Stephen J. Livingstone

Glacial geomorphology of the central sector of the last British-Irish Ice Sheet

Abstract Please click here to download the map associated with this article. We here present a glacial geomorphological map covering 11,800 km2, at a scale of 1:550,000, of the central sector of the last (Main Late Devensian) British-Irish Ice Sheet. The map is based on the 5 m resolution NEXTMap dataset. Six landform types have been mapped; subglacial lineations, hummocky terrain, ribbed moraine, meltwater channels, eskers and glaciofluvial sediment accumulations. The subglacial lineations have been further sub-divided into a series of ‘flow sets’ based on their morphology, conformity and length. Over 9,000 individual landforms have been identified within the study area, concentrated predominantly in the lowlands of the Vale of Eden, Solway and over the Tyne and Stainmore Gaps. A palimpsest geomorphic signature characterised by cross-cutting flow-sets is interpreted as evidence that dynamic, multiphase ice flow occurred throughout the Main Late Devensian (marine isotope stage 2) in response to migrating ice dispersal centres and ice divides. A relative chronology has been constructed and interpreted, based on the complex, cross-cutting flow signatures displayed throughout the region.

Understanding controls on rapid ice‐stream retreat during the last deglaciation of Marguerite Bay, Antarctica, using a numerical model

Using a one-dimensional numerical model of ice-stream flow with robust grounding-line dynamics, we explore controls on paleo-ice-stream retreat in Marguerite Bay, Antarctica, during the last deglaciation. Landforms on the continental shelf constrain the numerical model and suggest retreat was rapid but punctuated by a series of slowdowns. We investigate the sensitivity of ice-stream retreat to changes in subglacial and lateral topography, and to forcing processes including sea-level rise, enhanced melting beneath an ice shelf, atmospheric warming, and ice-shelf debuttressing. Our experiments consistently reproduce punctuated retreat on a bed that deepens inland, with retreat-rate slowdowns controlled by narrowings in the topography. Sensitivity experiments indicate that the magnitudes of change required for individual forcing mechanisms to initiate retreat are unrealistically high but that thresholds are reduced when processes act in combination. The ice stream is, however, most sensitive to ocean warming and associated ice-shelf melting and retreat was most likely in response to external forcing that endured throughout the period of retreat rather than to a single triggering ‘event’. Timescales of retreat are further controlled by the delivery of ice from upstream of the grounding line. Due to the influence of topography, modeled retreat patterns are insensitive to the temporal pattern of forcing evolution. We therefore suggest that despite regionally similar forcing mechanisms, landscape controls significant contrasts in retreat behavior between adjacent but topographically distinct catchments. Patterns of ice-stream retreat in the past, present and future should therefore be expected to vary significantly.

Glacial geomorphology of Marguerite Bay Palaeo-Ice stream, western Antarctic Peninsula

This paper presents a glacial geomorphological map of over 17,000 landforms on the bed of a major palaeo-ice stream in Marguerite Bay, western Antarctic Peninsula. The map was compiled using various geophysical datasets from multiple marine research cruises. Eight glacial landform types are identified: mega-scale glacial lineations, crag-and-tails, whalebacks, gouged, grooved and streamlined bedrock, grounding-zone wedges, subglacial meltwater channels, gullies and channels, and iceberg scours. The map represents one of the most complete marine ice-stream signatures available for scrutiny, and these data hold much potential for reconstructing former ice sheet dynamics, testing numerical ice sheet models, and understanding the formation of subglacial bedforms beneath ice streams. In particular, they record a complex bedform signature of palaeo-ice stream flow and retreat since the last glacial maximum, characterised by considerable spatial variability and strongly influenced by the underlying geology. The map is presented at a scale of 1: 750,000, designed to be printed at A2 size, and encompasses an area of 128,420 km2.

