By definition, ice shelves come into large-scale contact with the sea. The Southern Ocean water masses have a decisive influence on the stability and mass balance of the ice sheets. As an example, on the continental shelf of the southern Weddell Sea, where the sea floor slopes towards the land, highly saline, cold shelf water with a temperature of about minus 1.9 degrees Celsius flows just above the seabed for a distance of up to a thousand kilo­metres, reaching far under the Filchner-Ronne Ice Shelf. The ­further the water masses penetrate under the ice sheet, the more ­destructive they are for the ice. This is because the water sinks deeper with every metre that it travels towards the coast. The water pressure under the ice shelf therefore increases and, correspondingly, the freezing point of the water drops from minus 1.9 degrees Celsius to minus 2.5 degrees Celsius. The result of this change is that the cold shelf water deep under the ice sheet does not freeze, but releases its residual heat to the ice and cools down even further. The loss of heat has two consequences: First, the coldest water masses in the world, known as ice-shelf water, form underneath the ice shelf. Its initial temperature is minus 2.5 degrees Celsius. Second, the ice shelf melts from the bottom (basal melting) because of the heat released by the inflowing water. When the ice melts, freshwater is released, thus diluting the super-cold ice-shelf water. Its density decreases and it rises up to meet the underside of the ice shelf. It then flows back to the shelf-ice margin.
On its way there, the freezing point of the ice-shelf water continues to rise due to decreasing pressure. As a result, salt-free ice crystals form in the super-cold ice-shelf water, which then rise to attach to the underside of the ice and freeze there. Scientists refer to this new ice as marine ice. The remaining water masses continue to drift further. Measurements at the shelf-ice edge have revealed that the ice-shelf water flows out from beneath the ice with a temperature below minus 2.2 degrees Celsius, and that it ultimately becomes a part of the Antarctic Deep Water. Ice shelves are therefore intimately interconnected with the deep ocean. While the ocean regulates the thickness of the ice shelf, the ice shelf cools the migrating water masses of the shelf sea and contributes to ­driving the thermohaline circulation.

fig. 2.35 > When sea ice forms in the Weddell Sea, cold, highly saline shelf water is created that flows beneath the Filchner-Ronne Ice Shelf where it cools still further. It ultimately flows down the continental slope as super-cold ice-shelf water.