Estimating Ice Shelf Thickness in Grounding Zones of the Filchner-Ronne Ice Shelf with Tidal Flexure from ICESat-2

Faye M. Elgart , Brent Minchew

In the grounding zones of Antarctic ice sheets, grounded ice sheets become floating ice shelves at the triple junction of the ice, ocean, and bedrock. Ice downstream of the grounding line rises and falls with ocean tides while ice upstream of it does not, creating kilometers wide flexure zones. Ice thickness in the flexure zone cannot be well estimated by assuming it is in hydrostatic equilibrium. Here we develop and apply an observationally-constrained inversion based on the tidal flexure of ice shelves, using repeat-track ICESat-2 altimetry data and an elastic beam bending model, to infer ice thickness and thickness gradient close to grounding lines. We apply this method to Institute Ice Stream, Foundation Ice Stream, and Recovery Glacier on the Filchner-Ronne Ice Shelf (FRIS) and find that flexure-derived ice thickness can be locally up to approximately 10% greater than the hydrostatically derived ice thickness from BedMachine. We also find that ice in these three grounding zones is relatively stiff, with a mean effective Young’s modulus (E*) of 6.5 GPa. If ice in grounding zones around Antarctica is indeed thicker than commonly estimated, as found here, this may have far- reaching implications for ice sheet modeling.