Nicolas-Pierre Tribovillard

Geochemical study of organic-matter rich cycles from the Kimmeridge Clay Formation of Yorkshire (UK): productivity versus anoxia

Abstract In this contribution, we study two meter-scale cycles from the Kimmeridge Clay Formation (cored near Marton, Yorkshire) which shows cyclic organic matter (OM) distribution. Our aim is to try to understand the factors responsile for (OM) accumulation. The first cycle, called lower cycle, shows a total organic carbon (TOC) content between 1 and 10% whereas the second, called upper cycle, shows a TOC content varying between 5 and 35%. The geochemical composition (major elements and trace elements), the organic geochemistry (TOC, HI, palynofacies) and mineralogy of the sediments have been determined. In both cycles, the cyclicity is expressed through variations in the nature and in the relative abundance of the various types of organic-matter constituents. Furthermore, dilution effects by inorganic components of the sediment cannot account for the TOC cyclicity. For lower cycle, the Mo, V and U content is low and little variable as is the intensity of the oxidation which OM suffered from. This indicates that variations in phytoplanktonic productivity may be held responsible for the cyclicity in steady and mildly reducing redox conditions. In the upper cycle, cyclicity also appears to depend on productivity but variations in the concentration of Mo, V and U and in the oxidation state of the OM suggest the environment was temporarily more reducing. It is proposed that larger amounts of H 2 S were released into marine bottom waters as a result of initial OM decomposition, forced the oxic-anoxic boundary to rise in the water column and thus favoured OM storage. The main driving force for variations in the OM concentration was the productivity of organic-matter-walled phytoplankton. Redox conditions of the depositional environment could have had a positive action, but only by acting as a positive feedback effect.

Organic-rich biolaminated facies from a Kimmeridgian lagoonal environment in the French Southern Jura mountains—A way of estimating accumulation rate variations

Abstract During Kimmeridgian times, the present-day Southern Jura was a carbonate platform. In the palaeolagoon at the village of Orbagnoux, organic matter was deposited, giving birth to calcareous laminites, with organic carbon reaching 12%. Three facies may be distinguished; two types of laminites are encountered: one made of parallel laminae and the other made of undulating laminae. The laminites alternate with massive limestone beds, in which no internal structure may be seen. The lithology, sedimentology and the faunal, floral and organic content of laminites and of massive limestones was thoroughly investigated at different scales using various methods: field observation, transmitted-light and scanning electron microscopy, Rock Eval pyrolysis, biomarkers, infrared study of kerogen, palynofacies observation. It is suggested that the different types of laminites (parallel or undulating) may reflect variations in the accumulation rates, linked with the carbonate and organic matter productions. Undulating laminae would correspond to periods of lack of sedimentation whereas parallel laminae would reflect alternations of periods of sedimentation and periods of non-sedimentation. These variations might in turn reflect climatic changes affecting the emerged lands supplying the lagoon with detrital products (organic or inorganic). The accumulation and preservation of organic matter were made possible by the development of microbial mats, building “oxygen-tight” barriers at the water-sediment interface. These barriers separated overlying, oxygenated water from anaerobic sediments. These phenomena may explain one way in which potential hydrocarbon source-rocks can form under a (poorly ?) oxygenated water column.

Lack of organic matter accumulation on the upwelling-influenced Somalia margin in a glacial-interglacial transition

Abstract Monsoon-induced upwelling has been operating off Somalia for at least the last 160,000 years, making the NW Indian Ocean one of the most productive oceanic areas in the world. We have studied the sediments (recovered during the Indusom cruise of the R/V Marion-Dufresne ) from three different environments off this margin; a core from the highly productive surface waters (core MD85-674); a core where the oxygen minimum intersects the slope (core MD85-664); and a reference core from the deep basin with normal bottom oxygen levels and moderate primary production (core MD85-668). Though the first two environments are a priori favourable to organic matter (OM) accumulation, no marine OM enrichment can be observed in the cores. To understand this anomaly, emphasis is placed upon the transition from isotopic Stage 6 to Stage 5, because the end of the glacial Stage 6 is characterised by intensified upwelling and primary productivity in this region, whereas Stage 5 experienced a marked decrease in productivity. Various palaeoenvironmental markers, such as OM, V, Ni, Cu, Zn, Ba and P, have been used to identify periods of high productivity and/or bottom water oxygen-depletion. The results suggest that (1) the lack of marine OM in two settings favourable to organic carbon storage may be due to the nature of primary productivity. Coccolithophorid production would not be prone to OM accumulation, in contrast to the contribution of OM by diatoms or, particularly, naked phytoplankton (dinoflagellates or other peridinians); (2) Ba is not an accurate palaeoproductivity marker in this region although the oceanic environment would have been a priori suitable for the use of Ba as a proxy indicator; (3) the period of intensified upwelling in Stage 6 as well as the Stage 6-5 transition left no geochemical imprint on the sediments. This means that some upwelling systems may not be accompanied by marked enrichments in marine organic matter within the underlying sediments. In other words, the Somalia upwelling does not participate to the so-called biological pump and has no positive effect upon long-term carbon storage.