Assessing the contribution of hydrothermal alteration to element fluxes from volcanic island arcs using dissolved Ge/Si ratios in rivers

Volcanic island arcs display the highest silicate weathering rates on Earth, thereby playing an important role in global chemical fluxes to the ocean. However, in these regions, uncertainties remain regarding the significance of hydrothermal alteration in contributing to the chemical fluxes. This impedes accurate assessments of weathering fluxes and associated atmospheric CO2 consumption. Germanium, a trace element, behaves as a chemical analog to Si. However, it is fractionated relative to Si in surface processes. According to a few studies, the dissolved Ge/Si ratios of terrestrial hydrothermal springs are higher (2-1000 µmol/mol) than to those found in river waters worldwide (0.2-2.8 µmol/mol). This suggests that the Ge/Si ratio may be used as a tracer of hydrothermal inputs to rivers. However, additional work is needed to better constrain the range of Ge/Si values in terrestrial hydrothermal systems. Here we report the Ge/Si ratio values of hydrothermal spring and river waters from three different volcanic island arcs with active hydrothermal systems: La Soufrière volcano (Guadeloupe), Kawah Ijen volcano (Indonesia) and Aso volcano (Japan). La Soufrière’s hydrothermal system is characterised by chloride alkaline, acid-sulphate, and acid-sulphate-chloride waters, and the system from Aso by Ca-SO4 waters; pH value in waters from these two hydrothermal systems ranges between 3 and 9. Kawah Ijen’s magmatic hydrothermal system is characterised by acid-sulphate-chloride waters with extremely low pH values (< 0.5). The highest measured Ge/Si ratios (10-508 µmol/mol) were found in Kawah Ijen springs. The hydrothermal springs from La Soufrière and Aso displayed significantly lower Ge/Si values in the ranges, respectively, of 0.3-15 and 0.2-21 µmol/mol. The Ge/Si fingerprint of hydrothermal contribution to rivers is higher in hydrothermal system with extremely low pH (4-514 µmol/mol, Kawah Ijen) than in other sites (0.2-5 µmol/mol, La Soufrière; 0.2-2 µmol/mol, Aso). This study highlights that the use of Ge/Si ratio to assess hydrothermal contribution to rivers should carefully consider local variability of Ge/Si ratio in hydrothermal springs.