The methane oxidation rate in the Kuroshio and Oyashio Extension (KOE) region of the Northwest Pacific

The mixing zone of the Kuroshio and Oyashio Extension (KOE) is a critical carbon sink in the global ocean, significantly influencing global warming. However, the impact of ocean mixing on methane (CH4) biogeochemical cycling in the KOE remains poorly understood. In this study, we integrated station observations and isotope tracer incubations to examine the controls on methane methanotrophic activity in the dynamic KOE region (135.98°-151.99°N; 29.5-42.4°E) of the Northwest Pacific during cruises in 2021 (May-June) and 2022 (June-July). Samples were collected using 12 L Niskin bottles, equipped with an SBE 911 plus conductivity-temperature-depth (CTD) probe. CH4 samples were taken in 120 mL brown bottles following the standard dissolved gas sampling procedure, with 50 μL of saturated HgCl2 added to inhibit biological activity. Dissolved CH4 was measured using a cryogenic purge-and-trap system coupled to an Agilent GC-8890 gas chromatograph with a flame ionization detector (FID). Seawater samples for bacterial production (BP) and methane oxidation (MOx) rate measurements were gently introduced into 1 L polycarbonate bottles without bubbles or vortexing, and stored at 4°C in the dark before analysis. MOx rates were quantified using a radiotracer approach with 3H-labeled methane, and BP rates were measured with 3H-labeled leucine.