Stable carbon isotope ratios of dissolved methane in hydrothermal plumes in the central Indian Ridge

Abstract

Stable carbon isotopic composition (δ13C) of dissolved methane, along with its vertical distributions, were measured to trace the hydrothermal plume and identify the source and behavior of methane in the Central Indian Ridge (11 - 13°S). Significant hydrothermal plumes were observed at depths of 2500 - 3500 m. The concentration and δ13C of methane in the plumes (Sts. IR02 and IR03) ranged from 3.3 to 42.3 nmol kg-1 and -30.0 to -15.4 ‰, respectively. The concentration and δ13C of methane in the background seawater (St. IR01) ranged from 0.52 to 1.15 nmol kg-1 and -35.1 to -28.9 ‰, respectively. The δ13C of methane was highest in the center of the plumes at St. IR02 (-15.4 ‰) and St. IR03 (-17.8 ‰). The δ13C of methane in the source hydrothermal vents estimated using methane distribution and its stable isotopic composition was approximately -22 ‰. The results indicated that the methane was most likely derived from magmatic outgassing or the chemical synthesis of inorganic matter. The behavior of the methane can be explained by the relationship between the δ13C of methane and 1/[CH4]. If the behavior of methane was controlled by simple mixing with ambient seawater, the measured stable carbon isotope ratios of methane would fall on the mixing line between vent and ambient methane. However, if the behavior of methane was influenced by microbial oxidation, the measured stable carbon isotope ratios wou Significant hydrothermal plumes were observed at depths of 2500 - 3500 m. The concentration and δ13C of methane in the plumes (Sts. IR02 and IR03) ranged from 3.3 to 42.3 nmol kg-1 and -30.0 to -15.4 ‰, respectively. The concentration and δ13C of methane in the background seawater (St. IR01) ranged from 0.52 to 1.15 nmol kg-1 and -35.1 to -28.9 ‰, respectively. The δ13C of methane was highest in the center of the plumes at St. IR02 (-15.4 ‰) and St. IR03 (-17.8 ‰). The δ13C of methane in the source hydrothermal vents estimated using methane distribution and its stable isotopic composition was approximately -22 ‰. The results indicated that the methane was most likely derived from magmatic outgassing or the chemical synthesis of inorganic matter. The behavior of the methane can be explained by the relationship between the δ13C of methane and 1/[CH4]. If the behavior of methane was controlled by simple mixing with ambient seawater, the measured stable carbon isotope ratios of methane would fall on the mixing line between vent and ambient methane. However, if the behavior of methane was influenced by microbial oxidation, the measured stable carbon isotope ratios wou