Lithium isotopic fingerprints of sources and processes in surface waters of the Ebro River Basin (Spain)

Abstract

The Ebro River in north-eastern Spain is among the largest contributors of freshwater to the Mediterranean Sea and ends in the Ebro delta, one of the major wetlands in Europe. The bedrock of the Ebro River basin mainly consists of carbonate rocks and evaporites of Palaeozoic and Mesozoic age, and the river flows through several large cities, and agricultural and industrial areas. The Ebro outlet at Amposta was sampled once a month for a year (2006), and a field campaign in April of the same year sampled the Ebro along its main course as well as its principal tributaries. In the present study, the behaviour of Li and its isotopes was investigated at basin scale, with the objective of elucidating the processes controlling the lithium-isotope signatures of a large river draining mostly sedimentary bedrock. δ7Li values show a narrow range from +17.1 ‰ to +18.3 ‰ along the Ebro main stream, and between +16.3 and +18.9 ‰ at the outlet. In the major tributaries, the δ7Li values ranged from +12.9 ‰ to +20.9 ‰, with bedrock values ranging from +0.5 to +29.3 ‰. Comparing Li concentrations with Cl and SO4 ones, it appears that evaporite weathering plays an important role in controlling Li concentrations, but no anthropogenic agricultural or industrial influence on Li concentrations was detected. The Na/Li, Cl/Li and SO4/Li ratios clearly reflect the role of halite dissolution for some tributaries (Gallego, Ega and Aragon), gypsum dominating others (Guadalope, Matarrana, Huerva and Segre), and little influence of carbonate in all tributaries, the Ebro itself being a mixture of all tributaries. We tentatively applied the simple Rayleigh fractionation model, but most δ7Li values of the Ebro water samples plotted away from the fractionation line, reinforcing the major role of mixing processes in the Ebro basin, rather than fractionation processes during water/rock interactions. A comparison of δ7Li values and 87Sr/86Sr ratios further demonstrates the role of gypsum/anhydrite and limestone in the Ebro and its tributaries. Sr-isotopes show a non-negligible role of carbonate dissolution, generally considered to be weak in the control of the lithium cycle in catchments.