Fault-related basins as carbon reservoirs: Soil CO2 emissions in the SE Korean Peninsula

Abstract

Carbon dioxide (CO2) is one of the main components of greenhouse gases, and there is a great concern about emissions mainly from human activities. However, long term CO2 emissions during the Earth’s history are largely related to natural emissions by geological factors. Natural CO2 emissions are mainly at highly permeable passages below the surface (e.g., volcanoes or faults). Unlike volcanoes, which actively release mantle-derived volatiles, faults can be affected by CO2 of more diverse origins and are relatively lacking in research. Here, we note that it is important for CO2 emissions that faults as channels with high permeability take topographical control to form sedimentary basins and produce mature soils. To this end, we performed CO2 flux measurements in the Yangsan Fault System (YFS), Republic of Korea. As this fault is an active fault, topographical variations are clear and accessible. In the YFS, high spring CO2 fluxes (119.5 g m−2 d-1 to 77,699.5 g m−2 d-1) were observed in Sector 1, and high soil CO2 fluxes (0.5 g m−2 d-1to 1,240 g m−2 d-1) were observed along the Hyeongsan River in Sector 2. The carbon isotope composition indicates that the mantle contributes to fault-related spring CO2 (δ13C = -14.03 ‰ to −7.44 ‰), and soil CO2 comes from shallow biogenic sources (δ13C = –22.82 ‰ to −13.53 ‰) deposited in fault-forming basins. In addition, the helium isotope ratios indicative of the mantle contribution (3He/4He = 1.86 to 6.02 Ra) are shown in the gas samples of the springs. Furthermore, in a post-earthquake survey, we found temporal changes in soil CO2 flux values near the epicenter, which may be related to the earthquake. A simple calculation of overall CO2 emissions suggests that significant amounts of CO2 (6.2 Mt yr−1) are being emitted along the YFS. Therefore, this study reports both large-scale structural degassing and biological CO2 emissions from fault zones, providing implications for the role of sedimentary basins and soil types formed in fault zones.