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RESEARCH PRODUCT

Environmental change during the Early Cretaceous in the Purbeck-type Durlston Bay section (Dorset, Southern England): a biomarker approach.

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20 pages; International audience; The Purbeck-type section (Durlston Bay, Dorset, UK) exhibits littoral lagoonal to lacustrine facies. It shows a gradual climatic/environmental change from semi-arid conditions associated with evaporites at the Jurassic–Cretaceous transition, to a more humid climate at the end of the Berriasian. Though generally organic-poor (total organic carbon, TOC, <1.3%), the Durlston Bay section shows an organic-rich episode (TOC up to 8.5%) located at the transition from evaporitic to more humid facies. A biomarker study was performed in order to determine the origin of the organic matter (OM) in the section and see if changes in organic sources accompanied the general environmental change. The distribution of alkanes, hopanes and steranes indicates that the origin of the OM is mainly algal/bacterial and only changed moderately with the climatic evolution. The saline and anoxic bottom water conditions indicated for most of the samples point to the recurrence of salinity stratification within the basin. A high contribution of odd numbered C23–C31 n-alkanes from algaenan-containing freshwater algae, in particular Botryococcus, is noted for all the samples, despite the low abundance of these organisms in the corresponding palynofacies. This prominence probably results from the high resistance to degradation and the selective preservation of Botryococcus-related lipids. The subtle balance of aridity and freshwater inputs favoured both an abundance of Botryococcus and the development of anoxia, leading to the enhanced preservation of OM during the intermediate climatic episode, while the conditions were less favourable during the semi-arid and more humid episodes.