ROV Icefin at Ross Ice Shelf Grounding Zone: 5 km of ice, ocean, seafloor, and crevasse exploration

Ice shelf grounding zones are important regions where interactions between sediment, ice, and ocean combine to influence cavity geometry, ice flux, and water circulation. In collaboration with the New Zealand Antarctic Research Institute-funded Ross Ice Shelf Programme and the MBIE Antarctic Science Platform, from December 17-21 2019 we deployed Icefin, a remotely operated underwater vehicle (ROV), through a borehole into the ocean lens three kilometers downstream of the Ross Ice Shelf grounding zone (Kamb Ice Stream HWD-1 Site). Icefin is supported by the NASA Ross Ice Shelf and Europa Underwater Probe (RISE UP) project, and for this work was equipped with sensors to measure water velocity, salinity, temperature, oxygen, organics, turbidity, chlorophyll, and pH, along with multiple sonars and cameras.

We observed a 30 m thick water column stratified into two layers. The fresher upper layer was within 0.1°C of the freezing point and had slightly elevated turbidity and dissolved oxygen levels relative to the lower layer, which was more saline and warmer, within 0.3 °C of the freezing point. 1.5 km of ice flow parallel transects mapped five 35-50 m tall asymmetric crevasses. Investigation into a crevasse identified supercooled water and active marine ice formation in the upper 10 m. The entire ice base exhibited scalloped cusps indicative of melting, including one region of sediment-rich basal ice. The seafloor exhibited two dominant types- 1) fine sediment drape with sub-meter scale ridges above ice flow-parallel fluting, which was typical beneath meteoric ice regions, and 2) smooth, ridge free seafloor with abundant cm-scale debris beneath the sediment-laden basal ice region. Two 20-30 m wide sections of parallel, sub-meter tall seafloor ridges had similar strike and dimensions to basal crevasses, both transverse to ice flow. These observations help to understand grounding zone dynamics, and more specifically Kamb Ice Stream's stagnation history and potential to reactivate and increase the rate of sea level rise. Direct observations of marine ice formation in a basal crevasse inform ice shelf stability, modeling, and analogous processes on other ocean worlds.