Landscape response to climate forcing: Quantifying mechanisms and rates of erosion and weathering along an orographically-enhanced climate gradient.

摘要

Landscapes evolve in response to external forcings such as climate and tectonics that influence the pace and patterns of erosion and weathering processes. Furthermore, internal feedbacks between erosion and weathering regulate the expression of these forcings on upland soil-mantled landscapes. This thesis quantifies links between climate, erosion, weathering and hillslope form along a Sierra Nevada climate gradient. We quantify denudation rates and partition them into physical and chemical components using cosmogenic radionuclide (CRN) 10Be concentrations in saprolite and stream sediments, and zirconium concentrations in soil, saprolite and rock. Furthermore, we present an adapted set of equations to calculate these rates that specifically accounts for deep saprolite weathering not captured by CRN-derived surface denudation rates. We also quantify dominant soil production and transport processes through field observations and both profiles and inventories of fallout radionuclides 137Cs and 210Pb. Our data show clear patterns in erosion and weathering across a climate gradient, and together suggest that climate directly controls chemical weathering by temperature and water-availability control on reaction rates, and that climate influences erosion rates by modulating the efficacy of individual soil transport mechanisms and controlling pace and patterns of chemical weathering processes that reduce rock strength and increase erodability. These results also suggest that saprolite weathering, a process rarely factored into geomorphology studies, plays an important and previously unquantified role in the evolution of upland soil-mantled landscapes. Lastly, we examine morphometric expression of climate and dominant soil transport processes using high resolution laser altimetry data, erosion rates, and calculations of 1D sediment flux. Our tests of predicted functional relationships between hillslope morphology and erosion indicate that current transport laws may have limited applicability at studied Sierran sites and similar landscapes because they do not account for variations in chemical weathering with topography and erosion. Together, our findings are an important contribution to geomorphic understanding of landscape response to climate forcing, and identify previously unquantified controls on the evolution of soil mantled landscapes.