VARIATIONS IN CHEMICAL DEPLETION ACROSS AN ALTITUDINAL GRADIENT IN THE SIERRA NEVADA, CALIFORNIA

Chemical weathering supplies nutrients to soils and sediment to streams, and is an important part of many biogeochemical cycles. Spatial variations in the intensity of chemical weathering across a landscape may reflect variations in climate, topography, and vegetation. We present geochemical analyses of soils and bedrock from the eastern Sierra Nevada, California, where climate, topography, and vegetation vary markedly with altitude in a catchment spanning 2 km of relief. Geochemical mass balance methods yield estimates of the Chemical Depletion Fraction (CDF), which measures the intensity of chemical weathering in terms of the ratio of immobile element concentrations in bedrock and soil. As rocks weather, immobile elements become enriched in soils relative to parent bedrock, and the fraction of mass lost due to chemical erosion is reflected quantitatively in the CDF. Across our field sites, CDF decreases with increasing elevation, consistent with previously reported results from elsewhere in the region. The altitudinal trends in CDF correspond to variations in climate and topography: Higher elevations are colder, wetter, and steeper, with little vegetation; lower elevations are warmer, drier, and have gentler slopes and more vegetation. Several soils yield CDFs that are too low, likely reflecting incomplete characterization of parent material. The observed decrease in chemical weathering with elevation likely corresponds to an increase in physical weathering at higher elevations, and may help explain previously observed altitudinal trends in the sizes of sediment produced on hillslopes.