Ca isotopes in the Ebro River Basin: mixing and lithological tracer
Résumé
A large investigation of the Ebro River catchment was done in the past years regarding hydrogen, oxygen, lithium, boron, sulphur and oxygen from SO4 and strontium isotope measurements together with major and trace elements in the dissolved load of 25 river samples collected within the Ebro River Basin in Spain (Millot et al., Geophysical Research Abstracts, Vol. 14, EGU2012-2062, 2012). The Ebro River (928 km long, 85,530 km2 drainage basin) located in North-Eastern Spain rises near the Atlantic coast in the Cantabrian Mountains and flows into the western Mediterranean Sea through several large cities and agricultural, mining and industrial areas. The river is one of the largest contributors of freshwater in the Mediterranean Sea and ends in the Ebro delta, one of the most important wetlands in Europe. Bedrocks of the Ebro River Basin are mainly dominated by carbonates and evaporites from the Paleozoic and Mesozoic terrains. The Ebro river mainstream was sampled at Amposta one time per month between June 2005 and May 2006 and secondly, the Ebro River along its main course and its main tributaries were sampled during one field campaign in April 2006. The behaviour of Ca and its isotopes during water/rock interactions at the scale of a large river basin having various lithologies will be investigated in addition with Sr, S (SO4) and O (SO4) isotopes. One objective is to characterize the processes controlling the isotope signatures of a large river draining predominantly sedimentary bedrocks. The _44Ca ratio (_44/40 normalised to Seawater) ranged between -0.87 and -1.09″° along the Ebro main stream, increasing towards the delta as the Ca content increase. In Amposta, the _44Ca ratio ranged between -0.66 and -1.04″° and tends to decrease with the increasing discharge. These variations are very similar to those given by the 87Sr/86Sr ratios and Sr contents. For the tributaries, the _44Ca ratio ranged between -0.43 and -1.04″° whereas the anhydrite-gypsum bedrock display a range from -0.94 to -1.22 and the carbonate bedrock ranged from -1.04 to -1.39″°. Comparing Sr isotope ratios and Ca/Na ratios evidenced the role of anhydrites/halides weathering for some tributaries (Guadalope, Matarrana, Aragon, Ega), the role of carbonates/halides weathering for the others (Gallego, Cinca, Segre); the Ebro being a mix of both. Weathering of rock masks the seasalt signal, if any. As there is no Ca in halides, the comparison of the _44Ca and 87Sr/86Sr ratios further evidenced the role of anhydrites and carbonates for the Ebro and tributaries, highlight geochemical processes like carbonate oversaturation (Guadalope and Matarrana tributaries) and imprints the seasalt signal.