Short-Term Monitoring of Geogenic Soil CO2 Flux in a Non-Volcanic and Seismically Inactive Emission Site, South Korea

Abstract

Temporal changes of soil CO2 flux (FCO2) and soil CO2 concentration ([CO2](v)) were surveyed in a natural CO2 emission site to characterize the factors controlling the shortterm temporal variation of geogenic FCO2 in a non-volcanic and seismically inactive area. Due to a lack of long-term monitoring system, FCO2 was discontinuously measured for three periods: I, II at a high FCO2 point (M17) and. about 30 cm away. Whereas [CO2](v) was investigated at a point (60 cm depth) for all periods. A 2.1 magnitude earthquake occurred 7.8 km away and 20 km deep approximately 12 h before the period II. The negative correlation of FCO2 with air pressure suggested the non-negligible advective transport of soil CO2. However, FCO2 was significantly and positively related with air temperature as well, and [CO2](v) showed different temporal changes from FCO2. These results indicate the diffusive transport of soil CO2 dominant in the vadose zone, while the advection near the surface. Meanwhile [CO2](v) rapidly decreased while an anomalous FCO2 peak was observed during the period II, and the CO2 emission enhanced by the earthquake was discussed as a possible reason for the synchronous decrease in [CO2](v) and increase in FCO2. In contrast, [CO2](v) increased to 56.8% during the period. probably due to low gas diffusion at cold weather. In addition, FCO2 was low during the period. and showed different correlations with measurements compared to FCO2 at M17, implying heterogeneous CO2 transport conditions at the centimeter scale. The abnormal FCO2 observed after the earthquake in a seismically inactive area implies that the global natural CO2 emission may be higher than the previous estimation. The study result suggests a permanent FCO2 monitoring station in tectonically stable regions to confirm the impact of geogenic CO2 to climate change and its relation with earthquakes.