Melting at the Grounding Zone of Thwaites Glacier Observed by Icefin

From January 9-12 2020, ROV Icefin conducted the first long range robotic exploration of the grounding zone of Thwaites Glacier, as part of the ITGC MELT project. Icefin, an underwater vehicle designed for borehole deployments, conducted 5 missions amassing over 15km of continuous data collection with oceanographic, imaging and sonar sensors. Missions extended seaward over 3.2km from the grounding zone, where Icefin observed ice-seafloor contact, traversing a <1 m water column. Imagery and sonar revealed diverse basal ice conditions and complex geometries, including terraced features, both smooth ablated and cuspate melted surfaces, crevasses, various sediment laden layers, and interspersed clear freshwater ice accreted upstream of the grounding zone. These offer a window into upstream hydrological and glaciological conditions. The water column ranges from ~100m thick downstream, thinning to ~50m, and quickly narrowing in the last ~500m towards the grounding zone. Ocean conditions vary from moderately well-mixed near the grounding zone to highly stratified near the ice base at seaward locations. Generally subdued seafloor topography is roughly parallel to ice flow. Sediments range from fine grained downstream to course angular gravel distributed between larger boulders near the grounding zone; and much of the basal ice contains heavy sediment load, often size sorted. We also catalogued organisms from the seafloor to the ice-ocean interface, including ice-burrowing anemones.

From the combined Icefin oceanographic and ice profiling data, the emerging perspective is that topography along the ice-ocean interface evolves dramatically from the grounding zone. Features such as terraces that have been commonly associated with subglacial meltwater channels are instead found across the basal ice surface of the whole survey area. This indicates that in warm oceanographic settings the lateral advection of heat into the ice drives faster melting than vertical melting, which is suppressed by stratification along the flat ice base. These observations demonstrate that the ice-ocean interface is complex at scales and geometries not commonly observed, and imply that the topography of the ice base influences mass loss from Thwaites Glacier.