Circulation and ocean-ice shelf interaction beneath the Denman and Shackleton Ice Shelves

Ocean-driven basal melt can thin and weaken ice shelves, reducing buttressing and causing sea level rise. However, the processes driving melt in ice shelf cavities are challenging to observe. Here we report float observations spanning the ocean cavity beneath Shackleton Ice Shelf. Warm water at depth reaches the cavity beneath the Denman Glacier, driving strong melt. Beneath the rest of the Shackleton, water at the surface freezing point drives widespread but weak melt. The inferred circulation in the Denman cavity consists of four stacked overturning cells, each linked to basal melt driven by distinct water masses reaching the ice base at different depths. The float observations show that Denman is delicately poised: Small increases in thickness of the warm layer would allow more ocean heat to reach the ice, driving more rapid melt that could initiate unstable retreat, while the rest of the Shackleton is at present isolated from warm water.