Scientists are getting a much clearer picture of the retreat of the West Antarctic Ice Sheet over thousands of years, and of the forces driving it.
(From BBC News / by Jonathan Amos) — New research indicates that warm waters pulled up from the deep by strong winds sharply undercut glaciers from about 11,000 years ago to 7,500 years ago.
This incursion then stopped until it got under way again in the 1940s.
The findings are important because they inform our understanding about how the ice may respond in the future.
Today, the big glaciers that enter the ocean in a key sector called the Amundsen Sea Embayment are in a rapid withdrawal.
These ice streams, such as Pine Island and Thwaites glaciers, are colossal in scale – and their melting has become a significant contributor to global sea-level rise at around 1mm per decade.
The glaciers’ grounding lines – the places where they enter the ocean and become buoyant – are heading inland; as are the floating segments, or shelves, they push out in front themselves.
Dr Claus-Dieter Hillenbrand, a senior marine geologist at the British Antarctic Survey, explained: “We know today that the ice sheet in the Amundsen Sea is mainly influenced by this warm deep-water upwelling, which is very effectively melting the undersides of the ice shelves and weakening them, and because these shelves buttress the glaciers we therefore get the thinning of the glaciers, the acceleration in the flow speed of the glaciers and the retreat of their grounding lines.”
Dr Hillenbrand and colleagues have been examining the shells of tiny marine organisms called foraminifera recovered from ocean-floor sediments in the Amundsen Sea Embayment.
These shells have chemical “fingerprints” that record the nature of the water in which they were formed.
For example, the ratio of different trace metals in the shells says something about how cold or warm the water was. And the different types of carbon incorporated into the shells reveal information about the age of the water.
In Antarctica, the surface waters are “young” and cold; young in the sense that they are in contact with the atmosphere.
The deep waters, on the other hand, have not seen the surface in a long time and are therefore considered “old” – but they are relatively warm.
Piecing together the distribution and fingerprint pattern of the shells, Dr Hillenbrand’s team has been able to show that the warm deep-water would have welled up and spilled across the continental shelf in front of the glaciers to melt their fronts at the end of the last ice age – about 11,000 years ago.
Read the full story here: http://www.bbc.com/news/science-environment-40560236