The role of fresh water in driving ice shelf crevassing, rifting and calving

Meltwater is widely implicated in the disintegration of ice shelves and the lower density of fresh meltwater relative to salty seawater can reconcile observations and models of ice shelf rifting. Advanced numerical models predict that a seawater-filled basal crevasse will break through a shelf to form a rift only where extensional stresses equal those at the seaward shelf edge. However, rifts open in ice shelf interiors where resistance to flow at the sides and base of the ice shelf, or buttressing, is estimated to significantly reduce extensional stresses compared to those at the shelf edge. The stress in a shelf is often quantified in terms of a buttressing number and rifts are seen to open in areas where the estimated buttressing number is between 0.2 and 0.3. Numerical models assuming fresh meltwater-filled basal crevasses predict rift opening for buttressing numbers up to about 0.2. Published estimates indicate that about a quarter of the area of the Antarctic ice shelves are characterized by buttressing numbers less than 0.2. The present results indicate that those areas are directly vulnerable to rifting in the presence of abundant meltwater and that calving of those areas could drive further rifting. The numerical results suggest that rifting can be understood in terms of a horizontal force balance, justifying derivation of a simple analytic relation between the density of water infilling a rift and the maximum buttressing number allowing rifting. This analytic relation should hold for any spacing of crevasses or any vertical variations in ice rheology. Enforcing a horizontal force balance also allows derivation of an analytic relation between the heights of closely spaced ice shelf crevasses and buttressing numbers.