Marine hypoxia/anoxia as a source of CH₄ and N₂O

• Main Authors: M. I. Scranton, P. M. S. Monteiro, L. Farías, H. W. Bange, S. W. A. Naqvi, J. Zhang
• Format: Article
• Language: English
• Published: Copernicus Publications 2010-07-01
• Series: Biogeosciences
• Online Access: http://www.biogeosciences.net/7/2159/2010/bg-7-2159-2010.pdf
• ISSN: 1726-4170; 1726-4189

We review here the available information on methane (CH₄) and nitrous oxide (N₂O) from major marine, mostly coastal, oxygen (O₂)-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 O₂ levels to varying degrees. Organic matter supply to seafloor appears to be the primary factor controlling CH₄ production in sediments and its supply to (and concentration in) overlying waters, with bottom-water O₂-deficiency exerting only a modulating effect. High (micromolar level) CH₄ 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 O₂-deficiency (hypoxia to anoxia), CH₄ concentrations vary from a few to hundreds of nanomolar levels. Since coastal O₂-deficient zones are generally very productive and are sometimes located close to river mouths and submarine hydrocarbon seeps, it is difficult to differentiate any O₂-deficiency-induced enhancement from in situ production of CH₄ in the water column and its inputs through freshwater runoff or seepage from sediments. While the role of bottom-water O₂-deficiency in CH₄ formation appears to be secondary, even when CH₄ accumulates in O₂-deficient subsurface waters, methanotrophic activity severely restricts its diffusive efflux to the atmosphere. As a result, an intensification or expansion of coastal O₂-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 CH₄ budget. The situation is different for N₂O, the production of which is greatly enhanced in low-O₂ 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 O₂-deficient systems serve as strong net sources of N₂O to the atmosphere. This is especially true for coastal upwelling regions with shallow O₂-deficient zones where a dramatic increase in N₂O 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 N₂O 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 N₂O as well as insufficient data from key coastal regions.