CO₂ emissions related to the Yellowstone volcanic system 2. Statistical sampling, total degassing, and transport mechanisms

A stratified adaptive sampling plan was designed to estimate CO₂ degassing in Yellowstone National Park and was applied in the Mud Volcano thermal area. The stratified component of the sampling design focused effort in thermal areas and the adaptive component in high-flux regions, yet neither sampling technique biased the estimate of total degassing. Both diffuse soil fluxes (up to ∼30,000 g m^-2d^-1) and emission rates from thermal vents (up to 1.7 × 10⁸ mol yr^-1) were measured in thermal areas. Soil fluxes observed in most nonthermal regions were similar to values reported for conifer forests (≤ 15 g m^-2d^-1). However, through adaptive sampling, high-flux vegetated sites were identified in Mud Volcano that likely would not have been found if sampling was focused in obvious thermal or altered regions. A simple model applied to flux measurements suggests that ∼40% of the analyzed measurements were dominated by possible advective transport and ∼30% by diffusive transport. Isotopic signatures of soil CO₂ generally suggest a deep origin (δ¹³C = -2.3 to 0.0) in thermal areas and biogenic origin (δ¹³C = -20.5) in nonthermal, low-flux areas. Vent emissions accounted for ∼32-63% of the total degassing observed at Mud Volcano (2.4 to 4.0 × 10⁹ mol yr^-1). The largest source of error in the estimation of total degassing (factor of ∼2) resulted because the population distribution of thermal feature emissions was indeterminate. Total CO₂ emissions at Mud Volcano are comparable to other hydrothermal regions worldwide, suggesting that the Yellowstone volcanic system is likely a large contributor to global volcanic/metamorphic/hydrothermal (VMH) emissions.