Evidence for a grounding line fan at the onset of a basal channel under the ice shelf of Support Force Glacier, Antarctica, revealed by reflection seismics

Coen Hofstede,Daniel Steinhage,Angelika Humbert,Sebastian Beyer,Olaf Eisen,Emma C Smith,Tore Hattermann,Niklas Neckel,Ole Zeising,Hugh Corr,Veit Helm

doi:10.5194/tc-15-1517-2021

Abstract

Abstract. Curvilinear channels on the surface of an ice shelf indicate the presence of large channels at the base. Modelling studies have shown that where these surface expressions intersect the grounding line, they coincide with the likely outflow of subglacial water. An understanding of the initiation and the ice–ocean evolution of the basal channels is required to understand the present behaviour and future dynamics of ice sheets and ice shelves. Here, we present focused active seismic and radar surveys of a basal channel, ∼950 m wide and ∼200 m high, and its upstream continuation beneath Support Force Glacier, which feeds into the Filchner Ice Shelf, West Antarctica. Immediately seaward from the grounding line, below the basal channel, the seismic profiles show an ∼6.75 km long, 3.2 km wide and 200 m thick sedimentary sequence with chaotic to weakly stratified reflections we interpret as a grounding line fan deposited by a subglacial drainage channel directly upstream of the basal channel. Further downstream the seabed has a different character; it consists of harder, stratified consolidated sediments, deposited under different glaciological circumstances, or possibly bedrock. In contrast to the standard perception of a rapid change in ice shelf thickness just downstream of the grounding line, we find a flat topography of the ice shelf base with an almost constant ice thickness gradient along-flow, indicating only little basal melting, but an initial widening of the basal channel, which we ascribe to melting along its flanks. Our findings provide a detailed view of a more complex interaction between the ocean and subglacial hydrology to form basal channels in ice shelves.

Highlights

Ice shelf channels (Drews, 2015), known as channels (Alley et al, 2016), surface channels (Marsh et al, 2016) or M-channels (Jeofry et al, 2018b), are narrow, long channels on the surfaces of ice shelves
This is the case around the basal channel where the base of the ice shelf has significant topography, and the underlying water column varies in thickness if the seabed is flat
We investigated the characteristics of a subglacial channel continuing as a basal channel across the grounding line of the Support Force Glacier

Summary

Introduction

Ice shelf channels (Drews, 2015), known as channels (Alley et al, 2016), surface channels (Marsh et al, 2016) or M-channels (Jeofry et al, 2018b), are narrow (a few kilometres wide and 20–30 m deep mostly), long channels on the surfaces of ice shelves They are often remotely detected with satellite imagery like MODIS (Moderate Resolution Imaging Spectroradiometer; Scambos et al, 2007) or Landsat 8. These channels are a surface expression of a sub-ice-shelf channel (Le Brocq et al, 2013), known as a basal channel (Marsh et al, 2016; Alley et al, 2016, 2019) or U-channel (Jeofry et al, 2018b), most often aligned with the ice flow direction but occasionally migrating across the ice flow direction. Ice shelf channels potentially influence ice shelf stability, which in turn

Results

Discussion

Conclusion