Soil CO2 Emissions: a Proxy for Heat and Mass Flow Assessment, Rotokawa, New Zealand

Abstract

The quantification of the heat and mass flow between deep reservoir(s) and the surface is a significant challenge to the sustainable development and exploration of magma-hydrothermal systems. Here, we use high resolution measurement of carbon dioxide (CO2) flux and heat flow at the land surface to characterize the mass (CO2 and steam) and heat released from the magma-hydrothermal systems. Statistical and isotopic characterization of background sources of CO2 flux is utilized to reduce the level of uncertainty when deriving mass (emissions) and heat flow estimates from high temperature reservoirs. The soil gas and heat flow survey of the Rotokawa thermal area consisted of ~2,900 direct measurements of CO2 flux and soil temperature, and 60 soil gas carbon isotopes samples. Results indicate a total CO2 emission rate of 633 ± 16 t d-1 (2.5 km2) for Rotokawa. The measured thermal energy release at the Rotokawa thermal area was 37MWt and by combining the magmatic-hydrothermal sourced CO2 emission (constrained using stable isotopes) with reservoir H2O:CO2 concentration ratios and the enthalpy of evaporation; the projected reservoir mass flow was 120 Kg s-1 and the heat flow was 314 MWt. CO2 emissions are higher than previously published values by up to 570 td-1 at Rotokawa and while mass and heat flow values are within historical predictions. Paired assessment of the CO2 source using stable isotopes and statistical analysis of the CO2 flux has reduced the uncertainty when constraining the magma-hydrothermal CO2 emission from the reservoirs and has implications for blind geothermal system exploration.