Chemical weathering rates and strontium isotopes

Combining laboratory dissolution experiments, a field weathering study and geochemical modeling, I have developed a more rigorous quantitative relationship between silicate weathering rates and Sr isotopes. This work provides insights into using the marine Sr isotope record as an indicator of average global weathering rates in the geologic past and will help to improve the use of Sr isotopes as tracers of mineral weathering reactions in modern catchment studies. I have investigated a number of the factors that can affect both the total Sr flux to the oceans and the isotopic ratio of that flux. Plagioclase dissolution experiments reveal that plagioclase weathering, which is a major source of silicate Sr, is highly dependent on solution saturation state. Significant decreases in the Sr release rate are also observed as the weathering solution becomes increasingly saturated. Biotite dissolution experiments were also performed because biotite represents a major source of 87Sr. Slight changes in biotite dissolution rates can significantly affect the isotopic ratio of a weathering solution. These experiments were used to quantify the dependence of biotite weathering and Sr release rates on important factors such as pH, temperature and solution saturation state. The mineral dissolution experiments also provided insights into the extent to which disseminated calcite contributes to the Sr budgets of silicate weathering.; The field study compared the weathering rates of three common silicate lithologies in a setting where other factors that can affect weathering rates were held essentially constant. The relative overall weathering rates, CO 2 consumption rates and Sr release rates were measured for basalt, granite and tonalite. Both present day stream fluxes and long-term average rates based on soil depletion profiles were measured for all three rock types. The results of this field project can be used to predict the extent to which variations in the marine Sr isotope ratio are driven by changes in the paleogeology of the earth's surface.; Finally, I developed a model that predicts the effect of the emplacement and subsequent weathering of the Columbia River Basalt on the marine Sr record of the Miocene. Using a coupled fluid flow---mass transfer model to calculate the Sr flux resulting from the weathering of the young Columbia River basalts, I show that the dissolution of the reactive phases in the extensive continental flood basalt could have caused the significant inflection in the marine Sr isotope record that occurred approximately 15 million years ago. As this model is based on the kinetics and thermodynamics of mineral dissolution, it allows me to relate changes in the Sr record to changes in global CO2 consumption and chemical erosion rates.; Approaching the relationship between Sr isotopes and chemical weathering rates from a more quantitative perspective should improve the use of Sr isotopes as a tracer of chemical weathering rates both in modern environments and in the geologic past.