Premiers résultats d'une étude géophysique sur le flanc nord des glissements de Storegga (Norvège)

The Storegga slides, off Norway, are among the largest submarine slides ever known on a continental slope. The HYDRATECH cruise on N/O Le Suroît aimed at a high-resolution survey of an area at the northern boundary of the slides. This survey images in great detail the bottom simulating reflector (BSR) extent and properties, the various fluid escape structures and the sediment deformations. The combination and the quality of the data help to understand the still poorly constrained relationships between fluid escapes, gas hydrates and slope stability in the survey area. To cite this article: H. Nouzé et al., C. R. Geoscience 336 (2004).     

Altération latéritique du gîte aurifère d'Ovala (Gabon), et formation d'une anomalie superficielle de dispersion

Résumé Les sols de la région d'Ovala (Sud Gabon) présentent une anomalie superficielle en or, à l'aplomb d'une minéralisation de la roche-mère, latéritisée sur 50 m d'épaisseur. De bas en haut, le profil d'altération comprend trois horizons principaux: saprolitique, nodulaire et sablo-argileux. Cette altération est développée in-situ à partir de gneiss archéens et de roches métamorphiques protérozoïques du faciès schiste vert. Dans la roche fraîche, l'or est associé avec du quartz, de la pyrite, de la tourmaline et de la muscovite. Lorsque l'altération progresse, les particules aurifères sont libérées et chimiquement arrondies. Depuis la minéralisation primaire jusqu'à la surface, les mécanismes de dissolution amenuisent de plus les particules d'or, et les appauvrissent en argent. Une dispersion mécanique latérale, vers l'aval des versants, se superpose á l'évolution verticale. Ce mécanisme fournit les particules de plus petite taille, dans l'horizon superficiel. Des mécanismes á la fois chimiques et physiques sont á l'origine de la formation du halo supergène.

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In the southern part of Gabon, at Ovala, a surficial gold anomaly has been found in soils. This anomaly develops directly from an underlying fresh mineralized rock through a 50-m-thick lateritic profile. From the bottom to the top, this profile consists of saprolite, a nodular layer and a sandy-clayey layer. The weathering profile formed in situ from Archean gneisses and Proterozoic greenschist facies. In the fresh rock, gold is associated with quartz, pyrite, tourmaline and muscovite. With increasing weathering, the residual gold particles become free and chemically rounded. From the primary gold mineralization to the surface, dissolution processes make gold more and more fine and Ag-depleted. Lateral downslope mechanical dispersion superimposes on this vertical evolution. This process spreads out the finest gold particles through the sandy-clayey layer. Both chemical and physical processes are responsible for the formation of the supergene dispersion halo.

     

Coloring and preservation state of faunal remains from the neanderthal levels of Kůlna Cave,Czech republic

The preservation process of faunal remains from the Middle Paleolithic levels of Kůlna Cave was studied within a taphonomic and palaeodietary framework. A visual examination of bones and teeth showed that they display a wide variety of colors. Chemical analyses (C, N, Fe, Mn) and a combination of analytical techniques, such as Fourier transform infrared (FT‐IR), X‐ray diffraction (XRD), and electron spin resonance (ESR) spectrometry, were conducted on 13 fossil remains. The crystallinity indexes, IR‐SF, XRD‐CI, XRD‐L200, the IR area band (Amide I), and the ESR analyses showed that the bones were not burned at a temperature higher than 200°C. This implies that the bone remains were not used as fuel by Neanderthals at Kůlna Cave. Coloring in these bones is largely due to the presence of Mn and Fe. © 2006 Wiley Periodicals, Inc.     

Preliminary report on a dinosaur tracksite from Lower Cretaceous strata in Mount Lebanon

A dinosaur tracksite was discovered in Batha on the side of the road from Harissa to Ghosta (Keserwan, Lebanon). About ten trackways are exposed at the top of two beds of Lower Cretaceous limestones over an area of approximately 1000 m². These footprints were probably made by at least two dinosaur species, Sauropoda and either Theropoda or Ornithopoda. The site, which is the first record from Lebanon, should be protected to allow further scientific investigations.     

Acoustic monitoring of gas emissions from the seafloor. Part I: quantifying the volumetric flow of bubbles

Three decades of continuous ocean exploration have led us to identify subsurface fluid related processes as a key phenomenon in marine earth science research. The number of seep areas located on the seafloor has been constantly increasing with the use of multi-scale imagery techniques. Due to recent advances in transducer technology and computer processing, multibeam echosounders are now commonly used to detect submarine gas seeps escaping from the seafloor into the water column. A growing number of en-route surveys shows that sites of gas emissions escaping from the seafloor are much more numerous than previously thought. Estimating the temporal variability of the gas flow rate and volumes escaping from the seafloor has thus become a challenge of relevant interest which could be addressed by sea-floor continuous acoustic monitoring. Here, we investigate the feasibility of estimating the volumetric flow rates of gas emissions from horizontal backscattered acoustic signals. Different models based on the acoustic backscattering theory of bubbles are presented. The forward volume backscattering strength and the inversion volumetric flow rate solutions were validated with acoustic measurements from artificial gas flow rates generated in controlled sea-water tank experiments. A sensitivity analysis was carried out to investigate the behavior of the 120-kHz forward solution with respect to model input parameters (horizontal distance between transducer and bubble stream, bubble size distribution and ascent rate). The most sensitive parameter was found to be the distance of the bubble stream which can affect the volume backscattering strength by 20 dB within the horizontal range of 0–200 m. Results were used to derive the detection probability of a bubble stream for a given volume backscattering strength threshold according to different bubble flow rates and horizontal distance.     

