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

[1] We evaluate a comprehensive soil CO₂ 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 CO₂ in fractured terrain. CO₂ 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 CO₂ effluxes was observed to be higher at faulted SAF and CF sites, relative to comparable background areas. However, δ¹³C (−23.3 to −16.4‰) and Δ¹⁴C (75.5 to 94.4‰) values of soil CO₂ in both faulted and unfaulted areas are indicative of biogenic CO₂, even though CO₂ effluxes in faulted areas reached values as high as 428 g m⁻² d⁻¹. Profiles of soil CO₂ 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 CO₂ flow. Spectral and correlation analysis of SAF CO₂ efflux and meteorological parameter time series indicates that effects of wind speed variations on atmospheric air flow though fractures modulate surface efflux of biogenic CO₂. The resulting areal patterns in CO₂ 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.