Iron isotope constraints on diagenetic iron cycling in the Taixinan seepage area, South China Sea

Abstract

Authigenic pyrite is a common mineral in seepage areas, where it is mainly derived from sulfate-driven anaerobic oxidation of methane (SO4-AOM). However, reports on the iron isotopic composition of SO4-AOM generated pyrite are limited, and the mechanisms controlling the fractionation of iron isotopes are insufficiently understood. Here, we present new iron isotope data for pyrite and iron oxides from sediments of the Taixinan seepage and gas hydrate-bearing area (sites HD109 and GMGS2-16) and revisit published data for pyrite from the Shenhu area to unravel the impact of SO4-AOM on the iron isotope composition of pyrite. The δ56Fe values (relative to average igneous rock) of pyrite were found to vary from −0.86 to −0.56‰ for site HD109, and from −1.57 to −0.25‰ for site GMGS2-16. In contrast, the iron isotopic compositions of iron oxides from site GMGS2-16 reveal positive values throughout the core, spanning from +0.52‰ to +0.74‰. In addition, the extent of pyritization (Fepy/FeHR) of the sediment was investigated, revealing a variability between 0.01 and 0.29 for HD109, and between 0.00 and 0.36 for GMGS2-16. A previous study on the Shenhu seepage area recognized an enrichment of 56Fe in pyrite at (paleo-)sulfate-methane transition zones (SMTZ), which was regarded as a consequence of pronounced SO4-AOM. However, peaks in δ56Fepy values are not observed at (paleo-)SMTZs at sites HD109 and GMGS2-16 in this study. The negative δ56Fepy values are likely to reflect the dominance of dissimilatory iron reduction (DIR) during diagenesis in this area. In addition, for all samples of the Taixinan and Shenhu areas, their δ56Fepy values, and the extent of pyritization, a clear positive correlation becomes apparent, where both δ56Fepy values and Fepy/FeHR ratios from the Taixinan area are lower. Hence, it can be inferred that the availability of sulfide is among the key factors that affect the iron isotopic composition of pyrite. With respect to the Taixinan area, the low degree of pyritization was apparently insufficient to alter the iron isotopic composition of the iron sources (i.e., iron oxides) to a significant degree.