Molecular and isotopic characterization of biomarkers in the Frick Swiss Jura sediments: A palaeoenvironmental reconstruction on the northern Tethys margin

Abstract

Molecular and stable carbon isotope compositions of source-specific hydrocarbons have been used to reconstruct palaeoenvironmental conditions during deposition of the Middle Hettangian to Upper Sinemurian sediments on the northern epicontinental Tethys margin, Frick Swiss Jura. Increasing algal, cyanobacterial and phytoplanktonic (i.e., dinoflagellate) contributions associated with the 13C-enrichment of cyanobacteria derivatives (i.e., hopanes and monomethylalkanes) suggest enhanced primary productivity upsection. This is related to the 13C-enrichment of dissolved CO 2 in the upper layers and the progressive increase of depth and oxygenation of the water column. In the Middle Hettangian shallow-water environments (lagoon), the occurrence of green sulfur bacteria ( Chlorobiaceae) derivatives indicates that the lower part of the water column was strictly anoxic and rich in H 2S. Since these bacteria require very low light intensity to grow, these euxinic conditions may be extended up to the photic zone, allowing for anaerobic photosynthesis. Light penetration depth is most likely reduced by high productivity and/or turbidity in the photic zone. In these sediments, 13C-depleted hopanoids (−39.5‰) are most likely associated with phototrophic purple sulfur bacteria utilizing isotopically light organic carbon at the base of the aerobic zone. These purple sulfur bacteria may have consumed the H 2S used by Chlorobiaceae in the deeper layers and thus, sustained the algae and cyanobacteria productivity in the upper layers. The 13C-depleted carbonate (−13.3‰) may be partially related to the anaerobic oxidation of the organic matter during bacterial sulfate-reduction.