Production and transport of methane in oceanic particulate organic matter

Abstract

METHANE is an important component of the global carbon cycle¹ and a potent greenhouse gas^2,3. Surface ocean waters are typically supersaturated with dissolved methane relative to atmospheric equilibrium, presumably as a result of in situ microbial methane production^4–8. Because methanogenic bacteria are strict anaerobes⁹and surface ocean waters are highly oxygenated, the observation of methane supersaturation has been termed the 'oceanic methane paradox'¹⁰. Although methanogenic bacteria have been isolated from oceanic particulate matter, faecal pellets and zooplankton^11–14, no data are available on in situ rates of methane formation in these microenvironments. During a series of experiments in the North Pacific ocean, we have identified a previously unrecognized component of the oceanic methane cycle. We find that methane is associated with sinking particles, presumably as a dissolved constituent of the interstitial fluids of particulate biogenic materials, which exchanges with the water column as particles sink. This phenomenon provides a mechanism for the active transport in the water column of an otherwise passive, dissolved species. The particle-to-seawater methane flux that we measure is sufficient to replace all of the methane present in the upper water column in about 50 days and to produce the characteristic methane supersaturations in less than a month. We suggest that particulate production and transport may also be relevant to the redistribution and cycling of other bioreactive compounds in the marine environment.