Ice‐ocean interaction on Ronne Ice Shelf, Antarctica

Abstract

Detailed glaciological studies have been completed at 28 sites lying on an approximate flow line, extending 760 km across Ronne Ice Shelf. Parameters measured at each location include ice velocity, thickness, principal strain rates, surface elevation, temperature, and accumulation rate. The data have been used in a steady state model to derive the basal mass flux and the temperature profile with depth at each site, from the principles of mass and energy conservation. These calculations indicate basal melting in excess of 1 m yr−1 over the first 100 km of the flow line downstream of the grounding line, where the ice shelf is between 1200 m and 1600 m thick. The maximum melt rates in this region occur near the inland margin and exceed 4 m yr−1. Melting continues at a lesser rate over the next 200 km before freezing commences. Freezing then dominates up to the final 100 km before the ice front, resulting in the accumulation of a layer of basal sea ice up to 50 m thick. This is rapidly removed as melt rates increase to over 6 m yr−1 at the ice front. The cumulative effect of this pattern of basal accumulation and ablation is the wastage by melting of 85% of the mass discharged across the grounding line before it reaches the ice front. Supporting evidence for a layer of saline ice underlying the ice shelf is provided by the strength of basal radar reflections, observed during radio echo sounding of ice thickness. In the region where mass balance calculations suggest an accretion of basal sea ice, reflection coefficients are consistently low, ranging from −6 dB to −38 dB. Most of these weak reflections are believed to originate from the true base of the ice shelf, the additional energy loss being the result of increased attenuation of the radar signal within the saline layer. The derived pattern of basal melting and freezing is consistent with a simple model of sub‐ice‐shelf oceanic circulation, involving a deep thermohaline convection cell. Dense, saline water, which is formed over the continental shelf during winter when the sea surface freezes, drains into the deepest parts of the sub‐ice‐shelf cavity. At the inland margin, where this water mass comes into contact with the ice shelf, its temperature is 1°C above the local pressure freezing point. Melting of ice results, producing a buoyant outflow of cold, relatively fresh water, along the ice shelf base. Basal freezing occurs towards the ice front, where the ascending water becomes supercooled. This circulation has important implications for the production of Antarctic Bottom Water and for the response of the ice shelf to driving stresses, through the temperature dependent viscosity of ice.