Paleoceanographic changes during the early Cretaceous (Valanginian-Hauterivian): evidence from oxygen and carbon stable isotopes

摘要

We investigated Valanginian-Hauterivian bulk rock and belemnite samples from Vocontian Basin sections in southeastern France for their stable carbon and oxygen isotope signature. Firstly, because these sections permit calibration with high-resolution biostratigraphy based on ammonites and secondly because detailed isotope studies for the Hauterivian are lacking. The results show that δ~(13)C values for bulk rock decrease during the late Valanginian-early Hauterivian with 1‰ and increase again during the late Hauterivian with 1‰. The δ~(18)O signal for bulk rock samples is mostly disturbed by diagenesis. The belemnites show carbon and oxygen isotope values that are lower and higher than bulk rock samples respectively. We explain this as the result of the belemnites recording a deeper water signal with lighter δ~(13)C values and heavier δ~(18)O values, implicating colder water at greater depth. The overall preservation of the belemnites is very good and permits the construction of a paleo-temperature trend. This trend shows warm deeper water temperatures during the late early Valanginian (15 ℃) and progressively cooler temperatures during the late Valanginian and early Hauterivian (11 ℃). During the late Hauterivian temperatures increase again (13 ℃). We relate this cooling trend for deeper water to a second order sea level rise, which allowed for the exchange of cold Boreal and warm Tethyan water masses. The influx of cold nutrient rich water had a profound effect on carbonate producing biota along the northern margin of the Tethys during the Hauterivian leading to prolonged phases of condensation and platform destruction. During the early Hauterivian the carbonate system along the northern Tethyan margin shifted into a 'green water' mode of carbonate production. High rates of carbonate production under mesotrophic conditions, also observed in other parts of the world, meant that the global carbon cycle became buffered shown by the stable trends in carbon isotopes.