Characterization Of Greenland Ice Sheet Bed Conditions By Direct Measurement In A Network Of 36 Boreholes

Abstract:

Sliding dynamics of Greenland ice sheet are governed by basal conditions. The characteristics of the bed – for example, whether it consists of bedrock, deformable till, or some intermediary – are therefore key to interpreting historical ice volume variations, as well as understanding present and future responses of the ice sheet to warming climate. Here we report on the thermal and sediment characteristics of the ice-bed interface along an inward running transect of western Greenland ice sheet. We employ direct measurement of basal conditions in a network of 36 boreholes drilled through the ablation zone of the ice sheet. The holes vary from 100 m deep and located 1 km from the margin, to ~850 m deep and 46 km inland. We synthesize a suite of different measurements aimed at characterizing the bed conditions, including: bed penetrometer experiments; measurement of meter scale water pressure gradients, both natural and induced; sediment sampling; probing with an instrumented drill; borehole photography; in situ dye tracing; and, temperature and tilt sensors in basal ice.

The emergent view of bed and near-bed conditions is as follows: (a) a temperate layer of basal ice generally grows in thickness towards the margin, although with a high degree of spatial variability due to complex flow around topography. The warm basal layer is only ~10 m thick at 46 km inland, whereas near the margin the entire ice depth of ice is temperate; (b) sediment is commonly entrained into the lowermost few meters of ice, although rarely in high concentrations or with large grain sizes; (c) the bed has a strongly heterogeneous sediment cover consisting mainly of sand and gravel. Most clay particles, and perhaps some silt particles, have been flushed; (d) the sediment is decimeters thick in places, but not meters thick. Some locations have virtually no sediment whatsoever; (e) water pressure gradients imply that Darcy flow through permeable till layer is not a primary mechanism for water transport; (f) the observed relationship between water flux and effective pressure suggests that water does not flow through canals cut into sediment; (g) the drainage system behaves as a hard bed, with pressure gradients dictated by drainage system elements (e.g., cavities, orifices, conduits) that have developed with respect to bed topography.