Fossil Shells Tie an Ancient Carbon Dioxide Rise to the Southern Ocean

Nitrogen locked inside fossil plankton shells indicates that the Southern Ocean stored less carbon in the deep sea during the Early Eocene Climatic Optimum, a warm interval 53 to 49 million years ago with the highest atmospheric carbon dioxide levels of the past 66 million years. Simone Moretti and Alfredo Martínez-García of the Max Planck Institute for Chemistry in Mainz, Germany, and colleagues published the measurements in Nature Geoscience on Nov. 6, 2026.

The open-access paper tests an idea borrowed from the ice ages: that the biological pump, which carries carbon from the sunlit surface into the deep ocean, sets much of the swing in carbon dioxide. The authors write that the causes of the Eocene peak remain unclear, and that proposals such as volcanism and continental rifting cannot explain the highest levels reconstructed.

The team measured nitrogen isotopes in organic matter sealed inside shells of the plankton genus Subbotina. The shells came from four sediment cores drilled by the Ocean Drilling Program, two in the Southern Ocean and two at mid-latitudes, and the gap between the two sets tracks how completely Antarctic surface waters used their nitrate. The authors report that consumption fell by about 40% at the interval's peak in temperature and carbon dioxide, leaving more nutrients unused at the surface and less carbon heading below.

The authors fed that drop into a box model of ocean and atmosphere chemistry, where it raises carbon dioxide enough to explain at least half of the roughly 800-ppm rise reconstructed for the interval. That apportionment is their own and rests on two conditions they argue held in the Eocene: seawater that buffered added carbon dioxide poorly, and a weakened silicate weathering feedback, the slow rock-chemistry thermostat that would otherwise have pulled it back down.

What moved the nutrients is not settled. The paper weighs stronger overturning bringing up more nitrate against weaker plankton growth leaving it unused, leans toward the first, and says its conclusion holds either way. Records from the warm interglacials of the ice ages show the same pattern, with more unused nutrients in Antarctic surface waters.