Why are we so interested in the Amery Ice Shelf?

Each year about 2000 million tonnes of snow falls on Antarctica, a precipitation balanced by an outflow of glaciers. The Lambert Glacier, draining 16 percent of the East Antarctic ice area, feeds the Amery Ice Shelf, the largest in East Antarctica. Ocean water penetrates over 550km under the Amery, which thins as it flows towards Prydz Bay, and loses mass by calving of icebergs at its face. It is also diminished by melting where it meets ocean water underneath, an interaction with potential implications for the flow of the glaciers.

In Prydz Bay, near-freezing water at the ocean surface sinks to the bottom and then flows under the ice shelf (Figure 1). At the Lambert Glacier grounding point, almost 2500m below sea level, high pressure causes a lowering of the local freezing point, making incoming seawater warmer than the ice above, which it melts. This melting makes the seawater colder, fresher and less dense, so it rises along the base of the ice shelf until it reaches a point where it is colder than the local freezing point. Here ice crystals form and adhere to the underside of the ice shelf. This ‘marine ice’ is what gives some icebergs their distinctive green colour.

Some of this supercooled water flows out from under the ice shelf into Prydz Bay, where its interactions with ocean water and ice influences Prydz Bay circulation and thus the local ecosystem. It also contributes to formation of dense ‘Antarctic Bottom Water’ which ventilates the deep ocean.

Models of climate change project a warming of the water flowing under the ice shelf. This could significantly increase the rate of melting near the grounding line of the ice shelf, but implications for the Lambert’s flow, the formation of Antarctic water masses and Prydz Bay circulation are uncertain.

The Amery Ice Shelf Oceanographic Research (AMISOR) project aims to quantify the processes occurring under the Amery — in particular the heat and freshwater exchanges between the ocean and the ice shelf — allowing us to make robust models for both present and future conditions. For this we need observations of the Amery’s flow and thickness, the conditions underneath it by means of holes drilled through the ice shelf (see Amery Ice Shelf: home of the Troglodytes on the previous page) and the nature of the ocean at the front of the ice shelf.

The oceanographic program began in the 2000–01 summer, when top-to-bottom data on ocean temperature, salinity, oxygen, and nutrients at 24 locations along the face of the ice shelf were obtained and analysed on board Aurora Australis (Figure 2). Samples were also collected for US collaborators who will measure properties indicating the presence of helium gas, tritium and an isotope of oxygen (d18O) in the ice shelf water.

The temperature observations (Figure 3) and ocean currents measured aboard Aurora Australis showed super-cooled water flowing out from under the Amery Ice Shelf at depths of 200 to 400m. Measurements of temperature, salinity, oxygen and nutrient concentrations were repeated several days after initial observations to help estimate the extent of short-term variability.

To understand the seasonal evolution of the ocean properties and the circulation, seven moorings (Figures 2 and 4) recording ocean currents, temperature and salinity every hour were deployed along the front of the ice shelf. All were recovered late in the summer of 2001–02, when we repeated the temperature, salinity, oxygen and nutrient measurements and collected more samples for our US collaborators.

John Church, Antarctic CRC and CSIRO Marine Research; Nathan Bindoff, Antarctic CRC; John Hunter, Antarctic CRC; Mark Rosenberg, Antarctic CRC