Impact of Climate, Agriculture and Vegetation in the Sahel in the recent past : the CAVIARS Projet. [P-3330-53]

The semi-arid regions of the Earth are particularly vulnerable to wind erosion. The Sahelian region experienced contrasted climatic conditions during the last decades, with severe drought in the 70's and 80's and a relative re-greening in the recent years. Over the same period, changes in land use have occurred with an increase of the cultivated surfaces leading to a decrease of fallows and rangelands. As a result, a significant proportion of the land is bare or sparsely vegetated, and thus is not efficiently protected from the erosive action of wind. In this region, wind erosion tends to decrease the productive capacity of the soils whose fertility is already very low. In addition, the impact of wind erosion is expected to increase significantly in the near future (1) in relation with the expected changes in climate (in particular the modifications of precipitation and surface wind) and (2) in response to the increasing land use due to population increase and the related food needs. The aims of the CAVIARS project (Climate, Agriculture and Vegetation: Impacts on Aeolian ERosion in the Sahel) are to develop an integrated modeling tool to describe the evolution of wind erosion in the Sahel in connection with climatic and land use changes, to validate this tool in the current period by making the best possible use of the numerous data sets acquired in recent years over West Africa, and to test its ability to reproduce specific events (such as the drought in the Sahel) of the recent past (about the last 50 years). This project is based on a modeling approach of this recent past (hindcasts) that is justified by the need to ensure the robustness of the simulations with different forcings prior to any simulation of future scenarios. The proposed strategy is (1) to develop or optimize reliable modeling tools for quantifying the various terms (land use, changes in aridity...) responsible for changes in the intensity of wind erosion (2) to synthesize quality checked observations, that can be used as direct or indirect indicators of wind erosion (precipitation time series, changes in vegetation cover, atmospheric dust load,...) (3) to implement a validation strategy based on the quantification of wind erosion both locally, measured on grazed and cultivated plots, and at the regional and continental scales. (Texte intégral

Redox-driven mineral and organic associations in Jezero Crater, Mars

The Perseverance rover has explored and sampled igneous and sedimentary rocks within Jezero Crater to characterize early Martian geological processes and habitability and search for potential biosignatures1,2,3,4,5,6,7. Upon entering Neretva Vallis, on Jezero Crater’s western edge8, Perseverance investigated distinctive mudstone and conglomerate outcrops of the Bright Angel formation. Here we report a detailed geological, petrographic and geochemical survey of these rocks and show that organic-carbon-bearing mudstones in the Bright Angel formation contain submillimetre-scale nodules and millimetre-scale reaction fronts enriched in ferrous iron phosphate and sulfide minerals, likely vivianite and greigite, respectively. This organic carbon appears to have participated in post-depositional redox reactions that produced the observed iron-phosphate and iron-sulfide minerals. Geological context and petrography indicate that these reactions occurred at low temperatures. Within this context, we review the various pathways by which redox reactions that involve organic matter can produce the observed suite of iron-, sulfur- and phosphorus-bearing minerals in laboratory and natural environments on Earth. Ultimately, we conclude that analysis of the core sample collected from this unit using high-sensitivity instrumentation on Earth will enable the measurements required to determine the origin of the minerals, organics and textures it contains

Coupling d34S(SO42-) and (206Pb/207Pb) to discriminate the origin of trace metals in the urban Orge River, France. Goldschmidt Conference, Prague, République Tchèque (14-19 août 2011)

POSTE

salinity and organic amendment effects on methane production of a rain-fed saline paddy field.

International audienc

Building an isotopic hydrogeochemical indicator of anthropogenic pressure on urban rivers

International audienceThis study provides evidence on the sources and pathways of trace-metal contamination using isotopic tracers, δ34S of dissolved sulfate and 206Pb/207Pb of suspended particulate matter (SPM), in an urban French river. The trace-metal contamination of the Orge River watershed was correlated with a highly contrasted land-use pattern with mainly forests and agricultural lands upstream and a highly dense urban area (up to 8000 inhabitants/km2) downstream in the suburbs of the Parisian megacity. The increase in the sulfate concentrations (0.25-1 mmol/l) of the dissolved compartment (< 0.45 μm) monitored during a hydrological year (2010/2011) correlated with the dissolved sulfate δ34S and δ18O ratios (+ 2 to + 12 and + 6 to + 13‰, respectively). This result indicated that the runoff and sewage waters are important sources of water in the river, particularly for the highly urbanized sites. The lead enrichment factor in the SPM was observed to increase according to the urbanization gradient (1 to 6), and the associated 206Pb/207Pb ratio indicated that the signature of the lead-bearing particles transformed from a three-endmember system (2001 samples) to a two-endmember one (2010/2011 samples) over the last decade. These results suggest that gasoline-originated lead has disappeared from particulate fluxes since the lead additive prohibition in 2000. The lead-bearing species in the Orge River were also observed to originate mainly from urban activities, and the reactivity of selected urban lead-bearing species is discussed based on microscopic observations. Finally, coupling the δ34S and 206Pb/207Pb ratios provided a powerful indicator of urban influence as a function of both river hydrodynamics and physico-chemical conditions