Basal melting over Subglacial Lake Ellsworth and its catchment: insights from englacial layering

Neil Ross,Martin Siegert

doi:10.1017/aog.2020.50

Abstract

AbstractDeep-water ‘stable’ subglacial lakes likely contain microbial life adapted in isolation to extreme environmental conditions. How water is supplied into a subglacial lake, and how water outflows, is important for understanding these conditions. Isochronal radio-echo layers have been used to infer where melting occurs above Lake Vostok and Lake Concordia in East Antarctica but have not been used more widely. We examine englacial layers above and around Lake Ellsworth, West Antarctica, to establish where the ice sheet is ‘drawn down’ towards the bed and, thus, experiences melting. Layer drawdown is focused over and around the northwest parts of the lake as ice, flowing obliquely to the lake axis becomes afloat. Drawdown can be explained by a combination of basal melting and the Weertman effect, at the transition from grounded to floating ice. We evaluate the importance of these processes on englacial layering over Lake Ellsworth and discuss implications for water circulation and sediment deposition. We report evidence of a second subglacial lake near the head of the hydrological catchment and present a new high-resolution bed DEM and hydropotential model of the lake outlet zone. These observations provide insight into the connectivity between Lake Ellsworth and the wider subglacial hydrological system.

Highlights

The exploration, access and measurement of Antarctic subglacial lakes have the potential to address several significant scientific questions regarding Antarctic Ice Sheet history, life in extreme environments, and how subglacial hydrology can influence ice-sheet flow (Siegert, 2016)
A prominent feature of the radio-echo sounding (RES) data over Subglacial Lake Ellsworth (SLE) are buckles generated by up-ice flow over rugged subglacial topography (Ross and others, 2011b), which disrupt the layer stratigraphy and geometry, and limit uninterrupted layer picking in ice >500 m below the ice surface in some areas (Figs 1b, 2c and d)
Englacial reflections drape over the rugged topography with traceable layers over the Ellsworth Trough mapped at ice thicknesses up to double that observed over the topographic highs of the valley sidewalls and surrounding highlands and interfluves (Figs 2 and 3)

Summary

Introduction

The exploration, access and measurement of Antarctic subglacial lakes have the potential to address several significant scientific questions regarding Antarctic Ice Sheet history, life in extreme environments, and how subglacial hydrology can influence ice-sheet flow (Siegert, 2016). Considerable focus has been given to inputs and outputs associated with water flow across the ice-sheet bed (e.g. Wingham and others, 2006; Fricker and others, 2007; Wright and others, 2012; Fricker and others, 2016) but there has been relatively little investigation constraining basal melt of the overlying ice sheet at the ice/water interface. A characteristic of the ice sheet that can be exploited to constrain both the location and magnitude of basal melting is the englacial structure of the ice sheet, which can be imaged and characterised using radio-echo sounding (RES) (Siegert, 1999; Dowdeswell and Evans, 2004). Localised areas of high geothermal heat flux, englacial layers can be locally drawn down so that younger layers are found at greater than expected depths in the ice-sheet column, indicative of high rates of subglacial melt (Siegert and others, 2000; Gudlaugsson and others, 2016; Jordan and others, 2018)

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