Methane venting in gas hydrate potential area offshore of SW Taiwan: Evidence of gas analysis of water column samples

Abstract

Water column samples were collected systematically in several potential gas hydrate areas offshore of SW Taiwan for analysis of dissolved gases. Some these samples show unusually high dissolved methane concentrations at sites A, B, C, and H of cruise ORI-765. The profiles of helium concentrations in the dissolved gases of the water column also exhibit consistent results with an increasing trend toward the seafloor. The 3He/4He ratios range from 0.2 to 0.4 times that of the atmospheric air ratio after air correction, which fall in the range of typical crustal gas composition and are similar to those of on-shore mud volcanoes in SW Taiwan. This indicates that gases are venting actively from the seafloor in the region and may share similar gas sources to on-shore mud volcanoes. The venting gases are considered to have originated from dissociation of gas hydrates and/or a deeper gas reservoir.