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.