Presentation Information
[G-O-26]Origin of helium in surface sediments from the continental slope area of the East China Sea
*Tomohiro TOKI1, Ryusei Yonamine1, Takanori KAGOSHIMA2, Naoto TAKAHATA3 (1. University of the Ryukyus, 2. University of Toyama, 3. The University of Tokyo)
Keywords:
East China Sea,continental slope,surface sediment,helium isotope ratio
On the continental slope of the East China Sea, cold seeps, mud volcanoes, and methane concentration anomalies have been reported, and methane is released from the seafloor into the ocean. Methane in the atmosphere is a greenhouse gas tens of times more potent than carbon dioxide, so it is extremely important to quantitatively understand the mass budget in the Earth's surface environments. Many studies have been conducted to determine the origin of gases in sediments using a helium isotopic ratio (3He/4He). This study aims to examine the helium isotopic ratios and gas composition of surface sediments from the continental slope region of the East China Sea west of Amami-Oshima Island, to determine the origin of helium and the presence of methane, and to verify whether there are methane seep phenomena and they are related to hydrothermal systems. The obtained 3He/4He values ranged from 0.98 ± 0.04 RA to 1.09 ± 0.04 RA, and tended to decrease as increasing depth. The values in the shallowest layer is consistent with that of deep seawater in the East China Sea reported by previous studies. In the gas composition, the methane concentrations were lower than 1.5 μM, which is the detection limit of the present analytical method. The carbon dioxide concentrations were about 10 times higher than in seawater, which is more comparable to the properties of cold seep fluids than hydrothermal fluids. The origin of helium is calculated to be ~15% mantle-derived helium on average in surface sediments obtained in this study. The 4He flux was estimated to be 5.09 × 104 atoms/cm2/s, which is lower than those of hydrothermal systems and cold seeps in previous studies, suggesting the activity of gas emission is not so high. In the previous study, methane seep phenomena is reported to be distributed in discontineous patches around the studied site. The sediments obtained in this study may have also been taken from the areas between such patchy methane seeps.