Methane sources and sinks in the subtropical South Pacific along 17°S as traced by stable isotope ratiosOriginal Research Article

We analyzed the concentration and stable carbon isotopic ratio (δ^C-CH_4) of methane in the atmosphere and in dissolved methane in water column along 17°S in the subtropical South Pacific. Additionally, the hydrogen isotopic ratios (δD-CH_4) of some water samples were analyzed. The sea–air CH_4 flux is high in the eastern region and off the west coast of Australia, which is related to the high concentrations of dissolved CH_4 and high wind speeds. Moreover, there is a positive correlation between the CH4 and chlorophyll a concentrations at the surface. This consistency suggested that active CH_4 productions related to the primary production cause surface CH_4 accumulation. CH_4 shows a decrease in concentration and an increase in δ^C-CH_4 and δD-CH_4 values from the surface to the depth of about 1000 m. The relationship between δ^C-CH_4 values and CH_4 concentration indicates that the isotopic enrichment of CH_4 reflects microbial oxidation of CH_4 with isotopic fractionation during vertical transport via vertical sinking and/or zooplankton migration. East of 120°W, δ^C-CH_4 values at around 1000 m exceed − 30.0‰. The relationships among the δ^C-CH_4 values, CH_4 concentrations, and oxygen concentrations indicate that the ^C-enriched CH_4 originates not only from in situ CH_4 production and oxidation but also from CH_4 transported from the eastern margin off Peru. Furthermore, at a site near the Central Lau Spreading Centers in the Lau Basin, high δ^C-CH_4 values (up to − 21.4‰) are observed in the benthic water, suggesting a hydrothermal field source