Did an oceanic burp end the last ice age?

A huge and sudden belch of carbon dioxide from the depths of the ocean may have called time on the last ice age - but what does this mean for climate change today?

A huge carbon burp from the Southern Ocean may have helped end the last ice age according to scientists at the University of Cambridge who have been studying marine sediment core.

An international team – led by Dr Luke Skinner – radiocarbon dated shells left behind in sediment by tiny marine creatures called foraminifera (forams). By measuring how much carbon-14 (¹⁴C) was in the bottom-dwelling forams’ shell and comparing it with the amount of ¹⁴C in the atmosphere they were able to work out how long the CO₂ had been locked in the ocean.

“Our results show that during the last ice age, around 20,000 years ago, carbon dioxide dissolved in the deep water circulating around Antarctica was locked away for much longer than today,” said Skinner, “If enough of the deep ocean behaved in the same way, this could help to explain how ocean mixing processes lock up more carbon dioxide during glacial periods.”

Over the last two million years, the Earth has alternated between ice ages and warmer periods and – as suggested in the Milankovic theory – these changes are driven mainly by changes in the Earth’s orbit around the sun. However, changes in the Earth’s orbit could not have created a global energy imbalance without the help of large positive feedback loops – like changes in atmospheric CO₂.

The ocean is a large dynamic reservoir of carbon and changes in ocean circulation have long been expected to play a major role in changes in CO₂ – Antarctica is suspected to have been a centre of action because deep water can be lifted to the surface and release the CO₂ to the atmosphere.

Scientists think that more CO₂ is locked in the ocean during ice ages and that pulses or burps of the gas from the deep Southern Ocean helped trigger a global thaw every 100,000 years or so. They believe the size of these burps were roughly the same as the change in CO₂ experienced since the start of the industrial revolution.

If correct, we would expect to see large transfers of carbon from the ocean at the end of each ice age – most obvious in the relative concentrations of 14C in the ocean and atmosphere. The longer carbon is locked up in the deep sea, the less 14C it contains.

The research also illustrates how ocean circulation can change significantly over a relatively short space of time, which Skinner believes this has implications for climate change. “This has implications for proposals to pump carbon dioxide into the deep sea as a way of tackling climate change,” he said, “Such carbon dioxide would eventually come back up to the surface, and the question of how long it would take would depend on the state of the ocean circulation,” Skinner said.