Observation of Diffusive Methane Emission from Holocene Mud Deposits on the Continental Shelf Offshore Southeastern Korea

Abstract

Chun, J.-H.; Kim, Y.; Choi, J.-Y.; Kim, Y.J.; Lee, H.-J., and Kim, Y.H., 2020. Observation of diffusive methane emission from Holocene mud deposits on the continental shelf offshore southeastern Korea. In: Jung, H.-S.; Lee, S.; Ryu, J.-H., and Cui, T. (eds.), Advances in Geospatial Research of Coastal Environments. Journal of Coastal Research, Special Issue No. 102, pp. 127-136. Coconut Creek (Florida), ISSN 0749-0208.This study investigated gas-charged sediments found in Holocene mud deposits on the inner continental shelf offshore southeastern Korea. Headspace gases obtained from seven piston cores of Holocene mud deposits were analyzed using gas chromotography, and methane dissolved in the water column were analyzed using in situ methane sensors and gas chromotography. The methane concentration data from both sediments and water columns provide the evidences of diffusive methane emission from the inner continental shelf. The methane concentration of headspace gases ranged from 22.2 to 123.9 mM below the upper front of acoustic blanking zone (ABZ). The maximum anomaly of dissolved methane in the water column was 38.3 nM at a site 5 over rough seafloor in the ABZ, which is surrounded by water masses of relatively lower methane concentration. Dissolved methane concentrations were significantly lower (< 10 nM) in the upper water column (< 20 m water depth). Concentrations of dissolved methane within the whole water column in the gas-free zone (GFZ) were < 5 nM. These results imply that diffusive methane emission occurred from a rough seafloor with erosional features on the inner continental shelf at water depth of about 54 m; this diffusive methane emission was not related to high methane concentrations in piston core sediments of the shallow upper front of the ABZ.