Iridium in Natural Waters

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Abstract
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) × 108 to 5.7 (±0.8) × 108 atoms per kilogram. Rivers contain from 17.4 (±0.9) × 108 to 92.9 (±2.2) × 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 × 103 and 2 × 104 years.