9628724 Johnsson Large drainage basins are complex systems in which sediment is generated, transported, stored, and, after varying amounts of time, either buried beyond the depth of reworking or exported to the World's oceans. The rate at which sediment is generated in such drainage basins and the residence time of that sediment as it moves, grain by grain, down the fluvial transport system are poorly known; yet measuring these rate is prerequisits to understanding dynamic interactions between the solid Earth and the hydrosphere. Over the past decade, advances in mass spectrometry have allowed reliable measurement of extremely rare nuclides such as those produced as cosmic rays interact with rock. Because cosmogenic nuclide abundances (when normalized for exposure location) reflect the residence time of material within the uppermost several meters of Earth's surface, simple analytical models (beholden to a variety of assumptions) have been used to estimate "cosmogenic" exposure ages and erosion rates of bedrock outcrops and boulders. During the past two years, we have been funded by Hydrologic Sciences to test an interpretive model (Bierman and Steig, 1992, 1995; Brown et al., 1995; Granger et 1994a,b) which suggests that measurement of cosnogenic 10Be and 26A1 in sediments can be used to estimate the rate at which drainage basins erode and constrain the transport system, and out of the basin. Because our initial data (Bierman et apt, 1995, in review) and the data of others (Brown et al., 1995; Granger et al., l994a,b), suggest that cosmogenic nuclides are indeed useful tools by which to study hydrologic processes in small end moderate size drainage basins, we seek funding to expand our current research to larger scales and over longer tine frames. The overall objective of our proposed research is to determine at what scale and in what fluvial regimes, cosmogenic nuclides remain useful tools for 1) determining basin-scale, spatially-averaged erosion rates and 2) placing limits on the duration of sediment storage. In order to meet this objective, we seek funding for isotopic and other corollary measurements needed to determine the spatial variation in 10Be and 26A1 abundances in approximately 80 samples collected from locations throughout the drainage networks of four tectonically and climatically distinct fluvial systems.: the Ganges (Himalayas), the Apure (Andean Mountains), the Sagavanirktok (Alaska, North Slope) and the East Alligator (northern Australia). Testing the spatial and temporal variation of 10He and 26A1 abundance in sediments of some of the World's major rivers will constrain the scale of applicability of existing interpretive models, a significant advance in Isotope Geochemistry. The data we generate regarding the relative duration of sediment storage and model rates of basin scale denudation will be of fundamental significance in the fields of Sedinentology, Hydrology, end Geomorphology. In the sense that we cannot anticipate the outcome of our measurements, some might consider our proposal "high risk science In light of the success we and others have had using 10Be and 26A1 to estimate erosion rates and constrain sediment storage times in smaller catchments (Biermam et al., 1995, in review; Brown at al., 1995; Granger et el., 1994a,b), and because of the fundamental nature of the questions we are addressing, we feel such a "risk" is minimal and worth taking.
