Sedimentological imprint on subseafloor microbial communities in Western Mediterranean Sea Quaternary sediments

An interdisciplinary study was conducted to evaluate the relationship between geological and paleoenvironmental parameters and the bacterial and archaeal community structure of two contrasting subseafloor sites in the Western Mediterranean Sea (Ligurian Sea and Gulf of Lion). Both depositional environments in this area are well-documented from paleoclimatic and paleooceanographic point of views. Available data sets allowed us to calibrate the investigated cores with reference and dated cores previously collected in the same area, and notably correlated to Quaternary climate variations. DNA-based fingerprints showed that the archaeal diversity was composed by one group, Miscellaneous Crenarchaeotic Group (MCG), within the Gulf of Lion sediments and of nine different lineages (dominated by MCG, South African Gold Mine Euryarchaeotal Group (SAGMEG) and <i>Halobacteria</i>) within the Ligurian Sea sediments. Bacterial molecular diversity at both sites revealed mostly the presence of the classes <i>Alphaproteobacteria</i>, <i>Betaproteobacteria</i> and <i>Gammaproteobacteria</i> within <i>Proteobacteria</i> phylum, and also members of <i>Bacteroidetes</i> phylum. The second most abundant lineages were <i>Actinobacteria</i> and <i>Firmicutes</i> at the Gulf of Lion site and <i>Chloroflexi</i> at the Ligurian Sea site. Various substrates and cultivation conditions allowed us to isolate 75 strains belonging to four lineages: <i>Alpha-</i>, <i>Gammaproteobacteria</i>, <i>Firmicutes</i> and <i>Actinobacteria</i>. In molecular surveys, the <i>Betaproteobacteria</i> group was consistently detected in the Ligurian Sea sediments, characterized by a heterolithic facies with numerous turbidites from a deep-sea <i>levee</i>. Analysis of relative betaproteobacterial abundances and turbidite frequency suggested that the microbial diversity was a result of main climatic changes occurring during the last 20 ka. Statistical direct multivariate canonical correspondence analyses (CCA) showed that the availability of electron acceptors and the quality of electron donors (indicated by age) strongly influenced the community structure. In contrast, within the Gulf of Lion core, characterized by a homogeneous lithological structure of upper-slope environment, most detected groups were <i>Bacteroidetes</i> and, to a lesser extent, <i>Betaproteobacteria</i>. At both site, the detection of <i>Betaproteobacteria</i> coincided with increased terrestrial inputs, as confirmed by the geochemical measurements (Si, Sr, Ti and Ca). In the Gulf of Lion, geochemical parameters were also found to drive microbial community composition. Taken together, our data suggest that the palaeoenvironmental history of erosion and deposition recorded in the Western Mediterranean Sea sediments has left its imprint on the sedimentological context for microbial habitability, and then indirectly on structure and composition of the microbial communities during the late Quaternary

La spectrométrie de fluorescence X et la microsonde électronique, techniques complémentaires pour la détermination précise du chlore à l'état de trace dans les verres volcaniques formés au niveau des dorsales océaniques

