ASTRONOMICAL PACING OF METHANE HYDRATE DISSOCIATION DURING THE EARLY TOARCIAN

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 [1,2]. Associated features of this event include a 400-800% increase in global chemical weathering rate [2], an increase in seawater temperature, increased global organic carbon burial, and a mass extinction. In this study we have characterized the precise structure of the negative excursion at high resolution using carbon-isotope analyses of bulk organic carbon from 449 samples collected over a 12 m interval of stratigraphically complete organic-rich mudrocks from Yorkshire, UK. Our data reveal a -7 per mil excursion in organic carbon-isotope ratios during the exaratum ammonite Subzone over a ~7 m interval which comprises 3 separate, abrupt negative shifts of up to 3 per mil each. Each of these shifts occurs over <10 cm of strata. We interpret this stepwise excursion pattern as unambiguous evidence for 3 separate pulses of methane release from methane hydrates.

We have 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 cycles. Our new cyclostratigraphy allows us to constrain accurately the duration of each pulse of methane release to <2 kyr, with all 3 pulses occurring within a 60 kyr period. 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.

Further work is currently being undertaken to test the hypothesis that a switch from precession dominated climate forcing to obliquity dominated forcing occurred during or just after the event [3]. Such a switch could have occurred as a direct consequence of the massive environmental perturbation the Earth system suffered following methane release.

[1] Hesselbo et al., 2000. Nature, 406, 392-395 [2] Cohen et al. 2004. Geology, 32(2), 157-160 [3] Hinnov and Park, 1999. Phil. Trans. R. Soc. Lond. A., 357, 1831-1859