UC Irvine

We report methane (CH4) concentration and methane oxidation (MO x) rate measurements from the eastern tropical north Pacific (ETNP) water column. This region comprises low-CH4 waters and a depth interval (~200-760 m) of CH4 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 14C-CH4 (LL 14C) and 3H-CH4 (3H). The two tracers showed similar trends in MO x rate with water depth, but consistent with previous work, the LL 14C rates (range: 0.034-15 × 10-3 nmol CH4 L-1 d-1) were systematically slower than the parallel 3H rates (range: 0.098-4000 × 10-3 nmol CH4 L-1 d-1). Priming and background effects associated with the 3H-CH4 tracer and LL 14C 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 CH4 turnover within the OMZ that may permit CH4 derived from coastal sediments to travel great lateral distances. The MO x rate constants correlate with both CH4 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 CH4 sources and demonstrate the importance of water column MO x, even at slow rates, as a sink for CH4 that limits the ocean-atmosphere CH4 flux in the ETNP region.