Evaluation of the WAMME model surface fluxes using results from the AMMA land-surface model intercomparison project

The West African monsoon (WAM) circulation and intensity have been shown to be influenced by the land surface in numerous numerical studies using regional scale and global scale atmospheric climate models (RCMs and GCMs, respectively) over the last several decades. The atmosphere–land surface interactions are modulated by the magnitude of the north–south gradient of the low level moist static energy, which is highly correlated with the steep latitudinal gradients of the vegetation characteristics and coverage, land use, and soil properties over this zone. The African Multidisciplinary Monsoon Analysis (AMMA) has organised comprehensive activities in data collection and modelling to further investigate the significance land–atmosphere feedbacks. Surface energy fluxes simulated by an ensemble of land surface models from AMMA Land-surface Model Intercomparison Project (ALMIP) have been used as a proxy for the best estimate of the “real world” values in order to evaluate GCM and RCM simulations under the auspices of the West African Monsoon Modelling Experiment (WAMME) project, since such large-scale observations do not exist. The ALMIP models have been forced in off-line mode using forcing based on a mixture of satellite, observational, and numerical weather prediction data. The ALMIP models were found to agree well over the region where land–atmosphere coupling is deemed to be most important (notably the Sahel), with a high signal to noise ratio (generally from 0.7 to 0.9) in the ensemble and a inter-model coefficient of variation between 5 and 15%. Most of the WAMME models simulated spatially averaged net radiation values over West Africa which were consistent with the ALMIP estimates, however, the partitioning of this energy between sensible and latent heat fluxes was significantly different: WAMME models tended to simulate larger (by nearly a factor of two) monthly latent heat fluxes than ALMIP. This results due to a positive precipitation bias in the WAMME models and a northward displacement of the monsoon in most of the GCMs and RCMs. Another key feature not found in the WAMME models is peak seasonal latent heat fluxes during the monsoon retreat (approximately a month after the peak precipitation rates) from soil water stores. This is likely related to the WAMME northward bias of the latent heat flux gradient during the WAM onset.     

Regional geochemical reconnaissance in the Belgian Ardennes: Secondary dispersion patterns in stream sediments

Two geochemical stream sediment surveys were conducted in the Belgian Ardennes for Fe, Cu, Pb, Zn, Co, Ni and Mn. The geological setting is of quartzo-pelitic rocks of Early Devonian and Cambro-Ordovician age. One survey covers the Ourthe and Salm valleys, the other the Lesse and Lomme drainage basins. Results of these surveys show the geochemical similarity of the two regions. Anomalies have been ascribed to four factors: human activity, Fe-Mn enrichment, known mineralization and potential mineralization. Mean values of metal concentration and statistical dispersion are identical. This results from similar physico-chemical conditions which induce the same types of surficial metallic association. The stream environment is characterized by two main phases, an oxidizing phase where Fe-Mn oxides trap trace metals, and a reducing phase where organic matter and secondary sulphides are bound with Pb, Zn and Cu. In oxidizing conditions, Cu and Ni show a greater affinity for the Fe-oxides while Zn and especially Co are related to the Mn-oxides. Pb has only a weak relationship with the Fe-Mn oxides but is also associated with the organic matter and the secondary sulphides in the reducing environment. Consequently, one may propose the following scale of mobility: Zn > Ni-Mn > Cu-Co > Pb > Fe.     

Mineralogical evidence for a local volcanic origin of the parent material of Bermuda Quaternary paleosols

The alternation of carbonate deposits and paleosols compose the emerged part of the Bermuda archipelago. The pedological units present a complex and diversified mineralogy. Former studies demonstrated that the paleosols are not primarily a product of the unique dissolution of the surrounding carbonates, but contain a massive input of allochthonous non-carbonate detrital material. Researchers during more than the past three decades have attributed this flux of insoluble residues (IR) to Saharan dusts. We carried out systematic field and mineralogical analyses on the Quaternary paleosols from the Bermuda archipelago. Their mineralogical assemblage predominantly includes carbonates, clay minerals (kaolinite, chlorite and chlorite/vermiculite), phosphates, and aluminium and iron oxides/hydroxides. This assemblage is strikingly close to the mineralogy of the weathered volcanic substrate of Bermuda, but noticeably different from the mineralogy of Saharan dust. Moreover, we found volcanic lithoclasts in numerous paleosol profiles all over the archipelago and in all the recorded time intervals. We thus consider the volcanic seamount underlying Bermuda as the main source of non-carbonate minerals detected in the paleosols. This hypothesis further resolves the anomalous maturity of Bermudan paleosols compared to their southern counterparts in the Bahamas and Barbados.