CO2 degassing along the San Andreas Fault, Parkfield, California

The fluxes, concentrations, and carbon isotopic compositions of soil CO2 were measured along the Parkfield segment of the San Andreas fault (SAF). Single or double‐peak CO2 flux anomalies (> 18 g m−2 d−1) were observed along 12 of 16 fault‐crossing transects at five sites. Flux anomalies did not coincide with concentration anomalies. Flux anomalies paralleled the fault trace, suggesting zones of high diffusivity/permeability. One flux anomaly may have been accentuated by a 0.3 mm creep event. However, values of δ13C (‐23.7 to ‐21.6 ‰) and 14C (110.5 to 111.9 pmC) for soil gas CO2 are characteristic of CO2 of biogenic origin. The CO2 flux anomalies are therefore consistent with fault‐related biogenic gas flow and do not yield evidence for degassing of deeply derived CO2.