Southern Ocean CO2 release at the peak warmth of the Cenozoic

The Early Eocene Climatic Optimum (EECO) was an interval in Earth’s history when reconstructed temperatures and atmospheric CO2 concentrations were among the highest of the Cenozoic Era (i.e., the last 66 million years). The causes of the high CO2 levels remain unclear. The Southern Ocean, a major “leak” in the global ocean’s biological carbon pump that has been implicated in the CO2 changes over recent ice age cycles, may also have played a role earlier in the Cenozoic. Here, we report foraminifera-bound δ15N measurements that show higher surface nitrate concentrations in the Southern Ocean during peak EECO. The associated venting of CO2 from the deep ocean to the atmosphere would have been amplified by the low seawater pH buffering capacity of the Eocene. Previously proposed low silicate rock weatherability during the Eocene may have weakened the “weathering thermostat”, allowing the reconstructed change in biological pump efficiency in the Southern Ocean to explain at least half of the atmospheric CO2 increase observed during the EECO.