Depressurization experiment of pressure cores from the central Ulleung Basin, East Sea: Insights into gas chemistry

We performed depressurization experiments on pressure cores (PCs) recovered from Holes UBGH1-10B and UBGH2-11B, which were drilled during two separate expeditions in the Ulleung Basin (UBGH1 and UBGH2). Various lines of evidence, such as convex curvature in the P–V curves obtained by depressurization, as well as X-ray images and resistivity-at-bit (RAB) data, point to the presence of gas hydrates at these holes. We estimated gas hydrate saturation (Sh) at these holes to range from 5–23%, based on the amount of gas released from PCs. These Sh values are in agreement with those reported at other high gas flux settings. lar (C1/C2+) and isotopic (δ13CCH4, δDCH4, δ13CC2H6) indicators delineate a microbial source for the CH4 and C2H6. In Core UBGH1-10B-5R, the carbon isotopic fractionation factor between CO2 and CH4 (εc ≈ δ13CCO2 − δ13CCH4) was observed to increase from 50 to 68 as pressure was reduced during our experiments. These changes illustrate the isotope fractionation that results from the preferential release of 13C depleted CO2 during the early stages of depressurization. nstant carbon isotope separation (∼10–20‰) between CO2 in void gases and in the dissolved inorganic carbon (DIC) pool shown in cores from the Ulleung Basin, has also been observed in gas hydrate-bearing sediments elsewhere. It is important to consider this effect when using carbon isotopic data in carbon cycling models, specifically when unraveling metabolic pathways of methane generation. In situations where partial CO2 degassing of a core is suspected, we recommend the use of the carbon isotope ratio of DIC instead of the gas measurements to best delineate the methane cycling pathways in marine systems.