Articles | Volume 7, issue 7
Biogeosciences, 7, 2159–2190, 2010

Special issue: Hypoxia

Biogeosciences, 7, 2159–2190, 2010

  12 Jul 2010

12 Jul 2010

Marine hypoxia/anoxia as a source of CH4 and N2O

S. W. A. Naqvi1,2, H. W. Bange3, L. Farías4, P. M. S. Monteiro5, M. I. Scranton6, and J. Zhang7 S. W. A. Naqvi et al.
  • 1National Institute of Oceanography (Council of Scientific & Industrial Research), Dona Paula, Goa 403 004, India
  • 2Max-Planck Institut für Marine Mikrobiologie, Celsiusstrasse 1, 28359 Bremen, Germany
  • 3Forschungsbereich Marine Biogeochemie, IFM-GEOMAR, Düsternbrooker Weg 20, 24105 Kiel, Germany
  • 4Laboratorio de Procesos Oceanográficos y Clima (PROFC), Departamento de Oceanografía y Centro de Investigación Oceanográfica en el Pacífico Suroriental (COPAS), Universidad de Concepción, Casilla 160-C, Concepción, Chile
  • 5Department of Oceanography, University of Cape Town, Rondebosch, South Africa
  • 6School of Marine and Atmospheric Sciences, Stony Brook University, Stony Brook NY 11794, USA
  • 7State Key Laboratory of Estuarine and Coastal Research, East China Normal University, 3663 Zhongshan Road North, 200062 Shanghai, China

Abstract. 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.

Special issue
Final-revised paper