The Palaeocene–Eocene thermal maximum (PETM), a period of intense, global warming about 55 million years ago1, has been attributed to a rapid rise in greenhouse gas levels, with dissociation of methane hydrates being the most commonly invoked explanation2. It has been suggested previously that high-latitude methane emissions from terrestrial environments could have enhanced the warming effect3,4, but direct evidence for an increased methane flux from wetlands is lacking. The Cobham Lignite, a recently characterized expanded lacustrine/mire deposit in England, spans the onset of the PETM5 and therefore provides an opportunity to examine the biogeochemical response of wetland-type ecosystems at that time. Here we report the occurrence of hopanoids, biomarkers derived from bacteria, in the mire sediments from Cobham. We measure a decrease in the carbon isotope values of the hopanoids at the onset of the PETM interval, which suggests an increase in the methanotroph population. We propose that this reflects an increase in methane production potentially driven by changes to a warmer1,6 and wetter climate7,8. Our data suggest that the release of methane from the terrestrial biosphere increased and possibly acted as a positive feedback mechanism to global warming.
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We thank I. D. Bull and R. Berstan of the Bristol Node of the NERC Life Sciences Mass Spectrometry Facility (LSMSF) for analytical support; Alfred McAlpine plc, AMEC and Channel Tunnel Rail Link for access to the Cobham Lignite Bed, and S. Rose for making arrangements; J. Skipper and S. Tracey for help with initial sample collection and field discussions; and L. Kump for critical comments and advice on the ideas proposed here. We acknowledge funding support for this research from the Leverhulme Trust, and studentship support for L.H. from the NERC.
Author Contributions R.D.P. and M.E.C. contributed equally to this paper. M.E.C., J.J.H. and A.C.S. initially characterized the Cobham Lignite, with I.J.G., and led the subsequent more detailed characterization of the sediments and fossils conducted by D.S.S. R.D.P. led the biomarker analyses at Bristol, with most of the preparation and analyses being conducted by D.S.S., with training and assistance from L.H. N.V.G. conducted bulk organic matter isotopic measurements. R.D.P. wrote the paper, and all authors discussed the results and commented on the manuscript.
Reprints and permissions information is available at www.nature.com/reprints. The authors declare no competing financial interests.
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Pancost, R., Steart, D., Handley, L. et al. Increased terrestrial methane cycling at the Palaeocene–Eocene thermal maximum. Nature 449, 332–335 (2007). https://doi.org/10.1038/nature06012
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