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

## TL;DR

This study explores how methane and microbial activity interact in marine sediments, particularly in gas chimneys in the Ulleung Basin, to understand carbon cycling and hydrate stability.

## Contribution

The study reveals how methane flux and hydrate dynamics influence microbial communities and geochemical processes in gas chimneys and non-chimney marine sediments.

## Key findings

- Chimney sites had shallower sulfate–methane transition zones and evidence of hydrate dissociation compared to non-chimney sites.
- Microbial communities in hydrate-bearing sediments were dominated by JS1 and Lokiarchaeia, linked to methane oxidation and sulfate reduction.
- Functional predictions showed sulfate reduction as the dominant process in the sulfate–methane transition zone, with methanogenesis peaking at this zone.

## Abstract

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 (SO42−) 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. SO42− 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 SO42− reduction. Functional predictions, consistent with geochemical trends, showed SO42− 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.

The online version contains supplementary material available at 10.1186/s12866-025-04547-7.