A pronounced negative carbon-isotope (delta13C) excursion of approximately 5-7 per thousand (refs 1-7) indicates the occurrence of a significant perturbation to the global carbon cycle during the Early Jurassic period (early Toarcian age, approximately 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. 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 per thousand to -3 per thousand. 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 species.

The long-term stability of Earth's climate throughout the Phanerozoic stands in marked contrast to the dramatic fluctuations that have taken place on time scales as short as a few years, reflecting the high efficiency of longer-term climate regulation through negative feedbacks. A fundamental mechanism is thought to involve control of CO 2 in the ocean-atmosphere system through continental weathering, although unambiguous, high-resolution data supporting this hypothesis have hitherto not been available. Organic-rich mudrocks from Yorkshire, England, which were deposited during the Toarcian oceanic anoxic event (ca. 181 Ma, Early Jurassic), contain evidence of an exceptionally large excursion in the 187 Os/ 188 Os ratio of contemporaneous seawater, from ϳ0.4 to ϳ1.0. The most likely explanation for this excursion is that it resulted from a transient increase in global continental weathering rates of ϳ400%-800%. The Os isotope excursion coincided with a well-documented global ␦ 13 C excursion of ؊6‰ that affected all the major biospheric reservoirs of the time. Higher mean global temperatures caused global chemical weathering rates to increase substantially, while, in turn, chemical weathering was very effective in reducing the elevated levels of atmospheric CO 2 and the high temperatures to preexcursion levels.Keywords: osmium, weathering, Toarcian, oceanic anoxic event, strontium, methane hydrate. of this manuscript, and the Natural Environment Research Council and the Open University for financial support. We are grateful for journal reviews provided by S.P. Hesselbo and an anonymous reviewer.

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Rhenium and osmium isotope and abundance data have been obtained on precisely-located samples from three suites of immature, organic-rich mudrocks from Jurassic coastal outcrops in England. The data provide precise and accurate whole-rock ages of 207±12 Ma, 181±13 Ma and 155±4.3 Ma for suites of Hettangian, Toarcian (exeratum Subzone) and Kimmeridgian (sensu anglico, wheatleyensis Subzone) samples. These new Re-Os ages are indistinguishable, within the assigned analytical uncertainties, from interpolated depositional ages estimated from published geological timescales, and establish the importance of the ReOs dating technique for chronostratigraphic studies. Early-diagenetic pyrite nodules possess levels of Re and Os which are ~1-2 orders of magnitude lower than in the enclosing organicrich mudrocks, indicating that these elements had already been removed from sediment pore waters at the time of nodule formation. Thus the Re-Os isotope system in these organic-rich mudrocks has been closed since, or from very soon after, the time of sediment deposition.Because most of the Re (98%+) and Os (95-99.8%+) in the mudrocks is shown to be hydrogenous, the 187 Os/ 188 Os (i) of the samples is interpreted to be that of contemporaneous seawater. The data thereby provide the first estimates of the Os isotope composition of Jurassic seawater. During the earliest Jurassic (Hettangian), the seawater 187 Os/ 188 Os ratio was extremely unradiogenic (~0.15); it had increased to ~0.8 at the end of the Early Jurassic (Toarcian) ~20 Ma later, while in the Late Jurassic (Kimmeridgian) the seawater 187 Os/ 188 Os ratio was ~0.59. The most likely explanation for the unradiogenic Os isotope composition of Hettangian seawater is that the contribution of unradiogenic Os to the oceans from the hydrothermal alteration of oceanic crust greatly exceeded the input of radiogenic Os from the continents at that time. This interpretation is in line with observations suggesting that global weathering rates were low in the Hettangian, and that increased hydrothermal and volcanic activity preceded the break-up of Pangea. The Re/Os ratios of Hettangian mudrocks (and by inference, of contemporaneous seawater) are similar to those of mudrocks deposited at later times during the Jurassic, and argues against the unradiogenic Os in Hettangian seawater being derived from extraterrestrial meteoritic sources.

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[1] During the early Toarcian (∼183 Ma ago), a high rate of organic carbon burial globally over a brief interval of time has led to the recognition of a major oceanic anoxic event (OAE). A pronounced negative excursion in the carbon-isotope composition of marine organic matter, marine carbonate and terrestrial plant material is a key feature of this event but the precise timescale and cause(s) of this isotopic anomaly are debated. Associated with the negative carbon-isotope excursion is evidence for a coeval rise in seawater palaeotemperature, an increase in continental weathering rates, and the mass extinction of marine invertebrate species. The early Toarcian OAE provides evidence for the Earth's response during rapid climate change, and critical to our understanding of the event is a high-resolution timescale that allows us to quantify the rates, duration and lead/lag times of environmental processes. In this study, we present 2743 new high-resolution organic carbon, sulphur and carbonate concentration data from samples of well-preserved organic-rich mudrocks spanning the early Toarcian OAE in Yorkshire, UK. We have used these data to document the geochemical changes and significantly extend and refine the astronomical timescale across this event. Our detailed analysis of the relationship between astronomical forcing and carbon isotope changes in both Yorkshire and a section from Peniche, Portugal, indicates that astronomical forcing paced the timing of major shifts in d 13 C and hence climate in both sections. Our analyses also demonstrate that there was a marked increase in the relative strength of astronomical forcing recorded at the onset of the OAE, and that the recorded nature of astronomical forcing changed during the event. Both the Yorkshire and Peniche cyclostratigraphies suggest that one astronomical forcing parameter paced environmental change through the d 13 C event, and that this parameter was obliquity or precession.