ND AND SR ISOTOPIC MASS BALANCE OF DISSOLVED, SUSPENDED AND BED LOAD OF THE POWDER RIVER, WY AND MT: INSIGHTS INTO WEATHERING PROCESSES IN AN ARID WATERSHED

The Powder River flows north from headwaters in central Wyoming to its confluence with the Yellowstone River in eastern Montana. The 87Sr/86Sr ratio of dissolved Sr generally decreases downstream along the length of the river. The radiogenic 87Sr/86Sr of river water near the headwaters reflects the input of radiogenic Precambrian erosional debris from the Bighorn and Granite Mountains. Downstream the Sr isotopic ratio decreases because of the addition of Sr from younger, less radiogenic Phanerozoic sedimentary rocks. The 87Sr/86Sr ratio of the suspended sediment carried by the Powder River is more radiogenic than the dissolved Sr, but like the dissolved Sr, the isotopic ratio decreases downstream. This suggests that although the dissolved load appears to carry a larger contribution of less radiogenic Sr from soluble sources such as sulfate and carbonate, Sr in both dissolved and suspended loads becomes less radiogenic as lower 87Sr/86Sr dissolved Sr is added to the water and as radiogenic suspended sediment falls out of the water column and is replaced by local, less radiogenic material. The 87Sr/86Sr ratio of the bedload is more radiogenic than either dissolved or suspended load Sr, and the ratio does not correlate with sample location. These results imply that the sources of sand to gravel size bedload are not identical to those that supply Sr to the dissolved and suspended load.

The Nd content of the dissolved load is negligible compared to that in the suspended load (~25 ppm Nd) and bed load (~15 ppm Nd). Both suspended and bed load have Sm/Nd ratios suggesting that rare earth element (REE) sources to the river are from typical, light REE-enriched continental crust. The 143Nd/144Nd of the suspended sediment is slightly more radiogenic on average than that of the bed load, resulting in slightly older Nd depleted-mantle model ages for the bed load compared to suspended load (1.9 Ga vs 1.7 Ga). There is no variation in Sm-Nd isotopic characteristics as a function of location along the river, suggesting that Nd has been efficiently recycled from Precambrian basement into younger sedimentary rocks such that all sources of fine and coarse sediment carried by the Powder River have nearly indistinguishable Nd isotopic compositions.