Methane ocean waters

Figure 8.35 shows the redox state and acidity of the main types of seawaters. The redox state of normal oceanic waters is almost neutral, but they are slightly alkaline in terms of pH. The redox state increases in aerated surface waters. Seawaters of euxinic basins and those rich in nutrients (eutrophic) often exhibit Eh-pH values below the sulfide-sulfate transition and below carbonate stability limits (zone of organic carbon and methane cf figure 8.21). We have already seen (section 8.10.1) that the pH of normal oceanic waters is buffered by carbonate equilibria. At the normal pH of seawater (pH = 8.2), carbonate alkalinity is 2.47 mEq per kg of solution. 

More direct measurements of methane oxidation rates, particularly in wetlands and ocean waters, are needed. The use of stable isotope estimates of methane oxidation, which give an indication of total oxidation, should continue, but direct rate measurements using both " C-CH4 and H-CH4 should be a priority. Pulse-labeling experiments conducted through a growing season are needed to resolve the effect of plant phenology on methane emission. The methane oxidation threshold suggested by a number of open ocean rate measurements should be studied in open ocean samples from areas near and well removed from shelf vent sources. 

Hydrothermal and mantle contributions of methane are not significant. Oceanic surface waters are oversaturated with respect to methane, due to bacterial (methanogenic) activity in localized anaerobic environments, such as the digestive tracts of zooplankton, resulting in a net flux of methane to the air. Methane is similarly produced in freshwater environments. Deep ocean waters contain much lower methane concentrations than surface waters and the methane generated within anaerobic sediments is mostly oxidized by methanotrophes. Marine and lacustrine environments as a whole do not make a large contribution to the methane flux, but natural wetlands do. The bacterial... 

The metabolic coupling involved in AOM, produces sulfide and dissolved inoiganic carboa Both methane and sulfate needed for AOM, are available in large amounts where methane vents are present at the seafloor. In the case of Hydrate Ridge, gas hydrates provide an almost inexhaustible supply of methane and the ocean water constitutes a large sulfate reservoir. Here the anaerobic methane oxidation rate is large because of the conti-nuous supply of methane from deeper sediments. 

First, the minor, randomly occurring differences between adjoining points were smoothed out in the course of model calculation. At the same time, the upper part of the profile adapted more and more to the ocean water transition, while the lower part adapted to the low concentration of the sulfate-methane transition zone. After somewhat more than 20 years, an almost perfect adjustment to the measured profile was achieved without any other fitting technique (center diagram in Fig. 15.12). This core was obtained in 1994, and the sulfate profile was measured in the same year. The sediment avalanche consequently took place in the early 70s of the last century. The right diagram in Figure... 

Methane forms looser bonds than water, so its Y shape is farther south. For methane, Titan hes east of the fork, on the island of the map that imphes methane oceans and weather cycles. Venus, Earth, and Mars are all well north of methane s freezing line, so there is no possibihty of methane oceans this close to the sun. 

Hudson ED, Ariya PA (2007) Measurements of non-methane hydrocarbons, DOC in surface ocean waters and aerosols over the Nordic seas during polarstem cruise ARK-XX/1 (2004). Chemosphere 69 1474... 

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