The impact of vegetation on fractionation of rare earth elements(REE) during water–rock interaction

Previous studies on waters of a streamlet in the Vosges mountains (eastern France) have shown that Sr and rare earth elements (REE) principally originate from apatite dissolution during weathering. However, stream water REE patterns normalized to apatite are still depleted in light REE (LREE, La–Sm) pointing to the presence of an additional LREE depleting process. Speciation calculations indicate that complexation cannot explain this additional LREE depletion. In contrast, vegetation samples are strongly enriched in LREE compared to water and their Sr and Nd isotopic compositions are comparable with those of apatite and waters. Thus, the preferential LREE uptake by the plants at the root–water–soil (apatite) interface might lead to an additional LREE depletion of the waters in the forested catchment. Mass balance calculations indicate that the yearly LREE uptake by vegetation is comparable with the LREE export by the streamlet and, therefore, might be an important factor controlling the LREE depletion in river waters

Is subsurface geophysics as seismic and acoustic investigations a rescue to groundwater flow inversion?

Understanding subsurface flow, especially in partly karstified rock formations mainly housing water through a few preferential pathways, is still challenging. This point is the consequence of the poor accessibility of the subsurface and lack of accurate depictions of water bearing bodies and distributions. This notwithstanding, highly-resolved geophysical investigations bring new images of the subsurface.A 3-D seismic survey with shots and wave monitoring at the surface is carried out over a subsurface karstified reservoir located at the Hydrogeological Experimental Site (HES) of the University of Poitiers (France). Processing the 3-D data, in association with wave velocity calibration from vertical seismic profiles (VSP) recorded via geophones in wells, renders a 3-D velocity block. The velocity block is then converted into pseudo-porosity values revealing three high-porosity, presumably water-productive, layers, at depths of 35–40, 85–87, and 110–115 m.In addition, full wave acoustic logging (FWAL) can detect, close to wells, porous or open bodies that are too small for being captured by the spatial resolution of 3-D seismic images. A FWAL can also confirm or invalidate data from VSP recorded via hydrophones.The block of pseudo-porosities is compared to a different representation of the subsurface in the form of hydraulic conductivity distributions (or hydraulic diffusion) obtained by slug tests or by inversion of transient interference testing between wells. The inverted hydraulic conductivity maps do not match up the distribution of porous bodies identified by seismic data. This poses the question of guiding conventional inversions on the basis of a prior guess as the subsurface structure obtained via geophysical investigations

Is subsurface geophysics as seismic and acoustic investigations a rescue to groundwater flow inversion?

Understanding subsurface flow, especially in partly karstified rock formations mainly housing water through a few preferential pathways, is still challenging. This point is the consequence of the poor accessibility of the subsurface and lack of accurate depictions of water bearing bodies and distributions. This notwithstanding, highly-resolved geophysical investigations bring new images of the subsurface.A 3-D seismic survey with shots and wave monitoring at the surface is carried out over a subsurface karstified reservoir located at the Hydrogeological Experimental Site (HES) of the University of Poitiers (France). Processing the 3-D data, in association with wave velocity calibration from vertical seismic profiles (VSP) recorded via geophones in wells, renders a 3-D velocity block. The velocity block is then converted into pseudo-porosity values revealing three high-porosity, presumably water-productive, layers, at depths of 35–40, 85–87, and 110–115 m.In addition, full wave acoustic logging (FWAL) can detect, close to wells, porous or open bodies that are too small for being captured by the spatial resolution of 3-D seismic images. A FWAL can also confirm or invalidate data from VSP recorded via hydrophones.The block of pseudo-porosities is compared to a different representation of the subsurface in the form of hydraulic conductivity distributions (or hydraulic diffusion) obtained by slug tests or by inversion of transient interference testing between wells. The inverted hydraulic conductivity maps do not match up the distribution of porous bodies identified by seismic data. This poses the question of guiding conventional inversions on the basis of a prior guess as the subsurface structure obtained via geophysical investigations

U–Th and ¹⁰Be constraints on sediment recycling in proglacial settings, Lago Buenos Aires, Patagonia

