Understanding ice shelf processes
Ice shelves around Antarctica are thinning in response to climate change, with some, such as the Ross and Larson, calving huge icebergs, while others, such as the Amery, developing large rifts.
To understand the fundamental processes driving ice shelf thinning, Antarctic scientists will tackle the issue from above and below this season, deploying instruments on the surface of, and in the ocean beneath, the Amery and McMurdo ice shelves and on the Totten and Sørsdal glaciers.
By combining surface glaciological measurements with sub-ice oceanography, they hope to understand the ice shelves’ stability (ice gain versus loss) and their response to changes in environmental conditions, such as air and ocean temperatures and ocean currents.
Australian Antarctic Division ice-ocean modeller, Dr Ben Galton-Fenzi, said that in the past decade scientists have found that a significant cause of ice shelf thinning is through the interaction of the base of the ice shelf with the ocean (basal melting). However, he hopes the deployment of more modern technology will refine understanding of the fundamental processes involved, and inform the next generation of models.
“We have a crude idea of the drivers of basal melting and refreezing, which is reflected in our current models, but there are many details that we need to understand to get a more accurate picture of how the ocean interacts with the base of an ice shelf and drives grounding line retreat.”
Since 2010 the Amery Ice Shelf Ocean Research team has been monitoring a range of instruments deployed into two of six boreholes drilled on the Amery Ice Shelf (AM05 and AM06) and on the surface of the ice shelf in 2015 (see ice shelf graphic). This season they plan to retrieve the data and redeploy some of the surface instruments for a second year.
Instruments deployed into the boreholes include fibre optic distributed temperature sensors (DTS) that measure ice and ocean temperature at the base of the ice shelf, and acoustic doppler current profilers (ADCP), which measure the speed of water flowing beneath the ice shelf and ice shelf thinning due to basal melting. On the surface of the ice shelf are GPS to measure ice flow speeds and surface elevation changes, and two autonomous phase sensitive radio echo sounders (ApRES), which measure the thickness of the ice shelf with millimetre accuracy.
The ApRES instruments are part of a broader international project known by its acronym NECKLACE, which aims to deploy the instruments on all the major ice shelves around Antarctica (like a necklace). This will allow scientists to detect seasonal variability in basal melt rates and over time provide longer-term melt trends around the continent.
Contributing to this effort, Dr Galton-Fenzi plans to deploy up to six ApRES and six GPS instruments on the Totten Glacier (near Casey research station) this season. The Totten Glacier drains a sub-glacial basin containing enough ice to raise sea levels by 3.5 metres, and is a focus of Australian and international research through the ICECAP project (International Collaboration for Exploration of the Cryosphere through Aerogeophysical Profiling – Australian Antarctic Magazine 28: 12-15, 2015).
Recent research shows the Totten Glacier is susceptible to substantial amounts of interannual variability in basal melting and flow, and ICECAP research has found deep channels that could allow warm water to infiltrate the base of the glacier through to its grounding line, causing potentially destabilising melting and increasing this vast drainage basin’s contribution to sea level rise.
The ApRES and GPS instruments will be deployed on the Totten Glacier for six weeks over the 2016 summer, with a couple of sites left to overwinter, sending back information via satellite. The deployment sites will be along two glacier flow lines, from the point where the ice begins to float and further downstream, to measure flow speeds, thinning and melt-rates.
“This work will help us understand how the flow regime of the Totten varies with time, its basal melt rates, and how the ocean is driving melting,” Dr Galton-Fenzi said.
“Ultimately, it will allow us to provide a risk assessment of the likelihood of the collapse of the Totten over the next few hundred years.”
Similar instrument deployments occurred on the Sørsdal Glacier near Davis research station last summer by Dr Sue Cook of the Antarctic Climate and Ecosystems Cooperative Research Centre, as part of a project led by Dr Christian Schoof of the University of British Columbia, Canada. Dr Cook plans to redeploy ApRES and GPS instruments on the glacier again this season, as well as some pressure transducers to monitor lake formation on the surface of the glacier.
“In Greenland, lakes forming on the surface of the ice sheet are known to drain down to its base, changing how the ice sheet slides over the bedrock below,” Dr Cook said.
“So we want to test a similar theory in Antarctica – that surface meltwater ponding is occurring more frequently on outlet glaciers, and potentially reaching the bedrock and driving increased lubrication and acceleration of the glaciers.
“The pressure transducers will tell us how much water forms.”
Dr Cook will also conduct some seismic work on the glacier using a hammer and plate system.
“The radar of the ApRES can’t penetrate salt water, so we just get measurements of the ice thickness. But the seismic system can penetrate through the water column below, so you can map out the size of the ocean cavity,” she said.
“A common problem for many ice shelves is that we don’t know where the grounding line is or the bathymetry of the seafloor below it.
“The seismic work will allow us to identify this grounding line, as well as stratification [layers] in the water and the bathymetry of the seafloor, which both influence how the ocean interacts with the base of the ice shelf.”
To complete the East Antarctic ice shelf research this season, Institute for Marine and Antarctic Studies PhD student Ms Madelaine Rosevear will work with New Zealand scientist Dr Craig Stevens, to deploy oceanographic instruments through a borehole on McMurdo Ice Shelf. The data she collects will feed into a high resolution numerical model looking at the role of ocean mixing on the basal melting of ice shelves.
“Altogether, our study of ice sheets this season will provide good data on the fundamental processes that drive basal melting, as well as information about the bathymetry beneath ice shelves, ocean temperatures and the general state of the region,” Dr Galton-Fenzi said.
“We’ll put all that information into numerical models and look at regions potentially susceptible to rapid deglaciation, such as the Totten.
“From this we’ll be able to come up with better bounds on the problem and estimates of the uncertainty.”
Australian Antarctic Division
What is an ice shelf?
Ice shelves are a floating extension of the Antarctic ice sheet as it pushes seawards from its continental grounding line. As snow accumulates on the continent, its weight slowly pushes the ice sheet out over the ocean. Because ice shelves are in contact with the atmosphere above and the ocean below, they are the most vulnerable component of the Antarctic cryosphere and they will eventually calve icebergs from their fronts.
As ice shelves consist of ice that is already afloat, iceberg calving does not significantly affect sea level. However ice shelves have a buttressing effect, slowing the discharge of inland ice off the continent (via glaciers). As a result, changes in the shape and size of the ice shelf can affect the flow of their associated glaciers from the interior of the continent, outwards.