International audienceA numerical model of chemical weathering in soil horizons and underlying bedrock (WITCH) has been coupled to a numerical model of water and carbon cycles in forest ecosystems (ASPECTS) to simulate the concentration of major species within the soil horizons and the stream of the Strengbach granitic watershed, located in the Vosges Mountains (France). For the first time, simulations of solute con- centrations in soil layers and in the catchment river have been performed on a seasonal basis. The model is able to reproduce the con- centrations of most major species within the soil horizons, as well as catching the first-order seasonal fluctuations of aqueous calcium, magnesium and silica concentrations. However, the WITCH model underestimates concentrations of Mg2+ and silica at the spring of the catchment stream, and significantly underestimates Ca2+ concentration. The deficit in calculated calcium can be compensated for by dis- solution of trace apatite disseminated in the bedrock. However, the resulting increased Ca2+ release yields important smectite precipi- tation in the deepest model layer (in contact with the bedrock) and subsequent removal of large amount of silica and magnesium from solution. In contrast, the model accurately accounts for the concentrations of major species (Ca, Mg and silica) measured in the catchment stream when precipitation of clay minerals is not allowed. The model underestimation of Mg2+ and H4SiO4 concentrations when precipitation of well crystallized smectites is allowed strongly suggests that precipitation of well crystallized clay minerals is over- estimated and that more soluble poorly crystallized and amorphous materials may be forming. In agreement with observations on other watersheds draining granitic rocks, this study indicates that highly soluble trace calcic phases control the aqueous calcium budget in the Strengbach watershed
The French critical zone initiative, called OZCAR (Observatoires de la Zone Critique-Application et Recherche or Critical Zone Observatories-Application and Research) is a National Research Infrastructure (RI). OZCAR-RI is a network of instrumented sites, bringing together 21 pre-existing research observatories monitoring different compartments of the zone situated between "the rock and the sky," the Earth's skin or critical zone (CZ), over the long term. These observatories are regionally based and have specific initial scientific questions, monitoring strategies, databases, and modeling activities. The diversity of OZCAR-RI observatories and sites is well representative of the heterogeneity of the CZ and of the scientific communities studying it. Despite this diversity, all OZCAR-RI sites share a main overarching mandate, which is to monitor, understand, and predict ("earthcast") the fluxes of water and matter of the Earth's near surface and how they will change in response to the "new climatic regime." The vision for OZCAR strategic development aims at designing an open infrastructure, building a national CZ community able to share a systemic representation of the CZ , and educating a new generation of scientists more apt to tackle the wicked problem of the Anthropocene. OZCAR articulates around: (i) a set of common scientific questions and cross-cutting scientific activities using the wealth of OZCAR-RI observatories, (ii) an ambitious instrumental development program, and (iii) a better interaction between data and models to integrate the different time and spatial scales. Internationally, OZCAR-RI aims at strengthening the CZ community by providing a model of organization for pre-existing observatories and by offering CZ instrumented sites. OZCAR is one of two French mirrors of the European Strategy Forum on Research Infrastructure (eLTER-ESFRI) project.
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