Chimney-associated methane migration and hydrate dynamics influence SMTZ structure and microbial communities in deep-sea sediments

Gas chimneys within marine sediments function as preferential conduits for focused methane migration, significantly altering early diagenetic stratification and subsequent porewater geochemistry. A critical locus for these biogeochemical transformations is the sulfate–methane transition zone (SMTZ), where the anaerobic oxidation of methane is stoichiometrically coupled with sulfate reduction, regulating sedimentary carbon cycling. This study investigates the regulatory role of chimney-enhanced methane flux and gas hydrate dynamics on SMTZ depth and microbial community architecture within deep-sea sediments (water depths >2,000 m). We combined detailed porewater chemistry measurements, including hydrogen and oxygen isotope ratios of water, with DNA-based community profiling, and compared two chimney cores with a distal non-chimney core. The non-chimney core did not show a clearly defined SMTZ within the recovered interval. In contrast, the chimney cores showed a shallower and narrower SMTZ, consistent with stronger upward methane transport and tighter coupling between methane consumption and sulfate use. At one chimney site, a strong decrease in chlorinity together with shifts in water isotope ratios suggested gas-hydrate dissociation within the sediment. Microbial communities in hydrate-affected sediments were dominated by groups often associated with methane-rich and low-oxygen conditions, and additional increases in taxa linked to diverse carbon use suggest that high methane flow can broaden available energy and carbon pathways. Overall, these results support a feedback pattern in which focused methane transport and hydrate instability change the SMTZ and redox structure, which then shapes microbial community composition and, in turn, the chemical signals preserved in deep-sea sediment records.