Contrasting behaviour in geogenic degassing along an active segment of the East Anatolian fault zone (Türkiye)

Seismically active areas have long been recognised as hotspots of geogenic degassing. The present study investigated the Osmaniye Fault which belongs to the East Anatolian Fault Zone (EAFZ). Although EAFZ has been recently (February 2023) hit by the destructive Kahramanmaraş earthquake, the studied segment was not involved and is still accumulating stress. Three areas along this structure were selected with gas samples collected and analysed for their chemical and isotope compositions. Extensive CO₂-flux measurements were performed together with some soil gas Rn measurements in two of the areas. The three areas were characterised by very different degassing behaviour both in term of chemical and isotope composition and in term of CO₂ output. In the first (area A), signs of anomalous degassing are visible with several thousands of m² showing stunted or absent vegetation, strong alteration of the soils and extensive surface deposits of sulfur and sulfates efflorescences. The CO₂-flux measurements yielded a wide range of values (1.3-57,200 g m⁻² day⁻¹). Soil Rn activity was generally very low (0.1-1.7 kBq m⁻³). Soil gas chemistry indicates a mixing between atmospheric air and a CO₂-rich deep geogenic end-member (δ¹³C ∼ 0‰). Helium isotope composition indicates a high mantle contribution (R/R_A ∼ 6). Area B, on the contrary, shows very low soil flux values (0.4-57 g m⁻² day⁻¹), almost all compatible with organic activity in the soil, low Rn activity (0.3-5.6 kBq m⁻³) and strongly negative δ¹³C–CO₂ values (<-21.7‰). In area C, geogenic degassing can be recognised only as gas bubbling in a hyperalkaline (pH ∼ 12) spring. The gases comprise predominantly CH₄ of likely abiotic origin. The absence of deep-derived CO₂ suggests consumption either by carbonate precipitation or through reactions with H₂ produced during serpentinization processes occurring in the ultramafic rocks of the area. Deep origin of at least part of the gases is supported by the substantial contribution of mantle He (R/R_A ∼ 3). Estimation of the total CO₂ output of area A gave a value of about 20,000 t a⁻¹. Such output value, comparable with a quiescent volcanic system, further underscores the important contribution of geogenic degassing along active tectonic structures to the natural carbon cycle. Periodic and/or continuous monitoring of gaseous emissions in the same area would be useful to obtain possible precursory signals.