Erosional and climatic effects on long-term chemical weathering rates in granitic landscapes spanning diverse climate regimes
We used cosmogenic nuclide and geochemical mass balance methods to measure long-term rates of chemical weathering and total denudation in granitic landscapes in diverse climatic regimes. Our 42 study sites encompass widely varying climatic and erosional regimes, with mean annual temperatures ranging from 2 to 25 °C, average precipitation ranging from 22 to 420 cm·year -1, and denudation rates ranging from 23 to 755 t·km -2·year -1. Long-term chemical weathering rates range from 0 to 173 t·km -2 year -1, in several cases exceeding the highest granitic weathering rates on record from previous work. 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. A simple empirical relationship based on temperature, precipitation and long-term denudation rates explains 89-95% of the variation in long-term weathering rates across our network of sites. Our analysis shows that, for a given precipitation and temperature, chemical weathering rates increase proportionally with fresh-material supply rates. We refer to this as "supply-limited" weathering, in which fresh material is chemically depleted to roughly the same degree, regardless of its rate of supply from breakdown of rock. The temperature sensitivity of chemical weathering rates is two to four times smaller than what one would expect from laboratory measurements of activation energies for feldspar weathering and previous inter-comparisons 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.