Quantification of geothermal carbon dioxide fluxes using radiocarbon

Volcanic-hosted active hydrothermal systems degas carbon dioxide (CO 2 ) into the atmosphere. Understanding the rate and variability of degassing can provide insights into geothermal system behaviour, and the response of these systems to geothermal power generation. Flux measurements from the accumulation chamber method are commonly used to quantify the natural volcanic degassing rate to the surface, but these must account for co-located CO 2 flux from soil and vegetation respiration. Uncertainty in the respiration CO 2 flux can lead to significant uncertainties in the geothermal CO 2 flux, particularly for locations where the geothermal flux is modest. We use flux estimates from accumulation chamber measurements of CO 2 and radiocarbon ( 14 C) to partition the degassed CO 2 into geothermal and biogenic components. We demonstrate that at the Tauhara geothermal field (New Zealand), the biogenic CO 2 flux is significant and spatially variable. Further, the 14 C measurements can discriminate even tiny geothermal CO 2 contributions, allowing more robust mapping of the geographic limits of natural degassing. We also examine 13 C as an alternative partitioning method, and show that while it is generally effective, uncertainties in the end-member ∂ 13 C values and the presence of both C3 and C4 plants can make interpretation complex. • Geothermal and respiration CO 2 sources can be separated by isotopic measurements. • Radiocarbon gives definitive separation of CO 2 sources. • Stable isotopes can also separate sources with higher uncertainties, but are more readily measured. • Radiocarbon can identify geographic boundaries of surface geothermal CO 2 emissions.