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P.12 top paragraph: "The elements dissolved in the oceans can be equally vast, and even when the concentration is low the total can still be quite surprising. For example, although an element is present at only 5 p.p.m. in sea water, its total mass amounts to almost 7 trillion tonnes (the volume of the oceans is 1.37 billion billion cubic metres (1.37×10^18) hence 5 p.p.m., which is 5×10^-6, results in a total mass of 6.85 a 10^12 tonnes (a cubic metre of water weighs a tonne).) The lure of such vast amounts has tantalised people down the ages, and the amount of gold in the sea particularly so. At a concentration of only 10 parts per trillion (p.p.t.), even this amounts to more than 13 million tonnes. But gold is not the rarest metal in the sea. For example, the concentration of the metal holmium is only 0.4 p.p.t., amounting to only 550,000 tonnes in all. Even so, this is five times as much holmium as is estimated to be in land-based reserves in ores. The 10 most abundant elements in the oceans are given in the table." |