Marine hypoxia/anoxia as a source of CH4 and N2O

Abstract

We review here the available information on methane (CH sub(4)) and nitrous oxide (N sub(2)O) from major marine, mostly coastal, oxygen (O sub(2))-deficient zones formed both naturally and as a result of human activities (mainly eutrophication). Concentrations of both gases in subsurface waters are affected by ambient O sub(2) levels to varying degrees. Organic matter supply to seafloor appears to be the primary factor controlling CH sub(4) production in sediments and its supply to (and concentration in) overlying waters, with bottom-water O sub(2)-deficiency exerting only a modulating effect. High (micromolar level) CH sub(4) accumulation occurs in anoxic (sulphidic) waters of silled basins, such as the Black Sea and Cariaco Basin, and over the highly productive Namibian shelf. In other regions experiencing various degrees of O sub(2)-deficiency (hypoxia to anoxia), CH sub(4) concentrations vary from a few to hundreds of nanomolar levels. Since coastal O sub(2)-deficient zones are generally very productive and are sometimes located close to river mouths and submarine hydrocarbon seeps, it is difficult to differentiate any O sub(2)-deficiency-induced enhancement from in situ production of CH sub(4) in the water column and its inputs through freshwater runoff or seepage from sediments. While the role of bottom-water O sub(2)-deficiency in CH sub(4) formation appears to be secondary, even when CH sub(4) accumulates in O sub(2)-deficient subsurface waters, methanotrophic activity severely restricts its diffusive efflux to the atmosphere. As a result, an intensification or expansion of coastal O sub(2)-deficient zones will probably not drastically change the present status where emission from the ocean as a whole forms an insignificant term in the atmospheric CH sub(4) budget. The situation is different for N sub(2)O, the production of which is greatly enhanced in low-O sub(2) waters, and although it is lost through denitrification in most suboxic and anoxic environments, the peripheries of such environments offer most suitable conditions for its production, with the exception of enclosed anoxic basins. Most O sub(2)-deficient systems serve as strong net sources of N sub(2)O to the atmosphere. This is especially true for coastal upwelling regions with shallow O sub(2)-deficient zones where a dramatic increase in N sub(2)O production often occurs in rapidly denitrifying waters. Nitrous oxide emissions from these zones are globally significant, and so their ongoing intensification and expansion is likely to lead to a significant increase in N sub(2)O emission from the ocean. However, a meaningful quantitative prediction of this increase is not possible at present because of continuing uncertainties concerning the formative pathways to N sub(2)O as well as insufficient data from key coastal regions