François Baudin

A novel pathway of soil organic matter formation by selective preservation of resistant straight-chain biopolymers: chemical and isotope evidence

A resistant soil organic residue, “humin”, has been analyzed by solid-state 13C-NMR and pyrolysis followed by molecular and 13C determination of the pyrolysate alkane–alkene fraction. The results show the occurrence of highly aliphatic, straight-chain biopolymer material as a substantial part of soil organic matter. They confirm the hypothesis by which a part of soil organic matter can be formed by selective preservation of resistant highly aliphatic microbial polymers. This pathway is additional to the longstanding neogenesis hypothesis involving recondensation of small polar molecules such as amino acids, carbohydrates and phenols.

First record of the Early Toarcian Oceanic Anoxic Event from the Southern Hemisphere, Neuquén Basin, Argentina

Abstract: The first record of the Early Toarcian Oceanic Anoxic Event (c. 183 Ma) from the Southern Hemisphere is described from the Neuquén Basin, Argentina, identified chemostratigraphically on the basis of a relative increase in marine organic carbon and a characteristic negative carbon-isotope excursion (δ13Corg) in bulk rock and fossil wood. The negative excursion of −6‰ in bulk organic carbon (falling to −31.3‰) crosses the boundary of the tenuicostatum–hoelderi Andean ammonite Zones, equivalent to the tenuicostatum–falciferum/serpentinum zones of Europe. These data indicate that the Early Toarcian Oceanic Anoxic Event was a global phenomenon. Supplementary material: A detailed stratigraphic log, chemostratigraphic data and nannofossil data are available at http://www.geolsoc.org.uk/SUP18411.

Ammonoid and nannoplankton abundance in Valanginian (early Cretaceous) limestone^marl successions from the southeast France Basin: carbonate dilution or productivity?

A quantification of calcareous nannofossil and ammonoid absolute abundance was carried out in the Valanginian limestone–marl alternations of the Vergol section (Drome) located in the epicontinental Vocontian Basin (southeast France). This section is characterised by four centimetric organic carbon-rich layers (Barrande layers, B1–B4). The aim of this work is to reconstruct changes in the distribution of nutrients in the water column on the base of the analysis of fossil organisms at different levels of the food chain. Calcareous nannofossil abundance correlates negatively with carbonate content. Thus, calcareous nannoplankton probably did not produce the bulk of carbonates in the Valanginian of the studied section. Nannofossil abundance was more likely controlled by carbonate mud produced and exported from shallow platform environments to the basin. Variations of carbonate export, controlled by climate and hydrodynamics, are responsible for the limestone–marl alternations. The levels of highest abundance of nannoplankton are interpreted as mainly linked to a reduced accumulation rate and, to a minor extent, to variations in primary production. A dilution by allochthonous carbonate mud is further evidenced by the relationship between Total Organic Carbon (TOC) and calcium carbonate (CaCO3). TOC content decreases with increasing CaCO3 content, except for one of the organic carbon-rich layers (B4). Both taphonomy and dilution factors may have partly controlled the variations in macrofauna abundance. These can be more likely interpreted as a response to palaeoenvironmental changes of the various taxa with different palaeoecological affinities. The high abundance and dominance of bochianitids (orthocone ammonoids) in the layer B4 can be explained by their opportunistic behaviour and their deep-nektonic, possibly nektobenthic, mode of life. Their great capacity for vertical mobility may have permitted brief feeding incursions to the bottom waters when the trophic resources were higher in depth than in the surface waters. High trophic levels at the sea bottom may have also benefited the bivalves and gastropods which are very abundant in layer B4. The mode of life of different ammonoid families is also discussed.

Pyrolytic and spectroscopic study of a sulphur-rich kerogen from the ''Kashpir oil shales'' (Upper Jurassic, Russian platform)

Abstract The kerogen of an organic-rich sample, termed f top, from the Gorodische section (Russian platform) was studied using a combination of microscopic, spectroscopic and pyrolytic methods so as to examine its chemical structure, source organisms and formation pathway(s). This kerogen, which is mainly composed of orange gel-like, nanoscopically amorphous organic matter, exhibits a relatively high aliphatic character; organic sulphur is mainly present as di(poly)sulphides and alkylsulphides. The f top kerogen was chiefly formed via intermolecular incorporation of sulphur in algal or cyanobacterial lipids and carbohydrates. However, its formation also involved oxidative condensation via ether linkages. Comparison of f top sample with other S-rich kerogens points to a closer similarity with Monterey kerogens rather than with a kerogen from the bituminous laminites of Orbagnoux.

Évolution de la paléotempérature des eaux de la plate-forme russe au cours du Jurassique supérieur

The evolution of the paleotemperatures of the Russian Platform over the Late Jurassic are inferred from stable isotope measurements on belemnite rostra. The δ 18O data indicate temperature variations ranging from 7 to 18 °C during the Oxfordian and a stable temperature cal 17 °C over Kimmeridgian and Volgian. The results are compared to data from the literature in the frame of the paleogeographic reconstruction of Europe for the Late Jurassic.

