| Summary: | The melting of ice by ocean waters along the periphery of ice sheets is a major physical process driving their evolution in a warming climate. Using the fiber-optic-tethered Grounding line Remote Operated Vehicle (GROV), we explored the ice shelf cavity of Petermann Glacier, in Northwestern Greenland, in May 2023, using a novel interferometric multibeam sonar operating at 117 KHz with 360° viewing capability. The seafloor depth is uniform at 820 m and 200 m deeper than anticipated. At the ice shelf base, we find a succession of terraces interrupted by 20-40 m ice cliffs that have no signature at the surface, but are consistent with double-diffusive convection. The central melt channel deviates by ± 80 m from flotation, is smoother than indicated by the surface, and reveals asymmetric melt. The results demonstrate the fundamental importance of surveying the geometry of ice shelf cavities to document ice-ocean interaction. Funding provided by: National Aeronautics and Space Administration ROR ID: https://ror.org/027ka1x80 Award Number: 80NSSC20K1076 Funding provided by: National Aeronautics and Space Administration ROR ID: https://ror.org/027ka1x80 Award Number: 80NSSC23K0177 Petermann Glacier is a major outlet glacier in North Greenland (60°W, 81°N) that forms a 20-km wide and 45-km long floating ice shelf (Fig. 1). Petermann drains 4% of the Greenland Ice Sheet in area. The ice shelf undergoes massive ice melt in contact with the ocean waters that removes more than half of its ice thickness in the first 10 km. In 2010 and 2012, the ice shelf underwent two major calving events, which truncated its length by 25 km. In 2018-2022, the glacier grounding line retreated for the first time in this century, by 7 kilometers at the center, 4 km on the sides, and the glacier sped up by 100 - 150 m/yr. GROV: The``Grounding line Remote Operate Vehicle" (GROV) uses the Graaltech, torpedo shaped, X-300 Autonomous Underwater Vehicle (AUV), which is a portable, modular AUV, with an open control system, fully holometric five ...
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