Identifying blind geothermal systems with soil CO2 surveys

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 (15gCO2m−2day−1); (2) only 5.6% of the shallow (10cm) 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.