Methane Distribution, Production, and Emission in the Western North Pacific

The ocean is a source of atmospheric methane (CH4), but there are still large uncertainties in the estimations of global oceanic CH4 emission due to sparse data coverage. In this study, we investigated the spatial distribution and influencing factors of CH4 in the Western North Pacific (WNP) during May-June 2021. High-resolution continuous underway measurements showed that surface CH4 concentrations indicated an obvious spatial gradient with an increase from the south to the north due to the influence of water mixing between Kuroshio Extension (KE) and Oyashio. Surface water was generally oversaturated with respect to the atmospheric CH4, and high CH4 fluxes occurred in the Kuroshio-Oyashio transition region due to high productivity and intensive air-sea interaction, emphasizing the importance of the Kuroshio-Oyashio transition region in global oceanic CH4 emission. Vertically, subsurface CH4 maximums were observed around 50-300 m due to in situ production through multiple pathways, and their distributions in the water column were affected by subduction of North Pacific Intermediate Water (NPIW) and advective transport. Methylphosphonate (MPn) enrichment experiment and 16S rRNA gene sequencing showed that in subtropical region and Kuroshio-Oyashio transition region, Vibrio spp. might produce CH4 by degrading MPn. Although this process was inhibited by inorganic phosphorus and regulated by iron stress, it might be a potential source of CH4 in the oxygenated water in the WNP. Our results contribute to better constrain the global oceanic CH4 emission, and help understanding the role of biological and physical processes in regulating CH4 emission in the WNP. After carbon dioxide, methane (CH4) is the most potent greenhouse gas, contributing about 4%-9% of greenhouse effect. Open ocean is identified as a minor source of atmospheric CH4, releasing about 1 Tg CH4 into the atmosphere each year. The Western North Pacific (WNP) has intense air-sea interaction and complex hydrodynamic and biochemical processes due to the confluence of the Kuroshio and Oyashio. Based on analysis of dissolved CH4 and other parameters in the WNP, we found that the production and accumulation of CH4 depended on phytoplankton growth and degradation of organic matter, especially north of the axis of the Kuroshio Extension. The surface water was oversaturated with dissolved CH4 relative to atmosphere, and high sea-air fluxes of CH4 were observed in the Kuroshio-Oyashio transition region due to intensive sea-air interaction. In addition, we confirmed the existence of potential aerobic CH4 production pathways in the WNP, where CH4 is released into the water column as a product of degradation of methylphosphonate by Vibrio spp. and this would provide an explanation for CH4 oversaturation in oxygenated water. CH4 emission in the Western North Pacific is 0.5 Gg in spring, accounting for 0.5 parts per thousand of the annual global CH4 emission from the open ocean CH4 accumulated in water column characterized by high productivity and rapid remineralization north of the Kuroshio Extension axis Methylphosphonate can be decomposed to produce CH4 by Vibrio spp. and this process is regulated by iron stress and inorganic phosphorus