Methane dynamics in gas chimneys linking geochemical and microbial methane cycling in the Ulleung basin

Gas chimneys in marine sediments act as conduits for methane (CH₄) migration and influence geochemical and microbial processes. The sulfate-methane transition zone (SMTZ), where CH₄ is oxidized and sulfate (SO₄^2-) is reduced, plays a key role in carbon cycling. Temperate continental margins, such as the Ulleung Basin in the East Sea of Korea, are particularly sensitive to environmental changes that may influence methane flux and hydrate stability. We analyzed sediment cores from chimney (Sites P03, P04) and non-chimney (Site P01) structures to examine how CH₄ flux and hydrate dynamics shape SMTZ depth and microbial communities.

Geochemical analyses showed that the SMTZ was located deeper at the non-chimney site (P01), with stable salinity and isotopic composition, and no evidence of hydrate dissociation. In contrast, chimney sites (P03 and P04) had shallower SMTZs. Site P03 exhibited reduced salinity and chloride concentrations and enriched δD and δ¹⁸O values, indicating hydrate dissociation. SO₄^2- and CH₄ profiles further defined SMTZ depth, where their redox processes were spatially coupled. Carbon isotopic profiles from porewater and gas indicated active anaerobic methane oxidation near the SMTZ. In hydrate-bearing sediments, particularly at Site P03, microbial communities were dominated by JS1 and Lokiarchaeia, which are associated with CH₄ oxidation and SO₄^2- reduction. Functional predictions, consistent with geochemical trends, showed SO₄^2- reduction as the dominant microbial process in the SMTZ, while methanogenesis peaked at the zone and declined below. Chimney-specific taxa, including carboxydotrophic and fermentative bacteria, were also enriched at Site P03, suggesting adaptation to elevated CH₄ flux and diverse carbon sources.

This study enhances our understanding of carbon cycling in temperate marine sediments, where hydrate destabilization can significantly influence methane fate and microbial community responses.