Geothermal systems and carbon dioxide degassing: The Geysers-Clear Lake, and Dixie Valley regions of California and Nevada

Three studies examine the occurrence of CO₂ in surficial features above active geothermal systems to obtain information regarding the underlying geothermal reservoirs. The first study combines CO₂ flux measurements and geochemical and stable isotope analyses of geothermal fluids to examine changes occurring in a fumarolic area on the northern margin of the Dixie Valley geothermal field in Nevada. These data show that after a decade of fluid withdrawal, declines in reservoir pressure were responsible for boiling fluids that occurred above an outflow plume and led to formation of new fumaroles and localized areas of plant-kill. The second investigation uses carbon isotope results from calcite veins, organic carbon, CO₂ and CH₄ to show that carbon from Franciscan Complex and Great Valley Sequence rocks is the primary source for CO₂ in geothermal fluids across The Geysers-Clear Lake region in northern California. A SW-NE regional trend of increasing δ 13C-CO₂ values is attributed to variations in the local lithologies. The third study returns to the Clear Lake region to examine the CO₂ flux above a small liquid-dominated geothermal reservoir in what is now an EPA Superfund site. Flux data are used in combination with H₂S and Hg concentrations from gas analyses of vent emissions to provide information regarding the natural flux of these contaminants to the atmosphere.