Temporal variations in the marine Sr isotopic composition have been widely used to infer global changes in the rate of continental weathering. Marine isotope variations have been recently reported for Os (Pegram et al., 1992; Ravizza, 1993; Peucker-Ehrenbrink et al., 1995). In both the marine Sr and Os isotope records Cenozoic shifts toward more radiogenic isotope ratios are suggestive of intensified continental weathering. Unfortunately, little is known about the continental runoff of Os, the major single source of Os to seawater.
We have determined the Os-isotopic composition of continental surface waters and shallow groundwaters by analyzing freshwater Fe-Mn nodules and Fe-Mn bog-ores from northern Europe. Based on our results, 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 show intermediate values (187Os/186Os = 12.4-21.2), indicating mixture of North Sea water and continental freshwater. 187Os/186Os values as high as 21.2 in Fe-Mn nodules from the northern Baltic Sea require a similar fraction of Os in the Baltic Sea to be derived from the Precambrian shield areas compared with the fraction of Os entering the Baltic Sea from the Phanerozoic platform. Thus, Os differs from Sr, whose global riverine flux as well as the flux into the Baltic Sea are dominated by weathering of Phanerozoic carbonates, with little contribution from the radiogenic silicate portion of Precambrian shield areas. Variations in the Os isotopic composition of riverine input should therefore be more closely related to the age and composition of the non-carbonate fraction of weathering terrains. Runoff from Precambrian shield areas should be more important for the seawater Os budget than for the marine Sr budget. Isotope mass balance calculations for the Baltic Sea indicate that the concentration of soluble and dissolved Os in continental runoff is similar to the Os concentration of seawater. Extrapolating these data to infer the global runoff of Os, assuming a seawater Os concentration of 1 pg/l, yields ~48 kg/a, comparable with previous estimates (Pegram et al., 1994). The residence time of osmium 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, with radiogenic values characteristic of Precambrian shields. Short periodic fluctuations in the runoff from Precambrian shield areas, presently located at high latitudes, during periods of glaciations are thus more likely to affect the Os than the Sr isotopic composition of seawater. Enhanced erosion of radiogenic Precambrian shield areas during periods of glaciations could promote a more rapid increase in the marine Os isotope record, as seen in the inferred marine Os isotope record for the last 1-2 Ma.
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Ravizza, G., Earth Planet. Sci. Lett. 118, 335-348 (1993).