Methane in aging hydrothermal plumes

Methane concentration and the stable carbon isotopic composition (δ ¹³C-CH₄) were measured in aging hydrothermal plume waters originating at the Endeavour Segment, Juan de Fuca Ridge. CH₄ concentrations decreased systematically from 600 nM at on-axis stations to <11 nM at a distal station 15 km off axis; background CH₄ concentrations are <2 nM. CH₄ behaves nonconservatively in plume waters and does not correlate with conservative parameters such as temperature anomaly (Δθ), but is highly correlated with NH₄⁺ and total dissolvable Mn. δ ¹³C-CH₄ values for plume depth samples varies inversely and significantly (R² = 0.89) with methane concentrations normalized to Δθ. Some δ ¹³C-CH₄ values (+1.8 and +10.9 ‰) measured at the 15 km off-axis station are among the heaviest yet reported from a natural marine environment. Less than 5% of original hydrothermal methane remains in the plume at this station. The data are consistent with extensive microbial methane oxidation. A narrow range of fractionation factors (r_c ≈1.0072 to 1.0077) was calculated for the Endeavour samples. These fractionation factors are less than those reported by Coleman et al. (1981), but fall near the trend line of their r_c versus temperature data when extrapolated to plume water temperature (∼2 °C).