ABSTRACT

Within neutrally buoyant hydrothermal plumes derived from the Yonaguni Knoll IV hydrothermal field (24°51´ N, 122°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 (³He/⁴He), methane (CH₄) and its stable carbon isotope ratio (δ¹³C_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 from 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, ∼1,050 m and ∼1,200 m. The CH₄ concentrations and δ¹³C values for the 1,200-m plume ranged between 1,026 and 10 nmol/kg, and between -22.4 and +40.4‰ (the highest δ¹³C value yet reported for oceanic CH₄), respectively, indicating active microbial CH₄ oxidation accompanied by the δ¹³C increase for residual CH₄. The δ¹³C(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.