m thick. GZWs possess a transparent to chaotic acoustic character, which reflects the delivery of diamictic subglacial debris. Many GZWs contain seaward-dipping reflections, which indicate sediment progradation and wedge-growth through continued delivery of basal sediments. GZW formation is inferred to require high rates of sediment delivery to a fast-flowing ice margin that is relatively stable for probably decades to centuries. Although the long-term stability of the grounding zone is controlled by ice-sheet mass balance, the precise location of any still-stands is influenced strongly by the geometry of the continental shelf. The majority of high-latitude GZWs occur at vertical or lateral pinning points, which encourage grounding-zone stabilisation through increasing basal and lateral drag and reducing mass flow across the grounding zone.We thank TGS-NOPEC Geophysical Company ASA for permission to reproduce 2-D seismic reflection data from the northwest and northeast Greenland margins, and Phil O'Brien, Dag Ottesen and Michele Rebesco for the use of acoustic and bathymetric data modified for this work. We thank Rob Larter and one anonymous reviewer for their helpful comments

Lateral shear-moraines and lateral marginal-moraines of palaeo-ice streams

An understanding of the nature of sedimentation at ice-stream lateral margins is important in reconstructing the dynamics of former ice sheets and modelling the mechanisms by which sediment is transported beneath contemporary ice streams. Theories of the formation of ice-stream lateral moraines (ISLMs) have hitherto been based on a relatively limited number of terrestrial and marine examples. Here, an inventory of ISLMs is compiled from available studies, together with independent analysis of seismic-reflection and bathymetric datasets. The locations and dimensions of 70 ISLMs, alongside a synthesis of their key architectural and geomorphic characteristics, are presented. Two different types of ISLMs are identified. Type 1 ISLMs are up to 3.5 km wide and 60 m thick. They maintain a constant width, thickness and cross-sectional shape along their length. Type 1 ISLMs are interpreted and referred to as ice-stream $\textit{lateral shear-moraines}$ that form subglacially in the shear zone between ice streams and slower-flowing regions of an ice sheet. In contrast, Type 2 ISLMs are up to 50 km wide and 300 m thick. They are only identified close to the shelf break in the marine environment. Type 2 ISLMs exhibit an increase in width and thickness along their length and their distal slopes become steeper in a seaward direction. They contain internal dipping reflections that indicate sediment progradation away from the former ice stream. Type 2 ISLMs are interpreted and referred to as ice-stream $\textit{lateral marginal-moraines}$ that were formed at the lateral boundary between ice streams and seafloor terrain that was free of grounded ice. We suggest that, using bathymetric images and acoustic profiles, it is possible to differentiate between ice-stream lateral shear-moraines and lateral marginal-moraines in the geological record. This distinction is important for understanding the mechanisms of sediment transfer beneath ice streams and for making inferences about the conditions that existed beyond the lateral ice-stream margin at the time of lateral-moraine formation: slow-moving ice beyond lateral shear-moraines, and ocean water rather than ice beyond lateral marginal-moraines.Newnham College, Cambridge (Junior Research Fellowship)This is the author accepted manuscript. The final version is available from Elsevier via http://dx.doi.org/10.1016/j.quascirev.2016.08.02

Grounding-zone wedges on the West Greenland shelf imaged from multibeam and seismic data

Grounding-zone wedges (GZWs) are asymmetrical depocentres built up beneath the grounding-zone of marine-terminating ice streams and fast-flowing outlet glaciers through the delivery of soft, deforming subglacial till from up-glacier. They are typically tens of metres thick, tens of kilometres in length and usually form subdued transverse-to-flow ridges across the long-axes of fjords and troughs on high-latitude continental shelves (e.g. Shipp et al. 1999; Dowdeswell & Fugelli 2012; Batchelor & Dowdeswell 2014). The ridges, representing the relatively steeper ice-distal face of the wedge, usually appear as small scarps or steps in multibeam imagery. The wedges thin in an iceproximal direction, often becoming difficult to identify except by using acoustic-stratigraphic methods.This is the author accepted manuscript. The final version is available from the Geological Society of London via https://doi.org/10.1144/M46.6

