Osmium in the rivers

Abstract

There is a large uncertainty in our understanding of the behavior of osmium during weathering and transport into deep oceans and the osmium budget of the oceans. The problem stems chiefly from the lack of osmium data on the dissolved load in the rivers and in the estuaries. In this study, the concentration and isotopic composition of osmium have been determined in three North American rivers (the Mississippi, the Columbia, and the Connecticut) and one river draining central Europe and flowing into the Baltic Sea (the Vistula). Osmium concentration in the Mississippi and the Vistula is about 45 femto mol kg^(−1); it is about 14 and 15 femto mol kg^(−1) for the Connecticut and the Columbia, respectively. The ^(187)Os/^(186)Os ratios estimated for the Mississippi and the Vistula are 10.4 and 10.7, respectively. For the Connecticut and the Columbia ^(187)Os/^(186)Os = 8.8 and 14.4, respectively. Of all the rivers examined, the Mississippi is by far the largest, supplying ∼1.6% of the total annual world river flow. Its osmium isotopic composition is identical to the upper Mississippi valley loesses (Esser and Turekian, 1993a) indicating (1) congruent dissolution of the bedrock and (2) little or no impact of anthropogenic sources on the osmium isotopic composition of the dissolved load. The latter observation indicates that the upper limit of the anthropogenic input in the dissolved osmium load of the Mississippi outflow is about 250 g yr^(−1). While the osmium concentration of the Vistula is high the isotopic composition does not appear to have been affected by substantial pollution. The river data can be used to put limits on the mean residence time of osmium in the oceans (T_(Os) and on the osmium budget of the oceans. If the bulk river influx of dissolved osmium into the oceans is similar to that of the Mississippi, we get a value for the net riverine inflow of osmium of 1680 mol yr^(−1). If there were no sequestering of osmium in the estuaries, this would give a value of T_(Os) of = 1.3 × 10^4 years. As the estuaries may trap a significant amount of river osmium, this is a lower limit so that T_(Os) » 1.3 x 10^4 years. Using the iridium data by Anbar et al. (1996), the Os/Ir ratio for the Vistula is 2.8. We infer that there is no large fractionation between osmium and iridium during the transport of these elements from the continents into the oceans. It follows that the high Os/Ir ratio (∼22) observed in seawater must be related to the different rain out mechanisms of these elements in the estuaries or in the oceans.

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