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2 Volumina Jurassica

2 2006

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Jurassic cyclostratigraphy: recent advances, implications and problems

100%

Coe A. , Weedon G.

tom Vol. 4, no. 1

156-157

About 70% of the Jurassic is now covered by floating astronomical timescales based on the recognition of Milankovitch cycles. Astronomical timescales provide the highest resolution robust timescales over the tens of thousands to millions and even tens of millions of year timescales. This presentation will provide a summary of the status of the Jurassic astronomical timescale, including the outstanding problems and possible solutions. The oldest sea-floor magnetic anomaly pattern is Callovian. For earlier stages the scaling for the Geological Timescale 2004 (Gradstein et al. 2004), other than minimum estimates from direct counts of stratigraphic cycles, relied on a combination of c. 20 radiometric dates, the number of ammonite subzones and an assumption that the rate of change of seawater 87Sr/86Sr ratio for the Early Jurassic was linear over intervals of millions of years. However, this assumption has recently been questioned and thus there is an additional need to improve the cyclostratigraphic and radiometric databases. Cyclostratigraphy for much of the Early Jurassic has been completed using sections in England and the Alps. There is no cyclostratigraphy for the Bajocian and Bathonian. For the Late Jurassic, the existence of a sea-floor magnetic anomaly pattern together with recent and ongoing cyclostratigraphic and magnetostratigraphic studies on the same sections in the UK provide the potential to produce a high-resolution integrated timescale for the Callovian to Tithonian (c. 15 Ma duration). However, construction of the Late Jurassic timescale is complex because of the high number of magnetic reversals, the provincialism of the ammonites used for biostratigraphy and lack of agreement on the stages. Weedon et al. (2004) identified regular cycles in the Kimmeridge Clay Formation in England and used these to construct a floating 7.5 Ma astronomical timescale for the latest Oxfordian (as defined in the Tethyan province), Kimmeridgian and most of the Early Tithonian. Comparison of this astronomical timescale with the GTS2004 reveals that the Early Tithonian is c. 1 Ma (25%) longer according to the cyclostratigraphy. This mismatch may be resolved via better correlation of the magnetozones that were defined in France (Tethyan Province) and the Kimmeridge Clay Formation (Boreal Province). For the Oxfordian and Callovian a high-resolution magnetostratigraphy based on sections in the UK has recently been compiled. Work is currently being conducted to produce a floating astronomical timescale using exactly the same exposures.

The Early Toarcian environmental event

61%

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.