CO2 degassing along the San Andreas fault, Parkfield, California

The fluxes, concentrations, and carbon isotopic compositions of soil CO2 we re measured along the Parkfield segment of the San Andreas fault (SAF). Sin gle or double-peak CO2 flux anomalies (> 18 g m(-2) d(-1)) were observed al ong 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 delt a(13)C (-23.7 to -21.6 parts per thousand) and C-14 (110.5 to 111.9 pmC) fo r soil gas CO2 are characteristic of CO2 of biogenic origin. The CO2 flux a nomalies are therefore consistent with fault-related biogenic gas flow and do not yield evidence for degassing of deeply derived CO2.