BEYOND METHANE: TOWARDS A THEORY FOR THE PALEOCENE-EOCENE THERMAL MAXIMUM

Abstract

Extreme global warmth and an abrupt negative carbon isotope excursion during the Paleocene-Eocene Thermal Maximum (PETM) have been attributed to a massive release of methane hydrate from sediments on the continental slope [G.R. Dickens, J.R. O'Neil, D.K. Rea, R.M. Owen, Dissociation of oceanic methane hydrate as a cause of the carbon isotope excursion at the end of the Paleocene, Paleoceanography 10 (1995) 965-971.]. However, the magnitude of the warming (5 to 6 °C [J.C. Zachos, M.W. Wara, S. Bohaty, M.L. Delaney, M.R. Petrizzo, A. Brill, T.J. Bralower, I. Premoli-Silva, A transient rise in tropical sea surface temperature during the Paleocene-Eocene Thermal Maximum, Science 302 (2003) 1551-1554.,J.P. Kennett, L.D. Stott, Abrupt deep-sea warming, paleoceanographic changes and benthic extinctions at the end of the Paleocene, Nature 353 (1991) 225-228.]) and rise in the depth of the CCD (> 2 km; [J.C. Zachos, U. Rohl, S.A. Schellenberg, D. Hodell, E. Thomas, A. Sluijs, C. Kelly, H. McCarren, D. Kroon, I. Raffi, L.J. Lourens, M. Nicolo, Rapid acidification of the ocean during the Paleocene-Eocene Thermal Maximum, Science 308 (2005) 1611-1615.]) indicate that the size of the carbon addition was larger than can be accounted for by the methane hydrate hypothesis. Additional carbon sources associated with methane hydrate release (e.g. pore-water venting and turbidite oxidation) are also insufficient. We find that the oxidation of at least 5000 Gt C of organic carbon is the most likely explanation for the observed geochemical and climatic changes during the PETM, for which there are several potential mechanisms. Production of thermogenic CH4 and CO2 during contact metamorphism associated with the intrusion of a large igneous province into organic rich sediments [H. Svensen, S. Planke, A. Malthe-Sorenssen, B. Jamtveit, R. Myklebust, T.R. Eidem, S.S. Rey, Release of methane from a volcanic basin as a mechanism for initial Eocene global warming, Nature 429 (2004).] is capable of supplying large amounts of carbon, but is inconsistent with the lack of extensive carbon loss in metamorphosed sediments, as well as the abrupt onset and termination of carbon release during the PETM. A global conflagration of Paleocene peatlands [A.C. Kurtz, L.R. Kump, M.A. Arthur, J.C. Zachos, A. Paytan, Early Cenozoic decoupling of the global carbon and sulfur cycles, Paleoceanography 18 (2003).] highlights a large terrestrial carbon source, but massive carbon release by fire seems unlikely as it would require that all peatlands burn at once and then for only 10 to 30 ky. In addition, this hypothesis requires an order of magnitude increase in the amount of carbon stored in peat. The isolation of a large epicontinental seaway by tectonic uplift associated with volcanism or continental collision, followed by desiccation and bacterial respiration of the aerated organic matter is another potential mechanism for the rapid release of large amounts of CO2. In addition to the oxidation of the underlying marine sediments, the desiccation of a major epicontinental seaway would remove a large source of moisture for the continental interior, resulting in the desiccation and bacterial oxidation of adjacent terrestrial wetlands. © 2006 Elsevier B.V. All rights reserved.