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 C-13-enrichment of
cyanobacteria derivatives (i.e., hopanes and monomethylalkanes) suggest
enhanced primary productivity upsection. This is related to the
C-13-enrichment of dissolved CO2 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 H2S. 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,
C-13-depleted hopanoids (-39.5 parts per thousand) 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 H2S used by Chlorobiaceae
in the deeper layers and thus, sustained the algae and cyanobacteria
productivity in the upper layers. The C-13-depleted carbonate (-13.3
parts per thousand) may be partially related to the anaerobic oxidation
of the organic matter during bacterial sulfate-reduction. (c) 2006
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