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

Methane oxidation rates observed in a topsoil covering a retired landfill are the highest reported (45 g m day) for any environment. This microbial community had the capacity to rapidly oxidize CH(4) at concentrations ranging from 10 ppm (k = -2.37 h). The physiological characteristics of a methanotroph isolated from the soil (characteristics determined in aqueous medium) and the natural population, however, were similar to those of other natural populations and cultures: the Q(10) and optimum temperature were 1.9 and 31 degrees C, respectively, the apparent half-saturation constant was 2.5 to 9.3 muM, and 19 to 69% of oxidized CH(4) was assimilated into biomass. The CH(4) oxidation rate of this soil under waterlogged (41% [wt/vol] H(2)O) conditions, 6.1 mg liter day, was near rates reported for lake sediment and much lower than the rate of 116 mg liter day in the same soil under moist (11% H(2)O) conditions. Since there are no large physiological differences between this microbial community and other CH(4) oxidizers, we attribute the high CH(4) oxidation rate in moist soil to enhanced CH(4) transport to the microorganisms; gas-phase molecular diffusion is 10-fold faster than aqueous diffusion. These high CH(4) oxidation rates in moist soil have implications that are important in global climate change. Soil CH(4) oxidation could become a negative feedback to atmospheric CH(4) increases (and warming) in areas that are presently waterlogged but are projected to undergo a reduction in summer soil moisture.

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Rapid methane oxidation in a landfill cover soil.

A thermodynamic model for calculating methane solubility, density and gas phase composition of methane-bearing aqueous fluids from 273 to 523 K and from 1 to 2000 bar

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.

James B. Alcauskas

Papers

Solubility of methane in distilled water and seawater