Controls on weathering fluxes from hydrothermally-active volcanic regions : a Ge/Si ratio and Si isotopes perspective
(2020)
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- Volcanic regions significantly contribute to the global flux of solutes to the ocean and hence, to the consumption of atmospheric CO2. However, uncertainties remain regarding the variables which control riverine weathering fluxes in hydrothermally-active volcanic terranes, such as the source of acidity for weathering reactions, or the influence of seasonal runoff variability on the relative contribution from hydrothermal and meteoric processes. The aim of this PhD thesis is to better understand the factors controlling weathering fluxes in rivers draining hydrothermally-active volcanic regions by combining riverine element concentrations and two geochemical tracers, the germanium to silicon (Ge/Si) ratio and the stable silicon isotopes (δ30Si). First, a new method has been developed for the determination of δ30Si compositions of anion- and cation-rich water samples. This PhD study demonstrates that: (i) in thermal waters, volcanic gases, silica precipitation and the intensity of water-rock interaction are the major controls on the variability of Ge/Si ratios and δ30Si values, (ii) the riverine Ge/Si ratios and δ30Si values can be used as indicators of the source of acidity for weathering in volcanic terranes featuring hydrothermal manifestations, (iii) the seasonal change in riverine Ge/Si ratios and δ30Si values highlights short pulses of seasonal meteoric weathering contribution at high river discharge to be accounted for as they directly contribute to atmospheric CO2 consumption. Overall, this PhD study provides novel insights to improve our ability to estimate the atmospheric CO2 consumption associated to weathering in hydrothermally-active volcanic terranes.
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Gaspard, F. (2020). Controls on weathering fluxes from hydrothermally-active volcanic regions : a Ge/Si ratio and Si isotopes perspective. https://hdl.handle.net/2078.5/121485