Quantifying how Climate Affects Long-Term Rates of Weathering, Erosion, and Soil Development

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James W. KirchnerEAR-0000999 Physical erosion and chemical weathering are interdependent processes that sculpt mountainous landscapes, regulate the composition of soils, deliver sediment and solutes to aquatic habitats, and, over long timescales, help regulate global climate. Until recently, long-term rates of erosion and chemical weathering have been difficult to measure. As a result there is presently no clear consensus about how erosion and weathering affect the evolution of landscapes, soils, aquatic habitats, and climate. Recent advances now permit long-term average rates of physical erosion and chemical weathering to be inferred from the chemical composition of eroding sediment in watersheds. The concentrations of cosmogenic nuclides (26Al and 10Be) can be used to measure long-term rates of erosion. Chemical weathering rates can be inferred from erosion rates by comparing the concentrations of insoluble elements (such as Zr and Nb) in rocks and soils, which reflect the fraction of erosion that is accounted for by chemical weathering. This project will measure how chemical weathering rates vary with climate and physical erosion rates. Preliminary results show that climatic effects on weathering rates are small compared to erosional effects. However, the climatic effects are measurable, when the effects of erosion are explicitly accounted for. To expand on these results, six sites have been selected that span a wide range of climates (4 to 24 C in temperature; 10 to 420 cm/yr in precipitation). To minimize effects of variations in bedrock weathering susceptibility, this study will focus exclusively on granitic bedrock. At each of the six sites, 5 small watersheds will be selected, and samples of stream sediment, bedrock, and hillslope soils will be collected for analysis. Cosmogenic nuclide concentrations in the stream sediment will be used to measure long-term erosion rates. The fraction of erosion that is accounted for by chemical weathering than will be inferred by comparing the bulk chemical composition of soil and bedrock. This project will quantify how long-term rates of weathering and erosion vary with climate while, for the first time, explicitly measuring how chemical weathering and physical erosion interrelate. These results should contribute to better models of nutrient cycles and long-term climatic evolution, more accurate assessments of sediment and solute delivery to aquatic ecosystems, and a more quantitative understanding of soil development and landform evolution.