Nordvestfjord: A major East Greenland fjord system

This is the author accepted manuscript. The final version is available fromGeological Society of London via https://doi.org/10.1144/M46.4

Tunnel valleys of the central and northern North Sea (56°N to 62°N): Distribution and characteristics

© 2020 Elsevier B.V. The analysis of buried tunnel valleys in the North Sea can provide information about the past configuration and dynamics of the Scandinavian and British ice sheets and the processes by which sediment and meltwater were transported at the ice-sheet base. However, little is presently known about the distribution and characteristics of tunnel valleys in the Norwegian sector of the North Sea. Here we use an extensive database of 3D seismic and high-resolution magnetic data to map >2200 tunnel valleys in the Norwegian and British sectors of the North Sea between 56°N and 62°N. With the exception of the deep Norwegian Channel, in which evidence for tunnel valleys is absent, the geological setting of the North Sea is interpreted to have been conducive to tunnel-valley formation and preservation because of its poorly consolidated substrate and shallow water depths. The highest density of tunnel valleys is located in the central part of the North Sea where Quaternary sediments are thickest. The extreme length of some of the tunnel valleys, which are up to 155 km long, supports theories that tunnel valleys form in stages rather than catastrophically. Detailed analysis of the orientation of tunnel valleys and their relative age relationships within four representative subareas shows that tunnel-valley orientation varies significantly across the central and northern North Sea and between different generations of valleys. This suggests that the pattern of subglacial meltwater drainage in the central and northern North Sea was different between each deglacial event in which tunnel valleys were formed

Processes and patterns of glacier-influenced sedimentation and recent tidewater glacier dynamics in Darbel Bay, western Antarctic Peninsula

AbstractBathymetric data of unprecedented resolution are used to provide insights into former ice dynamics and glacial processes in a western Antarctic Peninsula embayment. An assemblage of submarine glacial landforms, which includes subglacially produced streamlined features and ice-marginal ridges, reveals the former pattern of ice flow and retreat. A group of more than 250 small (< 1–3 m high, 10–20 m wide) and relatively evenly spaced recessional moraines was identified beyond the margin of Philippa Glacier. The small recessional moraines are interpreted to have been produced during short-lived, possibly annual re-advances of a grounded ice margin during overall retreat. This is the first time that these features have been shown to be part of the assemblage of landforms produced by tidewater glaciers on the Antarctic Peninsula. Glacier-terminus changes during the last four decades, mapped from LANDSAT satellite images, were analysed to determine whether the moraines were produced during recent still-stands or re-advances of Philippa Glacier and to further investigate the short-term (annual to decadal) variability in ice-marginal position in tidewater glacier systems. The asynchronous response of individual tidewater glaciers in Darbel Bay is interpreted to be controlled mainly by local topography rather than by glacier catchment-area size.C.L. Batchelor was in receipt of a Junior Research Fellowship from Newnham College, Cambridge during this work. K.A Hogan and R.D Larter are supported by the Polar Science for Planet Earth programme funded by the Natural Environment Research Council, U.K

Submarine landforms reveal varying rates and styles of deglaciation in North-West Greenland fjords

