Microbial carbon isotope fractionation to produce extraordinarily heavy methane in aging hydrothermal plumes over the southwestern Okinawa Trough

Within neutrally buoyant hydrothermal plumes derived from the Yonaguni Knoll IV hydrothermal field (24 degrees 51' N, 122 degrees 42' E D = 1 370-1 385 m) in the southwestern Okinawa Trough back arc basin we obtained profiles of dissolved manganese (Mn), helium isotopes (He-3/He-4) methane (CH4) and its stable carbon isotope ratio (delta C-13(PDB)) in young to old plume waters We first mapped the spatial distribution of hydrothermal plumes by towing a fixed array of optical sensors (Miniature Autonomous Plume Recorders MAPRs) above the field We then made water column observations and samplings using a CTD (Conductivity Temperature and Depth sensors)-Carousel package with a transmissometer and Niskin-X bottles at five locations with distances between 0 6 and 6 1 km horn the hydrothermally active center so far discovered Vertical profiles of light transmission and chemical tracers indicated triple-layered plumes the centers of which were at depths of 700-800 m similar to 1 050 m and similar to 1 200 m The CH4 concentrations and delta C-13 value for the 1 200-m plume ranged between 1,026 and 10 nmol/kg, and between -22 4 and +40 4 parts per thousand (the highest delta C-13 value yet reported for oceanic CH4) respectively indicating active microbial CH4 oxidation accompanied by the delta C-13 increase for residual CH4 The delta C-13(CH4) was shown to be useful for tracing such a microbial plume whose CH4 concentration has already fallen to almost the background level By applying the Rayleigh distillation equation for a closed system we estimated the kinetic isotope fractionation factor of 1 012 for the CH4 oxidation process occurring within the hydrothermal plumes deeper than 1 000 m