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 terms of 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 net ‘sinks’ of atmospheric CO2 and others becoming sources 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, the 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,” explains Moy, before adding, “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. On this basis 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 CO2during the LGM and up until about 12,000 years ago. From then until roughly 4,000 years ago the region was a net source of CO2, prior to becoming a net sink once again.
Dr Andrew Moy, now a palaeoclimate scientist at the Australian Antarctic Division, was a visiting researcher at the Alfred Wegener Institute, Helmholtz Centre for Polar and Marine Research in Bremerhaven in 2005. Together with AWI biogeoscientist Dr Jelle Bijma, his focus was on questions concerning the global carbon cycle and climate change. At the time, Dr Bijma was the Coordinator of the EU-RP5 project C6 (“Carbonate Chemistry, Carbon Cycle and Climate Change”), the primary goal of which was to pursue a multi-proxy approach to reconstructing the ocean’s carbon history for the past 130,000 years. Much of the work presented in the new study was conducted or begun in the course of the project.
“Understanding the glacial-interglacial dynamics of natural carbon reservoirs in the ocean and the release of CO2 at the end of the glacial is indispensable to gauging the scale of global CO2 emissions today, and to considering potential solutions to the problem,” says Bijma. “After all, in the natural Earth system, the ocean is our thermostat.”
According to Moy, “There was increased biological productivity in this part of the Southern Ocean during the LGM, resulting in the draw-down of atmospheric CO2. 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 the region was a strong net source of CO2 during the deglaciation, before declining intermittently to reach a state of equilibrium with the atmosphere (neither a source nor a sink), about 4,000 years ago.
Moy believes the study will help improve geochemical models that explain glacial-interglacial variations in atmospheric CO2 change, and improve the modelling of future changes. As he explains, “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 regionally variable.”
Original Study
Andrew D. Moy, Martin R. Palmer, William R. Howard, Jelle Bijma, Matthew J. Cooper, Eva Calvo, Carles Pelejero, Michael K. Gagan & Thomas B. Chalk: Varied contribution of the Southern Ocean to deglacial atmospheric CO2 rise. Nature Geoscience, 21 October 2019, DOI:10.1038/s41561-019-0473-9