Seismic characterisation of subglacial conditions beneath the margin of the West Greenland Ice Sheet

Lead Research Organisation: Swansea University
Department Name: School of the Environment and Society

Abstract

The response of the Greenland Ice Sheet to a warming climate is of great significance for models of sea-level rise. As such, it receives significant attention from the media, among the general public and in government policy decisions. However, its behaviour is influenced by a number of complex processes, the interactions between which are not well understood. The ice sheet is therefore a focus of major scientific research campaigns, exploring the key influences on its stability and how it will evolve in the future. Recent research has suggested that water is a significant influence on ice sheet stability. Water that is present at the base of the ice sheet acts as a lubricant between the ice and the material beneath it, allowing the ice to move at a faster velocity. For the West Greenland Ice Sheet, a source of basal water is stored in surface meltwater lakes. These form at the start of the summer melt season, in June and July, and typically undergo one or more drainage events throughout July and August. It is believed that the water they contain drains through a network of cracks in the ice, eventually reaching the base of the ice sheet and causing a dynamic response. The drainage of these lakes is rather sudden (in one example, a lake with an area around 6 km2 was drained in under 2 hours) hence a large volume of water can be introduced to the bed of the ice sheet in a short space of time. Certain scientific groups have observed the drainage of a meltwater lake, and have detected increases in local ice dynamics very soon after; this clearly implies that the theory of meltwater reaching the bed of the ice sheet is correct. However, the delivery of meltwater has never been imaged directly; it has only been inferred from responses in other datasets. In this proposal, we intend to detect, delineate and quantify the passage of meltwater at the bed of the ice sheet in seismic geophysical data, which are widely used for this purpose in other areas of the earth sciences and commercial practice. The geophysical techniques we propose have been extensively applied to estimating the fluid content in the subsurface. Water modifies the physical properties of the subsurface to which seismic energy is particularly sensitive. We therefore intend to use geophysics to continually monitor the base of the ice sheet, throughout the melt-and-drainage season in July and August. We anticipate that our seismic data would show the basal water content to increase gradually, as an increasing number of meltwater lakes drain in the vicinity of the survey site. Our specific field site is located amongst a cluster of lakes and moulins (vertical or sub-vertical openings facilitating water drainage into the ice body), and located directly over an area of the ice sheet previously shown to respond to lake drainage events. We are therefore confident that if that response is due to the delivery of meltwater to the bed of the ice sheet, that meltwater will be detectable by our geophysical survey. Our observations will be complemented by a range of results from complementary scientific projects already underway at the site. This includes, e.g., airborne and ground-based radar, passive seismic and GPS assessment or monitoring, and a new catchment-scale model of surface-melt coupled dynamic feedbacks. Meltwater is an important influence on ice dynamics that, with continued global warming, is likely to become more significant in the future. Understanding that influence is therefore crucial for predicting how the Greenland Ice Sheet will respond to climate change, and the observations made during this study will undoubtedly contribute to our knowledge of ice dynamics.

Publications

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Description The project set out to discover the impact that rapid surface lake drainages have on the basal slip of the Greenland Ice Sheet. We planned to conduct repeat seismic reflection surveys that would image the base of the ice sheet before and after the drainage of a large surface lake, understood to puncture through the ice sheet before draining away at the ice-bed interface.
We did indeed conduct many seismic reflection surveys, and in fact more surveys than promised in the proposal, both upstream and downstream of the large surface lake. Our work discovered an abundance of subglacial sediment beneath the lake, and revealed profound changes in the slipperiness of those sediments. While the subglacial sediments upstream of the lake were soft and slippery, the sediments downflow of the lake were stiff and relatively sticky. These puzzling observations are consistent with a recently proposed hypothesis of sediment strengthening by rapid lake drainages, persistently altering sediments' stress regimes.
Our findings thus imply that the Greenland Ice Sheet may in fact be less sensitive to climatic forcing than expected so far: an increase in meltwater abundance would in fact act slow down, rather than speed up, the ice sheet. These inferences set a new agenda and must be confirmed by further seismic observations, ice sheet drilling and modelling.
Exploitation Route Our research is most significant to glaciologists and biological and climate scientists. We have published several papers based on this research in a major bid to disseminate it to these communities. We are keen that our findings are included in models that seek to predict the future contribution of the Greenland Ice Sheet to sea level rise - the latter being of great interest to policy makers and coastal protection planning and construction projects.
Sectors Construction,Environment,Government, Democracy and Justice