Flux and δ13C composition of diffuse carbon dioxide across Yellowstone: working towards an updated calculation of total diffuse carbon dioxide emissions from the caldera

Previous calculations of total diffuse carbon dioxide (CO₂) emissions from Yellowstone caldera (USA) have been based on flux measurements from thermally active, altered basins. However, our recent measurements show that thermally inactive, cold gas seeps can emit more diffuse CO₂ than thermally active basins. The contribution of these cold gas seep sites must be included in calculations of total diffuse CO₂ flux for better quantification of Yellowstone's role as a major degasser within the global CO₂ budget.

We used a PP Systems EGM 5 portable carbon dioxide analyzer to acquire over 1,300 CO₂ flux point measurements across 15 different sites in Yellowstone National Park during summer 2018 and 2019. Four of the sites measured were thermally inactive, cold gas seeps and the remaining sites were thermally active. Here we use the categorization of Vaughan et al. (2014) to define thermally active or inactive sites. We also measured the carbon isotope composition of diffuse CO₂ samples from all sites to characterize the potential source(s) of these emissions.

Our measurements of diffuse CO₂ flux agree with previous findings (Werner et al., 2003) that the highest average flux occurs in acid-sulfate soils (705 gm^-2day^-1) with the lowest average flux in neutral-chloride soils (18 gm^-2day^-1). All cold gas seep sites measured were in acid-sulfate dominated soil chemistries. The average diffuse CO₂ flux for these cold gas seeps was higher (837 gm^-2day^-1) than the average flux through acid-sulfate soils in thermally active basins (626 gm^-2day^-1). Carbon isotope compositions suggest that cold gas seeps have a similar magmatic source for diffuse CO₂ as thermally active sites. Our results suggest that soil chemistry is a better indicator of elevated diffuse CO₂ emissions than surficial heat flux. Thermally inactive altered soils comprise 64% of the total altered area in Yellowstone. Our inclusion of emissions from both thermally active and inactive altered soil results in a more accurate calculation of the total daily diffuse CO₂ emitted through Yellowstone. References (1) Vaughan et al. (2014) Provisional map of thermal areas in Yellowstone National Park, based on satellite thermal infrared imaging and field observations. USGS report 2014-5137; (2) Werner et al. (2003) CO₂ emissions from the Yellowstone volcanic system. G3.