CHEMICAL WEATHERING, PHYSICAL EROSION, AND CLIMATE: A COSMOGENIC PERSPECTIVE

Understanding the evolution of geochemical and geomorphic systems requires measurements of long-term rates of physical erosion and chemical weathering. Recently, cosmogenic nuclides such as 10Be and 26Al have become important new tools for measuring long-term denudation rates. Cosmogenic nuclides are produced in-situ within mineral grains by cosmic ray bombardment. Because this cosmic ray flux is attenuated by the mass overlying the sample, the accumulated cosmogenic nuclide concentration in a sample records how rapidly the overlying mass (and thus the cosmic ray shielding) has been removed, either by physical erosion or chemical weathering. Thus cosmogenic nuclides can be used to measure the total denudation flux (the sum of the rates of physical erosion and chemical weathering). We have recently shown how this total denudation flux can be partitioned into its physical and chemical components, using the enrichment of insoluble tracers (such as Zr) in regolith relative to its parent rock. Thus by combining cosmogenic nuclide measurements with the bulk elemental composition of rock and soil, we can measure rates of physical erosion and chemical weathering over 1000- to 10,000-year time scales.

We have measured long-term chemical weathering rates for 42 sites in granitic terrain, encompassing widely varying climates and denudation rates. Chemical weathering rates are highest at the sites with rapid denudation rates, consistent with strong coupling between rates of chemical weathering and mineral supply from breakdown of rock. The temperature sensitivity of chemical weathering rates is 2 to 4 times smaller than what one would expect from laboratory measurements of activation energies for feldspar weathering and previous intercomparisons of catchment mass-balance data from the field. Our results suggest that climate change feedbacks between temperature and silicate weathering rates may be weaker than previously thought, at least in actively eroding, unglaciated terrain similar to our study sites. To the extent that chemical weathering rates are supply-limited in mountainous landscapes, factors that regulate rates of mineral supply from erosion, such as tectonic uplift, may lead to significant fluctuations in global climate over the long term.