Recent changes in the dynamic condition of the Ross Ice Shelf, Antarctica

Variations in the amplitude of radar echoes from the bottom of the grid western half of the Ross Ice Shelf have been analyzed. Contrary to the results of a similar analysis performed for the grid eastern sector of the ice shelf, bands of low signal strength downstream from both Crary Ice Rise and the Siple Coast do not correlate with modern flow lines. The difference in direction between the radar bands downstream of Crary Ice Rise and the present velocity vectors and the absence of a comparable trend farther east suggest to us that the grounding line around Crary Ice Rise retreated within the last 1000 years. This hypothesis is reinforced by the observation of several domes and hollows in ice thickness downstream of Crary Ice Rise which are similar to a hollow now located in the wake of the ice rise and a dome on its eastern flank. We interprete this as evidence for a rapid increase in flow around the ice rise which carried downstream the ice topography formed around the ice rise. Similar but less detailed evidence found downstream of the Siple Coast suggests that there was a regional retreat of the West Antarctic grounding line.