Variations of U and Sr isotope ratios in Alsace and Luxembourg rain waters: origin and hydrogeochemical implications

Major and trace element concentrations, as well as Sr and U isotope ratios, were measured in rainwater samples collected in three different locations in Alsace (East of France) and Luxembourg: a mid-altitude mountain site (Aubure Environment HydroGeochemical Observatory), an urban site (Strasbourg) and a peri-urban site located in an area of well developed industrial activity (Esch-sur-Alzette in Luxembourg). Results highlight the quite high spatial and temporal variability of the chemical and isotopic characteristics of rainwater at the regional scale. They also suggest a quite systematic contribution of a local component in the chemical composition of rainwater. In urban and peri-urban sites, the local component is certainly linked to human activities, as it is well illustrated in this study with the Esch-sur-Alzette samples. On the other hand, for the Aubure site, i.e. a small forested watershed of mid-altitude mountain, data presented in this study demonstrate the influence of the vegetation on the chemical composition of rainwater for alkali and calc-alkali elements, as well as for the Sr isotope ratios. Such a result questions the reliability of the method classically used to estimate the rainwater contribution on the river chemical budget. In addition, data of the present study confirm the very low content of uranium in rainwater and demonstrate, especially through the U isotope analysis of Aubure rainwater, the negligible effect of rainwater on U budget of river waters. This work thus outlines the property of U to be a geochemical tracer specific of weathering fluxes carried by rivers. To cite this article: F. Chabaux et al., C. R. Geoscience 337 (2005).     

Magmatic zircon oxygen isotopes of 1.88–1.87 Ga orogenic and 1.65–1.54 Ga anorogenic magmatism in Finland

Summary Oxygen isotope ratios of igneous zircon from magmatic rocks in Finland provide insights into the evolution and growth of the Precambrian crust during the Svecofennian orogeny. These data preserve magmatic δ¹⁸O values and correlate with major discontinuities in the lower crust. Oxygen isotope ratios of zircon across the 1.88–1.87 Ga Central Finland granitoid complex (CFGC) range from 5.50‰ to 6.84‰, except for three plutons in contact with the adjacent greenstone and metasedimentary belts (δ¹⁸O(Zrc) = 7.60‰–7.78‰). There is a systematic variation in δ¹⁸O(Zrc) with respect to geographic location in the CFGC, ranging from 6.60±0.23‰ (σ) in the northeast to 5.90±0.40‰ in the west-southwest. These values correlate with a change in crustal thickness and shift in geochemical composition. The oxygen isotope composition of the 1.65–1.54 Ga rapakivi granites and related rocks in southern Finland show a decreasing trend from north to south, independent of their emplacement age. The southern anorogenic granite group has an average δ¹⁸O in zircon of 6.14±0.07‰ and the northern anorogenic group has an average δ¹⁸O in zircon of 8.14±0.59‰. This difference reflects the boundary between island arc terrains accreted during the Paleoproterozoic. Deceased     

Tetrahedral Plots of the Phase Relations for Basalts

The phase relations of quaternary systems are generally represented by projections onto ternary compositional planes. Such projections often obscure relationships that would only be evident in a three-dimensional tetrahedral plot. The tetrahedral plot requires that compositions of the minerals and melts be transformed into Cartesian coordinates. It is shown here how this transformation is carried out. The application is demonstrated by tetrahedral plots of experimental melt compositions of partially molten lherzolite. Furthermore, the plot can be used to evaluate whether or not a particular basaltic composition represents a primary melt. The methods are applicable to any four-component system.     

The source of calcium in wet atmospheric deposits: Ca-Sr isotope evidence

Tracing the origin of calcium in wet atmospheric deposits is important for environmental studies since it is a dominant acid neutralising cation and an essential nutrient. Here we present the first calcium isotope ratio measurements for rain and snow. Samples from northeastern and southern locations (France, Switzerland, Luxembourg, California and Japan) suggest a carbonate origin. Only at a local scale in a forested watershed (Vosges mountains, France) the Ca isotopic compositions of rainwater indicate that the rainwater becomes admixed with throughfall solution, which is a mixing product of Ca from dissolved atmospheric dust particles and leave excretions. If this proves to be widely true, the contribution of Ca from the canopy could be very important for the neutralisation of acid rain and of the corresponding soil system in forested watersheds. In such cases Ca and Sr cycles are decoupled in the lower atmosphere. In contrast to Sr isotopes, Ca isotopes can be applied to tracing processes occurring in the biosphere.     

