"Geomorphological record of a former ice stream to ice shelf lateral transition zone in Northeast Greenland

Understanding ice stream dynamics over decadal to millennial timescales is crucial for improving numerical model projections of ice sheet behaviour and future ice loss. In marine-terminating settings, ice shelves play a critical role in controlling ice-stream grounding line stability and ice flux to the ocean, but few studies have investigated the terrestrial lateral geomorphological imprint of ice shelves during deglaciation. Here, we document the terrestrial deglacial landsystem of Nioghalvfjerdsfjorden Glacier (79N) in northeast Greenland, following the Last Glacial Maximum, and the margin's lateral transition to a floating ice shelf. High-elevation areas are influenced by local ice caps and display autochthonous to allochthonous blockfields that mark the interaction of local ice caps with the ice stream below. A thermal transition from cold- to warm-based ice is denoted by the emplacement of erratics onto allochthonous blockfields. Below ~600 m above sea level (a.s.l.) glacially abraded bedrock surfaces and assemblages of lateral moraines, ‘hummocky’ moraine, fluted terrain, and ice-contact deltas record the former presence of warm-based ice and thinning of the grounded ice stream margin through time. In the outer fjord a range of landforms such as ice shelf moraines, dead-ice topography, and weakly developed ice marginal glaciofluvial outwash was produced by an ice shelf during deglaciation. Along the mid- and inner-fjord areas this ice shelf signal is absent, suggesting ice shelf disintegration prior to grounding line retreat under tidewater conditions. However, below the marine limit, the geomorphological record along the fjord indicates the expansion of the 79N ice shelf during the Neoglacial, which culminated in the Little Ice Age. This was followed by 20th century recession, with the development of a suite of compressional ice shelf moraines, ice-marginal fluvioglacial corridors, kame terraces, dead-ice terrain, and crevasse infill ridges. These mark rapid ice shelf thinning and typify the present-day ice shelf landsystem in a warming climate

Progressive ductile shearing during till accretion within the deforming bed of a palaeo-ice stream

This paper presents the results of a detailed microstructural study of a thick till formed beneath the Weichselian (Devensian) Odra palaeo-ice stream, west of Środa Wielkopolska, Poland. This SE-flowing ice stream was one of a number of corridors of faster flowing ice which drained the Scandinavian Ice Sheet in the Baltic region. Macroscopically, the massive, laterally extensive till which formed the bed of this ice stream lacks any obvious evidence of glaciotectonism (thrusting, folding). However, microscale analysis reveals that bed deformation was dominated by foliation development, recording progressive ductile shearing within a subhorizontal subglacial shear zone. Five successive generations of clast microfabric (S1 to S5) have been identified defining a set of up-ice and down-ice dipping Riedel shears, as well as a subhorizontal shear foliation coplanar to the ice-bed interface. Cross-cutting relationships between the shear fabrics record temporal changes in the style of deformation during this progressive shear event. Kinematic indicators (S-C and ECC-type fabrics) within the till indicate a consistent SE-directed shear sense, in agreement with the regional ice flow pattern. A model of bed deformation involving incremental progressive simple shear during till accretion is proposed. The relative age of this deformation was diachronous becoming progressively younger upwards, compatible with subglacial shearing having accompanied till accretion at the top of the deforming bed. Variation in the relative intensity of the microfabrics records changes in the magnitude of the cumulative strain imposed on the till and the degree of coupling between the ice and underlying bed during fast ice flow

Geomorphology and till architecture of terrestrial palaeo-ice streams of the southwest Laurentide Ice Sheet: a borehole stratigraphic approach

A multidimensional study, utilising geomorphological mapping and the analysis of regional borehole stratigraphy, is employed to elucidate the regional till architecture of terrestrial palaeo-ice streams relating to the Late Wisconsinan southwest Laurentide Ice Sheet. Detailed mapping over a 57,400 km2 area of southwestern Saskatchewan confirms previous reconstructions of a former southerly flowing ice stream, demarcated by a 800 km long corridor of megaflutes and mega-scale glacial lineations (Ice Stream 1) and cross cut by three, formerly southeast flowing ice streams (Ice Streams 2A, B and C). Analysis of the lithologic and geophysical characteristics of 197 borehole samples within these corridors reveals 17 stratigraphic units comprising multiple tills and associated stratified sediments overlying preglacial deposits, the till thicknesses varying with both topography and distance down corridor. Reconciling this regional till architecture with the surficial geomorphology reveals that surficial units are spatially consistent with a dynamic switch in flow direction, recorded by the cross cutting corridors of Ice Streams 1, 2A, B and C. The general thickening of tills towards lobate ice stream margins is consistent with subglacial deformation theory and variations in this pattern on a more localised scale are attributed to influences of subglacial topography including thickening at buried valley margins, thinning over uplands and thickening in overridden ice-marginal landforms

Submarine glacial-landform distribution across the West Greenland margin: a fjord-shelf-slope transect through the Uummannaq system (70-71 N)

Today, the Greenland Ice Sheet reaches the sea via a number of fast-flowing outlet glaciers that are fed by ice draining from huge interior basins (Rignot & Kanagaratnam 2006). At the Last Glacial Maximum (LGM), the ice sheet expanded to reach the continental shelf break around much of Greenland (Ó Cofaigh et al. 2013a). In the Uummannaq area at c. 70–71° N (Fig. 1a) there is now a 400 km distance between the terminus of Rink Glacier, which drains about 30 000 km2 of the ice sheet, and the shelf edge. This provides a transect from the modern glacier front, through a deep fjord system and adjacent cross-shelf trough, to the continental slope in Baffin Bay. The seafloor is now exposed along this transect and the landforms produced by past glacial activity can be examined using marine-geophysical methods. Deglaciation from the LGM was underway at the shelf edge in Uummannaq Trough by 14.8 kyr ago and from the mid-shelf by 10.9 kyr, and ice had probably retreated back into the fjord system by 9.3 kyr ago (Ó Cofaigh et al. 2013a; Roberts et al. 2013)

Subglacial meltwater channels in Marguerite Trough, western Antarctic Peninsula

Meltwater channels that have formed beneath glaciers and ice sheets have been observed on glaciated terrain, both terrestrial and submarine. Around Antarctica, channels incised into bedrock on the inner continental shelf have been found in several cross-shelf troughs that held major ice-streams during the last full-glacial (e.g. Pine Island Trough, Nitsche et al. 2014; Dotson-Getz Trough, Larter et al. 2009). The relatively widespread occurrence of meltwater channels has important implications for enhancing the flow of former ice streams over rough bedrock topography, yet questions remain regarding their timing and mode of formation. Meltwater channels from Marguerite Trough offshore of the western Antarctic Peninsula are described here (Fig. 1a)