DISENTANGLING CLIMATIC AND TOPOGRAPHIC CONTROLS ON CHEMICAL WEATHERING IN THE EASTERN SIERRA NEVADA

Chemical weathering impacts landscapes by reducing rock strength and sediment size, turning rock to soil, supplying nutrients to soils, and influencing long-term global climate through the drawdown of CO₂. Topographic and climatic controls on chemical weathering have been documented by previous work, but disentangling their effects is difficult because they covary in many mountain landscapes. However, the relative effects of climate and topography can be isolated by careful study design. Here we quantify weathering rates and intensity across small transects than span a range of local slope angle (2-30 degrees), which should drive erosion rates and residence time in the weathering zone. Each transect is separated by 200-300 m of elevation for a total range of nearly 2 km total relief, spanning a climatic gradient at the scale of entire catchments. We focus on two sites in the eastern Sierra Nevada, California, where previous work has suggested opposite trends in weathering intensity with elevation despite similar climate, bedrock, and vegetation. We hypothesize that local slope, and thus residence time in the weathering zone, is the primary control on chemical weathering intensity at these sites, rather than altitudinal differences in climate. We test this hypothesis by quantifying soil production and weathering rates using cosmogenic ¹⁰Be in saprolite and bulk geochemistry of soil, saprolite, and parent materials across our sites. If topographic controls on residence time are indeed the dominant control on weathering, we expect sites with steeper slopes to have faster soil production rates and lower weathering intensity, regardless of their elevation in the catchment. Our preliminary results on a single hillslope show that most saprolite samples with higher local slope values have higher soil production rates. By examining the relationships between local slope, soil production rates, and weathering, we aim to tease apart the relative controls of climate and topography on weathering rates and intensity in the Eastern Sierra Nevada.