The Southern Ocean played a critical role in the rapid atmospheric carbon dioxide increase during the last deglaciation that took place 20,000 to 10,000 years ago, according to a new report by Boston College geochemist Xingchen (Tony) Wang and an international team in the online edition of Science Advances.
In this new study, Wang and his coauthors analyzed deep-sea coral fossils from 20,000 to 10,000 years ago, when atmospheric carbon dioxide was on the rise.
By examining the chemical signatures of nitrogen and carbon in the coral fossils, the researchers revealed that ocean carbon sequestration decreased as phytoplankton failed to devour macronutrients supplied by upwelling currents in the Southern Ocean and trap carbon dioxide in the deep ocean.
As atmospheric carbon dioxide levels rise because of human activities—dominated by fossil fuel consumption—the findings raise questions about the ocean’s ability to absorb the anthropogenic carbon dioxide and the environmental consequences, including global warming, rising sea levels, and more frequent wildfires, according to Wang.
For all the carbon dioxide emitted by human activities since the industrial revolution, roughly 50 percent stayed in the atmosphere, with about a quarter absorbed by the ocean and approximately 25 percent sequestered by the ecosystem on land. To better predict the fate of anthropogenic carbon dioxide in the future, Wang and his collaborators have looked at the past variations in atmospheric carbon dioxide before any significant human activities, from 20,000 to 10,000 years ago when the Earth was moving out of the last ice age.
“A clearer understanding of carbon dioxide variations in the past provides important insights into the fate of anthropogenic carbon dioxide in the future,” said Wang, an assistant professor in the Department of Earth and Environmental Sciences.
By studying air bubbles trapped in ancient ice from Antarctic, scientists learned that the atmospheric carbon dioxide concentration during the ice ages was about 30 percent lower than the preindustrial level. This lower carbon dioxide level encouraged the growth of large ice sheets in North America and cooled the ice-age Earth. However, there has been strong debates about why the carbon dioxide concentration was lower during the ice ages. In a previous study led by Wang, he found strong evidence suggesting that the Southern Ocean was largely responsible for the lower concentrations of carbon dioxide during the ice ages.
Phytoplankton growth in the ocean, supported by macronutrients nitrogen and phosphorus, assimilates carbon dioxide from the atmosphere and transforms them into organic carbon. When these organisms die, their biomass sinks into the deep ocean and decomposes back to carbon dioxide. This process, called the “biological pump”, transfers carbon dioxide