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Continental warming preceding the Palaeocene–Eocene thermal maximum


Marine and continental records1 show an abrupt negative shift in carbon isotope values at 55.8 Myr ago. This carbon isotope excursion (CIE) is consistent with the release of a massive amount of isotopically light carbon into the atmosphere and was associated with a dramatic rise in global temperatures termed the Palaeocene–Eocene thermal maximum (PETM). Greenhouse gases released during the CIE, probably including methane, have often been considered the main cause of PETM warming. However, some evidence from the marine record suggests that warming directly preceded the CIE2,3,4, raising the possibility that the CIE and PETM may have been linked to earlier warming with different origins. Yet pre-CIE warming is still uncertain. Disentangling the sequence of events before and during the CIE and PETM is important for understanding the causes of, and Earth system responses to, abrupt climate change. Here we show that continental warming of about 5 °C preceded the CIE in the Bighorn Basin, Wyoming. Our evidence, based on oxygen isotopes in mammal teeth (which reflect temperature-sensitive fractionation processes) and other proxies, reveals a marked temperature increase directly below the CIE, and again in the CIE. Pre-CIE warming is also supported by a negative amplification of δ13C values in soil carbonates below the CIE. Our results suggest that at least two sources of warming—the earlier of which is unlikely to have been methane—contributed to the PETM.

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Figure 1: Stable carbon and oxygen isotope stratigraphy of Phenacodus third molar tooth enamel.
Figure 2: Expanded views of isotopic records near the CIE.
Figure 3: Relationships used to estimate δ 18 O SF /MAT slopes.


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We thank D. C. Fisher, G. F. Gunnell, G. R. Smith, L. Wingate, D. L. Dettman, W. J. Sanders, D. L. Fox, P. L. Koch, R. E. Blake, H. C. Fricke, G. J. Bowen, J. I. Bloch, F. A. Smith-McInerney, B. H. Wilkinson, T. Huston, R. Lange and Y. Zhang for their help and advice; S. L. Wing, A. Rountrey, and A. R. Wood for manuscript comments; I. S. Zalmout, D. M. Boyer, and P. Rose for field support. This work was supported by grants from the American Chemical Society (to P.D.G.), the National Science Foundation (to P.D.G.), the Geological Society of America (to R.S.), and the University of Michigan Department of Geological Sciences (to R.S.).

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R.S. sampled and pretreated enamel and ganoine, developed modified techniques for precipitation of Ag3PO4 with K.G.M., did most of the analytical work, and wrote most of this paper. P.D.G. provided specimens, primary stratigraphic data, guidance, and field support. K.G.M. and K.C.L. provided laboratory support and assistance interpreting results. All authors discussed the results and commented on the manuscript.

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Correspondence to Ross Secord.

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Secord, R., Gingerich, P., Lohmann, K. et al. Continental warming preceding the Palaeocene–Eocene thermal maximum. Nature 467, 955–958 (2010).

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