What is this ice: geophysical investigations of a thick accreted basal ice layer at HWD-2, Ross Ice Shelf, Antarctica

In the austral summer of 2017, the Aotearoa New Zealand Ross Ice Shelf programme drilled two boreholes through the ice shelf at 80°39' 9" S, 174°28' 2" E, approximately 450 km downstream of the grounding zone. The ice originates in the glacier-left margin of the flow band from Liv Glacier. Both boreholes encountered a thick (10s of m) layer of bubble-poor ice containing unevenly distributed sediment. In the present contribution, a suite of geophysical imaging surveys conducted around the borehole site are used to investigate the nature of this layer and test hypotheses regarding its origin.

Interpreted separately, the imaging methods lead to different conclusions regarding the character of the base of the ice shelf and none predict the accreted basal ice observed via the boreholes. The difference may be due to sensitivity of the different imaging methods to different attributes of the ice or localisation of the accreted ice. Relatively diffuse basal reflections in a grid of phase sensitive radar (pRES) soundings around the borehole site led us to anticipate basal melting. Both the thick layer of accreted glacier ice and a thin layer of platelet ice on the ice shelf base argue against this interpretation. Viewed using low frequency radar (7 MHz dipole antennas towed across the snow surface), a strong basal reflector rises upward as we profiled across flow, past the borehole site. The apparent thinning of the ice column is in excess of what would be predicted from the surface elevation change along the profile. These observations will be synthesized with seismic profiling carried out in 2015.

Hot water drilling was undertaken by the Victoria University of Wellington Hot Water Drilling initiative. We thank the hot water drilling team led by A. Pyne and D. Mendeno.