Scientists have detected what they think is the source of a huge carbon 'burp' that helped end Earth's last ice age.
Australian geoscientist Dr Stewart Fallon from the Australian National University in Canberra and colleagues report their findings today in Science.
As ice sheets melted at the end of the last ice age, around 19,500 years ago, CO2 gas in the atmosphere jumped by 50%.
Scientists have presumed this sudden influx came from a release of CO2 gas previously trapped in the ocean, but until now the evidence for this remained elusive.
Fallon and colleagues have now found evidence trapped in the shells of tiny organisms called foraminifera, that once lived in the ocean.
The researchers compared the carbon isotope ratio in surface- and bottom-dwelling foraminifera.
The findings suggest that during the last ice age, a pool of deep water developed in the Atlantic sector of the Southern Ocean that was much older than surface sea waters at that time.
"What appears to have happened is deep water was being isolated from the water above it," says Fallon.
Over thousands of years, as cold water dropped to the bottom of the ocean it slowly became enriched in dissolved CO2 from sinking decaying organic matter, the researchers say.
Eventually, says Fallon, the CO2 was released, possibly because retreating sea ice exposed a larger ocean surface area to bursts of wind.
He says water moved by the wind may have been replaced by deeper water below, leading to upwelling and more mixing from the bottom up.
The researchers say their findings support the hypothesis that glacial-interglacial increases in the concentration of atmospheric CO2 arise from rapid release of the gas sequestered in the deep sea, primarily the Southern Ocean.
Geologist Professor Mike Sandiford, director of the Melbourne Energy Institute at the University of Melbourne says the result is "interesting but controversial".
Recent radiocarbon analyses from Chilean sites failed to find evidence of the missing Southern Ocean CO2, he says.
"There are challenges in getting sufficient data to really point to [the missing source]," says Sandiford.
"What they've done here is show there was a dramatic change in C14 at this time, so we must have an ocean reservoir that was out of contact with the atmosphere for several thousand years. That's a much longer time than usual."
Fallon agrees more data is needed as the data gathered so far accounts for only half of the discrepancy in carbon.
"We need to get more information from additional deep sea cores to work this one out fully," he says.
But he says getting obtaining deep sea cores containing preserved foraminifera is tricky.
University of Sydney geologist Dr John You says the results are "important".
"Discussions of oceanic processes such as water circulation, mixing and upwelling/downwelling are most speculative since [during] glaciation ocean conditions could be different from the present," says You.
He says the research helps to "fill the gap" in the transition between glaciation and deglaciation between 25,000 to 10,000 years ago.