Journal
GEOCHEMICAL JOURNAL
Volume 44, Issue 6, Pages 477-487Publisher
GEOCHEMICAL SOC JAPAN
DOI: 10.2343/geochemj.1.0097
Keywords
hydrothermal plume chemistry; microbial methane oxidation; carbon Isotope fractionation; southern Okinawa Trough
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Funding
- MEXT [11308020, 17101001, 19253006]
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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
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