Although methanogenesis is considered a strictly anaerobic process, oxygen‐replete surface open‐ocean waters are usually supersaturated with methane (CH4), a phenomenon termed the oceanic methane paradox. Here, we report that abiotic methane photoproduction from chromophoric dissolved organic matter (CDOM) significantly contributes to this paradox. Methane photoproduction was observed during solar‐simulated irradiations of various waters collected along the land‐ocean continuum. Methane photoproduction rates decreased seaward, whereas its relative production efficiency and the methane‐to‐carbon‐monoxide (CO) photoproduction ratio (ΔCH4/ΔCO) both followed a reversed trend. Remote‐sensing modeling incorporating a ΔCH4/ΔCO–CDOM absorption relationship yielded an annual methane photoproduction of 118 Gg for the global open ocean, accounting for 20–60% of the open‐ocean methane efflux and being of comparable magnitude to the upper‐ocean methane microbial‐oxidation sink. The photodegradation of CDOM thus plays an important role in maintaining supersaturated methane concentrations in the oxygenated upper ocean and in sustaining oceanic methane emissions to the atmosphere.
As the second most important greenhouse gas with continuously increasing atmospheric concentrations, methane has attracted much attention during the last several decades (IPCC, 2013). The ocean is recognized as a natural source of atmospheric methane, however, estimates of oceanic emissions vary by more than an order of magnitude (0.4-18 Tg CH 4 year −1 ) (
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