Carbon monoxide degassing from seismic fault zones in the Basin and Range province, west of Beijing, China

Degassing of carbon monoxide (CO), which plays a significant role in the contribution of deep carbon to the atmosphere, commonly occurs within active fault zones. CO degassing from soil to the atmosphere in the Basin and Range province, west of Beijing (BRPB), China, was investigated by in-situ field measurements in the active fault zones. The measured concentrations of CO in soil gas in the BRPB ranged from 0.29 x 10(-6) to 1.1 x 10(-6) with a mean value of 0.6 x 10(-6), which is approximately twice as large as that in the atmosphere. Net fluxes of CO degassing ranged from -48.6 mg m(-1) d(-1) to 12.03 mg m(-2) d(-1). The diffusion of CO from soil to the atmosphere in the BRPB was estimated to be at least 7.6 x 10(3) ton/a, which is comparable to the corresponding result of about 1.2 x 10(4) ton/a for CO2. CO concentrations were spatially heterogeneous with clearly higher concentrations along the NE-SW trending in the BRPB. These elevated values of CO concentrations were also coincident with the region with low-velocity and high conductivity in deep mantle, and high Poisson's ratio in the crust, thereby suggesting that CO degassing from the soil might be linked to upwelling of the asthenospheric mantle. Other sources of CO in the soil gas are suggested to be dominated by chemical reactions between deep fluids and carbonate minerals (e.g., dolomite, limestone, and siderite) in country rocks. Biogenic processes may also contribute to the CO in soil gas. The spatial distribution patterns of CO concentrations are coincident with the stress field, suggesting that the concentrations of CO could be a potential indicator for crustal stress field and, hence is potential useful for earthquake monitoring in the BRPB.