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The Early Toarcian environmental event

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Coe A. , Cohen A. , Kemp D. , Pearce C. , Caswell B. , Skelton P.

tom Vol. 4, no. 1

155-156

A pronounced negative carbon-isotope excursion in marine organic matter, marine carbonate and terrestrial plant material during the Early Toarcian indicates a major and sudden perturbation to the global carbon cycle, which has been previously ascribed to the release of a large volume of methane from marine methane hydrates (Hesselbo et al. 2000, Cohen et al. 2004). Associated features of this event include evidence for a 400-800% increase in global chemical weathering rate (Cohen et al. 2004), a major increase in seawater temperatures, increased global organic carbon burial, a crisis in the primary producers and mass extinctions. We have characterized the precise structure of the carbon-isotope excursion at high resolution using analyses of bulk organic carbon from organic-rich mudrocks from Yorkshire, UK (Kemp et al. 2005). Our data record 3 separate, abrupt negative shifts of up to 3 per mil each. We interpret this stepwise excursion pattern as unambiguous evidence for 3 separate pulses of methane release from methane hydrates. Evidence from other recently published papers on this event in which the above interpretation has been questioned will be discussed. We have also obtained high-resolution calcium carbonate, sulphur and total organic carbon concentration data from the same section. These data have been analysed using spectral analysis and reveal cycles that we ascribe to astronomical precession. The stratigraphic phase relationship between the cyclostratigraphy and the 3 pulses of methane release also permits a direct causal link to be made between methane hydrate dissociation and astronomical climate forcing (Kemp et al. 2005). Our new cyclostratigraphy allows us to constrain accurately the duration of different parts of the environmental perturbation, including the onset and recovery periods. New Mo-isotope data that we have produced suggest that rapid changes in the redox state of the oceans occurred on very short (thousand year) timescales during the Early Toarcian. These changes in redox are directly linked in time to the three abrupt carbon isotope shifts. We are currently completing high-resolution palaeontological studies through this interval in order to better characterize the associated mass extinction event and to understand the life habits of the marine fauna that characterize the crisis interval.