CHEMICAL WEATHERING AND SOIL DEVELOPMENT IN A POST-GLACIAL SIERRAN CATCHMENT

Bedrock weathering contributes directly to the development of soils and ecosystems, turning rock into soil, liberating rock-derived nutrients for biota, and contributing to global carbon cycling. After glacial retreat, weathering of fresh bedrock forms saprolite and soils, and weathering rates and intensity should vary with both time and soil depth. Here we present weathering intensities from a down-valley transect in the Sabrina Lake Basin, in the Eastern Sierra Nevada, which was deglaciated at about 13 ka. Across 6 sites spanning 5 km and over 400 m of elevation, we measured bulk geochemistry in fresh rock, saprolite, and soils using X-ray Fluorescence. Using a mass-balance approach, we quantify weathering intensity in saprolite and soils across a range of soil depths from <10 to 30 cm. We find that, regardless of soil depth, saprolites are chemically indistinguishable from parent bedrock, and the majority of chemical weathering in this system occurs in soils. This suggests that physical rather than chemical weathering processes are almost entirely responsible for soil production at the soil-saprolite boundary, and rock-derived nutrients are derived from weathering within established soils. This work has important implications for biota, including microbes and plants that rely on rock-derived weathering products, and on landscape evolution in post-glacial landscapes.