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 short-term 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 Ⅲ 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 Ⅲ probably due to low gas diffusion at cold weather. In addition, FCO₂ was low during the period Ⅲ 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.