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

We investigated Valanginian-Hauterivian bulk rock and belemnite samples fro m Vocontian Basin sections in southeastern France for their stable carbon a nd oxygen isotope signature. Firstly, because these sections permit calibra tion with high-resolution biostratigraphy based on ammonites and secondly b ecause detailed isotope studies for the Hauterivian are lacking. The result s show that delta(13)C values for bulk rock decrease during the late Valang inian-early Hauterivian with 1 parts per thousand and increase again during the late Hauterivian with 1 parts per thousand. The delta(18)O signal for bulk rock samples is mostly disturbed by diagenesis, The belemnites show ca rbon and oxygen isotope values that are lower and higher than bulk rock sam ples respectively. We explain this as the result of the belemnites recordin g a deeper water signal with lighter delta(13)C values and heavier delta(18 )O values, implicating colder water at greater depth. The overall preservat ion of the belemnites is very good and permits the construction of a paleo- temperature trend. This trend shows warm deeper water temperatures during t he late early Valanginian (15 degrees C) and progressively cooler temperatu res during the late Valanginian and early Hauterivian (11 degrees C), Durin g the late Hauterivian temperatures increase again (13 degrees C). We relat e this cooling trend for deeper water to a second order sea level rise, whi ch allowed for the exchange of cold Boreal and warm Tethyan water masses. T he influx of cold nutrient rich water had a profound effect on carbonate pr oducing biota along the northern margin of the Tethys during the Hauterivia n leading to prolonged phases of condensation and platform destruction. Dur ing the early Hauterivian the carbonate system along the northern Tethyan m argin shifted into a 'green water' mode of carbonate production. High rates of carbonate production under mesotrophic conditions, also observed in oth er parts of the world, meant that the global carbon cycle became buffered s hown by the stable trends in carbon isotopes. (C) 2000 Elsevier Science B.V . All rights reserved.