Sea ice near an ice shelf

Abstract

In this thesis we investigate how supercooling in the ocean near an ice shelf affects the mass balance, crystal structure, and composition of sea ice.

Ice shelves around Antarctica can cause supercooled water to appear below coastal fast ice in winter. In some circumstances an open matrix of large, disordered ice crystals forms attached beneath the sea ice. Sea ice grows into this subice platelet layer to form platelet ice which displays a distinctive crystal structure.

New winter-long measurements of sea ice and the ocean from McMurdo Sound, Antarctica, show the near-surface ocean was supercooled from mid-July. By measuring sea ice crystal structure and the energy balance at the ice/ocean interface we show that a subice platelet layer formed when an oceanic heat sink was responsible for more than 25 percent of the rate of ice growth. Over four months, the ocean contributed 12 percent of the ice thickness.

Both salinity and stable isotope fractionation for sea ice depend on its growth rate, which we confirm. Variations in salinity and isotopic composition about their trends were not correlated. We provide new distributions for sea ice salinity and stable isotope values, which will be useful when developing new models of sea ice desalination and when measuring quantities linked to brine transport in sea ice. Brine channels were responsible for the shape of the salinity distribution, and for vertical persistence of higher salinity over at least 0.4 m. Platelet ice had slightly higher bulk salinity than columnar ice, probably due to changes in the connectivity of its microstructure.

We also investigate multi-year sea ice under compression by an ice shelf. Snow ice formation contributes to its mass balance and affects its salinity profile. A process of upwards seawater percolation occurs in summer that maintains the permeability of the multi-year ice around a critical value.

Finally our results can be applied to interpret observations of platelet ice around Antarctica, and we provide a climatology of the late-winter heat flux between the sea ice and the ocean across McMurdo Sound. This indicates a mean oceanic heat flux of -48 W m-2 in front of the McMurdo Ice Shelf.