Wide Range Variation of δ³⁴S Isotopes in the Çayırhan Oil Shales: Multiple Sulfur Sources and Environmental Influences

Geochemical and mineralogical analyses of the Miocene-aged Çayırhan Oil Shales (ÇOS) reveal a significant presence of sulfur, particularly abundant framboidal pyrite. The formation of framboidal pyrite typically occurs in reducing, sulfur-rich, and microbially active environments, indicating such conditions prevailed during deposition. Sulfur isotope analyses (δ³⁴S) of 25 samples show a wide range of values, from –6.37‰ to +26.55‰, with an average of +13.86‰. This broad range suggests the involvement of multiple sulfur sources, including marine sulfate, freshwater sulfate (potentially through plant uptake), and hydrothermal sulfur. Low δ³⁴S values point to microbial sulfate reduction and freshwater influence, while high values may reflect closed-system conditions or rapid sedimentation. Since framboidal pyrite generally forms via microbial sulfate reduction and subsequent FeS₂ precipitation, its presence in ÇOS strongly supports a biogenic origin. The wide δ³⁴S variation further indicates that environmental factors such as pH and oxygen fugacity (ƒO₂) influenced pyrite formation. According to the literature, positive δ³⁴S values are often linked to slight pH drops, while negative values are associated with higher ƒO₂ levels. In summary, framboidal pyrite formation in the Çayırhan Basin was controlled by a complex interplay of diverse sulfur sources and varying environmental conditions. The isotopic variability reflects both open and closed system behavior in a lacustrine setting, influenced by microbial activity, diagenesis, redox conditions, sedimentation rates, and pH changes.