ROCK TO SEDIMENT—SLOPE TO SEA WITH ¹⁰BE—RATES OF LANDSCAPE CHANGE

Measurements of cosmogenic nuclides, predominately ¹⁰Be, allow new insights into the ways in which and the rates at which sediment is generated, transported, and deposited over timescales ranging from 10³ to 10⁶ years. Samples from rock exposures are used to estimate erosion rates at points on the landscape, whereas samples of fluvial sediment provide estimates of basin-scale rates of denudation integrated over <1 to >10⁴ km². Nuclide data show that hilltop, bare rock outcrops erode more slowly than basins as a whole, suggesting the potential for relief to increase over time as well-drained outcrops grow higher. More elaborate experiments and interpretive models provide insight into the distribution of hillslope processes, including the bedrock-to-soil conversion rate, which appears to increase under shallow soil cover and then decrease under deeper soils. Changes in average nuclide activity down slopes can be used to estimate grain speed over millennia, suggesting, for example, that sediment on desert piedmonts moves, on average, decimeters to meters per year. In other cases, changes in nuclide activity down river networks or along shorelines can be interpreted with mixing models to indicate sediment sources. Sediment deposition rates in otherwise undateable deposits can now be estimated by analyzing samples collected from depth profiles. Over the past decade, the analysis and interpretation of cosmogenic nuclides has given geomorphologists an unprecedented opportunity to measure rates and infer the distribution of geomorphic processes across Earth's varied landscapes. Long-standing models of landscape change can now be tested quantitatively.