1. Department of Earth Sciences, Centre for Earth, Planetary, Space & Astronomical Research, The Open University, Milton Keynes MK7 6AA, UK
2. Geologisches Institut, Universität zu Köln, Zülpicher Strasse 49a, D-50674 Köln, Germany

Correspondence to: David B. Kemp¹ Correspondence and requests for materials should be addressed to D.B.K. (Email: d.kemp@open.ac.uk).

Abstract

A pronounced negative carbon-isotope (¹³C) excursion of 5–7 (refs 1–7) indicates the occurrence of a significant perturbation to the global carbon cycle during the Early Jurassic period (early Toarcian age, 183 million years ago). The rapid release of ¹²C-enriched biogenic methane as a result of continental-shelf methane hydrate dissociation has been put forward as a possible explanation for this observation^{1, 7, 8}. Here we report high-resolution organic carbon-isotope data from well-preserved mudrocks in Yorkshire, UK, which demonstrate that the carbon-isotope excursion occurred in three abrupt stages, each showing a shift of -2 to -3. Spectral analysis of these carbon-isotope measurements and of high-resolution carbonate abundance data reveals a regular cyclicity. We interpret these results as providing strong evidence that methane release proceeded in three rapid pulses and that these pulses were controlled by astronomically forced changes in climate, superimposed upon longer-term global warming. We also find that the first two pulses of methane release each coincided with the extinction of a large proportion of marine species⁹.

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