L'abondance relative du chlore (Cl) dans les roches silicatées, le désigne comme un indicateur géochimique potentiel de première importance. Cependant les teneurs de cet halogène dans les échantillons géologiques sont peu connues, principalement en raison de la difficulté de mesurer efficacement cet élément présent dans la plupart des cas à l'état de trace. Dans ce travail, la spectrométrie de fluorescence X et la microsonde électronique sont étroitement associées pour déterminer précisément le chlore dans les verres volcaniques formés au niveau des dorsales océaniques. La fluorescence X est utilisée en amont pour certifier les teneurs en chlore dans une série de verres volcaniques afin d'étalonner la microsonde. La détermination en fluorescence X se fait selon une procédure originale. Le chlore, élément volatil, est éliminé de l'échantillon finement broyé, par pyrolyse. Un étalonnage est réalisé avec des pastilles de poudre de la roche calcinée et dopée avec des quantités croissantes d'une solution standard de KCl. La concentration en chlore de l'échantillon est mesurée sur une pastille préparée avec la poudre non calcinée et lue sur cet étalonnage. Les concentrations de chlore trouvées en fluorescence X sont corrélées linéairement avec les intensités mesurées à la microsonde sur des fragments de verre des mêmes échantillons montés en lame mince. En raison de la volatilité du chlore, il s'avère nécessaire de définir judicieusement les conditions de mesure de la microsonde. L'optimisation des paramètres analytiques et la répétition des séquences d'analyse permettent d'obtenir des résultats avec une limite de détection abaissée à 26 ppm et une précision (déviation standard) meilleure que 30 ppm dans la gamme de concentration 0-700 ppm.The relative abundance of chlorine (Cl) in silicate rocks, point out this element as a potential geochemical indicator of first importance. However the contents of this halogen in geological sample are poorly known, mainly due to the difficulty to measure efficaciously this element present in most cases at trace level. In this work, X-ray fluorescence spectrometry and electron microprobe are closeness associated to determined precisely chlorine in volcanic glasses sampled on mid-occanic ridges. X-ray fluorescence is used first to certify chlorine amounts in a set of volcanic glasses to calibrate the electron microprobe. X-ray fluorescence determination is made according to an original procedure. Chlorine, volatile element, is eliminated from the finely rock powder sample by pyrolise. A calibration curve is realized with press pellets of calcinated rock powder and doped with increasing amounts of a KCl standard solution. Chlorine concentration in sample is measured on a press pellet prepare with un-calcinated rock powder and read on this calibration curve. Chlorine concentrations found in X-ray fluorescence are linearly correlated with microprobe intensities measured on a thin section of small separate glass ships from the same rock samples. Because chlorine volatitility, it is necessary to define judiciously electron microprobe measurement conditions. Optimization of analytical parameters and repetition of measurement cycles allow to obtain results with a detection limit lowered to 26 ppm and a precision (standard deviation) better than 30 ppm in 0-700 ppm concentration range

Séparation des métaux nobles (Pt, Au) d'une matrice géologique par chromatographie d'échange d'ions et détermination par spectrométrie de fluorescence X

Une procédure simple et relativement rapide pour déterminer l'or et le platine dans des matériaux géologiques divers est décrite. L'échantillon est attaqué par l'eau régale ; l'or et le platine sont solubilisés et collectés sur un filtre comportant une résine échangeuse d'ions fortement basique selon la technique du bain en milieu chlorhydrique faiblement acide. Les mesures sont effectuées directement sur le filtre par spectrométrie de fluorescence X à dispersion de longueur d'ondes (WD-XRF). La répétition des cycles d'analyse améliore la précision et la limite de détermination. Ainsi, nous démontrons que 35 ηg de métal retenu sur le filtre est la plus petite quantité d'or ou de platine mesurable. La procédure s'élargit au bismuth et au thallium qui accompagnent quantitativement les métaux nobles au cours des étapes de séparation et de fixation. La procédure est testée par la détermination de ces éléments dans quelques standards géochimiques et les concentrations trouvées sont comparées avec les valeurs publiées. Le taux de récupération varie de 80 à 100 % selon les cas.A relatively rapid and simple procedure to determine gold and platinum in various geological samples is described. The sample first is digested by aqua regia ; dissolved gold and platinum are adsorbed on a strongly basic anion-exchange membrane filter according to the batching mode fiom dilute HCl media. Analysis were performed directly on membrane filter by wavelength dispersive X-ray fluorescence spectrometry (WD-XRF). Repeated cycle countings strategy is employed to improve precision and the limit of determination. We demonstrate than aproximatively 35 ηg of metal adsorbed on membrane filter is the smallest quantity of gold or platinum mesurable. The procedure extend to bismuth and thallium which quantitatively accompanie noble metals during the separation and collection scheme. The procedure is evaluated by measuring the concentration of these elements in some geochemical reference standards and compared with published values. Recovev is in all cases comprise between 80 to 100%

Apport des techniques par rayons X à la caractérisation des encroûtements cobaltifères océaniques

Les techniques d'analyse par rayons X sont des outils particulièrement bien adaptés à la caractérisation minéralogique et géochimique des encroûtements d'oxydes de fer et de manganèse enrichis en cobalt. La diffraction X sert à identifier la faible cristallinité du matériau. La caractérisation géochimique des encroûtements est faite globalement par fluorescence X avec la détermination précise des teneurs moyennes d'une trentaine d'éléments majeurs et d'éléments traces. On utilise la microsonde électronique pour cartographier quantitativement la répartition des éléments dans les phases oxydes à l'intérieur des encroûtements et pour connaître la composition chimique des micro-sphérules d'origine cosmique qu'ils renferment.A range of X-ray analytical techniques contributes particularly well to the mineralogical and geochemical characterisations of cobalt-rich iron and manganese oxides crusts. X-ray diffraction is used to identify the weak crystallinity of the material. Bulk geochemical characterisation of the crusts is done by X-ray fluorescence for the precise determination of ca. thirty major and trace elements. Fine scale analysis is carried out by electron microprobe to quantitatively map the distribution of elements in oxide phases within the crusts. Microprobe work is essential to obtain the chemical composition of the embedded metallic micro-meteorites

