Ca and Sr isotope constraints on chemical weathering processes: A view through the Ebro river basin, Spain

The Ebro River (928 km long, 85,530 km2 drainage basin) in north-eastern Spain 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 to the Mediterranean and ends in the Ebro delta, one of the most important wetlands in Europe. Bedrock of the Ebro River basin mainly consists of carbonate rocks and evaporites of Paleozoic and Mesozoic age. The Ebro river mainstream was sampled monthly for a year (2006) near the outlet at Amposta, and a field campaign in April of the same year sampled the Ebro along its main course as well as its main tributaries. The behaviour of Ca and its isotopes-combined with that of Sr-isotopes-during water/rock interactions was investigated at the basin scale, with the objective of elucidating the processes controlling the isotope signatures of a large river draining mostly sedimentary bedrock. The delta 44Casw values (normalized to seawater) ranged from -0.87 to -1.09%o along the Ebro mainstream, decreasing toward the delta as the Ca content increased. At Amposta, the delta 44Casw ratio ranged between -0.66 and - 1.04%o, with a tendency to decrease with increasing discharge. These variations were similar to those given by the 87Sr/86Sr values and Sr contents. For the main tributaries, the delta 44Casw ratio in river waters ranged between -0.43 and - 1.04%o, whereas anhydrite-gypsum bedrock values ranged from -0.94 to -1.22%o and carbonate bedrock from -1.04 to -1.39%o. Comparing Sr-isotope values and Ca/Na ratios showed the role of anhydrite/halide bedrock weathering for some tributaries (Guadalope, Matarrana, Aragon, Ega), that of carbonate/halide bedrock weathering dominating the others (Gallego, Cinca, Segre), the Ebro mainstream being a mix of both. Bedrock weathering masks the sea-salt signal, if any. As halides contain no Ca, a comparison of delta 44Casw and 87Sr/86Sr values further demonstrated the role of anhydrite and limestone for the Ebro and its tributaries, highlighting geochemical processes like carbonate supersaturation in the Guadalope and Matarrana tributaries.

Automated summary

The Ebro River, one of the largest contributors of freshwater to the Mediterranean, flows through carbonate rocks and evaporites in its basin. Variations in Ca and Sr isotopes reflect the sources and geochemical processes of the river and its tributaries.