Sharp decrease in long-term chemical weathering rates along an altitudinal transect

Abstract

We used cosmogenic nuclide and geochemical mass balance methods to measure long-term rates of chemical weathering and physical erosion across a steep climatic gradient in the Santa Rosa Mountains, Nevada. Our study sites are distributed along a 2 km ridgeline transect that spans 2090 to 2750 m in altitude, and encompasses marked contrasts in both vegetative cover and snow depth, but is underlain by a single, roughly uniform, granodiorite bedrock. Cosmogenic nuclides in colluvial soils reveal that denudation rates vary by less than a factor of 1.4 (104–144 t/km 2/yr) along this transect. Bulk elemental analyses indicate that, relative to the parent rock, soils are less intensively weathered with increasing altitude, and show little evidence of weathering-related mass losses near the top of the ridge. Chemical weathering rates decrease rapidly with increasing altitude, both in absolute terms (from 24 to 0 t/km 2/yr) and as a fraction of total denudation rates (from 20 to 0%). Thus these results indicate an increasing dominance of physical erosion with altitude. The observed decrease in chemical weathering rates is greater than one would predict from the decrease in mean annual temperature using simple weathering kinetics, suggesting that weathering rates along our transect may also be affected by the progressive decline in vegetative cover and increase in snow depth with increasing altitude. These results, considered together with weathering rate measurements for a wide range of climates in the Sierra Nevada, USA, suggest that chemical weathering rates may be particularly sensitive to differences in climate at higher-altitude sites. Consistent with this hypothesis, chemical weathering rates fall virtually to zero at the highest sites on our transect, suggesting that sparsely vegetated, high-altitude crystalline terrain may often be characterized by extremely slow silicate weathering rates.