Abstract

The isotopic composition of osmium (Os) in continental surface waters and shallow ground waters has been determined by analyzing fresh-water Fe-Mn nodules and bog ores. The isotopic composition of the continental runoff from the Precambrian Baltic shield is much more radiogenic (187Os/186Os = 31.2–65.4) than the runoff from the Phanerozoic sedimentary platform to the south of the Baltic Sea (187Os/186Os = 8.6–11.0). Fe-Mn nodules from the Baltic Sea have intermediate values (187Os/186Os = 12.4–21.2), indicating mixture of North Sea water and continental fresh water. Isotope mass-balance calculations indicate that the concentration of Os in the continental runoff is similar to the Os concentration in seawater. Extrapolating these data to infer the global runoff yields ∼48 kg/yr, comparable with previous estimates. The residence time of Os in seawater relative to continental runoff is calculated at ∼32 ka. The oceanic residence time of Os relative to the bulk input to seawater is ∼16 ka, if the mean 187Os/186Os of the continental runoff is 16. The Os isotopic composition of the continental runoff is much more variable than previously suggested, 187Os/186Os values >30 being characteristic of Precambrian shields. Glaciation of old cratonic shields might therefore modulate riverine supply of this very radiogenic cratonic Os, linking accelerated increase in the marine Os isotope record to glaciations of Precambrian shields.

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