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Appendix: The Composition of Sea Water
Sea water is a basic ingredient in the chemistry of the earth's surface. The oceans cover 71% of the planet and have a combined volume of 1.4 x 1021 liters (3.3 x 108 cubic miles). The major role of the oceans in shaping the earth's geology, meteorology, and biology is a reflection of this statistical dominance. Any serious attempt to understand the chemical inputs to the planet's ecology must include a close look at sea water.
The chemical composition of sea water is remarkably constant considering the huge size of the reservoir. While there are significant variations in the total salt content of the surface waters, the relative proportions of the different ions are essentially constant throughout the world. The total ion content is affected by dynamic factors such as the local balance between evaporation and precipitation. Aside from variations due to differing dilution, however, the oceans, like the lower atmosphere, are chemically homogeneous.
The figure below shows literature values for the composition of sea water of average salinity, an oceanographic term for total ion content. The average value is 0.62 equivalents/liter, which corresponds to 3.5% by weight of dissolved salts. Local experience with sea water samples collected from shore stations on both American coasts has indicated that a value between 0.40 and 0.50 equivalents/liter is typical for these samples; obviously, samples collected directly off shore are susceptible to dilution by terrestrial fresh water sources.
Although the ions shown in the figure represent all but 0.3% by weight of the ion content of sea water, there are trace quantities of many others. An important lesson in chemical oceanography is the fact that elements present in minute amounts can have major environmental impact. Two reasons stand out for the importance of trace constituents. The simplest of these is the enormous volume of the oceans. To illustrate this point, it can be calculated that the oceans contain 8 x 106 tons of dissolved gold (value: about $1.1 x 1014) even though the gold content of sea water is only 6 x 10-10 % by weight. The second factor, which depends on the first, is the capacity of organisms to concentrate trace components from a large reservoir until they accumulate to substantial levels. Although this capability is essential with respect to scarce elements that are vital to metabolism, it also accounts for the sensitivity of ocean ecology to trace pollutants.
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