Strontium isotopic composition of the Po river dissolved load: Insights into rock weathering in Northern Italy

Abstract

Strontium isotopic composition (87Sr/86Sr) of river water provides insight on the nature of rocks involved in the weathering processes and important constraints for reconstructing silicate versus carbonate weathering in large scale basins. New data, presented in the context of a geochemical map of the Po river basin, highlight that the Po river water evolves from the upper part of the catchment, where 87Sr/86Sr approaching 0.7097 relates to the weathering of igneous and metamorphic silicate rocks, to the middle part, where less radiogenic compositions (87Sr/86Sr ∼ 0.7089) are driven by the confluence of the tributaries draining Mesozoic carbonate rocks of the South Alpine domain and from inflow of Apennine streams characterized by less radiogenic Sr signatures due to weathering of marly sediments and mafic magmatic rocks. Down flow, the Sr isotopic compositions of Po river water rises at 0.7091 either by the confluence of the last Apennine tributary (Panaro, 87Sr/86Sr ∼ 0.7095) and/or by the possible contribution from hyporheic exchanges. Po river isotopic signatures reveal slightly more radiogenic 87Sr/86Sr in comparison to other important Alpine river systems (e.g. Danube, Rhine and Rhône), likely suggesting that silicate rocks are comparatively contributing more weathering byproducts in the Po basin.