Wednesday, April 29, 2009

Ice sheets much more volatile and dynamic than previously thought, Tahiti corals show

Science Daily: Fossilised corals from tropical Tahiti show that the behaviour of ice sheets is much more volatile and dynamic than previously thought, a team led by Oxford University scientists has found. Analysis of the corals suggests that ice sheets can change rapidly over just hundreds of years – events associated with sea level rises of several metres over the same period. It also shows that a natural warming mechanism thought to be responsible for ending ice ages does not fit the timing of the end of the penultimate ice age, around 137,000 years ago.

…"It’s amazing just how rapidly these ‘melting’ – or ‘deglaciation’ – events occurred and how enormous the volumes of ice involved were," said Dr Alex Thomas, from the Department of Earth Sciences at Oxford University, lead author of the paper. "In the case of deglaciation after the penultimate ice age, before 137,000 years ago, we’re talking about ice sheets – that covered most of the USA and Canada and were up to five kilometres thick – simply vanishing."

The tropical paradise of Tahiti is an ideal place to study the sea level rises associated with deglaciation. This is because not only is it home to different species of corals that like to live at different depths but it is sinking at a constant rate which can be adjusted for when dating these corals, and it is far enough away from the ice sheets not to be affected by displacement or gravitational effects.

"Getting to these ancient fossilised corals without damaging the reef and local ocean life is far from easy," said Dr Thomas. "A robot submersible was sent to survey the ocean floor and placed a target which was used to guide down a drill from a shallow-draft drilling vessel with great precision and extract our cores. We only left a tiny hole behind that soon disappeared – something that was only possible because of the expertise of the Integrated Ocean Drilling Programme."…

Resolution and Adventure with fishing craft in Matavai Bay, painted by William Hodges in 1776, shows the two ships of Commander James Cook's second voyage of exploration in the Pacific at anchor in Tahiti.

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