Measuring long-term, mineral-specific weathering rates in diverse climatic settings

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ABSTRACTChemical weathering and physical erosion are interdependent processes that sculpt landscapes, regulate the composition of soils, and deliver solutes and sediment to streams. They also play critical roles in Earth's long-term climatic evolution. Silicate weathering is the long-term sink for atmospheric CO2. Hence, to the extent that silicate-weathering rates increase with temperature in natural settings (as theoretical considerations and laboratory experiments indicate that they should), weathering feedbacks will, over millions of years, buffer Earth's climate against large temperature shifts. To the extent that silicate-weathering rates depend on rates of mineral supply from physical erosion, tectonic forcing of erosion may affect Earth's long-term climatic evolution. Quantifying these mechanisms has remained difficult, however, because weathering rates of individual mineral phases have rarely been measured under field conditions.Recently developed methods now allow long-term rates of physical erosion and chemical weathering to be measured from the chemical and isotopic composition of actively eroding soils.Concentrations of cosmogenic nuclides in actively eroding soils can be used to infer their long-term denudation rates. We have recently shown that these denudation rate estimates can be combined with mass balance calculations (based on the bulk chemistry of soils and their parent bedrock), to yield measurements of long-term chemical weathering rates.Intellectual meritHere we propose to further extend these recent advances, by extending our cosmogenic methods to measure weathering rates of individual mineral phases. These methods should be widely applicable, because they do not require unusual field situations; by contrast, conventional soil mass-balance methods typically require non-eroding soils of known age and known initial composition. Thus these methods should provide an important new tool for measuring mineral weathering rates in the field.In previous work we have used cosmogenic nuclides to measure denudation rates and bulk chemical weathering rates in a widespread network of sites spanning diverse climatic regimes. Here we propose to measure the mineral phase composition of soils and parent materials at four of these sites, and thus quantify the weathering rates of each of the major mineral phases. Because this project leverages our previous measurements of cosmogenic nuclides and bulk geochemistry, it can be pursued very cost-effectively. Moreover, our proposed XRD, thin section, and microprobe analyses will use samples that we have already collected, thus eliminating field expenses.Across the proposed sites, mean annual temperatures span a range of 2 to 25C, and average precipitation spans a range of 67 to 420 cm/yr. This project aims to identify and quantify the effects of climate on long-term mineral weathering rates. Because long-term denudation rates have already been measured in previous work, and because bedrock mineral composition will be quantified as part of the proposed work, it should be possible to explicitly account for potentially confounding effects arising from site-to-site differences in rates of mineral supply from erosion of bedrock. Hence, these results should contribute to better models of nutrient cycles and long-term climatic evolution, and to a more quantitative understanding of mineral weathering and soil development processes.Broader impactsThis study will provide postdoctoral training for one research associate. Undergraduates will gain hands-on research experience through involvement in many phases of the project. Results from this work should find application in a wide range of fields, including geomorphology, geochemistry, and pedology.