Luminescence dating of ice‐marginal deposits in northern Germany: evidence for repeated glaciations during the Middle Pleistocene (MIS 12 to MIS 6)

The exact number, extent and chronology of the Middle Pleistocene Elsterian and Saalian glaciations in northern Central Europe are still controversial. This study presents new luminescence data from Middle Pleistocene ice‐marginal deposits in northern Germany, giving evidence for repeated glaciations during the Middle Pleistocene (MIS 12 to MIS 6). The study area is located in the Leine valley south of the North German Lowlands. The data set includes digital elevation models, high‐resolution shear wave seismic profiles, outcrop and borehole data integrated into a 3D subsurface model to reconstruct the bedrock relief surface. For numerical age determination, we performed luminescence dating on 12 ice‐marginal and two fluvial samples. Luminescence ages of ice‐marginal deposits point to at least two ice advances during MIS 12 and MIS 10 with ages ranging from 461±34 to 421±25 ka and from 376±27 to 337±21 ka. The bedrock relief model and different generations of striations indicate that the older ice advance came from the north and the younger one from the northeast. During rapid ice‐margin retreat, subglacial overdeepenings were filled with glaciolacustrine deposits, partly rich in re‐worked Tertiary lignite and amber. During MIS 8 and MIS 6, the study area may have been affected by two ice advances. Luminescence ages of glaciolacustrine delta deposits point to a deposition during MIS 8 or early MIS 6, and late MIS 6 (250±20 to 161±10 ka). The maximum extent of both the Elsterian (MIS 12 and MIS 10) and Saalian glaciations (MIS 8? and MIS 6) approximately reached the same position in the Leine valley and was probably controlled by the formation of deep proglacial lakes in front of the ice sheets, preventing a further southward advance.     

Energy partitioning over a semi‐arid shrubland in northern China

During the last decade, the widely distributed shrublands in northern China have shown significant signs of recovery from desertification, the result of widespread conservation practices. However, to support the current efforts in conservation, more knowledge is needed on surface energy partitioning and its biophysical controls. Using eddy‐covariance measurements made over a semi‐arid shrubland in northwest China in 2012, we examined how surface energy‐balance components vary on diurnal and seasonal scales, and how biophysical factors control bulk surface parameters and energy exchange. Sensible heat flux (H) exceeded latent heat flux (λE) during most of the year, resulting in an annual Bowen ratio (β, i.e. H/λE) of 2.0. λE exceeded H only in mid‐summer when frequent rainfall co‐occurred with the seasonal peak in leaf area index (LAI). Evapotranspiration reached a daily maximum of 3.3 mm day^?1, and summed to 283 mm yr^?1. The evaporative fraction (EF, i.e. λE/R_n), Priestley–Taylor coefficient (α), surface conductance (g_s) and decoupling coefficient (Ω) were all positively correlated with soil water content (SWC) and LAI. The direct enhancement of λE by high vapour pressure deficit (VPD) was buffered by a concurrent suppression of g_s. The g_s played a direct role in controlling EF and α by mediating the effects of LAI, SWC and VPD. Our results highlight the importance of adaptive plant responses to water scarcity in regulating ecosystem energy partitioning, and suggest an important role for revegetation in the reversal of desertification in semi‐arid areas. Copyright © 2015 John Wiley & Sons, Ltd.     

Diffusive mass exchange of non‐reactive substances in dual‐porosity porous systems – column experiments under saturated conditions

Diffusive mass exchange into immobile water regions within heterogeneous porous aquifers influences the fate of solutes. The percentage of immobile water is often unidentified in natural aquifers though. Hence, the mathematical prediction of solute transport in such heterogeneous aquifers remains challenging. The objective of this study was to find a simple analytical model approach that allows quantifying properties of mobile and immobile water regions and the portion of immobile water in a porous system. Therefore, the Single Fissure Dispersion Model (SFDM), which takes into account diffusive mass exchange between mobile and immobile water zones, was applied to model transport in well‐defined saturated dual‐porosity column experiments. Direct and indirect model validation was performed by running experiments at different flow velocities and using conservative tracer with different molecular diffusion coefficients. In another column setup, immobile water regions were randomly distributed to test the model applicability and to determine the portion of immobile water. In all setups, the tracer concentration curves showed differences in normalized maximum peak concentration, tailing and mass recovery according to their diffusion coefficients. These findings were more pronounced at lower flow rates (larger flow times) indicating the dependency of diffusive mass exchange into immobile water regions on tracers' molecular diffusion coefficients. The SFDM simulated all data with high model efficiency. Successful model validation supported the physical meaning of fitted model parameters. This study showed that the SFDM, developed for fissured aquifers, is applicable in porous media and can be used to determine porosity and volume of regions with immobile water. Copyright © 2015 John Wiley & Sons, Ltd.     

Correction to: Extreme waves induced by cyclone Nargis at Myanmar coast: numerical modeling versus satellite observations

Natural Hazards - A correction to this paper has been published: https://doi.org/10.1007/s11069-021-04687-9