Combining evidence of frozen and thawed beds to constrain geothermal flux: initial results from Hercules Dome, Antarctica

Few direct observations of geothermal flux exist in Antarctica because of the difficulty in reaching the ice/bed interface. Remotely sensed inferences of geothermal flux are often limited by lack of knowledge of the geology. The geothermal flux is an important constraint for glaciological models and critical for selecting ice core sites. The thermal state of the bed, when combined with ice temperature modeling, can be used to place either maximum and minimum constraints on the geothermal flux. If the bed is known to be frozen, e.g. a Raymond Arch exists, the maximum geothermal flux can be determined; if the bed is known to be melting, e.g. a subglacial lake exists, the minimum geothermal flux can be estimated. At Hercules Dome, phase sensitive radar indicates a vertical velocity pattern indicative of the Raymond effect and that the bed is frozen at a depth of approximately 1600 m while ~50 km away at a depth of 2400 m is the shallowest easily identified subglacial lake in the survey. Using 1D temperature modeling, the frozen bed at 1600 m indicates a maximum geothermal flux of ~85 mW m-2 while the lake at 2400 m depth indicates a minimum geothermal flux of ~65 mW m-2. These bounds constrain the geothermal flux at Hercules Dome, although we cannot exclude small scale variations in geothermal flux. These constraints are consistent with three remotely sensed estimates of 62, 68 and 66 mW m-2 (Shen et al., 2020; Stal et al., 2020; and Martos et al., 2017). Locations with topographic relief and both frozen and thawed beds, like Hercules Dome, can be used to help constrain geothermal flux beneath the Antarctic ice sheet.