At the end of the last ice age, atmospheric carbon dioxide levels shot up by nearly 50 per cent. But where did the CO2 come from? This long-standing climatic mystery has now been solved.

Climate scientists have suspected - but never been able to prove - that the CO2 was the result of a huge belch of gas from the oceans. They predicted that the ice age had slowed ocean circulation, trapping CO2 deep within it, and that warmer temperatures reversed this process.

Signs of stagnant CO2-rich water have now been discovered 3700 metres beneath the Southern Ocean's seabed, between Antarctica and South Africa.

Stewart Fallon of the Australian National University in Canberra and his colleagues collected samples from drill cores of the marine crust of tiny marine fossils called foraminifera. Different species of these lived at the surface and the bottom of the ocean. The chemical composition of their shells is dependent on the water they form in and how much CO2 it contains.

The team found that species of foraminifera living on the sea floor around the time of the ice age contained more carbon than those that floated at the surface (Science, DOI: link). They also found this discrepancy disappeared around 19,000 years ago, which is also when the ice sheets began to melt.

The findings could help predict how ocean circulation will affect atmospheric CO2 levels in future, says Will Howard of the University of Tasmania, Australia.