Seismic evidence of mechanically weak sediments underlying Russell Glacier, West Greenland

Abstract

Amplitude-versus-angle (AVA) analysis of a seismic reflection line, imaged 13km from Russell Glacier terminus, near the western margin of the Greenland ice sheet (GrIS), suggests the presence of sediment at the bed. The analysis was complicated by the lack of identifiable multiples in the data due to a highly irregular and crevassed ice surface, rendering deeper seismic returns noisy. A modified technique for AVA processing of glacial seismic data using forward modelling with primary reflection amplitudes and simulated multiple amplitudes is presented here. Our analysis demonstrates that AVA analysis can be applied to areas with noisy seismic returns and indicates that sediment underlies the seismic study site. Our data are inconsistent with the common assumption that the GrIS is underlain only by hard bedrock, but consistent with the presence of subglacial sediment with porosity between 30% and 40%. As analysis and modelling of ice-sheet dynamics requires a sound knowledge of the underlying basal materials, subglacial sediment should be taken into account when considering ice dynamics in this region of the GrIS.Peer reviewe

Recent Advances in Our Understanding of the Role of Meltwater in the Greenland Ice Sheet System

“…For example, there is compelling evidence that efficient drainage develops rapidly, even at high elevations [55], following lake drainage. Current models by contrast [65,69,73] typically require unrealistically long time scales to evolve large channels, as highlighted by Hoffman et al [83•]. One possible issue is that these models start their incipient channel growth from very small channel dimensions, to represent an initial inefficient drainage system, which ensures that channel growth by frictional wall-melting is extremely slow.…”

“…Recent surveys have reported the presence of subglacial till layers [73,74]. The significance of such layers will depend on how extensive, thick and deformable they are and, if they are pervasive, how they will impact ice dynamics in a warming world driven by the enhanced subglacial runoff.…”

“…Recent evidence [73][74][75] suggests that parts of the ice sheet are underlain by subglacial sediments. In such areas, efficient drainage where it exists is likely to be characterised by channels (or 'canals' [2]) incised both down in to the sediments as well as up into the ice, while inefficient drainage may occur via Darcian flow through the sediments [2].…”

Sensitive response of the Greenland Ice Sheet to surface melt drainage over a soft bed

“…The physical properties in the subglacial sediment model integrates recent geophysical observations, revealing where available that RG is underlain by a porous, mechanically weak sediment 34,35 , of similar character to tills produced by glaciers in Canada and Svalbard 37,45 (see Methods). The subglacial sediment model was coupled to a hydrological model, which has previously been used to calculate the routing and fluxes of water associated with the episodic drainage of subglacial lakes in Antarctica 46 , and is well suited for analysis of SGL drainage events.…”

“…In addition, by focusing explicitly on the character of the hydrological system, previous work has inherently assumed that the ice-bed interface consists of hard bedrock. However, thick subglacial sediments have been observed 34,35 and furthermore are known to exert firstorder control on flow in other glaciated regions [36][37][38][39][40][41] . To date, theoretical considerations on the implications of a soft sedimentary bed on GrIS dynamics are only starting to emerge 42 , but have never been implemented and tested in modelling studies.…”