THE VALANGINIAN (EARLY CRETACEOUS) POSITIVE CARBON-ISOTOPE EVENT AS RECORDED IN TERRESTRIAL ORGANIC MATTER

A stratigraphic, biostratigraphic and isotopic investigation has been performed on a Crimean section located on the Kacha River, Verkhorechie Village, SW Crimea. This clastic-dominated succession consists of a series of bioturbated inter-bedded shallow-marine silty sands, claystones and some oolitic sands. A published detailed study of the ammonite fauna has been undertaken and has revealed that the succession can be compared to standard Tethyan schemes. The lower part of the succession is dated on the basis of the ammonite fauna as Early Valanginian (otopeta–campylotoxus ammonite Zones), although this latter zone is highly condensed. A more expanded Late Valanginian is present (verrucosum, callidiscus and tauricum ammonite Zones), and is overlain by sand-dominated sediments of Early Hauterivian age. Throughout this section woody plant matter ranging in preservation from charcoal to coal has been collected and analyzed for stable carbon-isotope ratios. There is no correlation between state of preservation and carbon-isotope ratios. Carbon-isotope ratios range in the Early Valanginian from –24‰ to –22‰, and in the mid-verrucosum Zone values shift abruptly towards more positive values and peak at –18‰ in the lower callidiscus Zone. Wood carbon-isotope ratios decrease gradually through the remainder of the callidiscus Zone and return to pre-excursion values in the tauricum Zone. The remaining Hauterivian values fluctuate between –24‰ to –21‰. The structure, magnitude and timing of the terrestrial carbon-isotope curve is very similar to the marine carbonate curve (from +1‰ to +3‰) for the Valanginian. This would indicate, based on a delta-delta relationship between organic matter and carbonate, that there was very little change in atmospheric CO₂ concentrations during the Valanginian, and that the isotopic composition of the global carbon reservoir shifted. Future research on an Early Cretaceous (Valanginian–Hauterivian) interval from the Yatria River, sub-polar Urals, Western Siberia, which contains both belemnites and terrestrial wood fragments will provide another section from which to determine whether the delta-delta relationship for the Valanginian from Crimea is a global phenomenon or that it can be explained via latitudinal variation.