Volcanic and orbitally forced carbon release during the Middle Eocene Climatic Optimum

The drivers of the Middle Eocene Climatic Optimum (MECO) remain enigmatic. Here we report a high-resolution terrestrial MECO record from the Bohai Bay Basin, eastern China. The record shows episodic Hg enrichments and a volcanogenic Delta 199 Hg signature during the MECO, and an abrupt chemical weathering enhancement and negative delta 13 C org excursion (CIE) during the MECO peak warmth. A high-resolution age model constrains the MECO to ca. 40.59-40.18 Ma with the onset in the first similar to 320 k.y. Peak warmth and the CIE initiated at 40.27 Ma, corresponding to 405 k.y. and 100 k.y. eccentricity maxima, and lasted for similar to 90 k.y., reminiscent of the early Eocene hyperthermals. Our findings suggest that episodic volcanism contributed to gradual atmospheric p CO 2 rise, leading to the long duration of the MECO onset. Orbital forcing at 40.27 Ma, on top of CO2 2 forcing, caused the peak warmth and a positive carbon cycle feedback. The regional increase in weathering is inconsistent with evidence for weakened global continental weathering, suggesting strong spatial heterogeneity in the weathering response to MECO warming.