Ocean Carbon Storage across the middle Miocene: a new interpretation for the Monterey Event

The Miocene Climatic Optimum (MCO, 14-17 Ma) was ~3-4 °C warmer than present, similar to estimates for 2100. Coincident with the MCO is the Monterey positive carbon isotope (δ¹³C) excursion, with oceans more depleted in ¹²C relative to ¹³C than any time in the past 50 Myrs. The long-standing Monterey Hypothesis uses this excursion to invoke massive marine organic carbon burial and draw-down of atmospheric CO₂ as a cause for the subsequent Miocene Climate Transition and Antarctic glaciation. However, this hypothesis cannot explain the multi-Myr lag between the δ¹³C excursion and global cooling. We use planktic foraminiferal B/Ca, δ¹¹B, δ¹³C, and Mg/Ca to reconstruct surface ocean carbonate chemistry and temperature. We propose that the MCO was associated with elevated oceanic dissolved inorganic carbon caused by volcanic degassing, global warming, and sea-level rise. A key negative feedback of this warm climate was the organic carbon burial on drowned continental shelves.