The Valanginian isotope event: a complex suite of palaeoenvironmental perturbations.

17 pagesInternational audienceThe Valanginian records a severe crisis of carbonate systems, both on platforms and in the pelagic realm. This crisis is roughly concomitant with the Weissert Event, characterized by a positive δ13C excursion of about 2‰in marine carbonates. However, it is unclear if the response of these two carbonate systems to the global perturbations is contemporaneous, or if they react differently. For this purpose, accumulation rates of pelagic carbonates produced by nannofossils and of platform-derived carbonates have been quantified in a hemipelagic environment (the Vocontian Basin, SE France) that has the potential to record the reaction of both shallow-water and pelagic carbonate production to a major perturbation of the carbon cycle, testified by the isotope excursion. Also, changes through time of clay input have been measured in order to evaluate variations in continental weathering rates, and thus in continent-derived nutrient input and climatic conditions in terms of humidity/aridity. Accumulation rates of pelagic carbonates, of platform-derived carbonates and of clays were calculated on the basis of nannofossil absolute abundance and species-specific masses, of weight percent carbonate content of rocks, and a high-resolution timescale given by cyclostratigraphy. The onset of the positive δ13C excursion and the demise of the platforms testify for a synchronous, but inverse response of the shallow-water and pelagic carbonate systems to an increase in nutrient influx to marine waters. As carbonate production by nannofossils decreased during the Late Valanginian in time of still low platform production, the low temperature of marine waters coeval to a still high nutrient input recorded in the latest Late Valanginian possibly could explain that both platform and pelagic carbonate production contemporaneously decreased