Submarine shows cold salty water slows Sørsdal shelf melt

The first use of an Autonomous Underwater Vehicle (AUV) beneath the Sørsdal ice shelf, near Davis research station, has found that cold, saline water beneath the ice contributes to low rates of melting.

During six missions to the ice shelf in early 2019, the University of Tasmania’s seven-metre-long ‘yellow submarine’, nupiri muka, measured temperature, salinity and water currents along the calving front of the ice shelf, and up to 700 metres beneath it.

The AUV missions, led by researchers from the Institute for Marine and Antarctic Studies, Australian Antarctic Division and Australian Maritime College, also discovered a 1200-metre-deep seafloor trough running beneath the ice shelf, which could allow warm water to enter and increase the rate of future melting if ocean conditions change.

Ice shelf-ocean modeller, Dr David Gwyther, said the AUV’s observations were supported by numerical ocean modelling and satellite estimates of melting.

“The AUV found a cold and relatively saline environment adjacent to and beneath the ice shelf front, conducive to low rates of melting from the underside of the shelf,” Dr Gwyther said.

“This reflects observations from instruments left on the surface of the ice shelf that show weak melt rates of between 1.6 and 2.3 metres per year, which are in turn supported by numerical ocean model and satellite estimates of melting.

“This low rate of basal melting may be due to an 825 metre thick layer of cold, dense water that blocks the entry of warmer waters into the region.

“However, the deep trough that we identified could make the Sørsdal vulnerable to oceanographic changes that allow warm water to enter the cavity, including a reservoir of ocean heat detected less than 50 kilometres from the ice shelf.”

As well as the 1200 metre-deep trough, the AUV’s sub-bottom profiler and side-scan sonar detected a rugged and undulating sea floor, scoured and shaped by glacial erosion over the past 20,000 years.

Australian Antarctic Division glaciologist Dr Ben Galton-Fenzi said the use of the AUV provided a powerful tool which, when combined with more traditional sampling methods, help build a more complete picture of processes controlling the melting of ice shelves.

“Ocean-driven melting at the base of ice shelves is already the main contributor to mass loss from the Antarctic ice sheet,” he said.

“A reduction in the buttressing effect of ice shelves is a negative feedback that can lead to glacial acceleration and a further increase in their contribution to mean sea level.

“It is therefore vital that scientists continue to monitor and better understand the changes taking place underneath the Antarctic ice.

“The unique access and data collection capability provided by AUVs means they can play a key role in this global research effort, which we are contributing to in collaboration with our local and international colleagues.”

The Australian Antarctic Program research was published in the Journal of Geophysical Research . It was supported by the Australian Research Council’s Antarctic Gateway Partnership.

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Members of the AUV team at Davis research station standing beside the AUV on a trailer.
Members of the AUV team at Davis research station. (Photo: Andrew Harrison)

The AUV in the water at the front of the Sorsdal Glacier with a small support vessel behind it.

The AUV in a misty grey ocean with the Thwaites Glacier on the horizon.

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