Factors determining the spatial distribution of gas-charged sediments in the continental shelf off southeastern Korea

Abstract

A high-resolution seismic survey, piston coring, and water column measurements were conducted on the continental shelf off southeastern Korea to examine the gas-charged sediments and gas seepage in the water column. We examined the factors controlling the spatial distribution of the acoustic blanking zone in recent Korea Strait shelf mud (KSSM) deposits along the southeastern shelf. The molecular composition, methane, hydrogen, and carbon dioxide isotope values of headspace gases, and the geochemical composition of core sediments, were measured in 11 piston cores. Dissolved methane concentrations in the water column were observed at 6 of the 11 coring sites.Acoustic blanking of the recent KSSM deposits was limited to areas with water depths between 30 and 105m along the continental shelf, except line L-13 (133m depth). The flanks of the acoustic blanking zone can be classified as distinct, disturbed, and step types based on their configurations. The highest methane concentration in the headspace gases was about 2.7%, 2–4m below the seafloor, which matched the top of the acoustic blanking zone. No obvious methane was found in piston cores obtained from the gas-free zone. The δ13CCH4 ranged from −56.5 to −106.3‰ relative to Vienna Pee Dee Belemnite (V-PDB), and the δDCH4 ranged from −194 to −207‰ relative to Vienna Standard Mean Ocean Water (V-SMOW). These isotope values show that methane is produced by the microbial reduction of CO2. CH4 oxidation results in a significantly heavier carbon isotope fraction (from –91.2 to –56.5‰ in the 2m below the seafloor) with constant δ13CCO2 isotope fractionation. No obvious differences were seen in the total carbon (TC; 2.438–3.378%), total nitrogen (TN; 0.159–0.548%), total organic carbon (TOC; 1.139–2.078%), or CaCO3 (2.825–15.258%) in the continental shelf, whereas the TOC was high and the CaCO3 low distant from the Nakdong River. Distinct dissolved methane anomalies (21.87ppm) were seen in the water column in the acoustic blanking and gas seepage zones. This level was about 15 times higher than the dissolved methane level (1.5ppm) in the water column in the gas-free zone. These results indicate that the acoustic blanking zone accumulates biogenic methane. Methane seepage was observed in the water column throughout the acoustic blanking zone of the continental shelf off southeastern Korea.