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Large temperature drop across the Eocene–Oligocene transition in central North America


The Eocene–Oligocene transition towards a cool climate (33.5 million years ago) was one of the most pronounced climate events during the Cenozoic era1. The marine record of this transition has been extensively studied. However, significantly less research has focused on continental climate change at the time, yielding partly inconsistent results on the magnitude and timing of the changes2,3,4,5,6,7,8. Here we use a combination of in vivo stable isotope compositions of fossil tooth enamel with diagenetic stable isotope compositions of fossil bone to derive a high-resolution (about 40,000 years) continental temperature record for the Eocene–Oligocene transition. We find a large drop in mean annual temperature of 8.2 ± 3.1 °C over about 400,000 years, the possibility of a small increase in temperature seasonality, and no resolvable change in aridity across the transition. The large change in mean annual temperature, exceeding changes in sea surface temperatures at comparable latitudes9,10 and possibly delayed in time with respect to marine changes by up to 400,000 years, explains the faunal turnover for gastropods, amphibians and reptiles, whereas most mammals in the region were unaffected. Our results are in agreement with modelling studies that attribute the climate cooling at the Eocene–Oligocene transition to a significant drop in atmospheric carbon dioxide concentrations.

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Figure 1: Location map showing the sampling sites.
Figure 2: ‘Box and whiskers’ plots for enamel δ 18 O.
Figure 3: High-resolution bone carbonate δ 18 O and δ 13 C plotted versus time for the time interval 36–32 Myr ago.

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This work was supported by the NSF. Fossil tooth samples were provided by the University of Florida (UF), the Denver Museum of Nature and Science, and the American Museum of Natural History. Fossils sampled from the UF were collected on public lands under the auspices of a US Forest Service permit or on private lands with the kind permission of the leaseholders B. and R. Toomey. We thank E. Tappa and W. Straight for facilitating analyses, D. Fox and J. Mintz for their help in the field, and B. Beasley, M. Jin, K. Carpenter and R. Graham for allowing the sampling of bones and teeth. Author Contributions A.Z. and M.J.K. conceived the project, collected samples and data, and led write-up. B.J.M. collected tooth samples at Toadstool Park. D.O.T. helped collect bone samples and refine chronology and stratigraphy at Toadstool Park. All authors discussed and commented on this work.

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Correspondence to Matthew J. Kohn.

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Supplementary information

Supplementary Information

This file contains supporting material divided into the following categories: Supplementary Methods, Supplementary Figure 1, Supplementary Discussion, Supplementary Tables 1-9, and Supplementary Notes. The Supplementary Methods section briefly describes the chemical and analytical procedures employed to analyze the stable isotope composition of the bones and teeth. The Supplementary Figure shows that the in vivo relationship between δ18OCO3 and δ18OPO4 in tooth enamel is maintained in the fossil samples suggesting therefore the absence of significant diagenetic isotopic alteration. The Supplementary Discussion section presents a detailed explanation of the calculations and equations used to calculate MATs, ΔMAT, and MARTs. Supplementary Table 1 provides the stratigraphic position and age of the time-points used to calculate the age of the fossil bone samples. Supplementary Tables 2-9 provide the raw complete dataset of the study. Finally, the Supplementary Notes list the references cited in all the previous SI sections. (PDF 282 kb)

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Zanazzi, A., Kohn, M., MacFadden, B. et al. Large temperature drop across the Eocene–Oligocene transition in central North America. Nature 445, 639–642 (2007).

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