At the end of the last glacial period about 20,000 years ago, atmospheric CO2 concentrations rose in several steps. Radiocarbon measurements from a marine sediment core suggest that the upwelling of carbon-rich waters in the Southern Ocean contributed to the CO2 rise.
Giuseppe Siani of IDES-CNRS-Université de Paris-Sud, France, and colleagues measured the radiocarbon ages of surface and sea-floor-dwelling foraminifera from a marine sediment core in the Southern Ocean, with the idea that a smaller age difference indicates more mixing between the surface and the waters below. They found three periods of enhanced mixing during the deglaciation. Using independently dated layers of volcanic ash within the core, they compared their records with reconstructions of atmospheric CO2 concentrations from Antarctic ice cores. They found that the two younger periods of enhanced mixing coincided with a steep increase in atmospheric CO2 levels, and the oldest with a small, temporary rise.
The team concluded that enhanced vertical mixing in these intervals brought CO2-rich waters from the deep ocean to the surface, allowing CO2 to escape from these upwelled waters to the atmosphere.
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Newton, A. Southern upwelling. Nature Geosci 6, 989 (2013). https://doi.org/10.1038/ngeo2031