Methane oxidation in the eastern tropical North Pacific Ocean water column

We report methane (CH 4 ) concentration and methane oxidation (MO x ) rate measurements from the eastern tropical north Pacific (ETNP) water column. This region comprises low‐CH 4 waters and a depth interval (~200–760 m) of CH 4 supersaturation that is located within a regional oxygen minimum zone (OMZ). MO x rate measurements were made in parallel using tracer‐based methods with low‐level 14 C‐CH 4 (LL 14 C) and 3 H‐CH 4 ( 3 H). The two tracers showed similar trends in MO x rate with water depth, but consistent with previous work, the LL 14 C rates (range: 0.034–15 × 10 −3  nmol CH 4 L −1  d −1 ) were systematically slower than the parallel 3 H rates (range: 0.098–4000 × 10 −3  nmol CH 4 L −1  d −1 ). Priming and background effects associated with the 3 H‐CH 4 tracer and LL 14 C filtering effects are implicated as the cause of the systematic difference. The MO x rates reported here include some of the slowest rates measured in the ocean to date, are the first rates for the ETNP region, and show zones of slow CH 4 turnover within the OMZ that may permit CH 4 derived from coastal sediments to travel great lateral distances. The MO x rate constants correlate with both CH 4 and oxygen concentrations, suggesting that their combined availability regulates MO x rates in the region. Depth‐integrated MO x rates provide an upper limit on the magnitude of regional CH 4 sources and demonstrate the importance of water column MO x , even at slow rates, as a sink for CH 4 that limits the ocean‐atmosphere CH 4 flux in the ETNP region.