sediment column above Sulfate Methane Transition Zone the Dongsha area, South China Sea Raw sequence reads. sediment column above Sulfate Methane Transition Zone the Dongsha area, South China Sea

In this study, a short sediment core (330cm) collected from Dongsha seep area, Northern South China Sea, was examined by the geochemical and microbial methods. The calculated depth of SMTZ in this area is 1036.5cmbs, which is much deeper than the length of this sediment core. The pore water DIC (6.97~32.88 mM) had an increase trend with sediment depth increase, while the value of δ13C-DIC (-30.07~-4.07‰) showed an decrease pattern. Moreover, the concentration profiles of H2S, δ34S-sulfate, δ34S-pyrite and Fe(II), as well as Fe(III), all indicate an active S and Fe redox cycling in this core. Notably, both of sulfate and Fe(III) decrease trends correlated well with those of DIC and δ13C-DIC, indicating the occurrence of AOM related to sulfate and Fe(III) reduction in this sediment column. On the other hand, high-throughput sequencing data showed that a large number of bacteria (Shewanella, Sulfurvorum and Holamonas) and archaea (MCG, MBGB, and ANME-1b), which are the potential methane-, S- and Fe-related microorganisms, were detected in the clone libraries of the sediments. In addition, sulfate reduction functional genes (sat and cysJI), sulfur oxidation functional gene (soxABCYV), and functional genes related to iron metabolism (fbpABC and sitABCD) were also detected based on metagenomics analysis. By integrating these geochemical and molecular data, we suggest the occurrence of an active biogeochemical CH4-S-Fe cycling in the zone above SMTZ under the cold seep environments with high methane flux.