Quantification of geothermal carbon dioxide fluxes using radiocarbon

Abstract

Volcanic-hosted active hydrothermal systems degas carbon dioxide (CO₂) 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 CO₂ flux from soil and vegetation respiration. Uncertainty in the respiration CO₂ flux can lead to significant uncertainties in the geothermal CO₂ flux, particularly for locations where the geothermal flux is modest. We use flux estimates from accumulation chamber measurements of CO₂ and radiocarbon (¹⁴C) to partition the degassed CO₂ into geothermal and biogenic components. We demonstrate that at the Tauhara geothermal field (New Zealand), the biogenic CO₂ flux is significant and spatially variable. Further, the ¹⁴C measurements can discriminate even tiny geothermal CO₂ contributions, allowing more robust mapping of the geographic limits of natural degassing. We also examine ¹³C as an alternative partitioning method, and show that while it is generally effective, uncertainties in the end-member ∂¹³C values and the presence of both C3 and C4 plants can make interpretation complex.