Constraining sulfur and iron cycling in seep systems: insights from authigenic pyrite signatures

Seep activity has emerged as a critical area of research for understanding biotic communities and elemental cycling (e.g., carbon, sulfur and iron) in deep extreme environments. While previous studies have predominantly focused on sulfur isotopes in pyrite as a proxy for seep activity, recent advancements highlight the potential of iron isotopes as an additional indicator. However, the detailed mechanism of how sulfur and iron coupled in seep environment is still unclear and thus hamper the further constraint of seep activity by these two key elements. For this reason, the authors analyzed 25 samples from three sediment cores which are collected from seepage sites in the Taixinan Basin (core 973–4) and the Pearl River Mouth Basin (cores GG03 and Z22-3) in the Northern South China Sea. By using SEM-EDS analysis and in-situ sulfur (δ³⁴S) and iron (δ⁵⁶Fe) isotope measurements, also combining with results from previous studies, the evolution patterns of sulfur and iron in seep environments is explored. The δ³⁴S values of pyrite in core 973–4 range from −18.79 ‰ to 27.26 ‰, indicating a closed-system seep activity. In contrast, pyrite in cores GG03 and Z22-3 exhibit significantly negative δ³⁴S values (−49.75 ‰ to −46.29 ‰ and −53.88 ‰ to −37.11 ‰, respectively), characteristic of more open-system seep activities. Additionally, the δ⁵⁶Fe values of pyrite are consistently negative across all cores (−1.39 ‰ to −0.31 ‰ in core 973–4, −1.59 ‰ to −0.24 ‰ in core GG03, and −1.10 ‰ to −0.20 ‰ in core Z22-3), suggesting the absence of a heavy iron isotope pool. By integrating these findings with previous results, this study demonstrates that the δ⁵⁶Fe values of pyrite are influenced by the position of the sulfate-methane transition zone, diverse microbial reduction processes, and varying iron sources. Therefore, this study provides valuable data to decipher how coupled sulfur and iron evolving in seep environments by pyrite, enhancing the further understanding of elemental cycles in extreme settings.