The Upper Jurassic Calcaires en Plaquettes Formation (French Jura mountains) exhibits an alternation of laminated and massive limestones. The organic-rich (TOC 4-12%) laminated limestones consist of submillimetric light-coloured and dark-coloured laminae. Morphologically the laminites can be divided into an undulated and a parallel type. It has been suggested that microbial mats played an important role in the formation of both these types of laminites (Tribovillard et al., 1992). We tested this hypothesis by analysing the free and sulphur-bound biomarkers in four carefully isolated laminae (light/parallel, dark/parallel, light/undulated, dark/undulated), and in a massive limestone sample.
Both the free and sulphur-bound biomarkers in all samples are dominated by extended hopane carbon skeletons. This demonstrates that cyanobacteria were important primary producers during deposition of all these sediments. Also, all samples contain large amounts of sulphur-bound isorenieratane which testifies to the presence of photosynthetic green sulphur bacteria (Chlorobiaceae). These Chlorobiaceae are obligate anaerobes which live under extremely low light intensities. Their presence in the depositional environment points to an anoxic water column extending into the photic zone, and thus excludes the formation of a (cyanobacterial) microbial mat. Chroman ratios of all samples indicate that the salinity of the upper water column was not elevated relative to normal seawater. Terrestrial input was small, as revealed by the relatively low amounts of free long-chain odd-numbered n-alkanes.
Biomarker distributions of the limestone with undulated laminae differ substantially from those of the limestone with parallel laminae. Organic sulphur compounds, in particular those with linear carbon skeletons, are much more abundant in the latter. Differences between biomarker distributions of light-coloured and dark-coloured laminae of the same type (undulated or parallel) are less pronounced, and these mainly concern the relative abundance of sulphur-bound n-alkanes. Organic matter of the massive limestone closely resembles that of the light-coloured parallel laminae, suggesting a similar depositional environment. At present, 13C contents of individual compounds are being determined in order to further elucidate the different sources of organic matter.
Tribovillard N. et al., Palaeogeogr., Palaeoclimatol., Palaeoecol. 99, 163-177 (1992).