Evidence for a complex Valanginian nannoconid decline in the Vocontian basin (South East France)

Abstract

The Early Cretaceous is punctuated by widespread biocalcification crises. These are characterized by decrease in the carbonate platform growth and, in the pelagic realm, by a decline in Nannoconus relative abundance in the calcareous nannofossil assemblages. The Valanginian Nannoconus decline started before the positive δ13C excursion characterizing the Weissert Event. The nannoconid decline is investigated in two sections of the Vocontian Basin, La Charce and Vergol, which are biostratigraphically well-constrained and contain well-preserved calcareous nannofossils. Absolute and relative abundances of Nannoconus show a polyphased decline, with a first decrease in the interval from the Campylotoxus to the Verrucosum Ammonite Subzones before the positive δ13C shift, and a second decrease from the end of the Peregrinus to the base of the Radiatus Ammonite Zones concomitant with the long-term decrease in δ13C. These two declines are separated by an important increase in the Nannoconus abundance from the Verrucosum to the Peregrinus Ammonite Subzones concomitant with a slight short-term decrease of δ13C. Biometric analysis shows size changes of N. steinmannii and N. kamptneri along the nannoconid decline. The patterns of abundances and size changes seem to be related to paleoenvironmental changes, mainly characterized by a temperature decrease and variable nutrient supply. The described Valanginian nannoconid recovery seems to occur during the most intense phase of the cooling event and is recorded in the Vocontian basin as well as in the Atlantic and Tethys Oceans. These spatial correlations suggest a supra-regional character for the Valanginian nannoconid recovery event best recorded in the Vocontian Basin and at low latitudes.