Geochemistry of the Hollister Ridge: relation with the Louisville hotspot and the Pacific–Antarctic Ridge

International audienceThe Hollister Ridge is located on the western flank of the Pacific–Antarctic Ridge (PAR), between the Udintsev fracture zone (FZ) and the Eltanin fault system. It is a linear aseismic structure, 450 km long, oblique with respect to the PAR. Data show that the most recent activity is located in the central part of the chain, which can be considered as being still volcanically active. Both major/trace element and isotopic data suggest that some interaction occurred between the Pacific–Antarctic Ridge and the Hollister Ridge. The source of the Hollister Ridge samples has its own geochemical characteristics. The geochemical variations observed along the ridge can be explained by mixing between two major end-member components: (1) a PAR depleted source, and (2) a Hollister enriched source. A small contribution (20% maximum) of Louisville plume material is likely to exist in the middle of Hollister Ridge. These data unequivocally reject the possibility that the Hollister Ridge could be the present location of the Louisville hotspot. Ages and geochemistry data support the idea of an influence of intraplate deformation as a probable cause of the origin of the Hollister Ridge

Ferromanganese nodule fauna in the Tropical North Pacific Ocean: species richness, faunal cover and spatial distribution

The poorly known ferromanganese nodule fauna is a widespread hard substratum community in the deep sea that will be considerably impacted by large-scale nodule mining operations. The objective of this study was to analyze the spatial distribution of the fauna attached to nodules in the Clarion-Clipperton Fracture Zone at two scales; a regional scale that includes the east (14°N, 130°W) and the west (9°N, 150°W) zones and a local scale in which different geological facies (A, B, C and west) are recognizable. The fauna associated with 235 nodules was quantitatively described: 104 nodules from the east zone (15 of facies A, 50 of facies B and 39 of facies C) and 131 nodules from the west zone. Percent cover was used to quantify the extent of colonization at the time of sampling, for 42 species out of the 62 live species observed. Fauna covered up to 18% of exposed nodule surface with an average of about 3%. While species richness increased with exposed nodule surface, both at the regional and at the facies scales (except for facies A), total species density decreased (again except for facies A). When all nodules were included in the statistical analysis, there was no relation between faunal cover and exposed nodule surface. Nevertheless, faunal cover did decrease with exposed nodule surface for the east zone in general and for both facies B and C in particular. Species distributions among facies were significantly different but explained only a very small portion of the variance (5%). We identified two groups of associated species: a first group of two species and a second group of six species. The other species (34) were independently distributed, suggesting that species interactions play only a minor role in the spatial distribution of nodule fauna. The flux of particulate organic carbon to the bottom is the only major environmental factor considered to vary between the two zones within this study. We conclude that the higher species richness and higher percent faunal cover of the east zone can be partially attributed to greater food availability derived from surface inputs. Moreover, the surfaces of facies B and C nodules had a complex, knobby micro-relief, creating microhabitat heterogeneity that may also have contributed to the greater species richness observed in the east zone

The control of weathering processes on riverine and seawater hafnium isotope ratios

Hafnium 176Hf/177Hf isotope ratio variations in marine records are thought to reflect changes in continental weathering through time, but the behavior of Hf in rivers, and during weathering, is not well understood. Here, we present 176Hf/177Hf data for rivers, bedrock, soils, and leaching experiments for the Moselle basin, Vosges, France. These data strongly suggest that the 176Hf/177Hf composition of river waters is controlled by preferential dissolution of accessory phases (i.e., apatite, sphene) versus more resistant minerals (e.g., K-feldspar) and linked to the intensity of silicate weathering. Estimates for the global isotopic composition of riverine Hf suggest that the ocean Hf budget may be dominated by river input, and variations seen in marine records can be directly related to changes in silicate weathering intensity. © 2006 Geological Society of America