Ocean currents did not raise atmospheric carbon dioxide during the Holocene

New Delhi: Ocean currents, bringing carbon-rich deep waters up to the ocean's surface and thought to release marine carbon into the air, did not contribute to increasing atmospheric carbon dioxide (CO2) levels over the past 11,000 years, researchers say.

Instead, a combination of biological, geological and chemical cycles redistributing nutrients and carbon in the ocean and on land may have influenced this rise, the researchers proposed in a study led by the University of Bristol (UK) and the Nanjing University (China).

"Our research challenges assumptions about the role of ocean circulation in the carbon cycle during the Holocene," said lead author Tianyu Chen, professor of Marine Geochemistry at Nanjing and Bristol.

Ocean currents have been thought to contribute to increasing global levels of carbon dioxide in the air over the past 11,000 years, or the Holocene period.

Atmospheric CO2 levels in this period rose from an intial 260 parts per million by volume (ppmv) by 20 ppmv over the course of around 5,000 years, a rise equivalent to around 150 gigatons of CO2.

The researchers in this study carbon-dated deep-sea corals from the Drake Passage, located between South America and Antarctica, as well as the Reykjanes Ridge, south of Iceland, from the Holocene. These corals thrived during this period on seamounts, or underwater mountains rising above ocean floor.

These corals live at water depths stretching 1,900 metres within Antarctic circumpolar waters and North Atlantic Deep Water.

The radiocarbon data surprisingly suggested relatively stable ventilation in both these regions, indicating insignificant disturbances in the polar ocean mixing between surface and deep water over the long-term.

This further suggested, the researchers said, that the North Atlantic and Southern Ocean circulation alone did not drive the rise of atmospheric CO2 levels during the Holocene.

They have published their findings in the journal Nature Geoscience.

"By providing radiocarbon evidence for the stability of these (biogeochemical) processes, we pave the way for a deeper understanding of the complex interactions between the ocean and the Earth's climate system," said Chen.

Explaining the radioactive analysis of corals, co-author Joseph Stewart, Research Fellow in Geochemistry at Bristol, said, "Thanks to the decay of radioactive isotopes within their skeletons, deep-sea corals effectively contain two 'clocks.' "Trace amounts of uranium-238 within the corals gradually decays to thorium-230 allowing us to accurately assess their age by measuring this isotope ratio.

"The second clock, radiocarbon (carbon-14) also slowly decays away, however it predominantly enters the oceans via the atmosphere.

"The radiocarbon content of corals therefore tells us how long radiocarbon-depleted deep waters have been out of contact with radiocarbon-rich surface waters.

"By using both 'clocks' together we were able to assess the rates of ocean ventilation during this key interval." The study said that it represented an important step forward in unravelling the relationship between ocean circulation, carbon cycling, and climate dynamics during the Holocene.