Ice shelf calving due to shear stresses: observing the response of Brunt Ice Shelf and Halloween Crack to iceberg calving using ICESat-2 laser altimetry, satellite imagery, and ice flow models

Full-thickness ice shelf fractures, known as rifts, create tabular icebergs that can reduce ice shelf extent and thereby jeopardize overall marine ice sheet stability. Low confidence in the scientific understanding of ice shelf calving processes results in uncertainties in ice sheet evolution. Here, we investigate rift evolution and calving on the Brunt Ice Shelf, East Antarctica, using ICESat-2 laser altimetry, on-ice Global Navigation Satellite System receivers, satellite imagery, and ice flow models. We find that shear stresses played a decisive role in rift propagation, widening, calving, and stabilization during the period surrounding the calving of Iceberg A-74 from the North Rift on the Brunt Ice Shelf. The North Rift propagated along a fracture path that was optimally oriented to maximize shear stresses. Following the calving of A-74, reduced contact with local pinning points at the McDonald Ice Rumples reduced shear stresses and the opening rate of the Halloween Crack. We use historical observations and satellite imagery to investigate the calving cycle and demonstrate that the calving of Iceberg A-74 closely mimics previous calving events. Shear stresses likely play a role in tabular iceberg calving events on other ice shelves. Yet horizontal shear stresses are in the null space of most ice shelf calving laws. We propose modifications to widely used calving laws that improve their sensitivity to the shearing mode of fracture.