Marine primary producer community during the mid-Cretaceous oceanic anoxic events (OAEs) 1a, 1b and 1d in the Vocontian Basin (SE France) evaluated from triaromatic steroids in sediments

Desmethyl and A-ring methyl triaromatic (TA) steroids, including dinosteroids, were analyzed in the sediments deposited during the mid-Cretaceous oceanic anoxic events (OAEs) 1a (Goguel), 1b (Jacob, Kilian and Paquier) and 1d (Breistroffer) from the Vocontian Basin, SE France. These TA steroids were identified by GC–MS using non-polar (DB5-HT) and mid-polar (VF-200 ms) columns; their peaks are more clearly separated by the mid-polar column. The triaromatic dinosteroid indices (TADS and TADS′) are defined as alternative indicators for reconstructing dinoflagellate production. In these relationships, the TADS′ values tend to increase in the samples that have higher abundances of the co-eluted 3-methyl TA steroids. The TADS values are lower in the Goguel, Jacob and Kilian samples but increase in the Paquier and Breistroffer samples, and are especially consistently higher in the Paquier. These results suggest that dinoflagellates flourished under the stratified and eutrophic conditions in surface waters during the Paquier deposition (OAE1b). Moreover, an inverse correlation between the TADS and C₂₇ TA steroid (TAS) values is noticeable in the samples from the Kilian and Paquier deposits. Thus, the C₂₇ TA steroids in these sediments, which originated from C₂₈ sterols, may be primarily derived from an ecological competitor of dinoflagellates. We propose that their competitors are possibly coccolithophorids and/or prasinophyte algae. During the mid-Cretaceous hothouse period, coccolithophorids/prasinophytes had the potential to flourish under more oligotrophic conditions, whereas dinoflagellates were dominant as primary producers under eutrophic conditions.