The Miocene Climatic Optimum (MCO; ca. 17–14.7 Ma) represents one of several major interruptions in the long-term cooling trend of the past 50 m.y. To date, the processes driving high-amplitude climate variability and sustaining global warmth during this remarkable interval remain highly enigmatic. We present high-resolution benthic foraminiferal and bulk carbonate stable isotope records in an exceptional, continuous, carbonate-rich sedimentary archive (Integrated Ocean Drilling Program Site U1337, eastern equatorial Pacific Ocean), which offer a new view of climate evolution over the onset of the MCO. A sharp decline in δ18O and δ13C at ca. 16.9 Ma, contemporaneous with a massive increase in carbonate dissolution, demonstrates that abrupt warming was coupled to an intense perturbation of the carbon cycle. The rapid recovery in δ13C at ca. 16.7 Ma, ∼250 k.y. after the beginning of the MCO, marks the onset of the first carbon isotope maximum within the long-lasting “Monterey Excursion.” These results lend support to the notion that atmospheric pCO2 variations drove profound changes in the global carbon reservoir through the MCO, implying a delicate balance between changing CO2 fluxes, rates of silicate weathering, and global carbon sequestration. Comparison with a high-resolution δ13C record spanning the onset of the Cretaceous Oceanic Anoxic Event 1a (∼120 m.y. ago) reveals common forcing factors and climatic responses, providing a long-term perspective to understand climate–carbon cycle feedbacks during warmer periods of Earth’s climate with markedly different atmospheric CO2 concentrations.

You do not have access to this content, please speak to your institutional administrator if you feel you should have access.