The Earth's climate abruptly warmed by 5-8 • C during the Palaeocene-Eocene thermal maximum (PETM), about 55.5 million years ago 1,2 . This warming was associated with a massive addition of carbon to the ocean-atmosphere system, but estimates of the Earth system response to this perturbation are complicated by widely varying estimates of the duration of carbon release, which range from less than a year to tens of thousands of years. In addition the source of the carbon, and whether it was released as a single injection or in several pulses, remains the subject of debate 2-4 . Here we present a new high-resolution carbon isotope record from terrestrial deposits in the Bighorn Basin (Wyoming, USA) spanning the PETM, and interpret the record using a carbon-cycle box model of the ocean-atmosphere-biosphere system. Our record shows that the beginning of the PETM is characterized by not one but two distinct carbon release events, separated by a recovery to background values. To reproduce this pattern, our model requires two discrete pulses of carbon released directly to the atmosphere, at average rates exceeding 0.9 Pg C yr −1 , with the first pulse lasting fewer than 2,000 years. We thus conclude that the PETM involved one or more reservoirs capable of repeated, catastrophic carbon release, and that rates of carbon release during the PETM were more similar to those associated with modern anthropogenic emissions 5 than previously suggested 3,4 .Global climate warming from the late Palaeocene through early Eocene was punctuated by several transient hyperthermal events 2 . The most prominent of these events, the PETM, was characterized by a 5-8 • C temperature increase, a 3-6 negative carbon isotope excursion (CIE), and profound shifts in biotic communities 2 . The PETM CIE reflects a massive release of 13 C-depleted carbon to the ocean-atmosphere system at the event's onset. The source(s) and mechanism(s) of carbon release remain controversial, with hypotheses including thermal dissociation of marine methane hydrates, widespread oxidation of organic carbon, igneous intrusion into organic-rich sediments, and bolide impact 2 . Improved constraints on the pattern and pace of PETM carbon-cycle change, particularly during the event's onset, are needed to further evaluate these mechanisms. However, recent publications paint widely divergent pictures of the CIE onset, ranging from slow, protracted (20,000 year) carbon release 4 to a single, massive sub-decadal carbon burst 3 .We recovered new sedimentological and isotopic records from approximately 375 m of strata spanning the Palaeocene-Eocene boundary in two boreholes at Polecat Bench (PCB, Bighorn Basin, Wyoming; Fig. 1) as part of the Bighorn Basin Coring Project (BBCP). Fluvial deposits of the Willwood Formation, which contains the PETM at this site, consist of mudrocks with moderate to strong pedogenic overprinting, heterolithic deposits with weak pedogenic overprinting, and channel and sheet sandstones 6 (Fig. 1). Authigenic palaeosol carbonate is present as semi-round...
