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Persistence of carbon release events through the peak of early Eocene global warmth

Abstract

The Early Eocene Climatic Optimum (53–50 million years ago) was preceded by approximately six million years of progressive global warming1. This warming was punctuated by a series of rapid hyperthermal warming events triggered by the release of greenhouse gases1,2,3,4,5,6,7. Over these six million years, the carbon isotope record suggests that the events became more frequent but smaller in magnitude3,5,6,7. This pattern has been suggested to reflect a thermodynamic threshold for carbon release that was more easily crossed as global temperature rose, combined with a decrease in the size of carbon reservoirs during extremely warm conditions8,9,10,11. Here we present a continuous, 4.25-million-year-long record of the stable isotope composition of carbonate sediments from the equatorial Atlantic, spanning the peak of early Eocene global warmth. A composite of this and pre-existing7,12 records shows that the carbon isotope excursions that identify the hyperthermals exhibit continuity in magnitude and frequency throughout the approximately 10-million-year period covering the onset, peak and termination of the Early Eocene Climate Optimum. We suggest that the carbon cycle processes behind these events, excluding the largest event, the Palaeocene–Eocene Thermal Maximum (about 56 million years ago), were not exceptional. Instead, we argue that the hyperthermals may reflect orbital forcing of the carbon cycle analogous to the mechanisms proposed13,14 to operate in the cooler Oligocene and Miocene.

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Figure 1: Hyperthermals throughout the late Palaeocene to middle Eocene.
Figure 2: Coherence of bulk δ13C and δ18O across the Early Eocene Climate Optimum.
Figure 3: Conceptual threshold model results.

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Acknowledgements

S.K.T. thanks D. Lunt for his generous help with setting up the threshold model. This research used samples and data provided by the ODP. ODP (now IODP) is sponsored by the US NSF and participating countries under the management of JOI, Inc. Financial support for this research was provided by a National Science Foundation International Research Fellowship (to S.K.T.).

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S.K.T. developed the bulk carbonate stable isotope records with the assistance of C.D.C., designed and ran the threshold model experiments, and created the composite δ13C record and method for identifying hyperthermals. S.K.T. and P.F.S. wrote the manuscript. S.K.T. and R.D.N. developed Fig. 1. P.F.S. produced Supplementary Fig. 6. All authors contributed to the final text.

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Correspondence to Sandra Kirtland Turner.

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The authors declare no competing financial interests.

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Kirtland Turner, S., Sexton, P., Charles, C. et al. Persistence of carbon release events through the peak of early Eocene global warmth. Nature Geosci 7, 748–751 (2014). https://doi.org/10.1038/ngeo2240

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