Implications of marginal weakening for grounding line migration and marine ice sheet (in)stability

Abstract:

The stability of a grounding line on reverse-sloping bedrock has been the subject of much attention and debate. In the absence of ice shelf buttressing or lateral flow variations, such a marine ice sheet configuration has been shown to be unconditionally unstable. Numerous studies have speculated that recent grounding line retreat in West Antarctica indicates that a runaway instability is already underway for this portion of the ice sheet. However, modeling studies have shown that a grounding line can be stabilized on a reverse-sloping bed in the case of strong lateral flow convergence. One factor that has not been addressed in modeling studies of the marine ice sheet stability is fracturing and weakening of ice stream and ice shelf shear margins, processes that are known to be operating in many places. It has been speculated that marginal weakening should reduce the stress carried across shear margins, leading to reduced buttressing and thus allowing accelerated flow and grounding line retreat. Here, we test this hypothesis using an ice sheet model and a previously-used, idealized marine ice sheet configuration with strong lateral flow convergence. We first confirm earlier modeling studies that a stable grounding line position can be found on reverse-sloping bedrock. We then explore perturbations of this stable position caused by allowing the shear margins to damage according to a new, generalized constitutive framework for ice deformation and weakening. Constitutive parameters for marginal weakening are taken from assimilated velocity observations for several different ice shelves. We demonstrate that marginal weakening is at least as important as basal lubrication or ice shelf thinning for perturbing the grounding line. Model projections of marine ice sheet evolution that do not account for marginal weakening are therefore likely to underestimate both the rate of grounding line retreat and ice flux.