Identifying blind geothermal systems with soil CO₂ surveys

Highlights: • The semi-blind Ngatamariki geothermal field was surveyed for soil CO₂ flux and δ¹³C. • CO₂ flux values in isolation do not correctly identify areas of geothermal upwelling. • δ¹³CO₂ can distinguish even small additions of geothermal CO₂ to the soil efflux. • δ¹³CO₂ can identify areas of geothermal upwelling despite a low CO₂ flux. - Abstract: Diffuse soil CO₂ flux surveys are a widely applied approach for delineating zones of elevated heat and mass transfer in areas with geothermal surface features including hot-springs, mud pools, and geysers. However, many geothermal systems are capped by relatively impermeable layers that diminish the surface expression of potential resources present at depth. Here, we report diffuse soil CO₂ flux survey results with complementary δ¹³CO₂ values and shallow soil temperatures for the Ngatamariki geothermal system (Taupo Volcanic Zone, New Zealand), a largely surface blind system, in an effort to explore the utility of such data as indicators of magmatic carbon emission in geothermal systems with low CO₂ flux. Our results include that: (1) the majority (54%) of the soil CO₂ flux measurements are below background levels (15 gCO₂ m⁻² day⁻¹); (2) only 5.6% of the shallow (10 cm) soil temperatures exceed 30 °C; and (3) no correlation is present between soil temperature and CO₂ flux at Ngatamariki. These results belie the fact that a recently developed geothermal resource is present beneath Ngatamariki. Yet, δ¹³CO₂ values when interpreted in the context of soil gas CO₂ concentrations, demonstrate that the magmatic-hydrothermal system beneath Ngatamariki can be distinguished from soil-zone carbon sources using diffuse soil gas geochemical tracers. Modeling the Ngatamariki CO₂ system as a simple mixture of geothermal biogenic and atmospheric end-members allows for the apportionment of CO₂ flux at different locations across the field. Based on these findings, we suggest that delineating potential geothermal sites and assessing the relative contributions of mixed fluid end-members is possible even with low diffuse CO₂ flux and limited surface expression of geothermal systems at depth