Multidecadal Basal Melt Rates and Structure of the Ross Ice Shelf, Antarctica, Using Airborne Ice Penetrating Radar. Issue 3 (17th March 2020) - Das, Indrani
Abstract: Basal melting of ice shelves is a major source of mass loss from the Antarctic Ice Sheet. In situ measurements of ice shelf basal melt rates are sparse, while the more extensive estimates from satellite altimetry require precise information about firn density and characteristics of near‐surface layers. We describe a novel method for estimating multidecadal basal melt rates using airborne ice penetrating radar data acquired during a 3‐year survey of the Ross Ice Shelf. These data revealed an ice column with distinct upper and lower units whose thicknesses change as ice flows from the grounding line toward the ice front. We interpret the lower unit as continental meteoric ice that has flowed across the grounding line and the upper unit as ice formed from snowfall onto the relatively flat ice shelf. We used the ice thickness difference and strain‐induced thickness change of the lower unit between the survey lines, combined with ice velocities, to derive basal melt rates averaged over one to six decades. Our results are similar to satellite laser altimetry estimates for the period 2003–2009, suggesting that the Ross Ice Shelf melt rates have been fairly stable for several decades. We identify five sites of elevated basal melt rates, in the range 0.5–2 m a −1, near the ice shelf front. These hot spots indicate pathways into the sub‐ice‐shelf ocean cavity for warm seawater, likely a combination of summer‐warmed Antarctic Surface Water and modified Circumpolar Deep Water, and are potential areas of ice shelf weakening if the ocean warms. Plain Language Summary: Antarctic ice shelves are the floating extensions of the grounded ice sheet. The ice shelves provide forces that slow the flow of grounded ice into the ocean, thus helping to stabilize the ice sheet. Mass loss from ice shelves occurs by basal melting and calving of icebergs. Here, we present a novel method for estimating basal melting using airborne ice penetrating radars. We used data collected from an airborne survey of the Ross Ice Shelf to measure the thickness of a bottom unit of the ice shelf that is made of pure ice originating from the grounded ice sheet. The change in thickness of this bottom unit was used to estimate basal melting. These new melt rates are similar to satellite‐derived estimates, indicating that the Ross Ice Shelf has been fairly stable over the past several decades. Most of the melting is concentrated at the ice shelf front at five localized hotspots with melt rates of about half to two meters per year. The elevated melting in the hot spots is likely caused by inflows of warm ocean water through pathways provided by bathymetry and thinner ice, and indicates regions of potential future weakness of the ice shelf to ocean change. Key Points: Airborne ice penetrating radar survey (2015–2017) reveals three‐dimensional internal structure of the Ross Ice Shelf Changing thickness of a basal ice unit along ice flowlines is used to estimate multidecadal basal melt rates Elevated melt rates at hot spots along the ice front indicate pathways for warm seawater into the sub‐ice‐shelf ocean cavity