Tiny fossils reveal 25,000 years of Southern Ocean carbon history

A reconstruction of 25,000 years of South Ocean carbon chemistry, using micro-fossils buried in sediments, shows sub-Antarctic waters have played a key role in regulating atmospheric carbon dioxide since the Last Glacial Maximum (LGM).

Chemical changes measured in micro-fossil shells, as well as sediments, showed that different regions of the Southern Ocean varied in their circulation, chemistry and biological productivity, during the last glacial-interglacial cycle.

This resulted in regional variations in the exchange of carbon dioxide (CO₂) between the atmosphere and the ocean, with some parts of the Southern Ocean becoming a net 'sink' of atmospheric CO₂ and others a source of the gas.

The research, published in Nature Geoscience today, was undertaken by Australian Antarctic Division palaeoclimate scientist, Dr Andrew Moy, and an international team from Australia, United Kingdom, Germany and Spain.

"The Southern Ocean currently takes up more atmospheric CO₂ than any other ocean, and it has played a crucial role in regulating past atmospheric CO₂," Dr Moy said.

"However, the physical, biological and chemical variables that control this ocean-atmosphere CO₂ exchange during glacial-interglacial cycles are not fully understood."

To help fill this knowledge gap, the research team measured the chemical composition of microscopic 'foraminifera' shells in sediment samples collected from 3,000 metres below the ocean's surface, in the 'Indo-Pacific' sector of the Southern Ocean, south of Tasmania.

From this they were able to reconstruct dissolved CO₂ levels in surface waters and compare them to CO₂ levels measured in Antarctic ice cores.

They found that Southern Ocean surface waters in the Indo-Pacific region were a net 'sink' for atmospheric CO₂ during the LGM and up until about 12,000 years ago, when they became a net source of CO₂.

"There was increased biological productivity in this part of the Southern Ocean during the LGM, resulting in the draw-down of atmospheric CO₂," Dr Moy said.

"At the same time there was reduced upwelling and exchange of CO₂-rich deep-waters with the surface ocean.

"As the Earth moved from the LGM to a warmer interglacial period, changes in the strength of the biological pump in these waters, and increased upwelling and subsequent release of stored CO₂ from the deep-ocean, contributed to a rise in atmospheric CO₂."

A similar study in the 'Atlantic sector' of the sub-Antarctic Southern Ocean showed that region was a strong net source of CO₂ during the deglaciation, before declining intermittently to be in equilibrium with the atmosphere (neither a source nor a sink), about 4000 years ago.

Dr Moy said the research will help improve geochemical models that explain glacial-interglacial variations in atmospheric CO₂ change, and improve modelling of future change.

"Current models tend to assume the physical, biological and chemical variables that control the CO₂ exchange process between the ocean and the atmosphere are uniform across the Southern Ocean. But this and other new research shows that these processes are variable," he said.