Source and behavior of dissolved methane in hydrothermal plumes revealed by concentration and stable carbon isotope measurements at newly discovered venting sites on the Central Indian Ridge (11 - 13°S)

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 and -30.0 to -15.4 ‰, respectively. The concentration and δ13C of methane in the background seawater (St. IR01) ranged from 0.5 to 1.2 nmol/kg 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 -19 ‰. The results indicated that the methane was most likely derived from magmatic outgassing or the chemical synthesis of inorganic matter. We used the relationship between δ13C and methane concentration to examine the behavior of methane at the plume stations. In the IR03 plume, simple physical mixing was likely the major process controlling the methane profile. In the IR02 plume we interpret a more complicated relationship as resulting from microbial oxidation as well as physical mixing. The differences in the methane behavior between the two areas (St. IR02 vs. IR03) miSignificant 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 and -30.0 to -15.4 ‰, respectively. The concentration and δ13C of methane in the background seawater (St. IR01) ranged from 0.5 to 1.2 nmol/kg 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 -19 ‰. The results indicated that the methane was most likely derived from magmatic outgassing or the chemical synthesis of inorganic matter. We used the relationship between δ13C and methane concentration to examine the behavior of methane at the plume stations. In the IR03 plume, simple physical mixing was likely the major process controlling the methane profile. In the IR02 plume we interpret a more complicated relationship as resulting from microbial oxidation as well as physical mixing. The differences in the methane behavior between the two areas (St. IR02 vs. IR03) mi