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Abstract
Earth’s surface is the boundary where the atmosphere, hydrosphere, biosphere, and lithosphere interact. Physical erosion and chemical weathering, driven by those interactions, constantly reshape Earth’s topography and modify the chemistry of Earth’s upper crust by delivering sediment and solutes to river networks. These properties of Earth’s surface—its morphology and chemical composition—are of wide interest because they are crucial to the functioning of Earth’s environmental system. What factors set the pace of erosion and weathering? How sensitive are erosion and weathering to those factors? What determines how quickly river networks reorganize? In this thesis, I present my work on the quantitative controls on erosion, weathering, and channel network evolution in three chapters. Each chapter focuses on a different aspect of the main topic and contains new field and laboratory measurements.
In Chapter One, I explore the control and mechanism of drainage divide migration. Key to this are measurements of cosmogenic 10Be in stream sediment, which I use to infer erosion rates in 54 drainage basins in the Qilian Shan of northeast Tibet. My measurements reveal a near-exponential relationship between cross-divide differences in erosion rate and channel-head values of the topographic metric χ, which previous studies have suggested is a measure of geometric disequilibrium between drainage basins. My work provides rare empirical support for the hypothesis that cross-divide differences in χ can reflect the direction and rate of divide migration, and thus may be a useful tool in efforts to understand the controls on divide migration. In Chapter Two, I investigate the influence of climate and dust on soil chemical erosion rate. I measure soil chemical erosion rates, dust deposition rates, and soil climate along a steep climatic gradient at San Jacinto Mountain (SJM) in California, which spans nearly 3 km in elevation and is home to the largest range in mean annual temperature (~18°C) in a single unglaciated lithology in the United States. The measurements are consistent with a weak joint control of temperature and soil moisture on soil chemical erosion, and they reveal that dust is a secondary but non-negligible contributor to soil composition and mass. In Chapter Three, I investigate the sensitivity of erosion rates to topography and climate. I analyze a compilation of basin-averaged 10Be-derived erosion rates across SJM, consisting of 19 previously published erosion rates and 10 newly measured erosion rates. These data reveal a strong topographic and weak climatic control on erosion rates. They also show that SJM’s most prominent drainage divide is migrating south, and that SJM’s high-elevation, low-relief plateau is shrinking under migration from its northern, western, and southern boundaries. Collectively, the results in these chapters represent new efforts to quantify and better understand the controls of erosion, weathering, and drainage basin evolution.
