By Jonathan Amos
Science reporter, BBC News, Vienna
Tiny they may be, but fossil diatoms discovered deep under the ocean floor are revealing new details about Antarctica's warmer past.
The single-celled algae were pulled up by the Antarctic Geological Drilling (Andrill) Program, which has been operating from the Ross Ice Shelf.
Some are new to science; others would normally only be expected in waters with higher temperatures than today.
Scientists say the diatoms will help them understand future climate changes.
"Andrill's basic aim is to try to recover sedimentary records close to the Antarctic ice sheet that tell us about the history of the ice and the way that it interacts with the global climate system," explained Dr Tim Naish, from New Zealand's Institute of Geological and Nuclear Sciences and one of the project's chief scientists.
"This is of real importance as we look at the scenarios predicted with greenhouse gas increases and global warming, and we grapple with what the future changes might be for the Antarctic ice sheet."
He was reporting Andrill's progress here at the European Geosciences Union General Assembly meeting.
Dr Naish said this season's drilling had successfully retrieved a 1,285m-long core of rock - the longest core drilled anywhere on the Antarctic margin, and a record of past climate conditions that probably spans some 10 million years into the past.
Sea-saw
The core's sediment particles give an indication of the amount of ice that was in the Ross area over time - whether the shelf was overhead, withdrawn, or even so thick that it was actually grounded and scraping along the bed.
Preliminary analysis has already thrown up some remarkable insights into the behaviour of the shelf, which today covers an area the size of France.
At least 60 marked fluctuations are recorded, with the dominant state being one that is thicker than currently seen.
And the project's researchers have been fascinated by layers thick with diatoms.
The presence of some species normally associated with waters much further to the north indicates large areas of the shelf have undergone major retreat, with the ice possibly even withdrawing on to the Antarctic landmass.
"Our initial interpretations tell us that there were times when it was very cold and the ice was very big - and those conditions were in the youngest and the oldest part of the core," said Ross Powell, from Northern Illinois University, US.
"And then in between - over the period of Earth history we call the Pliocene - we are looking at something that was much warmer, when the ice was much more dynamic, going backwards and forwards; and in between the ice being there, there was open water with the diatoms coming in."
About 3.5 million years ago, it was very much warmer than today - about 2-3C warmer; but in the temperature region climate models suggest greenhouse gas emissions may be taking us again.
"Perhaps this is the window into which we are heading," said Dr Naish.
Tie together
Scientists are keen to know how sensitive the ice on Antarctica is to warming.
The Ross Ice Shelf, the world's largest ice shelf, would be vulnerable to an influx of warm ocean water; and researchers believe its demise would be an important precursor to the eventual collapse of the entire West Antarctic Ice Sheet (WAIS).
Although the WAIS is the smaller of the two great ice bodies on the continent, its disappearance alone could raise worldwide sea levels by an estimated six metres.
The Andrill team is now working to correlate its work with climate data gathered in other parts of the world.
These include deep-ocean geologic cores, which provide information about global temperature and past CO2 levels, and global sea-level records, which can be inferred from sedimentary deposits and erosion surfaces on continental shelves.
And the Andrill data itself provides an important reference for all the other research that has been done around the White Continent, says David Harwood, from the University of Nebraska, US.
"Lots of research around Antarctica - in the mountains, in coastal areas - has helped us to identify pieces of information, but they've given us only glimpses and the age control has not been great for all these other deposits," he told BBC News.
"We can now tie together other pieces of information from around Antarctica into a chronology that is the best continuous record we have to date."
Andrill will cut a new core next season that goes further back in time to the Middle Miocence (about 17 million years ago), when the Earth was again much warmer than today, warmer even than the Pliocene.
