Shallow soil CO₂ flow along the San Andreas and Calaveras Faults, California

Citations Over TimeTop 14% of 2003 papers

Abstract

We evaluate a comprehensive soil CO 2 survey along the San Andreas fault (SAF) in Parkfield, and the Calaveras fault (CF) in Hollister, California, in the context of spatial and temporal variability, origin, and transport of CO 2 in fractured terrain. CO 2 efflux was measured within grids with portable instrumentation and continuously with meteorological parameters at a fixed station, in both faulted and unfaulted areas. Spatial and temporal variability of surface CO 2 effluxes was observed to be higher at faulted SAF and CF sites, relative to comparable background areas. However, δ 13 C (−23.3 to −16.4‰) and Δ 14 C (75.5 to 94.4‰) values of soil CO 2 in both faulted and unfaulted areas are indicative of biogenic CO 2 , even though CO 2 effluxes in faulted areas reached values as high as 428 g m −2 d −1 . Profiles of soil CO 2 concentration as a function of depth were measured at multiple sites within SAF and CF grids and repeatedly at two locations at the SAF grid. Many of these profiles suggest a surprisingly high component of advective CO 2 flow. Spectral and correlation analysis of SAF CO 2 efflux and meteorological parameter time series indicates that effects of wind speed variations on atmospheric air flow though fractures modulate surface efflux of biogenic CO 2 . The resulting areal patterns in CO 2 effluxes could be erroneously attributed to a deep gas source in the absence of isotopic data, a problem that must be addressed in fault zone soil gas studies.

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