Nankai Trough, Japan Trench and Kuril Trench: geochemistry of fluids sampled by submersible <i>Nautile</i>

Deep-water samples collected during the Kaiko project are often associated with biological communities located on geological structures favorable to fluid venting. The evidence of fluid venting are the temperature anomalies, the decrease in sulfate concentrations, the content in methane and the lowC1(C2 +C3) ratio of light hydrocarbons. Because of large dilution by ambiant seawater during sampling it is difficult to compute the composition of the advected end-member pore fluid. Part of this fluid should originate in the “petroleum window”, i.e. at temperature about 60°C. Modeling the upward flow of water, taking into account the anomalies of temperature measured on the seafloor and the geochemical anomalies, leads to non-steady-state advection of the pore fluid. The occurrence of a deep component in the fluid has implications for the geological and tectonic models of the subduction zones off Japan

Signature isotopique et chimique des précipitations (pluies et pluviolessivats) en Guyane française

Au cours de 2 crues survenues le 24 mai 1992 et le 15 mai 1993 sur 2 bassins versants, nous avons étudié la composition isotopique et chimique des précipitations (pluies et pluviolessivats) ainsi que leurs variations temporelle et spatiale. Les bassins étudiés (d'environ 1,5 ha) sont situés près de la ville de Sinnamary (Guyane Française) et sont proches l'un de l'autre (200 m). Un des bassins (bassin B) est recouvert par une forêt primaire, tandis que le second (bassin A) a été défriché et transformé en prairie (Digitaria swazilendensis, programme ÉCÉREX Orstom-CTFT). Le dispositif expérimental est composé de 31 pluviomètres sur le bassin B et de 3 pluviomètres sur le bassin A. Les hauteurs d'eau précipitées lors des événements étudiés sont importantes (environ 60 mm sur le bassin A). La hauteur d'eau précipitée est homogène spatialement sur le bassin A, alors qu'elle est très hétérogène sur le bassin B. La teneur instantanée des précipitations en18O est très variable temporellement, mais reste homogène spatialement, sur les 2 bassins. L'interception de la pluie par la canopée déstructure donc la hauteur d'eau précipitée sous forêt, mais pas sa signature isotopique. Le 24 mai 1992, nous avons observé une dilution de la composition chimique de la pluie et une diminution de son pH au cours du temps. Les pluviolessivats sont généralement plus concentrés que la pluie et leur pH est plus tamponné. Nous n'avons pas observé de corrélation entre la composition chimique de la pluie ou des pluviolessivats et l'intensité des précipitations. La variabilité spatiale de la composition chimique des pluviolessivats, étudiée lors de l'averse principale du 24 mai 1992, est très importante et 31 pluviomètres semblent insuffisants pour estimer précisément les apports au sol. L'effet de masse est respecté le 24 mai 1992, mais n'est pas visible le 15 mai 1993. La comparaison de l'évolution des teneurs intégrées en Cl- et en18O montre que l'événement pluvieux du 24 mai 1992 est issu d'une masse d'air unique, alors que celui du 15 mai 1993 est issu de plusieurs masses d'air différentes. On remarque également que la teneur intégrée en18O des pluviolessivats est légèrement supérieure à celle de la pluie en milieu ouvert. En l'absence d'évaporation (la composition isotopique des pluviolessivats est alignée sur la droite locale des eaux météoriques), cela s'explique par un mélange entre la pluie directe et de l'eau de pluie plus ancienne, retenue sur la canopée et de composition isotopique différente.Geochemical hydrograph separation methods are frequently employed because they allow one to determine the origin (spatial or temporal) of water that contributes to creating floods. This approach, based on mass balance equations, requires a good knowledge of the geochemical (isotopic and chemical) signals of the reservoirs that contribute to the flood. However, geochemical signals in precipitation, an obvious reservoir, may vary strongly over time. In forested watersheds, throughfall - and not direct rain - make up the input signal. The geochemical signal of throughfall may be different from that of rain and it may vary temporally and spatially. In order to clarify the use of geochemical tracers for hydrograph separation, we studied the isotopic (δ18O, δ2H) and chemical composition of precipitation (rain and throughfall) in two watersheds, as well as the spatial and temporal variations of this precipitation during two runoff events that occurred on May 24, 1992 and on May 15, 1993. The studied watersheds are located near the city of Sinnamary (French Guyana), 120 km south-west of Cayenne. They are small in size (1,5 ha) and close to each another (200 m). One basin (hereafter named B basin) is covered by primary forest, whereas the other (hereafter named A basin) was cleared and turned into grassland (Digitaria swazilendensis, ÉCÉREX program, supported by Orstom-CTFT). The climate is tropical-humid, characterised by high mean annual temperatures (26°C), which slightly varied from month to month, and high mean annual precipitation (3500 to 3900 mm.yr-1). Precipitation occurred primarily during the main wet season, centred around May and June, and during the secondary wet season from December to January. Given the small distance between the watersheds, the differences noted between the rain collected in the A basin and the throughfall collected in the B basin (amount, geochemical signal) were attributed to the forest cover (leaching, interception,...). The monitoring equipment consisted of 31 rain gauges in the B basin and 3 rain gauges in the A basin. Rainfall was important for the two studied rain events (about 60 mm in basin A). Average rainfall in the A basin is characterised by low coefficients of variation, whereas average water inputs in the B basin showed high coefficients of variation. Thus, the amount of incoming water was spatially homogeneous in basin A, but heterogeneous in basin B. In both basins, the instantaneous δ18O value for precipitation varied considerably over time, but it was still spatially homogeneous in both watersheds with the average δ18O value showing a low coefficient of variation. This result means that the interception of the rain by the canopy destroyed the structure of the precipitation amounts under the forest, but not the structure of its isotopic signal. On May 24, 1992, we noted a dilution of the chemical content of the rain and a decrease in its pH over the course of the event. The chemical contents of the throughfall were on the whole more concentrated than in the incident rain and the pH more buffered. We did not note any correlation between the chemical content of rain or throughfall and the intensity of precipitation. The chemical composition of throughfall, studied during the main shower on May 24, 1992, exhibited considerable spatial variation and 31 rain gauges did not seem to be enough to precisely estimate the amount reaching the soil. A continuous depletion in heavy isotopes (18O,2H) and some chemical species (e.g., Cl-) was noted for the first episode (May 24, 1992) but not for the second (May, 15, 1993). This depletion may be explained by water vapour condensation outside the Rayleigh distillation, or by mixing of different air masses. The comparison between the evolution of integrated values of δ18O and the integrated Cl- content versus the amount of accumulated precipitation proved that the rain event of May 24, 1992, was generated by a single air mass whereas the event of May 15, 1993 was generated by several air masses. We also noted that the integrated value of δ18O for throughfall was slightly more concentrated than the content of rain. In the absence of evaporation (the isotopic composition of the throughfall corresponded to the local meteoric line), this enrichment suggests that direct rain mixed with older water that was stored in the canopy and had a different isotopic composition.This study showed that the intensity and the geochemical signal of precipitation (rain and throughfall) vary greatly on a temporal scale in a tropical environment. It also showed that the amount of incoming water varied spatially under a forest cover, as did its geochemical (isotopic and chemical) signal. In order to achieve a stream hydrograph geochemical separation, it is necessary to collect the precipitation (rain and throughfall) with a short time step. It is also necessary to collect the throughfall across a concentrated network of rain gauges

Origin of fluids and related barite deposits at vent sites along the Peru convergent margin

The venting at the northern Peru convergent margin, unlike at other margins, has produced large barite deposits, which have not been observed outside the vents. The 87Sr/86Sr isotopic ratios of the fluids are more radiogenic than seawater. To explain these elevated values, we propose either the influence of a fluid characterized by a more radiogenic signature originating from the continent, or a reaction between seawater and the underlying continental metamorphic basement. The presence of this nonlocal radiogenic component is marked more strongly on the 87Sr/86Sr ratios measured in the barite deposits. We assume that the fluid sampled at the venting site and the fluid responsible for the barite deposit sampled at the same site originated from the same source, i.e., the Paleozoic metamorphic basement of the Andean continental margin

Synthetic Biology of Proteins: Tuning GFPs Folding and Stability with Fluoroproline