An understanding of the former configuration and dynamics of the Greenland Ice Sheet is needed to provide a context for modern observations, to constrain numerical models and to predict the likely future ice-sheet response to climatic change. Whereas previous geophysical investigations of the North-West Greenland margin have focused on the mapping of full-glacial and deglacial landforms on the mid to outer shelf, relatively little is known about more recent ice-sheet dynamics on the inner shelf and in the fjords. We present swath-bathymetric data from the inner shelf and fjords of North-West Greenland. Streamlined subglacial landforms, including ice-sculpted bedrock and mega-scale glacial lineations, reveal the direction of Late Quaternary ice flow through fjords and across the inner shelf. Landforms that are transverse to the former ice-flow direction, including small recessional moraines, major moraine ridges and grounding-zone wedges, show the locations of former still-stands in the grounding zone during regional deglaciation and terminus readvances linked to the Little Ice Age. The distribution of submarine glacial landforms in the inner fjords suggests that the outlet glaciers of North-West Greenland experienced varying rates and styles of ice retreat during the late Holocene, which was probably controlled mainly by fjord water depth.Inner fjords that have contemporary water depths of less than 350 m contain series of small recessional moraines, which indicate the slow retreat of a grounded ice margin. Small recessional moraines are generally absent from inner fjords with water depths of more than 350 m, which are interpreted to have experienced more rapid ice retreat during the late Holocene.During this work, C.L. Batchelor was in receipt of a Junior Research Fellowship at Newnham College, Cambridge. The bathymetric data that was collected during JR175 of the RRS James Clark Ross to West Greenland in 2009 was funded by Natural Environment Research Council (NERC) grant NE/D001951/1 to C. Ó Cofaigh

Delicate seafloor landforms reveal past Antarctic grounding-line retreat of kilometers per year.

A suite of grounding-line landforms on the Antarctic seafloor, imaged at submeter horizontal resolution from an autonomous underwater vehicle, enables calculation of ice sheet retreat rates from a complex of grounding-zone wedges on the Larsen continental shelf, western Weddell Sea. The landforms are delicate sets of up to 90 ridges, 10 kilometers per year) are inferred during regional deglaciation of the Larsen shelf. If repeated today, such rapid mass loss to the ocean would have clear implications for increasing the rate of global sea level rise

Submarine Moraines in Southeast Greenland Fjords Reveal Contrasting Outlet-Glacier Behavior since the Last Glacial Maximum

Knowledge of the past behavior of the outlet glaciers of Southeast (SE) Greenland is necessary to understand and model spatial differences in the response of the Greenland Ice Sheet (GIS) to climatic changes. Here we use bathymetric data to map the distribution of more than 50 major moraines in SE Greenland fjords. Inner-fjord moraines are widespread along the SE Greenland margin, occurring in 65% of the surveyed fjords. We identify, for the first time, nine midfjord moraines that span the approximately 150-km-long eastern margin of the Julianehåb Ice Cap. In contrast, midfjord moraines are generally absent from the deeper and wider fjords of the SE GIS. The variable distribution of midfjord moraines along the SE Greenland margin reveals contrasting behavior of the SE GIS and the eastern Julianehåb Ice Cap during the last deglaciation, which probably reflects differences in fjord geometry and exposure to ocean heat

Sea-floor and sea-ice conditions in the western Weddell Sea, Antarctica, around the wreck of Sir Ernest Shackleton's Endurance

AbstractMarine-geophysical evidence on sea-floor morphology and shallow acoustic stratigraphy are used to examine the substrate around the location at which Sir Ernest Shackleton's ship Endurance sank in 1915 and on the continental slope-shelf sedimentary system above this site in the western Weddell Sea. Few signs of turbidity-current and mass-wasting activity are found near or upslope of the wreck site, and any such activity was probably linked to full-glacial higher-energy conditions when ice last advanced across the continental shelf. The wreck is well below the maximum depth of iceberg keels and will not have been damaged by ice-keel ploughing. The wreck has probably been draped by only a few centimetres of fine-grained sediment since it sank in 1915. Severe modern sea-ice conditions hamper access to the wreck site. Accessing and investigating the wreck of Endurance in the Weddell Sea therefore represents a significant challenge. An ice-breaking research vessel is required, and even this would not guarantee that the site could be reached. Heavy sea-ice cover at the wreck site, similar to that encountered by Agulhus II during the Weddell Sea Expedition 2019, would also make the launch and recovery of autonomous underwater vehicles and remotely operated vehicles deployed to investigate the Endurance wreck problematic.The Flotilla Foundation Marine Archaeological Consultants Switzerlan