Carbon dioxide in water and seawater: the solubility of a non-ideal gas

Gas hydrates are naturally ocurring solids consisting of water molecules forming a lattice of cages, most of which contain a molecule of natural gas, usually methane. The present article discusses three important aspects of gas hydrates: their potential as a fossil fuel resource, their role as a submarine geohazard, and their effects on global climate change. 70 refs., 16 figs., 1 tab.

Gas hydrates—geological perspective and global change

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.

Calculation of the thermodynamic and transport properties of aqueous species at high pressures and temperatures: Standard partial molal properties of organic species

Field and laboratory studies of anoxic sediments from Cape Lookout Bight, North Carolina, suggest that anaerobic methane oxidation is mediated by a consortium of methanogenic and sulfate-reducing bacteria. A seasonal survey of methane oxidation and CO2 reduction rates indicates that methane production was confined to sulfate-depleted sediments at all times of year, while methane oxidation occurred in two modes. In the summer, methane oxidation was confined to sulfate-depleted sediments and occurred at rates lower than those of CO2 reduction. In the winter, net methane oxidation occurred in an interval at the base of the sulfate-containing zone. Sediment incubation experiments suggest both methanogens and sulfate reducers were responsible for the observed methane oxidation. In one incubation experiment both modes of oxidation were partially inhibited by 2-bromoethanesulfonic acid (a specific inhibitor of methanogens). This evidence, along with the apparent confinement of methane oxidation to sulfate-depleted sediments in the summer, indicates that methanogenic bacteria are involved in methane oxidation. In a second incubation experiment, net methane oxidation was induced by adding sulfate to homogenized methanogenic sediments, suggesting that sulfate reducers also play a role in the process. We hypothesize that methanogens oxidize methane and produce hydrogen via a reversal of CO2 reduction. The hydrogen is efficiently removed and maintained at low concentrations by sulfate reducers. Pore water H2 concentrations in the sediment incubation experiments (while net methane oxidation was occurring) were low enough that methanogenic bacteria could derive sufficient energy for growth from the oxidation of methane. The methanogen-sulfate reducer consortium is consistent not only with the results of this study, but may also be a feasible mechanism for previously documented anaerobic methane oxidation in both freshwater and marine environments.

Field and laboratory studies of methane oxidation in an anoxic marine sediment: Evidence for a methanogen-sulfate reducer consortium

Bunsen solubility coefficients for methane in distilled water
and in seawater at three salinities were determined with an
estimated accuracy of 0.5 YO. The experimental data were
fit by the least-squares method to an equation established
by Weiss. A table of Bunsen solubility coefficients covering
the temperature range -2' to 3OoC and the salinity range
0-40 parts per thousand was calculated from the fitted
equation. For seawater of salinity 34%0, the Bunsen
coefficients ranged from 0.04489 at O°C to 0.02368 at
3OoC. Solubility values for distilled water were in agreement
with those reported by Bunsen and Claussen and Polglase
but were 34% higher than those of Winkler and Morrison
and Billet.

Thomas E. Crozier

Papers

Solubility of methane in distilled water and seawater

Solubility of hydrogen in water, sea water, and sodium chloride solutions

Determination of mercury in seawater by radioactivation

Mercury vertical distribution at two locations in the eastern tropical Pacific Ocean