Hydrothermal activity above Loihi submarine volcano was characterized by water column distributions of methane, pH and helium-3. It was found that the southern Loihi summit is almost covered with hydrothermal plumes, which have anomalously high concentrations of methane (maximum: 569 × 10-6 cm3 kg-1) accompanied by high concentrations of helium-3 and low pH values (minimum: 7.18). The plumes consist of two layers: a shallow plume (about 200 m above the summit) and a deep plume (about 100 m above the summit), probably derived from different hydrothermal vents. The shallow and deep plumes showed different CH4 3He and CH4 pH ratios with the same 3He pH ratio, which implies that methane concentrations differ between the hydrothermal end members for the two plumes. The variation of methane between the end members is suggested to result from inter-vent inhomogeneity of bacterial activities that consume or produce methane within the vents. Comparison of the CH4 3He ratios of the two plumes with the previous data for Loihi and other submarine hydrothermal areas confirms that the Loihi hotspot has one to two orders of magnitude smaller CH4 3He value than those of the East Pacific Rise and the Galapagos spreading centers.
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