Abstract
This review shows that thermodynamic and kinetic constraints largely prevent large-scale methanogenesis in the open ocean water column. One example of open-ocean methanogenesis involves anoxic digestive tracts and fecal pellet microenvironments; methane released during fecal pellet disaggregation results in the mixed-layer methane maximum. However, the bulk of the methane in the ocean is added by coastal runoff, seeps, hydrothermal vents, decomposing hydrates, and mud volcanoes. Since methane is present in the open ocean at nanomolar concentrations, and since the flux to the atmosphere is small, the ultimate fate of ocean methane additions must be oxidation within the ocean. As indicated in the Introduction and highlighted in Table 3, sources of methane to the ocean water column are poorly quantified. There are only a small number of direct water column methane oxidation rates, so sinks are also poorly quantified. We know that methane oxidation rates are sensitive to ambient methane concentrations, but we have no information on reaction kinetics and only one report of the effect of pressure on methane oxidation. Our perspective on methane sources and the extent of methane oxidation has been changed dramatically by new techniques involving gene probes, determination of isotopically depleted biomarkers, and recent 14C-CH4 measurements showing that methane geochemistry in anoxic basins is dominated by seeps providing fossil methane. The role of anaerobic oxidation of methane has changed from a controversial curiosity to a major sink in anoxic basins and sediments. © 2007 American Chemical Society.
Highlights
Through seeps, vents, and mud volcanoes emitting methanerich fluids or methane-rich bubbles
Methanol can be produced by bacterial degradation of lignins or pectin, while methylated amines can be produced by decomposition of choline, creatine, and betaine.[117]
This review shows that thermodynamic and kinetic constraints largely prevent large-scale methanogenesis in the open ocean water column
Summary
Any discussion of oceanic methane biogeochemistry should place the ocean in the context of the global methane budget. A geochemical budget is a flux balance (or a mass balance) that provides a useful means of partitioning and estimating the magnitudes of sources and sinks. The first global methane budget, a net atmospheric budget, was based on available flux measurements and estimates from a variety of sources.[22,23] The natural radiocarbon (14C) content of atmospheric methane was used to partition the budget between recent biogenic and fossil sources. The atmospheric mixing ratio increase and recent field measurements were reviewed by Cicerone and Oremland,[1] who concluded that the atmospheric increase was genuine and proposed a revised methane budget based on new information on sources and sinks. Seasonal time series observations at fixed stations were used as a constraint in an inverse model, and several likely global methane budget scenarios were proposed by Fung et al.[32]
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