Environment at the Grounding Zone of the Whillans Ice Stream-Ross Ice Shelf, West Antarctica

Abstract:

Grounding zones where grounded ice sheets transition to floating ice shelves, are the primary gateways through which the Antarctic Ice Sheet loses mass to the ocean. In these environments, ice, ocean, meltwater and sediment meet and interact, influencing both the ice sheet and ocean circulation beneath the ice shelf. Here, we report on conditions near the grounding zone of the Whillans Ice Stream, which feeds into the Ross Ice Shelf. Cameras and instruments lowered through an access borehole to the ocean cavity beneath the ice shelf found a 10m-thick water column comprising an upper layer of colder ice shelf water formed from mixing between meltwater with the lower layer of warmer higher salinity shelf water. This style of stratification is typical of large ice shelves, but it was uncertain whether it existed so near the grounding zone, where stronger tidal currents and/or strong subglacial stream discharges could mix the water column. Salinity and temperature of the water suggest it formed from sea ice production in the Western Ross Sea, with minimal modification beneath the ice shelf. This source region is distinct from waters previously observed at the nearby J-9 borehole, illustrating the importance of the sub-ice shelf bathymetry in steering circulation between the ocean and the grounding zone. Preliminary data suggest an active exchange of heat and nutrients between the grounding zone and the open ocean, despite being separated by 600km. Thus life found near the grounding line is probably not an isolated oasis, but may instead be part of a much broader ecosystem that spans the ice shelf. Although sea ice formation presently maintains water in the sub-ice shelf cavity near the surface freezing point, buffering many larger ice shelves from gradual ocean warming, these findings suggest that even grounding zones of extensive ice shelves may respond quickly to abrupt changes in ocean circulation, such as that observed in the Amundsen Sea.