Methane in aging hydrothermal plumes

Methane concentration and the stable carbon isotopic composition (delta(13)C-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 kin off axis background CH4 concentrations are <2 nM. CH4 behaves nonconservatively in plume waters and does not cor,relate with conservative parameters such as temperature anomaly (Deltatheta), but is highly correlated with NH4+ and total dissolvable Mn, delta C-13-CH4 values for plume depth samples varies inversely, and significantly (R-2 = 0.89) with methane concentrations normalized to Deltatheta, Some delta C-13-CH4 values (+ 1.8 and + 10.9 parts per thousand) measured at the 15 kin 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) approximate to 1.0072 to 1.0077) was calculated for the Endeavour samples. These fractionation factors a-re 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 (similar to2 degreesC). Copyright (C) 2002 Elsevier Science Ltd.