Tectonic and climatic control of physical erosion rates and chemical weathering rates in the Sierra Nevada, California, inferred from cosmogenic nuclides and geochemical mass balance.

摘要

The relationships among erosion, weathering, climate, and tectonic uplift have remained uncertain, because long-term rates of erosion and weathering have rarely been measured. Here, I present cosmogenic nuclide data that quantify landscape-scale denudation rates from topographically diverse catchments at seven climatically distinct sites, all in Sierra Nevada granites. I also present a new geochemical technique for measuring long-term chemical weathering rates and apply this technique to a subset of the Sierra catchments. Together, these data quantify how rates of physical erosion and chemical weathering interrelate, and how they vary with topography and climate.;In chapter one, I show that two different patterns emerge from the cosmogenic nuclide measurements of denudation rates. At three sites, denudation rates and hillslope gradients are strongly correlated, increasing with proximity to faulting and canyon incision, which appear to have locally accelerated base-level lowering rates, and thus increased denudation rates by up to 15-fold. At four other sites, far from faults and canyons, denudation rates are more uniform and less sensitive to gradients. These measurements show that contrasts in denudation rates cannot be inferred from gradients alone, because landscapes can evolve such that steep and gentle slopes erode at similar rates.;In chapter two, I use the cosmogenic data to quantify how denudation rates are affected by site-to-site differences in average precipitation (spans 20--180 cm/yr) and mean annual temperature (spans 4--15°C). Average denudation rates vary by less than 2.5-fold across the sites and are not correlated with precipitation or temperature, indicating that climate only weakly regulates non-glacial denudation rates in mountainous granitic terrain.;In chapter three, I show how long-term chemical weathering rates can be measured using a mass balance technique that combines denudation rates, inferred from cosmogenic nuclides, with dissolution losses, inferred from the rock-to-soil enrichment of insoluble elements. I apply this technique to 22 catchments that span a wide range of denudation rates and climates. Chemical weathering rates correlate strongly with denudation rates but only weakly with climate, implying that, by regulating denudation rates, tectonic uplift may be an important regulator of weathering rates.