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Melted Pleistocene ice lid let out a lot of CO2

The proliferation of antarctic sea ice during the last ice age slowed the exchange of CO2 between the ocean and atmosphere. But then the sea ice melted.
By Brooks Hays   |   Updated Jan. 4, 2016 at 7:14 PM
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CAMBRIDGE, England, Jan. 4 (UPI) -- New analysis suggests the melting of an "ice lid" at the end of the last age enabled the release of large amounts of carbon dioxide from the Southern Ocean.

The new study, published in the journal PNAS, is an effort by scientists at Cambridge University to model the ancient evolution of the Southern Ocean. Specifically, researchers wanted to find out how changes in density, temperature and salinity of ocean water precipitated the rise of CO2 levels in the atmosphere above.

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Previously, researchers thought a reduction in the density of deep Southern Ocean waters allowed for the release of CO2. But the new model suggests deep water density didn't decrease until 5,000 years after CO2 levels rose.

"We set out to test the idea that a decrease in ocean density resulted in a rise in CO2 by reconstructing how it changed across time periods when the Earth was warming," lead study author Jenny Roberts, a PhD student in earth sciences at Cambridge, explained in a press release. "However what we found was not what we were expecting to see."

Researchers were able to reconstruct the changing chemical composition of the ocean waters by analyzing microscopic shelled animals buried in ancient sediment.

Though their models showed periods of cold corresponded with deep, dense ocean water, a density decrease failed to explain the release of CO2 at the end the Pleistocene era.

"Before this study there were these two observations, the first was that glacial deep water was really salty and dense, and the second that it also contained a lot of CO2, and the community put two and two together and said these two observations must be linked," Roberts said. "But it was only through doing our study, and looking at the change in both density and CO2 across the deglaciation, that we found they actually weren't linked. This surprised us all."

Instead, researchers hypothesized that sea ice played a predominant role in trapping and releasing CO2. The Southern Ocean is one of the most active regions for the exchange of CO2 between ocean and atmosphere. But the proliferation of antarctic sea ice during the last ice age slowed this exchange.

Scientists believe it was the rapid melting of sea ice as the atmosphere warmed at the end of the last ice age that precipitated the massive release of carbon dioxide.

"Although conditions at the end of the last ice age were very different to today, this study highlights the importance that dynamic features such as sea ice have on regulating the climate system, and emphasises the need for improved understanding and prediction as we head into our ever warming world," concluded Roberts.

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