Total diffuse CO2 flux from Yellowstone caldera incorporating high CO2 emissions from cold degassing sites

Abstract Previous calculations of total diffuse carbon dioxide (CO2) emissions from Yellowstone National Park have been based on flux measurements from thermally active, altered basins. However, our recent measurements show that thermally inactive, cold degassing sites can emit more diffuse CO2 per km2 than thermally active basins. The contribution of these cold degassing sites must be included in calculations of total diffuse CO2 flux for adequate quantification of Yellowstone's role as a major contributor within the global CO2 budget. We acquired 1342 diffuse CO2 flux point measurements across 19 different sites in Yellowstone National Park during summer 2018 and 2019. Eleven of the measured sites are thermally active with heat flux elevated above background, four sites are vegetation sites, and four sites are thermally inactive, cold degassing sites outside the caldera. Our measurements of diffuse CO2 flux agree with previous findings that the highest average flux occurs in acid sulfate soils (706 ± 79  gm−2 day−1, average for both thermally active and cold degassing sites) with lower diffuse CO2 flux in travertine (189 ± 98  gm−2 day−1) and neutral-chloride (19 ± 8  gm−2 day−1) soils. All cold degassing sites measured were in acid sulfate dominated soil chemistries. The average diffuse CO2 flux for these cold degassing sites was higher (840 ± 163  gm−2 day−1) than the average flux through acid sulfate soils in thermally active basins (625 ± 79  gm−2 day−1). We also measured the carbon isotope composition of diffuse CO2 soil gas samples from most sites to characterize the potential source(s) of these emissions. The majority of carbon isotope values from cold degassing sites are within the typical range of values for Yellowstone fumaroles. This suggests that cold degassing sites have a similar magmatic source for diffuse CO2. Previous calculations of the total daily diffuse CO2 emitted through Yellowstone National Park use the arithmetic average flux per soil chemistry and extrapolate that value across the entire park using only thermally active altered soils. Instead, we calculate the total CO2 flux per site in tons CO2 per km2 per day using Gaussian Geostatistical Simulations through the Geostatistical Analyst toolbox in ArcGIS. To account for the large component of cold degassing CO2, we extrapolate the average emission rate per soil chemistry to all hydrothermally altered soils rather than only thermally active soils across the entire area of Yellowstone National Park. We calculate a total deeply sourced (biogenic flux removed) CO2 flux of 24 ± 12 kt CO2 per day, with nearly two-thirds of that flux estimated to be from cold degassing acid sulfate soils. Cold diffuse degassing sites can be significant sources of magmatic carbon emissions and this has implications for the relationship between the flux of CO2 and heat as well as for global contributions from mantle CO2 emissions.