Airborne Radar Reveals Multi-Decadal Basal Melt Rates for Ross Ice Shelf, Antarctica

Accurate quantification of ice shelf mass balance is key to assessing the long term stability of the Antarctic Ice Sheet. Although the most extensive thinning currently is occurring in the Amundsen Sea sector of West Antarctica, other ice shelves experience changes in thickness on time scales from annual to ice age cycles. Here, we focus on Ross Ice Shelf. An 24-year record (1994-2018) of satellite radar altimetry shows substantial variability in ice shelf height on interannual time scales, complicating detection of potential long-term climate-change signals in the mass budget of Ross Ice Shelf. Variability of radar signal penetration into the ice-shelf surface snow and firn layers further complicates assessment of mass changes.

In this work, we investigate the mass balance of Ross Ice Shelf using aerogeophysical data from the ROSETTA-Ice IcePod. We use two ice-penetrating radars: a 2 GHz unit that images fine-structure close to the ice surface and a 188 MHz radar to identify the ice shelf base. We have identified a dominant internal layer that persists from the grounding line to the ice shelf front. Based on layer continuity, we conclude that this layer must be the horizon between the continental ice, delivered to the ice shelf by outlet glaciers, and snow accumulation once the ice is afloat. We use the Lagrangian change in thickness of this layer, after correcting for strain rates derived using modern InSAR velocities, to estimate multidecadal averaged basal melt rates. This is a novel way to quantify basal melt, avoiding the confounding impact of spatial and short-term temporal variability in surface accumulation and firn densification processes.

This method allows us to estimate multi-decadal variability in basal melt rates dating back to the comprehensive surface-based RIGGS survey observations in 1975, which provided estimates of ice thickness, strain rates, surface accumulation and velocity for Ross Ice Shelf. We compare our strain rate distribution with the RIGGS measurements to quantify changes in strain rates and basal mass balance from 1975 to 2016. Using the radar derived thickness change corrected for strain variations, we identify regions that are undergoing enhanced melting and provide a comprehensive understanding of how ocean processes may have impacted the base of Ross Ice Shelf in recent decades.