Fracture thresholds of glacier ice during cauldron collapse and surface loading

Iceberg calving, in which blocks of ice break off the seaward end of a glacier or ice sheet, accounts for a sizable fraction of the glaciological contribution to sea level rise. Fracture and calving remain challenging to include in ice sheet models. Many current models use a threshold stress criterion to determine where ice should calve away. Existing constraints on the fracture threshold stress of ice are derived from laboratory experiments and in-situ observations of lake ice, both of which are at fundamentally different scales and stress regimes from glacial ice. Here, we use a suite of models at various levels of complexity—linear elastic, viscoelastic, and particle-based—to characterize the stress regime in fracturing glacier ice. We simulate surface deformation associated with cauldron collapse (rapid removal of subglacial water) and variable surface loading (e.g. filling a surface meltwater pond). We show that our model scheme reproduces the magnitude and pace of the 2015 Eastern Skaftá Cauldron collapse, Vatnajökull Ice Cap, Iceland, and we deduce a threshold stress from the observed distribution of intact and fractured ice. Preliminary results suggest that surface tensile stresses in intact ice exceeded typical fracture threshold stress values. Our analysis has implications for iceberg calving parameterization as well as simulation of meltwater-ponding-induced ice surface deformation.