Understanding patterns of erosion and sedimentation in mountains is important for Earth scientists and land-use managers alike. Over management timescales, eroded sediment affects water quality, influences the amount and quality of aquatic habitat in rivers, and determines the lifespan of both natural and manmade reservoirs by setting the pace of sedimentation within them. Over longer, geologic timescales, eroded sediment also influences landscape erosion by providing rivers with the tools they need to cut into underlying bedrock and thereby adjust to changes in climate and tectonic forcing. Thus, the erosion of sediment is both the product of and a key driving force behind landscape change. Work funded by this grant will develop new methods for interrogating sediment about where it comes from, how it is generated, and how fast it moves across landscapes. This should permit progress on understanding erosional processes and how they influence landscape response to climatic and tectonic forcing.
Geologists have long recognized that sediment contains a wealth of information about its journey from intact rock on hillslopes to the jumbles of particles that cover modern riverbeds and fill ancient sedimentary deposits. Extraction of this information has become increasingly sophisticated with recent technological and methodological advances. As a result, understanding of surface processes has become increasingly quantitative. For example, rates of erosion from slopes can now often be measured from cosmogenic nuclides, which build up in sediment grains when they are near Earth's surface. Meanwhile, recent studies have shown that the clockwork-like buildup of radiogenic helium in the mineral apatite can be used as a fingerprint of where sediment comes from on catchment slopes. Here, this apatite-helium tracing technique will be used together with cosmogenic nuclides in a completely new way, to simultaneously unveil spatial variations in both the erosion rates and sizes of sediment produced on hillslopes by bedrock weathering. This marks an important advance in sediment tracing; until now, there was no way to quantify how the sizes of eroded sediment vary over catchment scales. Preliminary results show a connection between the elevation of slopes and the sizes of sediment that they produce by weathering and erosion. Higher-elevation slopes, which are colder and less vegetated, produce coarser sediment. This points to climate as a key regulator of sediment size. The proposed research will test the new sediment tracing approach in a steep catchment and explore hypotheses about linkages between climate and the sizes of eroded sediment. Expected outgrowths of this research include fresh insight on common, but as-yet incompletely understood downstream trends in landscapes, such as: fining in grain size in mountain streambeds; shifts from braided to meandering channel forms; and changes in aquatic habitats and the organisms that populate them.
