Abstract
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.
Highlights
The Miocene Climatic Optimum (MCO, 14–17 Ma) was ~3–4 °C warmer than present, similar to estimates for 2100
We suggest that the broad Monterey carbon isotope excursion (MCIE) represents a negative feedback process akin to Cretaceous Oceanic Anoxic Events (OAEs), which operated in a warmer-than-present icehouse world while the positive carbon isotope excursions (CM events) within the Monterey carbon excursion likely reflect a positive carbon cycle feedback process associated with orbital scale variability in marine productivity
Ocean Drilling Program (ODP) Site 761 T. trilobus boron to calcium (B/Ca), Mg/Ca, and δ13Cp records span a 5-million-year time slice beginning from the onset of the MCO, through its main body and into the MMCT (Fig. 1)
Summary
The Miocene Climatic Optimum (MCO, 14–17 Ma) was ~3–4 °C warmer than present, similar to estimates for 2100. 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. This hypothesis cannot explain the multi-Myr lag between the δ13C excursion and global cooling. An ideal time interval to explore the controls on long-term atmospheric CO2 and pinpoint such carbon cycle feedbacks is the Miocene Climatic Optimum (MCO)—a period of sustained global warmth ~3 °C warmer than modern with reduced continental ice volume occurring ca. Climate Transition (MMCT)ushered in a cooler and apparently more stable icehouse climate mode in a series of cooling or ice growth steps, as evidenced by benthic oxygen isotope records[17,21]
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