Osmium isotopic compositions and concentrations have been determined for deltaic and continental shelf sediments from three major rivers (the Amazon, Changjiang, and Mississippi) and for two loesses from the upper Mississippi River Valley. The loesses have heavy mineral and clay compositions characteristic of Mississippi river and delta sediment. Concentrations range from 15 to 120 pg Os/g, and Os-187/Os-186 ratios range from 8.2 to 10.9. Sediments which are polluted (surface sediment from the Changjiang Estuary) or which have scavenged metals from seawater (Gulf of Mexico slope surface sediment and distal Amazon Shelf sediment) have high osmium concentrations and less radiogenic isotopic com-positions. Purely terrigenous sediments have 15-90 pg Os/g, and Os-187/Os-186 = 10.0-10.9. North American loess and Mississippi Delta sediment osmium isotopic compositions are indistinguishable. We believe that Os-187/Os-186 = 10-11 is a good average for currently eroding upper continental crust-these sediments have neodymium isotopic compositions close to the global mean for river sediment. Terrigenous Os-187/Os-186 is significantly more radiogenic than seawater Os-187/Os-186 (8.6), requiring a nonriverine source of osmium to seawater. Terrigenous osmium is also significantly more radiogenic than bulk pelagic clay osmium. We use the improved estimate of terrigenous Os-187/ Os-186 to recalculate the flux of extraterrestrial osmium to Pacific marine sediment. If the residence age of osmium in an upper crustal region is similar to the neodymium model age of a sediment derived from that region, a model-dependent upper crustal Re-187/Os-186 ratio can be calculated. For the model ages of the sediments analyzed in this study (T(dm) = 1.5-1.7 Ga), model Re-187/Os-186 ratios are 340-360 and can be used to correct for bias resulting from the preferential erosion of young crust. The Os-187/Os-186 ratio of 2.2 Ga crust with Re-187/Os-186 = 400 is 14 +/- 2. We have also used the data from this study to estimate upper crustal osmium and rhenium concentrations of 50 pg/g and 390 pg/g, respectively.
