Carbon isotope variations in inorganic carbon materials: Implications for mud volcanic carbon cycling in the northern Tianshan fold zone, Xinjiang, China

Abstract Carbon isotopes of inorganic carbon-bearing materials (gaseous CO 2 , dissolved inorganic carbon (DIC) and solid carbonates) sampled from four mud volcanoes in the northern Tianshan fold zone were investigated to study the mud volcanic carbon cycling process. The analysed carbon isotopes showed 13 C enriched characteristics, the δ 13 C values of CO 2 , DIC and solid carbonates were in the range of 10.99–32.11‰, 17.02–36.22‰, and −7.28–12.40‰, respectively. The 13 C enrichment of CO 2 could be attributed to carbon isotope fractionation during the partial reduction of CO 2 by methanogenic bacteria. The positive δ 13 C values in DIC may be caused by the carbon isotope fractionation during CO 2 -water-rock interactions. The mixture of carbonate precipitates from DIC in the water and carbonate minerals from the host rock resulted in a relatively less positive δ 13 C value in carbonates of the mud samples. The reduction of CO 2 by methanogenic bacteria could be the primary mechanism for the 13 C enrichment of inorganic carbon during the carbon cycling process in the mud volcanic systems in the northern Tianshan fold zone.