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Turner, SK, Sexton PF, Charles CD, Norris RD.  2014.  Persistence of carbon release events through the peak of early Eocene global warmth. Nature Geoscience. 7:748-751.   10.1038/ngeo2240   AbstractWebsite

The Early Eocene Climatic Optimum (53-50 million years ago) was preceded by approximately six million years of progressive global warming(1). This warming was punctuated by a series of rapid hyperthermal warming events triggered by the release of greenhouse gases(1-7). Over these six million years, the carbon isotope record suggests that the events became more frequent but smaller in magnitude(3,5-7). 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 conditions(8-11). 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-existing(7,12) 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 proposed(13,14) to operate in the cooler Oligocene and Miocene.