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7.9: Gases Dissolved In Seawater

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    Gases Dissolved In Seawater

    Oxygen concentrations (O2) in air is about 21% oxygen, in water it is a tiny fraction of 1%. This large difference in oxygen concentrations forces oxygen to dissolve into water along the boundary between air and water. When wind blows creating waves, it increases the surface area, allowing more diffusion to occur.

    Carbon dioxide (CO2) is much more soluble in water than oxygen, but concentrations in the atmosphere are comparatively very low. When dissolved in water it becomes a bicarbonate ion (-HCO3), so carbon dioxide readily diffuses into the atmosphere if it is not consumed in the production of calcium carbonate (CaCO3). Biological respiration releases -HCO3 which combines with available dissolved calcium which the organism either excretes or incorporates into its skeletal structure if environmental conditions are warm enough for CaCO3 to persist. One of the gravest concerns about the burning of fossil fuels is that it is increasing the concentration of carbon dioxide in seawater, so organisms that produce carbonate shells and skeletons are negatively impacted.

    Solubility of gases in water at one atmosphereFigure 7.28. Solubility of gases in water is affected by temperature. Gases are much more soluble in cold water than warm water. In contrast, most solid materials (organic and inorganic; examples salt and sugar) are more soluble in hot water. The solubility of elements in seawater is complex and depends on many factors including pH (acid-base), eH (oxidation-reduction), temperature, pressure, and interactions between other compounds dissolved in seawater.

    Methane (CH4) has very low solubility in seawater, however, it is very abundant in sediments rich in organic mater. In cold settings, methane, carbon dioxide, and water form an unusual form of ice called a methane hydrate (Figures 7-29 and 7-30).

    A clathrate is a compound in which molecules of one component are physically trapped within the crystal structure of another, in this case CO2 and CH4 are trapped in the crystal structure of ice under certain pressure and temperature condition that exist on the seafloor in cold water, mostly on the outer continental shelves and slopes in polar regions. Global warming of the oceans can cause the release of tremendous amounts of CO2 and CH4 from the seafloor, contributing to anoxia conditions, with possible catastrophic consequences. Read about the "Clathrate Gun Hypothesis" (Wikipedia).

    Methane-ice clathrate structure Methane clathrate burns despite being ice.
    Figure 7.29. Methane-ice clathrate structure Figure 7.30. Methane-ice clathrates will burn!

    Sulfur dioxide (SO2) is extremely soluble in water. Sulfur dioxide is a gas that smells like rotten eggs. It is released in large quantities by volcanic eruptions, forest fires, and by burning coal and petroleum. SO2 is extremely soluble in water where it combines with water molecules to form sulfate ions (-HSO4). When concentrated (when water is removed), the solution becomes sulfuric acid (H2SO4). When concentrated by evaporation of seawater (where dissolved calcium and other metallic ions are present) sulfate ions precipitates as the salts gypsum (CaSO4·2H2O), anhydrite (CaSO4), epson salt (MgSO4·7H2O), and other salt minerals.

    Figure 7.31. Gypsum