Investigation of the hydrothermal vents using biogeochemical tracers (ATP and dissolved methane) in the northeastern Lau Basin

Abstract

Lau Basin (15-20˚N, 174-179˚N) is located in the subduction boundary of Pacific Plate and India-Australia Plate between southeastern Fiji and western Tonga. This area is characterized by the active back arc basin associated with relatively shallow water depth and by wedge shaped geographic setting. The cruise was conducted for searching hydrothermal vents at an off-axis seamount with caldera structure in the northern Fonualei Rift and Spreading Center (FRSC), Lau Basin in December 2005. We checked the concentrations of ATP and dissolved methane for all the stations. For the ATP, most stations show high concentration of ATP in deep water (>1000m, especially 1000-1500m) with a range of 0.90 to 13.71 ng/l (average: 3.97 ng/l) (Figure 3). Moreover, the stations that located center and south part of seamount with caldera structures are showed high concentrations of ATP. On the other hand, we can find high concentration (2-6 times higher than normal seawater) of dissolved methane in all the stations around 1000-1500m water depth. This suggests that the input of methane to the ambient water occurred within whole the study area. The thickness of plume-affected water layers is higher than 500m (Figure 5). Such thick layer implies that the input activity of methane from hydrothermal vent is vigorous in this area. Our data suggest that the hydrothermal plume can be detected by ATP and dissolved methane concentrations.