Hydrothermal and magmatic contributions to surface waters in the Aso caldera, southern Japan: Implications for weathering processes in volcanic areas

Abstract

The role of hydrothermal fluids in the dissolution of primary minerals in volcanic areas, hotspots of silicate weathering at the global scale, is not well quantified. This study aims to determine the hydrothermal contribution to the lateral export of solutes from the Aso caldera, southern Japan. A set of indicators for the hydrothermally related contribution is systematically discussed. Spring, groundwater and river water samples were collected along the two main rivers of the caldera, and their major and trace element concentrations, as well as the stable isotopic compositions of water (δ2Η and δ18O), dissolved inorganic carbon (δ13C), and sulphate (δ34S and δ18O) were analysed. Hydrothermal springs in the study area are associated with a substantial sulphate contribution of magmatic origin. In contrast, the hydrochemistry of non-hydrothermal springs reflects surface water-soil interactions. In particular, non-hydrothermal waters have significantly higher selenium to sulphate ratios than hydrothermal waters. The selenium to sulphate ratios allow to distinguish two different sulphur sources in the Aso catchment: hydrothermal waters impacted by the oxidation of sulphides or controlled by disproportionation reactions of SO2, and non-hydrothermal sulphate. Overall, the results show that hydrothermal waters strongly influence the sulphur budget, accounting for 67 to 91% of the total sulphate flux at the caldera outlet. The dissolution of magmatic CO2 and SO2 contribute to >60% of the observed weathering fluxes in this volcanic area. This suggests that magmatic gases and hydrothermal fluids should be considered for the estimation of biogeochemical budgets at the regional and global scales. However, the temporal variation of the magmatic activity and its effect on the chemistry of waters have to be investigated in future work.