Proline residues affect protein folding and stability via cis/trans isomerization of peptide bonds and by the C(gamma)-exo or -endo puckering of their pyrrolidine rings. Peptide bond conformation as well as puckering propensity can be manipulated by proper choice of ring substituents, e.g. C(gamma)-fluorination. Synthetic chemistry has routinely exploited ring-substituted proline analogs in order to change, modulate or control folding and stability of peptides.In order to transmit this synthetic strategy to complex proteins, the ten proline residues of enhanced green fluorescent protein (EGFP) were globally replaced by (4R)- and (4S)-fluoroprolines (FPro). By this approach, we expected to affect the cis/trans peptidyl-proline bond isomerization and pyrrolidine ring puckering, which are responsible for the slow folding of this protein. Expression of both protein variants occurred at levels comparable to the parent protein, but the (4R)-FPro-EGFP resulted in irreversibly unfolded inclusion bodies, whereas the (4S)-FPro-EGFP led to a soluble fluorescent protein. Upon thermal denaturation, refolding of this variant occurs at significantly higher rates than the parent EGFP. Comparative inspection of the X-ray structures of EGFP and (4S)-FPro-EGFP allowed to correlate the significantly improved refolding with the C(gamma)-endo puckering of the pyrrolidine rings, which is favored by 4S-fluorination, and to lesser extents with the cis/trans isomerization of the prolines.We discovered that the folding rates and stability of GFP are affected to a lesser extent by cis/trans isomerization of the proline bonds than by the puckering of pyrrolidine rings. In the C(gamma)-endo conformation the fluorine atoms are positioned in the structural context of the GFP such that a network of favorable local interactions is established. From these results the combined use of synthetic amino acids along with detailed structural knowledge and existing protein engineering methods can be envisioned as a promising strategy for the design of complex tailor-made proteins and even cellular structures of superior properties compared to the native forms

Grands bassins fluviaux périatlantiques : Congo, Niger, Amazone

L'or a été analysé en phase dissoute et en suspension lors d'une crue de l'Amazone (juin 1989). Les contributions dues aux principaux affluents brésiliens de l'Amazone ont aussi été étudiés. Les concentrations sont élevées et dépassent 100 pM au niveau des zones inondable (Varzea). Les flux correspondants sont compris entre 2 et 6 g Au/s. Le rôle des espèces organiques transitoires, produits lors de l'inondation du Varzea, est discuté en relation avec les problèmes de transport de l'or. Les résultats acquis et ceux déjà obtenus sur le Congo permettent de préciser le cycle géochimique de l'or. En particulier un nouveau temps de résidence de l'or dans les océans est calculé : 140 ans. (Résumé d'auteur

Nouvelles méthodes de détermination des principales espèces ioniques du soufre dans les eaux naturelles

Nous avons mis au point une méthode potentiométrique de dosage des espèces du soufre dans les eaux sulfurées basées sur l'emploi du chlorure mercurique comme réactif et d'une électrode spécifique des sulfures comme électrode indicatrice. On détermine ainsi les sulfures, les polysulfures, les organo-sulfurés, les thiosulfates, les sulfites. Le soufre élémentaire est déterminé à part après extraction. Cette méthode est facilement employable sur le terrain et donne de bons résultats avec les eaux sulfurées chaudes. Sa précision est supérieure d'un ordre ou deux ordres de grandeur à la méthode iodométrique

Contributions à l'histoire du Sénégal

Cet article analyse tout d'abord l'organisation sociale, économique et politique de l'Etat soninké du Gajaaga au début du 19eme siècle : agriculture, artisanat et commerce, aristocratie guerrière et familles islamisées, hiérarchie des groupes statutaires, rapports de dépendance et contre-pouvoirs. Il envisage ensuite l'impact de l'expansion commerciale française sur cette société, qui l'a profondément perturbée avant même la conquête coloniale de 1858 : mise en place d'un nouveau système commercial et "traite légitime", exaspération des conflits au Gajaaga et dans tout le haut Sénégal, lutte contre El Haaj Umar et les Soninké qui se sont, partiellement, ralliés au jihad. (Résumé d'auteur