Soil δ13C-CO2 and CO2 flux in the H2S-rich Rotorua Hydrothermal System utilizing Cavity Ring Down Spectroscopy

Diffuse soil CO2 flux and stable carbon isotope surveys facilitate interpretation of the nature and structure of hydrothermal systems. Although soil CO2 flux measurements are completed in the field, isotopic carbon in CO2 (i.e., δ13C-CO2) analyses are more commonly performed in a stable isotope mass spectrometry laboratory, thereby, increasing the effort, time and resources required to complete the survey. Cavity Ring Down Spectroscopy (CRDS) overcomes these challenges by enabling both CO2 isotopic and flux analyses in field settings. However, H2S interference has limited CRDS applications in H2S-rich hydrothermal settings. Here, we assess the application of a mobile CRDS system equipped with a simple Cu wire trap to complete δ13C-CO2 and CO2 flux surveying in the relatively high H2S Rotorua Hydrothermal System (RHS), North Island, New Zealand. Laboratory experiments demonstrate that the Cu trap was able to remove H2S (~250 ppmv) enabling accurate and precise δ13C-CO2 measurements. In the RHS, the Cu trap successfully removed H2S concentrations during in situ soil gas analyses. Carbon dioxide fluxes determined using the CRDS were found be 102% of those values produced by a traditional soil gas flux meter (i.e., Licor). Carbon dioxide flux determined by CRDS was similar to previous studies in the RHS; however, zones of high CO2 flux in central Kuirau Park have moved ~100-200 m in the past twelve years. Isoscapes of δ13C-CO2 for the RHS were constructed from our soil gas survey results and were used to assess the apportionment of magmatic-hydrothermal versus biogenic CO2 release. Our findings demonstrate that a CRDS enabled system allows for faster in field survey measurement, lower cost, and reactive sampling for both δ13C-CO2 and CO2 flux measurement in H2S-enriched hydrothermal systems.