Iridium, commonly used as a tracer of extraterrestrial material, was measured in rivers, oceans, and an estuarine environment. The concentration of iridium in the oceans ranges from 3.0 (± 1.3) x 108 to 5.7 (±0.8) x 108 atoms per kilogram. Rivers contain from 17.4 (± 0.9) x 108 to 92.9 (± 2.2) x 108 atoms per kilogram and supply more dissolved iridium to the oceans than do extraterrestrial sources. In the Baltic Sea, ~75% of riverine iridium is removed from solution. Iron-manganese oxyhydroxides scavenge iridium under oxidizing conditions, but anoxic environments are not a major sink for iridium. The ocean residence time of iridium is between 2 x 103 and 2 x 104 years.
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