Northeast sector of the Greenland Ice Sheet to undergo the greatest inland expansion of supraglacial lakes during the 21st century

The formation and rapid drainage of supraglacial lakes (SGL) influences the mass balance and dynamics of the Greenland Ice Sheet (GrIS). Although SGLs are expected to spread inland during the 21st century due to atmospheric warming, less is known about their future spatial distribution and volume. We use GrIS surface elevation model and regional climate model outputs to show that at the end of the 21st century (2070–2099) approximately 9.8 ± 3.9 km3 (+113% compared to 1980-2009) and 12.6 ± 5 km3 (+174%) of meltwater could be stored in SGLs under moderate and high representative concentration pathways (RCP 4.5 and 8.5), respectively. The largest increase is expected in the northeastern sector of the GrIS (191% in RCP 4.5 and 320% in RCP 8.5), whereas in west Greenland, where the most SGLs are currently observed, the future increase will be relatively moderate (55% in RCP 4.5 and 68% in RCP 8.5).

Manual mapping of drumlins in synthetic landscapes to assess operator effectiveness

Mapped topographic features are important for understanding processes that sculpt the Earths surface. This paper presents maps that are the primary product of an exercise that brought together 27 researchers with an interest in landform mapping wherein the efficacy and causes of variation in mapping were tested using novel synthetic DEMs containing drumlins. The variation between interpreters (e.g. mapping philosophy, experience) and across the study region (e.g. woodland prevalence) opens these factors up to assessment. A priori known answers in the synthetics increase the number and strength of conclusions that may be drawn with respect to a traditional comparative study. Initial results suggest that overall detection rates are relatively low (34–40%), but reliability of mapping is higher (72–86%). The maps form a reference dataset.

Discovery of relict subglacial lakes and their geometry and mechanism of drainage

Recent proxy measurements reveal that subglacial lakes beneath modern ice sheets periodically store and release large volumes of water, providing an important but poorly understood influence on contemporary ice dynamics and mass balance. This is because direct observations of how lake drainage initiates and proceeds are lacking. Here we present physical evidence of the mechanism and geometry of lake drainage from the discovery of relict subglacial lakes formed during the last glaciation in Canada. These palaeo-subglacial lakes comprised shallow (<10 m) lenses of water perched behind ridges orientated transverse to ice flow. We show that lakes periodically drained through channels incised into bed substrate (canals). Canals sometimes trend into eskers that represent the depositional imprint of the last high-magnitude lake outburst. The subglacial lakes and channels are preserved on top of glacial lineations, indicating long-term re-organization of the subglacial drainage system and coupling to ice flow.

The periodic topography of ice stream beds: Insights from the Fourier spectra of mega-scale glacial lineations

Ice stream bed topography contains key evidence for the ways ice streams interact with, and are potentially controlled by, their beds. Here we present the first application of two–dimensional Fourier analysis to 22 marine and terrestrial topographies from 5 regions in Antarctica and Canada, with and without mega-scale glacial lineations (MSGLs). We find that the topography of MSGL-rich ice stream sedimentary beds is characterized by multiple, periodic wavelengths between 300 and 1200 m and amplitudes from decimeters to a few meters. This periodic topography is consistent with the idea that instability is a key element to the formation of MSGL bedforms. Dominant wavelengths vary among locations and, on one paleo ice stream bed, increase along the direction of ice flow by 1.7±0.52% km-1. We suggest that these changes are likely to reflect pattern evolution via downstream wavelength coarsening, even under potentially steady ice stream geometry and flow conditions. The amplitude of MSGLs is smaller than that of other fluvial and glacial topographies, but within the same order of magnitude. However, MSGLs are a striking component of ice stream beds because the topographic amplitude of features not aligned with ice flow is reduced by an order of magnitude relative to those oriented with the flow direction. This study represents the first attempt to automatically derive the spectral signatures of MSGLs. It highlights the plausibility of identifying these landform assemblages using automated techniques and provides a benchmark for numerical models of ice stream flow and subglacial landscape evolution.

