Marine paleoecological variations during the mid-Cretaceous oceanic anoxic event 1a in the Vocontian Basin, southeastern France

We analyzed hopanoid and steroid biomarkers as well as marine palynomorphs in sediments from the Goguel level (oceanic anoxic event [OAE] 1a) of the Vocontian Basin in the Sauzeries section, southeastern France, to reconstruct the paleoecological conditions prevailing in sea surface waters. The 2-methylhopanoid index (2-MHI) values are enriched in the upper Goguel level, implying that the producers of these compounds, possibly oligotrophic alphaproteobacteria and/or Synechococcus cyanobacteria, highly contributed to bacterial production under oligotrophic conditions. Palynomorph analysis shows that acritarchs such as sphaeromorphs and pteromorphs are a characteristic group among the marine palynomorphs in the Goguel samples; therefore, the contribution of prasinophyte algae was possibly high among marine primary producers during OAE 1a in the Vocontian Basin. We suggest that prasinophyte algae and 2-methylhopanoid producers flourished as the main primary producers during the deposition of the Goguel level, which corresponds to the lower part of the C6 chemostratigraphic segment of OAE 1a. Moreover, the 2-MHI values increase in the upper half of the Goguel level, implying that the contribution of the 2-methylhopanoid producers was higher than in lower part and/or the producers were severely stressed by nutrient depletion. Low production of 2-methylhopanoid producers and prasinophyte algae under oligotrophic condition suggest that ocean stratification, rather than high productivity, caused anoxia in Vocontian Basin during OAE1a.