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Southern Ocean’s Role in End of Last Ice Age

Research published in Nature has concluded that carbon stored in an isolated reservoir deep in the Southern Ocean reconnected with the atmosphere, driving a rise in atmospheric CO2 and an increase in global temperatures that ended the last ice age.

“The ocean currently contains about 60 times more carbon than the atmosphere,” said Dr Gianluca Marino of The Australian National University. “In natural conditions it is the main driver of carbon dioxide variations.”

Atmospheric carbon dioxide levels have fluctuated from around 185 ppm during the most recent ice age to 280 ppm during warmer periods such as the last millennium. Carbon dioxide levels have now risen to nearly 400 ppm.

Marino and British co-workers reconstructed ancient carbon dioxide levels by studying the shells of ancient marine organisms that inhabited the surface of the ocean thousands of years ago in order to trace its carbon content.

“We found that very high concentrations of dissolved CO2 in surface waters of the Southern Atlantic Ocean and the eastern equatorial Pacific coincided with the rises in atmospheric CO2 at the end of the last ice age, suggesting that these regions acted as sources of CO2 to the atmosphere,” Marino explained.

“Just like the way the oceans have stored around 30% of humanity’s fossil fuel emissions over the last 100 years or so, our new data confirms that natural variations in atmospheric CO2 between ice ages and warm interglacials are driven largely by changes in the amount of carbon stored in our oceans,” added co-author Dr Gavin Foster of the University of Southampton.

“While a reduction in communication between the deep sea and the atmosphere in this region potentially locks carbon away from the atmosphere into the abyss during ice ages, the opposite occurs during warm interglacial periods,” explained joint lead author Dr Miguel Martínez-Botí of the University of Southampton.

“While our results support a primary role for the Southern Ocean processes in these natural cycles… other processes operating in other parts of the ocean, such as the North Pacific, may have an additional role to play,” Foster said.