Interactions Between Ice and Ocean Near an Ice Shelf Grounding Line

We describe a series of high precision, high temporal resolution measurements of surface deformation and ice thickness change made near the grounding line of Rutford Ice Stream, West Antarctica. Our observations span two periods of two to three weeks each, separated by one year. We measured surface deformation by GPS survey of a triangular pattern of markers, and changes in ice thickness using a phase-sensitive radar sounding system. Near the point of maximum tidal flexure we observed a tidal component to the horizontal strain of up to ∼0.1x10^-3. Hence the magnitude of the cyclical (predominantly semi-diurnal) strain rate far exceeds that of the steady, long-term deformation rate, which we observe to be of ∼1x10^-3 yr^-1. We made direct measurements of the vertical strain rate in the upper 500-600 m of the ice column from the relative displacement of internal reflectors visible in the radar records. Once again we see a tidal component superimposed on a long-term trend, and both of these are reflected in our measurements of total ice thickness. Overall the ice column is subject to a net vertical strain of up to ∼0.01x10^-3 over half a tidal cycle. The long-term trend in thickness differs from that implied by the long-term vertical strain rate. We attribute the difference to basal melting of 1.18+/-0.03 m yr^-1. There appears to be no seasonal variation in the melt rate, nor any change between times of spring and neap tides. The measured melt rate is consistent with an earlier estimate of that required to maintain the ice shelf locally in equilibrium (1.2+/-0.5 m yr^-1).