Oceanic methane biogeochemistry.

Measurements of dissolved methane in the ocean have been available for only about 50 years. Methane measurements in sediments, where concentrations are millimolar, were first reported in the mid-1950s, while measurements of methane in ocean waters, where concentrations are nanomolar, were first reported in the late 1960s. Methane is the most abundant hydrocarbon in the atmosphere, where it plays an important role in tropospheric atmospheric chemistry. Further, methane is an important greenhouse gas. Atmospheric time series observations over the past two decades have documented an increase in the atmospheric mixing ratio of methane, and a great deal of activity has focused on the cause and climate consequences of this increase. The ocean contributes a relatively small amount of methane to the global net atmospheric budget, and it cannot be expected to play a role in the contemporary atmospheric methane increase. Our interest in methane in the ocean is understanding the balance between the enormous reported methane additions from continental shelf and slope sediments and the microbial oxidation reactions that must occur in sediments and the water column to produce the low nanomolar concentrations observed in the bulk of the ocean volume. A number of poorly quantified external sources contribute methane to the ocean water column. The source processes include microbially-mediated diagenesis of sediment organic matter, abiotic production of methane through the serpentinization reaction, a rock/water reaction occurring in hydrothermal systems associated with the midocean ridges and spreading centers, leaks from near-surface petroleum deposits, and decomposition of methane clathrate hydrates. These contributions enter the ocean water column through coastal runoff, by diffusion from organic-rich anoxic sediments, and * E-mail: [email protected]. Telephone: 949-824-2986. Fax: 949-8243874. 486 Chem. Rev. 2007, 107, 486−513