Undervalued CO2 emissions from soil to the atmosphere in seismic areas: A case study in Tangshan, North China

Abstract

A large quantity of CO₂ produced in the Earth's interior is emitted to the atmosphere via soil diffusion, especially in active tectonic areas. Due to the lack of extensive in situ measurements, however, estimations of soil CO₂ output have been poorly constrained thus far, leading to the perception that soil CO₂ seems to be a marginal source of global carbon emissions. Here, the contribution of soil CO₂ to the atmosphere is discussed based on soil degassing rates measured at 187 sites in the Tangshan seismic area, North China. The measured degassing rates ranged from 9.04 g m⁻²d⁻¹ to 230.42 g·m⁻²d⁻¹, with an average of 87.46 g·m⁻²d⁻¹, suggesting that high degassing rates are common throughout the region. Carbon isotopic results show that the soil CO₂ comes mainly from the deep-seated carbonates and shallow biogenetic processes. Using the threshold value of the data population (96.20 g·m⁻²d⁻¹), the background and anomalous areas are distinguished. We find that anomalous degassing areas overlap well with epicenters of earthquakes with magnitudes greater than 5. The total annual CO₂ output in anomalous areas was estimated to be 38 Mt. This extremely high value can be attributed to the enlarged degassing areas and enhanced CO₂ emissions induced by regional active faults and frequent seismic activities. Our results indicate that the impact of soil CO₂ emissions in seismic regions should receive increased attention.