RAPID: Direct observations of the Nansen Ice Shelf front to constrain ocean-ice interactions and ice rheology

Iceberg calving contributes roughly half of the net freshwater flux from the Antarctic ice sheet to the Southern Ocean, with impacts on the local, regional, and global oceanography. One potentially dominant driver of calving is through wave erosion at the fronts of ice shelves. This erosion results in underwater ice “benches” that jut out beyond the above-water ice cliff. These benches often lead to upward bending of the ice shelf frontal region. While upward bending is well documented for many Antarctic ice shelves, the underwater benches have not been observed directly. The upward bending may alternatively be due to internal stresses that arise from temperature differences in the ice, thus challenging the bench-driven mechanism. The central component of this project is to produce 3-D maps of the full cliff front, covering both underwater sections (using multibeam sonar) and above-water sections (using photogrammetry), at an Antarctic ice shelf for the first time. This will shed light on the fundamental processes that drive bending and calving and make it possible to distinguish between the two proposed bending mechanisms. The insights gained will help constrain future ice sheet and sea level projections, as well as build understanding of ice properties and processes that may be applicable to icy planets beyond Earth. Satellite lidar data show remarkable variability in the bending of ice shelf fronts surrounding Antarctica: some areas bend upward by more than 10 meters, and others bend downward by that much. This is contrary to classic theory that predicts only downward shelf-edge bending. One idea to explain uplift is that melting of the shallow part of the shelf by warm ocean waters leaves a bench of submerged, protruding ice that buoyantly pushes up the front. A different idea is that vertical variations in the viscosity of ice, not treated in classical studies, result in an internal bending moment that can gradually drive upward shelf-edge bending after a calving event. To differentiate between these mechanisms, this project uses a ship-mounted multibeam sonar to image the submerged face along sections of the Nansen Ice Shelf front, and to image the above-water ice-front shape using aerial and ship-based cameras and structure-from-motion techniques. The Nansen Ice Shelf provides an ideal location to study these processes because it exhibits spatially variable surface uplift, is thin enough at the ice front (~150 m) to safely image the entire depth and exhibits the conditions necessary for both bending processes (bench-induced and internal moment-induced). The project will produce the first in situ observations of ice-shelf-front morphology, answering the question of what controls the upward bending across Antarctica’s ice-shelf fronts. Furthermore, integrating ship-based sonar measurements with concurrent oceanographic and land-based measurements provides a unique opportunity to link ice-front geometry to both ocean forcing and ice dynamics. Together, these observations and accompanying modeling efforts not only offer a way to differentiate between shelf bending models, but also to place new constraints on ocean-ice interactions and ice rheology. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.