What is an ice shelf?
The snow that falls on Antarctica (on average only 150 mm water equivalent per year) gradually compresses under its own weight to form glacier ice. This flows slowly at first (less than 10 metres per year), then more rapidly via ice streams through bedrock valleys toward the coast (several hundred metres per year), where it discharges directly into the ocean from the coastal ice cliffs or via vast floating slabs of ice known as glacier tongues (individual streams), or ice shelves (where several streams coalesce together). Ice shelves may be hundreds of kilometres across, and hundreds of metres of thick, as they spread across coastal embayments calving icebergs at their seaward margins.
Why do we study them?
Ice shelves are in contact with the atmosphere above, and the ocean below, making them the most vulnerable component of the Antarctic cryosphere. As they consist of ice that is already afloat, any change in their volume does not directly affect sea level. However they have a buttressing effect, slowing the discharge of inland ice off the continent, so that changes in ice shelf morphology (shape and size) can affect the flow of grounded ice from the interior of the continent.
What does it mean to me?The complete collapse of an ice shelf can lead to more rapid discharge of continental ice, which does contribute to increased sea level. This has already been shown to be the case with the loss of smallish ice shelves in the Antarctic Peninsula region. As a large proportion of the world population is located in near-coastal environments rising sea level has the potential for major economic and social impact. Water mass modification beneath ice shelves may also have a role in bottom water formation and this, as well as the freshwater distribution from the melting of ice shelves and icebergs, has a profound influence on ocean circulation patterns. This may have a major influence on regional climate as well as impact on important oceanic ecosystems and fish stocks.
The Amery Ice Shelf
The massive inland Lambert Glacier Basin, which occupies nearly 16% of the East Antarctic ice sheet, channels its flow into the Amery Ice Shelf, whose calving front occupies a mere 2% of the total East Antarctic coastline. The 550 km long, 60,000 km2 Amery Ice Shelf (AIS) is the major embayed shelf in East Antarctica. More than 50% of the ice flowing into the Amery is lost through basal melting processes within the first few hundred kilometres of it becoming afloat, some of which refreezes as marine ice to the base further north. The remaining loss (necessary to maintain a state of near equilibrium of the ice sheet) calves as icebergs from the front. This loss occurs slowly at first by continual shedding of small icebergs and ice cliff collapse from the shelf front. Major iceberg (30 km x 30 km) discharge occurs sporadically on a 40-50 year cycle as the ice shelf front grows, spreads, thins, and weakens into Prydz Bay. A "loose tooth" formation has appeared along the north central calving front of the Amery as rifts have opened and widened over the last decade or so, providing the prospect of the next major iceberg calving over the next couple of years (the last such event was mid-1960s).
How do we study it?
The Amery Ice Shelf Ocean Research (AMISOR) project aims to directly measure ocean characteristics, seawater circulation and the melt-freeze processes occurring at the base of the shelf using boreholes through the ice into the underlying ocean cavity. These are created using a purpose built hot water drill system to melt its way right through the shelf. Sampling is done whilst the borehole is kept open, with collection of short ice cores and sea floor sediment records, measurement of seawater properties throughout the water column, and video recording of the borehole walls, ocean cavity, and sea floor. Long-term monitoring instruments are then allowed to freeze in place to record annual cycles in the ice and seawater below.
What are we finding?
The cores and video records are giving an insight into life beneath the ice. The marine ice (so called because it is ever so slightly saline) that freezes to the base of the shelf, traps organisms floating in the water column – chiefly sea ice type diatoms from Prydz Bay that are swept under the shelf by ocean currents. Digital video recorded a krill swimming by at a depth of 775 m below sea level near the sea bed, where a complex benthic assemblage dominated by filter feeders, but also including deposit feeders, grazers and at least one species of predator-necrophage was found to exist in a deep, dark environment nearly 100 km from the open ocean in front of the shelf.
The above video footage taken through the Amery Ice Shelf boreholes shows (a) bryozoan species (large structure at left) and calcareous tubes; (b) mucous net filter feeders (right of centre); (c) white sponge (top left) and urchin (lower left) (d) sponge (centre) and hyoids (top right).
What happens next?
The project is linked with others investigating the ice shelf flow and mass budget, and the ocean circulation north of the shelf. All this information is used either as boundary condition input, or to validate model output of the ice shelf and sub-shelf ocean circulation, which are essential to predict the future response of the system under various global warming scenarios.
- Fauna flourish under honeycomb ice (Australian Antarctic Magazine 12: 32-33, 2007)