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 (δ 13 C) excursion, with oceans more depleted in 12 C relative to 13 C 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 CO 2 as a cause for the subsequent Miocene Climate Transition and Antarctic glaciation. However, this hypothesis cannot explain the multi-Myr lag between the δ 13 C excursion and global cooling. We use planktic foraminiferal B/Ca, δ 11 B, δ 13 C, 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.

Despite recent advances, the link between the evolution of atmospheric CO2 and climate during the Eocene greenhouse remains uncertain. In particular, modelling studies suggest that in order to achieve the global warmth that characterised the early Eocene, warmer climates must be more sensitive to CO2 forcing than colder climates. Here, we test this assertion in the geological record by combining a new high-resolution boron isotope-based CO2 record with novel estimates of Global Mean Temperature. We find that Equilibrium Climate Sensitivity (ECS) was indeed higher during the warmest intervals of the Eocene, agreeing well with recent model simulations, and declined through the Eocene as global climate cooled. These observations indicate that the canonical IPCC range of ECS (1.5 to 4.5 °C per doubling) is unlikely to be appropriate for high-CO2 warm climates of the past, and the state dependency of ECS may play an increasingly important role in determining the state of future climate as the Earth continues to warm.

A rapid and large injection of isotopically light carbon into the ocean-atmosphere reservoirs is signaled by a negative carbon isotope excursion (CIE) at the Paleocene-Eocene boundary ~56 m.y. ago. To better understand the extent of ocean warming and acidification associated with the carbon injection we generated elemental and isotopic records of surface and thermocline planktonic foraminifera across the Paleocene-Eocene boundary from an expanded section along the Mid-Atlantic coastal plain, New Jersey (USA). Ocean temperatures (derived from magnesium/calcium paleothermometer) document a lag in thermocline warming relative to surface waters, implying a progressive deepening of the mixed layer in addition to global warming. A similar magnitude of acidification (as recorded by boron/calcium, B/Ca) on the shelf compared with open ocean sites confirms widespread acidification of the surface ocean. An increase in seawater alkalinity after the CIE, as recorded by B/Ca in planktonic foraminifera, likely played an important role in neutralizing the added carbon, possibly minimizing benthic extinction along the shelf.