International audienceThe estimation of sediment transfer times remains a challenge to our understanding of sediment budgets and the relationships between erosion and climate. Uranium (U) and thorium (Th) isotope disequilibria offer a means of more robustly constraining sediment transfer times. Here, we present new uranium and tho-rium disequilibrium data for a series of nested moraines around Lago Buenos Aires in Argentine Patagonia. The glacial chronology for the area is constrained using in situ cosmogenic 10 Be analysis of glacial outwash. Sediment transfer times within the periglacial domain were estimated by comparing the deposition ages of moraines to the theoretical age of sediment production, i.e., the comminution age inferred from U disequilibrium data and recoil loss factor estimates. Our data show first that the classical comminution age approach must include weathering processes accounted for by measuring Th disequilibrium. Second, our combined data suggest that the pre-deposition history of the moraine sediments is not negligible, as evidenced by the large disequilibrium of the youngest moraines despite the equilibrium of the corresponding glacial flour. Monte Carlo simulations suggest that weathering was more intense before the deposition of the moraines and that the transfer time of the fine sediments to the moraines was on the order of 100-200 kyr. Long transfer times could result from a combination of long sediment residence times in the proglacial lake (recurrence time of a glacial cycle) and the remobilization of sediments from moraines deposited during previous glacial cycles. 10 Be data suggest that some glacial cycles are absent from the preserved moraine record (seemingly every second cycle), supporting a model of reworking moraines and/or fluctuations in the extent of glacial advances. The chronological pattern is consistent with the U-Th disequilibrium data and the 100-200 kyr transfer time. This long transfer time raises the question of the proportion of freshly eroded sediments that escape (or not) the proglacial environments during glacial periods

Contribution à l'étude du cycle géochimique du Bore

Cette étude se place dans le cadre de notre compréhension des cycles géochimiques à l interface entre le minéral et le vivant. Le bore trouve une place particulière dans ce domaine de réflexion grâce à ses propriétés physico-chimiques qui le rendent à la fois sensibles aux réactions minéralogiques et biologiques. Le développement de techniques d analyses du bore a permis de dégager les grandes lignes de son comportement (bio)-géochimique à trois échelles différentes. À l échelle sol/plante, le cycle du bore en solution est fortement dépendant de l activité biologique saisonnière. Le fort écart de signature isotopique du bore entre le compartiment végétal ( +30 ) et minéral ( -20 ) permet de quantifier la contribution respective de chacun de ses processus et met en évidence l interdépendance entre les flux d altération et le recyclage biologique. À l échelle du bassin versant, la signature géochimique du bore dans les rivières reflète principalement des effets conjoints de la lithologie et des processus d altération. À l échelle d un aquifère charbonneux de taille régionale, il a été proposé que le cycle du bore est contrôlé par des réactions de complexation avec des surfaces de type organique. La double fonctionnalité du bore comme traceur à la fois des processus lithologiques et biologiques est donc mise en avant et prouve le potentiel de cet élément comme reflet des équilibres et échanges entre compartiments naturels à différentes échelles.This work aims at improving our understanding of the geochemical cycles at the interface between the mineralogical and the biological world. Boron appears to be a good candidate in this field of investigation because of its particular physicochemical properties. Development and improvement of analytical techniques allow the study of boron geochemical signature at several scales. At the soil/plant scale, the boron cycle is strongly linked to the seasonal biologic activity. The great difference of isotopic composition between the biological ( +30 ) and the mineralogical reservoir ( -20 ) allows us to quantify the relative contribution of both processes and points out the feedback between weathering mechanisms and vegetation cycling. At the watershed scale, weathering processes mainly control the dissolved boron geochemistry in the river without noticeable impact of vegetation. In a coal bed aquifer (regional scale), we suggest that complexation reactions onto organic surfaces regulate the boron isotopic signature. The double feature of boron as a tracer of both weathering and biological processes are emphasized in this work and prove the importance of the information given by this isotopic system when applied to natural systems at different scales.STRASBOURG-Géologie (674822251) / SudocSudocFranceF