Investigating the Geometry and Conditions for Basal Accretion of Englacial Plumes

Abstract:

Large englacial features at the base of the ice sheet have been observed in recent radio echo sounding data in both Antarctica and Greenland. In Greenland such features are found in many places, especially in North Greenland, where high availability in basal water is likely. In the interior of Antarctica such features are observed in the Dome A region, a slow flowing region, where the alpine like relief of the Gamburtsev subglacial mountains allows for basal melt in the deeply incised valleys. All these observed englacial features resemble plumes, which can, in terms of their geometric structure, be reproduced by modelling internal isochrone layers by including basal ice-accretion. However, to produce such large freeze-on features sufficiently high basal accretion rates are needed.

Here we investigate the conditions required, in terms of water flux and bed slope, to obtain the needed basal accretion rates through the process of Clausius-Clapeyron cooling of basal water. Furthermore, we examine the geometrical aspect of the plumes and the related englacial isochrone pattern to understand the relationship with ice flux. We use the 3-D numerical model BASISM to calculate temperature, melt water flux and ice flux. We find that for most areas where plumes are found the relationship between water flux and steep ascending slopes in the basal topography along flow is sufficient to explain the accretion rates through the process of Clausius-Clapeyron cooling. Further, we find that the plume height for a given accretion rate is inversely related to the ice flux, matching areas with reduced local ice flux showing high plumes. Since ice rheology affects the ice flux and in addition, might be affected by basal freeze-on, we use the ELMER/ICE model to explore the effect of ice fabric evolution and internal temperature variation.