Spatial Distribution of Dissolved Methane Over Extreme Oceanographic Gradients in the Subtropical Eastern South Pacific (17° to 37°S)

Methane (CH 4 ) is one of the most powerful greenhouse gases with the capacity to influence the Earth's radiative budget as well as contribute to atmospheric chemistry. Natural oceanic production makes up to ∼4% of the overall global CH 4 emissions, however, there is uncertainty around the accuracy of this value due to a lack of accurate measurements. Such is the case in the Subtropical Eastern South Pacific Ocean (SESP), a region with pronounced chlorophyll‐a and oxygen gradients, which in turn affect the microbial CH 4 cycling. This study was conducted during spring‐summer (2014–2016) in the SESP. The region (∼17°–37°S/71°–110°W) is separated into (i) eutrophic, (ii) mesotrophic, and (iii) oligotrophic areas, according to oceanographic and biogeochemical criteria. The SESP presents high CH 4 zonal variability with levels ranging from 0.63 to 33.4 nmol L −1 , corresponding to 29% and 1,423% saturation, respectively. High CH 4 concentrations (>1,000% saturation) are observed in the narrow eutrophic area subjected to coastal upwelling. These conditions clearly differ to those observed in the extended oligotrophic subtropical gyre (∼100% saturation). Furthermore, CH 4 also tends to accumulate in the mesotrophic area (with upto 1,423% saturation), where oceanographic conditions as stratification, mesoscale eddies and island mass effect could trigger the presence of a microbial biomass that may be able to induce CH 4 regeneration. The CH 4 efflux is estimated to be between 0.13 and 19.1 µmol m −2  d −1 (mean ± SD = 4.72 ± 4.67) and the SESP has an emission rate of ∼87.9 Gg CH 4  yr −1 .