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 CO 2 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 −2 d −1 ) and emission rates from thermal vents (up to 1.7×10 8 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 −2 d −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 CO 2 generally suggest a deep origin (δ 13 C = −2.3 to 0.0) in thermal areas and biogenic origin (δ 13 C = −20.5) in nonthermal, low‐flux areas. Vent emissions accounted for ∼32–63% of the total degassing observed at Mud Volcano (2.4 to 4.0×10 9 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 CO 2 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/metamoohic/hydrothermal (VMH) emissions.