Carbon released from the ocean helped end the last ice age

Heard Island. Image: Kate Kiefer/Australian Antarctic Division

Scientists have found a release of carbon dioxide from deep in the ocean helped warm the planet and bring it out of the last ice age.

The findings, published in the journal Nature, will help scientists understand how the ocean affects the carbon cycle and climate change.

“The ocean currently contains about 60 times more carbon than the atmosphere – in natural conditions it is the main driver of carbon dioxide variations,” said Dr Gianluca Marino from The Australian National University (ANU) Research School of Earth Sciences.

“Carbon can exchange rapidly between the ocean and the atmosphere.”

Atmospheric carbon dioxide levels fluctuate from around 185 parts per million (ppm) during the most recent ice age, to 280 ppm during warmer periods such as the last millennium. Since 1850 carbon dioxide levels have risen to nearly 400 ppm.

The team of scientists from UK and Australia reconstructed ancient carbon dioxide levels by studying levels of the element boron in shells of microfossils recovered from the ocean floor, and compared them with the atmospheric carbon dioxide levels previously measured in ice cores from Antarctica.

The team found that at the end of the last ice age carbon dioxide dissolved in the ocean from tiny organisms in the South Atlantic Ocean and eastern equatorial Pacific became much higher than the atmospheric levels at the time.

Mixing between the surface and the deep ocean was weaker during the last ice age and this helped carbon remain trapped at the bottom of the ocean for millennia.

The team concluded that at the end of the last ice age, the carbon reserves deep in the Southern Ocean were re-released into the atmosphere.

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