Terrestrial origin for accreted basal ice in Ross Ice Shelf flow bands

Abstract:

Ice shelves are composites of ice from several sources, including terrestrial glaciers and ice streams, surface snow accumulation and basal accretion. Here we use a combination of direct and remote observation to investigate the origin of basal accreted ice in a flow band in the central region of the Ross Ice Shelf (RIS) in West Antarctica. Lessons learned from that site are applied to remotely observed features across other flow bands in the region.

Mismatches between radar-imaged basal reflections and ice-shelf draft predicted using surface height have been interpreted to indicate marine ice, acreted to the base of the floating ice. We observed similar features around the site of two Aotearoa New Zealand Ross Ice Shelf programme boreholes drilled at ~80°39’ 9” S, 174°28’ 2” E in the austral summer of 2017. The site is approximately 450 km downstream of the grounding zone. Footage of the borehole walls revealed a sharp transition from opaque, bubble-rich meteoric ice to bubble-poor ice containing sparsely distributed sediment, about 60 m from the base of the ~360 m thick ice shelf. Sediment recovered from the basal ice has a terrestrial origin. We conclude from this that the basal accreted ice layer originated on land, where basal meltwater rose and froze into basal crevasses.

These direct observations provide a framework in which similar ice-shelf basal features can be interpreted. Using low-frequency radar profiles of numerous flow bands in this region, we show that multiple origins are likely and can be deduced using englacial features.