Evidence for Limited Atmospheric CO ₂ Rise at the Miocene Climatic Optimum

The Miocene Climatic Optimum (MCO; 16.9–14.7 Ma) was a relatively warm period with atmospheric CO 2 averaging ∼500 μatm but CO 2 change across its onset is poorly documented. We present a record of algal 13 C‐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 13 C‐fractionation within the MCO than in the interval below the MCO. Based on various models to convert this signal to CO 2 , including a new statistical model based on core top and culture data, these records imply a CO 2 rise of ∼100–150 μatm across the MCO onset, consistent with previously published information.