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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...

Little is known about controls on river avulsion at geological time scales longer than 10 4 years, primarily because it is difficult to link observed changes in alluvial architecture to well-defined allogenic mechanisms and to disentangle allogenic from autogenic processes. Recognition of Milankovitch-sale orbital forcing in alluvial stratigraphy would provide unprecedented age control in terrestrial deposits, and also exploit models of allogenic forcing enabling more rigorous testing of allocyclic and autocyclic controls. The Willwood Formation of the Bighorn Basin is a lower Eocene fluvial unit distinctive for its thick sequence of laterally extensive lithological cycles on a scale of 4 to 10 m. Intervals of red palaeosols that formed on overbank mudstones are related to periods of relative channel stability when gradients between channel belts and floodplains were low. The intervening drab, heterolithic intervals with weak palaeosol development are attributed to episodes of channel avulsion that occurred when channels became superelevated above the floodplain. In the Deer Creek Amphitheater section in the McCullough Peaks area, these overbank and avulsion deposits alternate with a dominant cycle thickness of ca 7Á1 m. Using integrated stratigraphic age constraints, this cyclicity has an estimated period of ca 21Á6 kyr, which is in the range of the period of precession climate cycles in the early Eocene. Previous analyses of three older and younger sections in the Bighorn Basin showed a similar 7 to 8 m spacing of red palaeosol clusters with an estimated duration close to the precession period. Intervals of floodplain stability alternating with episodes of large-scale reorganization of the fluvial system could be entirely autogenic; however, the remarkable regularity and the match in time scales documented here indicate that these alternations were probably paced by allogenic, astronomically forced climate change.

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Warming and Shrinking In most mammals, individual body sizes tend to be smaller in warmer regions and larger in cooler regions. Secord et al. (p. 959 ; see the Perspective by Smith ) examined a high-resolution 175,000-year record of equid fossils deposited over a past climate shift—the Paleocene-Eocene Thermal Maximum—for changes in body size. Using oxygen isotopes collected from the teeth of co-occurring mammal species to track prevailing environmental temperature, a clear decrease in equid body size was seen during 130,000 years of warming, followed by a distinct increase as the climate cooled at the end of the period. These results indicate that temperature directly influenced body size in the past and may continue to have an influence as our current climate changes.

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The Willwood Formation of the Bighorn Basin (Wyoming, USA) is a thick succession of upper Paleocene and lower Eocene fl uvial-fl oodplain sandstones and mudstones. Reddish paleosols, formed on the fl oodplain mudstones, alternate rhythmically on various scales with heterolithic intervals of small-channel sandstones and mudstones showing weak pedogenesis. Spectral analysis of redness in the Willwood successions at Polecat Bench and Red Butte reveals signifi cant spectral peaks corresponding to cycle thicknesses of ~8 and ~3 m. The ~8 m cycle refl ects distinct clusters of 3-5 paleosols. Age constraints show that the period of this cycle closely matches the ~21 k.y. climatic precession cycle. The ~3 m cycle corresponds to individual paleosols, with a period of 7-8 k.y. This period is similar to millennial-scale sub-Milankovitch cycles found in marine and lacustrine successions of Pliocene-Pleistocene age. Precession and millennial-scale climate variations probably affected paleosol development through cyclic changes from predominantly overbank to predominantly channel-avulsion deposition, with the latter periodically halting soil formation because of high sediment accumulation. A new age model was developed for the Paleocene-Eocene carbon isotope excursion (CIE) at Polecat Bench, based on the precessional origin of paleosol clusters. The main body of the CIE spans ~5.5 precession cycles, or ~115 k.y., and the recovery tail of the CIE spans 2 precession cycles, or ~42 k.y. This outcome is consistent with, and independently confi rms, recent estimates of CIE duration based on deep-sea cores.