Quantifying non-thermal silicate weathering using Ge/Si and Si isotopes in rivers draining the Yellowstone Plateau Volcanic Field, USA
In active volcanic regions, high-temperature chemical reactions in the hydrothermal system consume CO 2 sourced from magma or from the deep crust, whereas reactions with silicates at shallow depths mainly consume atmospheric CO 2 . Numerous studies have quantified the load of dissolved solids in rivers that drain volcanic regions to determine chemical weathering rates and atmospheric CO 2 consumption rates. However, the balance between thermal and non-thermal components to riverine fluxes in these areas remains poorly constrained, hindering accurate estimates of atmospheric CO 2 consumption rates. Here we use the Ge/Si ratio and the stable silicon isotopes (δ 30 Si) as tracers for quantifying non-thermal silicon contributions in rivers draining the Yellowstone Plateau Volcanic Field, USA. The Ge/Si ratio (µmol.mol −1 ) was determined for seven thermal water samples (183 ± 22), eight rivers (35 ± 23) and six creeks flowing into Yellowstone Lake (5 ± 3) during base flow and during peak water discharge following snowmelt. The δ 30 Si value (‰) was determined for thermal waters (−0.09 ± 0.04), Yellowstone River at Yellowstone Lake outlet (1.91 ± 0.23) and creek samples (0.82 ± 0.29). The calculated atmospheric CO 2 consumption associated with non-thermal waters flowing through Yellowstone's rivers during peak discharge is ∼3.03 ton.km −2 .yr −1 , which is ∼2% of the annual mean atmospheric CO 2 consumption in other volcanic regions. This study highlights the significance of quantifying seasonal variations in chemical weathering rates for improving estimates of atmospheric CO 2 consumption rates in active volcanic regions.