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

Abstract

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 (C 1/C 2+ ratios ⩾ 600) and isotopic (δ 13C CH4, δD CH4 and δ 13C C2H6) indicators point to a microbial source of methane and ethane. Similarly, at all sites the δ 13C CH4 and δ 13C CO2 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 δ 13C CH4. At the background sites, the carbon isotope fractionation factor ( ε c = δ 13C CO2 − δ 13C CH4) for all gases ranges from 56–66, consistent with values predicted for methanogenesis driven predominantly by microbial CO 2 reduction. However, ε c values measured in the two acoustic chimney sites that contain gas hydrate are significantly lower (30–46). We conclude that these abnormally small ε c values reflect two phase fluid transport at gas hydrate bearing sites. The composition of the hydrocarbons sampled below the SMTZ at these sites (higher C 2+ abundance and δ 13C 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 ε c 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.