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 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. This study investigates three sediment cores from the Taixinan Basin (core 973-4) and the Pearl River Mouth Basin (cores GG03 and Z22-3) in the Northern South China Sea. Using SEM-EDS analysis and in-situ sulfur (δ34S) and iron (δ56Fe) isotope measurements, the evolution 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 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 researches, 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. This study provides valuable insights into the sulfur and iron cycles in both modern and ancient seep environments, enhancing the further understanding of biogeochemical processes in extreme settings.