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 CO2 sourced from magma or from the deep crust, whereas reactions with silicates at shallow depths mainly consume atmospheric CO2. Numerous studies have quantified the load of dissolved solids in rivers that drain volcanic regions to determine chemical weathering rates and atmospheric CO2 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 CO2 consumption rates. Here we use the Ge/Si ratio and the stable silicon isotopes (delta Si-30) as tracers for quantifying non-thermal silicon contributions in rivers draining the Yellowstone Plateau Volcanic Field, USA. The Ge/Si ratio (mu mol.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 delta Si-30 value (parts per thousand) 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 CO2 consumption associated with non-thermal waters flowing through Yellowstone's rivers during peak discharge is similar to 3.03 ton.km(-2).yr(-1), which is similar to 2% of the annual mean atmospheric CO2 consumption in other volcanic regions. This study highlights the significance of quantifying seasonal variations in chemical weathering rates for improving estimates of atmospheric CO2 consumption rates in active volcanic regions.