Physical Conditions of Fast Glacier Flow: 2. Variable Extent of Anisotropic Ice and Soft Basal Sediment From Seismic Reflection Data Acquired on Store Glacier, West Greenland

C Hofstede,T J Young,S H Doyle,B Hubbard,A Diez,O Eisen,A Hubbard,P Christoffersen

doi:10.1002/2017jf004297

Abstract

Outlet glaciers of the Greenland Ice Sheet transport ice from the interior to the ocean and contribute directly to sea level rise because discharge and ablation often exceed the accumulation. To develop a better understanding of these fast‐flowing glaciers, we investigate the basal conditions of Store Glacier, a large outlet glacier flowing into Uummannaq Fjord in west Greenland. We use two crossing seismic profiles acquired near the centerline, 30 km upstream of the calving front, to interpret the physical nature of the ice and bed. We identify one notably englacial and two notably subglacial seismic reflections on both profiles. The englacial reflection represents a change in crystal orientation fabric, interpreted to be the Holocene‐Wisconsin transition. From Amplitude‐Versus‐Angle (AVA) analysis we infer that the deepest ∼80 m of ice of the parallel‐flow profile below this reflection is anisotropic with an enhancement of simple shear of ∼2. The ice is underlain by ∼45 m of unconsolidated sediments, below which there is a strong reflection caused by the transition to consolidated sediments. In the across‐flow profile subglacial properties vary over small scale and the polarity of the ice‐bed reflection switches from positive to negative. We interpret these as patches of different basal slipperiness associated with variable amounts of water. Our results illustrate variability in basal properties, and hence ice‐bed coupling, at a spatial scale of ∼100 m, highlighting the need for direct observations of the bed to improve the basal boundary conditions in ice‐dynamic models.

Highlights

Mass loss from the Greenland Ice Sheet (GrIS) consists of two main components: meltwater runoff and ice discharge into the surrounding ocean (e.g., Fürst et al, 2015)
Whereas meltwater production and runoff is well represented in global sea level prediction (Intergovernmental Panel on Climate Change, 2013), the contribution to sea level rise (SLR) resulting from ice discharge and dynamic thinning is still relatively poorly understood (e.g., Csatho et al, 2014)
Parallel and transverse to the ice flow, we identified three seismic events separating four different media within and beneath Store Glacier

Summary

Introduction

Mass loss from the Greenland Ice Sheet (GrIS) consists of two main components: meltwater runoff and ice discharge into the surrounding ocean (e.g., Fürst et al, 2015). The most recent estimate from 2009 to 2012 is a mass loss of 378 Gt/a equivalent to an estimated sea level rise (SLR) of 1.1 mm/a over the period 2009–2012 (Enderlin et al, 2014; Van den Broeke et al, 2016), which makes the GrIS the single largest contributor of the global cryosphere to SLR Half of this ice loss is attributed to dynamic thinning with the other half explained by surface melting and runoff (Csatho et al, 2014; Rignot & Mouginot, 2012; Van den Broeke et al, 2009). Several possible mechanisms have been suggested as to the cause of increased dynamic thinning They can generally be divided into two end-members: A warmer atmosphere leading to increased surface melt which partly drains to the base, thereby enhancing sliding, or a warmer ocean interacting with the ice sheet’s marine-terminating glaciers, causing melt, thereby reducing the back force and increasing outlet ice flow. This, in turn, increases the driving force of outlet glaciers

Results

Discussion

Conclusion