Adjacent releases of mantle helium and soil CO2 from active faults: Observations from the Marmara region of the North Anatolian Fault zone, Turkey

Abstract

Volatile releases, in terms of CO2 efflux and noble gas of mantle origin, are investigated in the Marmara region of the seismically active North Anatolian Fault (NAF) zone. A total of 180 CO2 efflux measurements (1 to 309 g m(-2) d(-1)) were carried out along four transects across the NAF. Spatial variability of surface CO2 effluxes was higher at faulted sites, relative to comparable background areas. The delta C-13 values of soil CO2 are -24.0 to -15.6 parts per thousand without correlation with CO2 efflux values, which suggest that CO2 efflux anomaly is caused by fault-related biogenic gas flow. Furthermore, 36 gas and water samples from spring sites were investigated for He-3/He-4 ratios which resulted in a range from 0.26 R/R-A to 4.22 R/R-A. Large mantle helium contributions were observed from several sources along the NAF. The high CO2 efflux sector on the Gazikoy-Saros fault along the NAF coincides with the area of high mantle helium flux. This shows that fault zones play an important role both in transferring mantle helium to the surface and in causing fault-related biogenic gas flow.