1. Organic Geochemistry Unit, Bristol Biogeochemistry Research Centre, School of Chemistry, University of Bristol, Cantock's Close, Bristol BS8 1TS, UK
2. Department of Geology, Royal Holloway University of London, Egham, Surrey TW20 0EX, UK
3. Palaeontology Department, Natural History Museum, Cromwell Road, London SW7 5BD, UK
4. Department of Geology, The Field Museum, 1400 S. Lake Shore Drive, Chicago, Illinois 60605, USA

Correspondence to: Richard D. Pancost¹ Correspondence and requests for materials should be addressed to R.D.P. (Email: r.d.pancost@bristol.ac.uk).

Abstract

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 effect^{3, 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 warmer^{1, 6} and wetter climate^{7, 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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