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A team of researchers have discovered that the increased levels of CO2 in the atmosphere during the last ice age did not result from reduced density of the deep ocean as previously thought.
In the first study of its kind, the team, made up of researchers from The University of Nottingham, the University of Cambridge and the British Antarctic Survey, were able to disprove the theory that the CO2 (that came from the depths of the ocean), was released because deep water became less dense, able to overturn, and ventilate its dissolved carbon to the atmosphere.
The full research paper ‘Evolution of South Atlantic density and chemical stratification across the last deglaciation’ can be found online at NERC – Science of the Environment.
What caused the rise in CO2?
At the end of the last Ice Age, around 18,000 years ago, ice sheets disappeared on North America and dramatically reduced in size on Antarctica. During this time period, sea levels rose and global temperatures warmed due to an increase in the amount of CO2 in the atmosphere.
The cause of this rise in atmospheric CO2 during the end of the Ice Age — the last ‘deglaciation’ — has been a puzzle to scientists since its discovery in the early 1980s. It has been widely believed to be related to changes in carbon storage in the deep ocean, but the exact mechanisms responsible for releasing the CO2 has remained an open question.
During this study, scientists have reconstructed the changes in the density of the South Atlantic across the last deglaciation and have found evidence that the deep ocean lost its carbon to the atmosphere before it became less dense, and so disproving density as the cause of the loss.
These results suggest that other mechanisms, besides changes to deep ocean density stratification, were responsible for the ocean to atmosphere transfer of carbon.
Reconstruction
The research team deployed a novel approach to address this problem. They reconstructed deep ocean carbon by measuring the carbon isotopic value of foraminiferal shells — the remains of single-celled marine organisms that lived at the sea floor thousands of years ago. They reconstructed density by combining chemical signatures in the shells for temperature and salinity, both of which control the density of water.
Dr Sev Kender, a Research Fellow from The University of Nottingham’s School of Geography, and a researcher on the project, said: “At this period in time, we know that there was a huge rise in CO2 in the atmosphere and we know that the earth warmed. There is a lot of evidence that much of the extra carbon came from the deep ocean, but very little on precisely how that carbon reached the atmosphere.
“One theory is that the deep water in the southern ocean was more dense and didn’t circulate very easily, and that through the thawing, the density of the deep ocean reduced and subsequently made it easier for the CO2 to come to the surface. Through this study we were able to see that although the density did decrease during this period, it didn’t decrease until after the carbon was lost from the deep sea.
“The findings of this study are quite stunning, as they demonstrate that the loss of carbon from the deep ocean did not coincide with a change in the ocean density profile, which is a logical assumption. Instead, other mechanisms must have been responsible, such as surface ocean stratification or sea ice presence. These results help us to better understand the physical and chemical behaviour of the ocean during a past global warming and CO2 release event, ultimately allowing us to better constrain how it may behave in the future.”
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