Scientists think they’ve found the source of a huge carbon dioxide ‘burp’ which helped to end the last ice age 18,000 years ago.
It’s the first concrete evidence to support suspicions that carbon dioxide was more efficiently locked away in the deep ocean during the last ice age, turning it into a more ‘stagnant’ carbon repository.
And it casts doubt on the wisdom of sequestering carbon in the deep seas as a way of combating climate change.
Working on a marine sediment core recovered from the Southern Ocean floor between Antarctica and South Africa, the international team led by Dr Luke Skinner of the University of Cambridge radiocarbon dated shells left behind by tiny marine creatures called foraminifera.
By measuring how much carbon-14 was in the bottom-dwelling forams’ shells, and comparing this with the amount in the atmosphere at the time, 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,” says Dr 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.”
Ice ages have come and gone over the past two million years, mainly driven by alterations in the Earth’s orbit around the sun.
But these alterations could only have had such significant effects if there were substantial positive feedbacks. Changes in atmospheric CO2 were clearly one of the most important of these – but what drove these changes in CO2 remained uncertain.
It appears that changes in ocean circulation must have played a major role in motivating these large changes in CO2. The Southern Ocean around Antarctica is expected to have been an important centre of action, because this is where deep water can be lifted up to the sea surface and ‘exhale’ its CO2 to the atmosphere.
Scientists think more CO2 was locked up in the deep ocean during ice ages, and that pulses or ‘burps’ of CO2 from the deep Southern Ocean helped trigger a global thaw every 100,000 years or so. The size of these pulses was roughly equivalent to the change in CO2 experienced since the start of the industrial revolution.
“Our findings underline the fact that the ocean is a large and dynamic carbon pool. This has implications for proposals to pump carbon dioxide into the deep sea as a way of tackling climate change, for example,” says Dr Skinner.
“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, as illustrated by the last deglaciation.”