Paleofluvial and subglacial channel networks beneath Humboldt Glacier, Greenland

The identification of subglacial drainage systems can inform our understanding of past and present hydrological processes, landscape evolution, and ice dynamics. Here, we present evidence from satellite imagery, digital elevation models, and radio-echo sounding data for a series of channelized networks with contrasting paleofluvial and subglacial origins beneath Humboldt Glacier, northern Greenland. A >250-km-long, dendritic paleofluvial channel network beneath the northern portion of Humboldt is interpreted as a preglacial feature. Roughly linear channels beneath the southern portion of Humboldt, which display a similar distribution to tunnel valleys found on the beds of former ice sheets, are likely to have been eroded by subglacial meltwater routed along the ice-sheet bed. We suggest that basal meltwater is actively being routed down both the paleofluvial and subglacially formed channel networks to the coast. Inheritance of the preglacial channel network may have influenced the present-day location and dynamics of Humboldt Glacier and enhanced selective erosion at its down-glacier end.

Submarine glacial-landform distribution along an Antarctic Peninsula palaeo-ice stream: a shelf–slope transect through the Marguerite Trough system (66–70° S)

The Antarctic Peninsula comprises a thin spine of mountains and islands presently covered by an ice sheet up to 500 m thick that drains eastward and westward via outlet glaciers (Davies et al. 2012). The peninsula has undergone recent rapid warming, resulting in the collapse of fringing ice shelves and the retreat, thinning and acceleration of marine-terminating outlet glaciers (e.g. Pritchard & Vaughan 2007). At the Last Glacial Maximum (LGM), the ice sheet expanded to the continental shelf break around the peninsula, and was organized into a series of ice streams that drained along cross-shelf bathymetric troughs (O Cofaigh et al. 2014). Marguerite Bay is located on the west side of the Antarctic Peninsula, at about 66–70° S (Fig. 1). A 12–80 km wide and 370 km long trough extends across the bay from the northern terminus of George VI Ice Shelf to the continental shelf edge. Extensive marine-geophysical surveys of the trough reveal a suite of glacial landforms which record past flow of an ice stream which extended to the shelf edge at, or shortly after, the LGM. Subsequent retreat of the ice stream was underway by c. 14 ka ago and proceeded rapidly to the mid-shelf, where it slowed before accelerating once again to the inner shelf at c. 9 ka (Kilfeather et al. 2011). Fig. 1. Regional bathymetry and shelf architecture of the Marguerite Trough shelf–slope system, Antarctic Peninsula (AP). The location of subsequent figures is shown. ( a ) Multibeam-bathymetric coverage of the Marguerite Trough system. Light grey is grounded ice; dark grey is floating ice. Inset: location of study area on the Antarctic Peninsula (red box; map from IBCSO v. 1.0). Regional bathymetry from IBCAO v. 3.0. Arrows denote perspective of oblique views in Figures 2a and 3a. ( b ) 130 km along-dip seismic-reflection profile showing Antarctic …

Near-margin ice thickness and subglacial water routing, Leverett Glacier, Greenland

ABSTRACT Ice thickness measurements near the margin of the Greenland Ice Sheet (GrIS) are relatively sparse, presenting issues for modeling ice-flow dynamics, ice-sheet change, and subglacial hydrology. We acquired near-margin ice thickness data at Leverett Glacier, west Greenland, using a highly portable, low power, ground-penetrating radar operating at 10–80 MHz. Ice-thickness measurements, to a maximum of 270 m, were incorporated into the BedMachine model of ice thickness, created using mass conservation methods. The new data significantly modified the modeled ice thickness, and hence bed elevation and routing of subglacial water, in both the Leverett and adjacent Russell Glacier. Although the revised modeled basal topography and subglacial hydrology are consistent with observations, our new data unrealistically reduced the overall size of the Leverett Glacier hydrological catchment. Additional ice-thickness measurements are therefore required to realistically constrain subglacial topography and subglacial hydrological routing in this area. Our work improves understanding of the basal topography and the subglacial hydrology of Leverett Glacier, with implications for glacier dynamics and assessments of water piracy between catchments in the marginal zone of the GrIS, and for the interpolation of ice-thickness grids using mass conservation methods.