Acoustic sounder measurements of the vertical velocity of volcanic jets at Stromboli Volcano

We have used a Sodar (acoustic doppler sounder) to characterize the behavior of volcanoes, and in particular, to measure the vertical velocities of volcanic jets. We have analyzed more than 100 explosions on Stromboli volcano (Eolian islands). First results show vertical speeds close to the vent ranging from 20 m/s to 80 m/s. The uncertainty in the measurements is close to 10 %, due mainly to the inaccurate knowledge of the sound speed in volcanic jets. This is small, compared to other volcanological methods. This method is well suited for monitoring temporal variations during an eruption and should be particularly interesting for the study of the eruptive dynamics of lava fountains. The Doppler spectrum in explosions appears to be related to different types of echoes and contains information on particle sizes in the jet.

Origin of contrasting features and preservation pathways in kerogens from the Kashpir oil shales (Upper Jurassic, Russian Platform)

Abstract A kerogen, termed aBS base , from the Gorodische section (Russian Platform) was studied using a combination of microscopic, spectroscopic and pyrolytic methods in order to determine its chemical structure, source organisms and formation pathway(s). This kerogen was mainly formed via degradation-recondensation of phytoplanktonic material. Selective preservation and natural sulphurisation pathways only played a minor role, whereas a substantial contribution of ether linked lipids was noticed, revealing large oxygen cross-linking. Such observations allowed us to put forward, for the first time to the best of our knowledge, a substantial role for the oxidative reticulation pathway in the formation of a kerogen. Comparision with a previously studied sample from the same outcrop revealed contrasting features which reflect differences in preservation pathways triggered by different depositional conditions.

Palaeogeography of radiolarite and organic-rich deposits in Mesozoic Tethys

Siliceous and marine organic-rich deposits are sometimes associated, sometimes separate in space and time; however, both are generally accepted to be the result of high planktonic productivity. Among the siliceous marine deposits, the phtanite family facies is distinguished from the radiolarite family facies by several characteristics: They contain organic material and as a result are blackish (vs red/green for radiolarite facies), their time of deposition corresponds with strong faunal modifications and they are deposited generally in shallower environments. A palaeogeographic analysis of locations of Tethyan biosiliceous and marine organic-rich rocks, both resulting from a high planktonic palaeoproductivity, for three Mesozoic high sea-level intervals, Toarcian, Kimmeridgian and Cenomanian, show: (a) during Jurassic times these Tethyan deposits were dissociated, the siliceous deposits being closer to open ocean waters than the organic-rich ones. This is a common disposition in modern upwelling systems and suggests a common process; (b) during Cretaceous times these Tethyan deposits were often associated, i.e. both occur at the same site, and are probably the result of a different process from that in the Jurassic.

Mapping and geochemical characterization of the Toarcian organic matter in the Mediterranean Tethys and Middle East

Organic matter from Toarcian outcrops and boreholes in several basins around the Mediterranean Sea and Middle East has been studied. Rock-Eval pyrolysis, elemental analyses of kerogens and gas chromatography of chloroform extracts within these different basins have been used to determine the amount, petroleum potential and type of the organic matter. The results have been used to compile a mapping of the organic matter for the Toarcian stage, which shows heterogeneities in the distribution and type of organic matter: 1. (1) Marine organic matter (type II) occurs in different environments of deposition. Whereas high contents (>5% total organic carbon) correspond to thick deposits within the West European realm (Great Britain, North Sea, Paris Basin and Germany), the organic matter is less abundant in the Mediterranean area where lower concentrations (1–2% TOC), preserved in Lower Toarcian thin levels, are limited only to restricted basins (northern Italy, Greece). 2. (2) The predominance of continental organic matter (type III) along the northern margin of the Tethys corresponds to a deltaic environment. 3. (3) There is a predominance of altered organic matter within the carbonated platforms around the Mediterranean Tethys. These results, supplemented with the data issuing from the literature, add a geochemical dimension to the paleogeography of the Tethys.

Occurrence of proteinaceous moieties in S- and O-rich Late Tithonian kerogen (Kashpir oil Shales, Russia)

The polar fraction, isolated from the off-line pyrolysate at 400°C of a Late Tithonian, sulphur- and oxygen-rich, kerogen was examined via Raney Nickel desulphurization and TMAH thermochemolysis. Important information on this kerogen, not accessible via conventional pyrolysis, was thus obtained: (i) its structure is not simply based on alkyl skeletons cross-linked by ether and (poly)sulphide bridges, (ii) TMAH thermochemolysis afforded direct evidence of the survival of proteinaceous moieties in this 140 million years old kerogen and (iii) encapsulation within an aliphatic organic matrix was probably the main pathway reponsible for such a conspicuous preservation, also possibly favoured by the presence of numerous sulphur links.