Evidence for Limited Atmospheric CO₂ Rise at the Miocene Climatic Optimum

The Miocene Climatic Optimum (MCO; 16.9–14.7 Ma) was a relatively warm period with atmospheric CO2 averaging ∼500 μatm but CO2 change across its onset is poorly documented. We present a record of algal 13C-fractionation covering the Early to Middle Miocene (∼18.5–15 Ma), with a gap at the MCO onset, from Ocean Drilling Program Site 959 in the tropical Atlantic. Bulk marine organic carbon, alkenones and the dinoflagellate cyst species Spiniferites ramosus all show higher 13C-fractionation within the MCO than in the interval below the MCO. Based on various models to convert this signal to CO2, including a new statistical model based on core top and culture data, these records imply a CO2 rise of ∼100–150 μatm across the MCO onset, consistent with previously published information.