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.

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Oceanic methane biogeochemistry.

. 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.

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Marine hypoxia/anoxia as a source of CH4 and N2O

During three measurement campaigns on the Baltic and North Seas, atmospheric and dissolved methane was determined with an automated gas chromatographic system. Area-weighted mean saturation values in the sea surface waters were 113 ± 5% and 395 ± 82% (Baltic Sea, February and July 1992) and 126 ± 8% (south central North Sea, September 1992). On the bases of our data and a compilation of literature data the global oceanic emissions of methane were reassessed by introducing a concept of regional gas transfer coefficients. Our estimates computed with two different air-sea exchange models lie in the range of 11-18 Tg CH4 yr-1. Despite the fact that shelf areas and estuaries only represent a small part of the world's ocean they contribute about 75% to the global oceanic emissions. We applied a simple, coupled, three-layer model to numerically simulate the time dependent variation of the oceanic flux to the atmosphere. The model calculations indicate that even with increasing tropospheric methane concentration, the ocean will remain a source of atmospheric methane.

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Methane in the Baltic and North Seas and a Reassessment of the Marine Emissions of Methane

Microbial production and oxidation of methane in deep subsurface

Coastal areas such as continental shelves, estuaries, deltas, fjords and lagoons can release high amounts of nitrous oxide (N2O) and methane (CH4) to the atmosphere. However, estimates of trace gas emissions are often biased by incomplete spatial and temporal coverages. Based on a compilation of literature data, the distributions of N2O and CH4 in European coastal areas (i.e. Arctic Ocean, Baltic Sea, North Sea, northeastern Atlantic Ocean, Mediterranean Sea, Black Sea) were reviewed and their emissions to the atmosphere reassessed. Maximum N2O saturations were found in estuarine systems, whereas the shelf waters, which are not influenced by freshwater plumes, are close to equilibrium with the atmosphere. This implies that N2O is mainly formed in estuarine systems. European coastal waters are a net source of N2O to the atmosphere (0.33–0.67 Tg N year−1) with the major contribution coming from estuarine/river systems and not from open shelf areas. European shelf areas contribute significantly (up to 26%) to the global oceanic N2O emissions. CH4 saturations show a high temporal and spatial variability with maximum values in estuarine/fjord systems. European coastal areas are a source of atmospheric CH4 (0.35–0.75 Tg C year−1) and contribute significantly to the overall global CH4 oceanic emissions. However, this estimate still seems to be a severe underestimation since CH4 fluxes from estuaries and shallow seeps are not adequately represented. Future N2O and CH4 emissions from coastal areas strongly depend on the degree of eutrophication of coastal waters and might increase in the future.

Nitrous oxide and methane in European coastal waters

Methane distributions were measured in the Southern Bight of the North Sea during March 1989 as part of the North Sea Program of the United Kingdom Natural Environment Research Council. Methane concentrations in the open North Sea were at or near solubility equilibrium with the atmosphere, but near the coast of the Netherlands, in the plume of the Rhine, extremely high methane values (as high as 120 times saturation) were observed. Both the Rhine and Scheldt rivers, and possibly coastal sediments, seemed to be significant regional sources of methane. Methane oxidation represented a relatively small sink for methane in the North Sea compared to gas loss to the atmosphere. This was due, in part, to the fact that methane oxidizing bacteria within the river plume appeared to be oxidizing the gas at close to their maximal rate, although offshore bacteria were able to increase oxidation rates as methane concentrations increased. Thus our studies suggested that bacterial methane oxidation kinetics were saturated at much lower methane concentrations in plume or estuarine waters than in open ocean situations. If the Rhine and Scheldt are the sources of the methane found in the Southern Bight, river concentrations of methane must be high (order of 50–5000 nM). Although rivers and coastal zones may not represent a major atmospheric source of methane, they can be very important as methane sources to the coastal ocean.

Methane fluxes in the southern North Sea: the role of European rivers

Methane is an important greenhouse gas and plays a major role in tropospheric chemistry. Therefore its global budget has received considerable attention. On aspect of the global methane cycle which remains poorly understood is the role played by the oceans and rivers as sources of methane to the atmosphere. We recently conducted a study of methane in the Southern Bight of the North Sea, both before and during the spring bloom, in which we examined the distribution of methane as well as sinks for methane such as oxidation by bacteria and loss to the atmosphere. In most of the Southern North Sea during March and April 1989, waters were at or near saturation with the atmosphere, and both biological oxidation and gas exchange rates were low. However in a narrow zone along the Dutch coast, sampled during the March cruise, extremely high methane concentrations (up to 360 nM) were observed. In general elevated methane levels correlated will with low salinities. Linear extrapolations of the CH[sub 4]/Salinity surves to S = O were made to estimate the concentration in the riverine endmember(s). Results suggested the presence of two methane sources (of about 50 nM and about 5000 nM). Using a modelmore » developed by the Delft Hydraulics Laboratory, we estimate river concentrations of about 70-15- nM if the Rhine is the major source and of 6000 nM if the Scheldt outflow were most important. These river concentrations are within the range of observe values for a variety of other rivers suggesting river outflow may represent an important local methane source.« less

K. McShane

Papers

Methane fluxes in the southern North Sea: the role of European rivers

Methane fluxes in the Southern North Sea: The role of European rivers