Inferences on gas transport based on molecular and isotopic signatures of gases at acoustic chimneys and background sites in the Ulleung Basin

We present results of a comparative study of hydrocarbons collected at acoustic chimney sites and nearby non-chimney (background) sites in the Ulleung Basin (offshore Korea). At all sites, molecular (C1/C2+ ratios ⩾ 600) and isotopic (δ13CCH4, δDCH4 and δ13CC2H6) indicators point to a microbial source of methane and ethane. Similarly, at all sites the δ13CCH4 and δ13CCO2 have minimum values at the sulfate–methane transition zone (SMTZ) throughout the basin. These data document intense carbon cycling at this horizon, whereby anaerobic oxidation of methane (AOM) at the SMTZ generates a dissolved inorganic carbon pool depleted in 13C that is subsequently reduced to generate the observed minima in δ13CCH4. background sites, the carbon isotope fractionation factor (εc = δ13CCO2 − δ13CCH4) for all gases ranges from 56–66, consistent with values predicted for methanogenesis driven predominantly by microbial CO2 reduction. However, εc values measured in the two acoustic chimney sites that contain gas hydrate are significantly lower (30–46). We conclude that these abnormally small εc values reflect two phase fluid transport at gas hydrate bearing sites. The composition of the hydrocarbons sampled below the SMTZ at these sites (higher C2+ abundance and δ13C enriched methane) indicates a deep source origin for the microbial gas, which we postulate migrates upwardly and sustains the gas hydrate formation near the seafloor. If so, abnormally small εc values can be used as a potential geochemical indicator for the migration of methane in the gas phase and the consequent development of gas hydrate accumulations in shallow marine systems.