The contribution of continental volcanic arcs to global weathering fluxes: Insights from the Southern Andes

Abstract

Rivers draining volcanic regions are key to the global flux of solutes towards the ocean and in the consumption of atmospheric CO₂ through silicate rock weathering. In active volcanic regions, degassing and hydrothermal inputs can be an additional supply to weathering fluxes from high-temperature water-gas-rock reactions. Multiple surface hydrothermal manifestations and substantial degassing characterize continental volcanic arcs, and their contribution to the silicate rock weathering budget has been overlooked in comparison with volcanic islands. Here we use as a model the Southern Andes for investigating weathering of continental volcanic arcs. The Southern Andes is one of the most globally active volcanic provinces with >200 hydrothermal manifestations, high rainfall > 2000 mm/yr, numerous catchments feeding larger rivers draining toward the Pacific Ocean, and predominantly mafic volcanic rocks. We sampled twenty-two rivers and sixteen hot springs at high and low discharges from different volcanic watersheds in the Southern Andes. We found that more than half of the rivers were influenced by hydrothermal inputs using hydrothermal tracers Ge/Si and δ¹¹B independently, with both tracers agreeing in most cases. Furthermore, using the chloride budget and Ge/Si ratios as high-T° weathering tracers we estimate that influence of hydrothermal activity on the dissolved silicate load ranges from 0.2 to 81 % with a mean of 11 %. As a result, we present a first estimation of chemical weathering fluxes in the Southern Andes, with silicate weathering export rates ranging from 33 to 386 ton/km²/yr across the studied catchments. Using the silicate export rates, we calculate that the total carbon consumption via weathering of the total Southern Andes area is 0.11 × 10¹² mol/yr. This figure is equivalent to 0.0013 Pg C/yr which amounts to around 1 % of the global carbon consumption by the weathering of silicate rocks on the continents. This study is the first assessment of chemical fluxes in rivers of the Southern Andes and underscores the importance of studying large-scale continental volcanic arcs to improve our understanding of the feedback of silicate rock weathering in the global carbon cycle.