Concentration and stable carbon isotopic composition of dissolved methane in hydrothermal plumes at the Central Indian Ridge

Abstract

The concentration and stable carbon isotopic composition (δ13C) of dissolved methane were measured to trace hydrothermal plume and to identify the source and behavior of methane in the Central Indian Ridge, 11 - 13°S. We observed significant hydrothermal plumes in the depth of 2500 - 3500 m. The concentration and δ13C of methane in the plumes (Stn. IR02 and IR03) ranged from 3.34 to 42.33 nmol kg-1 and from -30.0 to -15.4 ‰, respectively. The concentration and δ13C of methane in the background seawater (Stn. IR01) ranged from 0.52 to 1.15 nmol kg-1 and from -35.1 to -28.9 ‰, respectively. The δ13C of methane was the heaviest in the center of plumes at Stn. IR02 (-15.4 ‰) and IR03 (-17.8 ‰). The estimated δ13C of methane in hydrothermal vents was around -20 ‰. The results indicated that methane was most likely derived from magmatic outgassing or chemical synthesis of inorganic matters. The behavior of methane was explained by the relationship between δ13C of methane and 1/[CH4]. The behavior of methane was mainly controlled by the physical mixing and diffusion at Stn. IR03, whereas the behavior of methane at Stn. IR02 was controlled by the microbial oxidation as well as by the physical mixing and diffusion. The topography of Stn. IR02 was the form of basin, unlike Stn. IR03. Thus, at Stn. IR02, because the methane could not be quickly mixing and diffusion with ambient seawater,ificant hydrothermal plumes in the depth of 2500 - 3500 m. The concentration and δ13C of methane in the plumes (Stn. IR02 and IR03) ranged from 3.34 to 42.33 nmol kg-1 and from -30.0 to -15.4 ‰, respectively. The concentration and δ13C of methane in the background seawater (Stn. IR01) ranged from 0.52 to 1.15 nmol kg-1 and from -35.1 to -28.9 ‰, respectively. The δ13C of methane was the heaviest in the center of plumes at Stn. IR02 (-15.4 ‰) and IR03 (-17.8 ‰). The estimated δ13C of methane in hydrothermal vents was around -20 ‰. The