Cenozoic CO2: from the deep ocean to the atmosphere

The Cenozoic is a time of climatic extremes: abrupt events and state changes pepper the transition from Hothouse warmth to the Pleistocene Icehouse, and these evolving climate regimes are accompanied by major changes in ocean chemistry and biota.  CO₂ is thought to play a critical role in environmental change throughout this era, but despite recent progress, there is still much to learn on the Cenozoic evolution of the ocean-atmosphere CO₂ system.  To address this, we present new reconstructions of ocean pH and atmospheric CO₂ spanning the late Cretaceous to the Pleistocene, based on the boron isotope composition of benthic and planktic foraminifera.  These are accompanied by improved constraints on the secular evolution of seawater chemistry, which are critical for accurate and precise determination of ocean pH and atmospheric CO₂ from boron isotopes.  Using the most reliable data and updated calculation routines, we find close coupling between the CO₂ system of the deep ocean, the atmosphere, and climate over the last 66 million years.  Our data also highlight intervals of dynamic changes in the carbon cycle, such as the transition into the Early Eocene Climatic Optimum, where we suggest a novel link between changes in ocean circulation, redox, and the sulphur and carbon cycles.  Overall, our data demonstrate the persistence of CO₂’s control on the climate system across varying boundary conditions, and the influence of both the long-term carbon cycle and shorter-term ocean biogeochemical cycling on Earth’s climate.