THE VALANGINIAN CARBON ISOTOPE EVENT - A 1ST EPISODE OF GREENHOUSE CLIMATE CONDITIONS DURING THE CRETACEOUS

Lower Cretaceous pelagic carbonates outcropping along the Southern Alps of northern Italy provide a record of Tethyan palaeoceanography as well as of low frequency fluctuations in the global carbon cycle. The carbonate C-isotope stratigraphy established at five selected localities in the Southern Alps allows an accurate picture to be drawn of the duration and amplitude of the Valanginian C-isotope event. Delta-C-13 values near 1.25-1.50 parts per thousand determined in Berriasian and lower Valanginian sediments are replaced by more positive-delta-C-13 values near 3 parts per thousand in the late Valanginian. The carbonate C-isotope excursion ends in the early Hauterivian with values fluctuating between 1.5 parts per thousand and 2 parts per thousand. The carbonate C-isotope excursion is accompanied by a positive excursion in the total organic carbon C-isotope curve. The Valanginian C-isotope excursion identified in Tethyan sediments correlates with a C-isotope excursion recorded in the western North Atlantic, in the Gulf of Mexico, and in the Central Pacific (DSDP Sites 534, 391, 535 and 167). By analogy with the Aptian stage, also marked by a significant positive C-isotope excursion, the time of positive-delta-C-13 values is regarded as a time of accelerated carbon cycling coupled with increased burial rates of organic carbon and detrital material in oceanic sediments. A warm and humid climate, possibly coupled with a high atmospheric CO2 content and a high global sea-level, may have triggered the acceleration of the global carbon cycling. In this case the Valanginian C-isotope event would reflect a first episode of Greenhouse Earth conditions during the Cretaceous.