Variable carbon isotope fractionation expressed by aerobic CH4-oxidizing bacteria

Carbon isotope fractionation factors reported for aerobic bacterial oxidation of CH₄(α_CH4-CO2) range from 1.003 to 1.039. In a series of experiments designed to monitor changes in the carbon isotopic fractionation of CH₄ by Type I and Type II methanotrophic bacteria, we found that the magnitude of fractionation was largely due to the first oxidation step catalyzed by methane monooxygenase (MMO). The most important factor that modulates the (α_CH4-CH3OH) is the fraction of the total CH₄ oxidized per unit time, which strongly correlates to the cell density of the growth cultures under constant flow conditions. At cell densities of less than 0.1 g/L, fractionation factors greater than 1.03 were observed, whereas at cell densities greater than 0.5 g/L the fractionation factors decreased to as low as 1.002. At low cell densities, low concentrations of MMO limit the amount of CH₄ oxidized, while at higher cell densities, the overall rates of CH₄ oxidation increase sufficiently that diffusion of CH₄ from the gaseous to dissolved state and into the cells is likely the rate-determining step. Thus, the residual CH₄ is more fractionated at low cell densities, when only a small fraction of the total CH₄ has been oxidized, than at high cell densities, when up to 40% of the influent CH₄ has been utilized. Therefore, since Rayleigh distillation behavior is not observed, δ¹³C values of the residual CH₄ cannot be used to infer the amount oxidized in either laboratory or field-studies. The measured (α_CH4-CH3OH) was the same for both Type I and Type II methanotrophs expressing particulate or soluble MMO. However, large differences in the δ¹³C values of biomass produced by the two types of methanotrophs were observed. Methylosinus trichosporium OB3b (Type II) produced biomass with δ¹³C values about 15‰ higher than the dissimilated CO₂, whereas Methylomonas methanica (Type I) produced biomass with δ¹³C values only about 6‰ higher than the CO₂. These effects were independent of the magnitude of the initial carbon isotope fractionation caused by MMO and were relatively constant despite changing ratios of assimilatory to dissimilatory carbon transformation by the organisms. This suggests that the difference in biomass carbon isotopes is primarily due to differences in the fractionation effect at the formaldehyde branch point in the metabolic pathway, rather than assimilation of CO₂ by Type II methanotrophs.     

Enrichment of redox-sensitive trace metals (U,V, Mo,As) associated with the late Hauterivian Faraoni oceanic anoxic event

The Faraoni Level is a short-lived oxygen-deficient event that took place during the latest Hauterivian. In order to improve our understanding of the palaeoenvironmental conditions that occurred during this event, we have analysed the contents of several redox-sensitive trace elements (U, V, Mo, As, Co, Cd, Cu, Zn, Ni, Pb, Cr) from bulk limestone samples of late Hauterivian–early Barremian age from three reference sections. U, V, Mo and As show consistent and significant enrichments during the Faraoni event whereas the other redox-sensitive trace elements analysed here are not systematically enriched. In order to explain this discrepant behaviour, we propose that the Faraoni Level was deposited during a period of anoxic conditions near the sediment–water interface. The distinctive peaks in U, V, Mo and As contents are traceable throughout the three studied sections and represent a good correlation tool which helps to identify the Faraoni Level and its equivalents in the western Tethyan realm and outside of the Tethys. For example, a peak in U contents in upper Hauterivian sediments of the northwestern Pacific realm (ODP leg 185, site 1149) may well be an expression of the Faraoni event in this particular basin.     

General oil-geological problems in Venezuela

A regional study, particularly of the Late Mesozoic facies development in Venezuela and the northeastern corner of Colombia, is of material assistance in outlining the continental frame-work bordering the Guiana Shield in the general Caribbean area. The habitat of the oil is outlined geologically, horizontally as well as vertically, with regard to both oil source rock and oil migration. Some aspects of regional migration and alteration of Cretaceous oil are discussed and a certain zonal regularity in the lateral distribution of the oil is observed regionally in the major Venezuelan basins.     

Enregistrements sismologiques de l'explosion sur le site de l'usine AZF (Toulouse,France)Seismological records of an explosion at the AZF chemical complex in Toulouse,France

This study presents a detailed analysis of the seismic records of a strong explosion that occurred on 21 September 2001 at a chemical complex located south of Toulouse, France, and provoked important damages. The explosion, which is equivalent to a 3.4 magnitude earthquake, has been recorded at most of the stations of the National Seismological Network, as well as at a station under test at the ‘Observatoire Midi-Pyrénées’, 4.2 km away from the epicentre. The main seismic phases are interpreted using the known crustal structures, and a modelling with synthetic seismograms is performed. To cite this article: A. Souriau et al., C. R. Geoscience 334 (2002) 155–161.     

Coarse carbonate breccias as a result of water-wave cyclic loading (uppermost Jurassic – South-East Basin, France)

In the uppermost Jurassic of the central part of the South-East Basin of France, an association of lime mudstone beds, calcarenite beds and coarse carbonate breccia bodies form an informal stratigraphical unit called the 'Barre Tithonique'. In the 'Barre Tithonique', gradual transitions from lime mudstone or calcarenite to breccia show different stages of deformation leading to progressive brecciation of the original lithologies. The study of the breccia facies, and the observed gradual transitions as a whole, document a new early diagenetic process in carbonate environments, resulting from water-wave and seabed interaction. Water-wave induced brecciation and its abundance in the 'Barre Tithonique' indicate that sea–seabed interaction was significant. Comparison with modern studies of the mechanics of wave–seabed interaction suggests that water depth was less than 200 m. It is demonstrated that sedimentary features such as channel-like structures, previously interpreted as being the result of erosion and deposition of mud-flows, were in fact produced by wave-induced, in situ reworking of lime mud, without any significant unidirectional flow or gravity induced displacement.     

Potential fields and their transformations in applied geophysics

Sans résumé

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Potential fields and their transformations in applied geophysics Baranov wladimir: Geoexploration monographs—series 1-n^o 6. (120 pages-32 figures-18 tables), gebründer borntraeger. Berlin-Stuttgart 1975