Environment Counts | Causes of Antarctica ice loss found to be complex
Author: Geoff Zeiss – Published At: 2012-07-14 22:27 – (1116 Reads)
Nearly 90% of Antarctica’s ice loss is discharged through ice shelves, floating glaciers, into the ocean. The ice shelves also affect the flow of the adjacent grounded ice. Recent examples of sudden break-up events along the Antarctic Peninsula show that some ice shelves appear to be highly sensitive to a changing climate. Direct measurements of ocean properties below the ice are sparse and the rates and mechanisms by which oceanic heat reaches the ice are not understood. This study of the Fimbul Ice Shelf (FIS) connects basal melting below the FIS both to solar forcing at the surface and to the coastal current dynamics that determine deep water fluxes, but strongly indicates that there is no simple relationship between basal melt rates and deep ocean temperatures or continental shelf width. GEOPHYSICAL RESEARCH LETTERS, VOL. 39, L12605 2012
Three general modes of heat supply for basal melting have been suggested.
Mode 1 – the base of the ice shelf is exposed to waters with temperatures at the surface
freezing point, which form a heat source for basal melting at depth, because the freezing point of seawater decreases with increasing pressure.
Mode 2 – melting is caused by direct access to an external warm water source due to a warm inflow at depth.
Mode 3 – similar to mode 2 but or due to the interaction of seasonally warmer surface waters with the ice front.
The Norwegian Antarctic Research Expedition in the austral summer of 2009/10 drilled the first hot water drill holes through the main body of the Fimbul Ice Shelf (FIS), in order to directly access the ocean below.
The FIS is typical for the ice shelves along the coast of the Eastern Weddell Sea, where only a narrow continental shelf separates the glaciated coast from the warm water of
the coastal current.
Two years of oceanic observations below the Fimbul Ice Shelf were examined in order to identify the major heat sources for basal melting. The study found that water within the ice shelf cavity is mainly composed of cold Winter Water, which appears to predominately enter the cavity across the eastern sill. In addition, two water masses with temperatures above the freezing point occasionally access the cavity, providing heat for basal melting: (i) intermittent bursts of Modified Warm Deep Water and (ii) fresh Antarctic Surface Water that flushes parts of the ice base with temperatures above freezing during late summer and fall. Unmodified Warm Deep Water is not observed.
GEOPHYSICAL RESEARCH LETTERS, VOL. 39, L12605