USE OF ISOTOPIC ANALYSIS TO DISTINGUISH BETWEEN BIOLOGICAL AND GEOTHERMAL SOIL CO2 FLUX AT TAUHARA AND TE MIHI GEOTHERMAL AREAS

Abstract

Soil CO2 flux measurements allow the identification of faults and near surface heat flow in geothermal areas. As CO2 is the major component of typical geothermal gases, and is readily detectable, it is the most appropriate component to focus on. However, a current limitation of the CO2 flux technique is the overlap between the magnitude of biological and geothermal CO2 flux in survey areas; this overlap makes the two sources difficult to distinguish and can give ambiguous survey results. This study demonstrates the use of a laser-based optical absorption technique (Cavity Ring-Down Spectroscopy, Picarro G2132) to determine the stable carbon isotope composition of gas samples collected from the accumulation chamber of a portable soil diffuse CO2 flux meter (West Systems, Italy). Isotope samples were collected from the accumulation chamber during normal CO2 flux surveying at the Tauhara and Te Mihi (Hot Hill) geothermal areas, Taupo. This allowed both the magnitude of CO2 flux, and the relative proportions of biological and geothermal CO2 present to be determined. This combination of measurements provides a powerful approach to distinguish geothermal from biological CO2 flux where the magnitude of CO2 flux alone is ambiguous.