EESS Winter Seminar Series - Kate Maher, Professor, Geological and Environmental Sciences, Stanford University
EESS Winter Seminar Series - Kate Maher, Professor Geological and Environmental Sciences, Stanford University, "The Last Rainfall Maximum in the western U.S. based on uranium isotopes in soil materials", Abstract: There are very few quantitative measures of past rainfall. To address this problem, we developed an approach that uses U isotopes in soil minerals to reconstruct past infiltration and precipitation over approximately the last 100,000 years. The approach uses high spatial resolution secondary ion mass spectrometry (Stanford/USGS SHRIMP-RG) to determine the age and U isotopic composition of secondary minerals formed in soils. Using this information, we develop a reactive transport model that links the U isotopes to the infiltration flux through the soil, and ultimately to precipitation. Records for several sites across the western U.S., including the Great Basin and Mojave Desert provinces, indicate a maximum in precipitation between 25 and 30 ka, just before the Last Glacial Maximum (19-25 ka) and nearly 10 ka before the regional lake highstands. This timing and magnitude of these changes place new constraints on the interpretation of lake and oxygen isotope records in the region. An overview of the approach and a discussion of the implications for hydrology of the western U.S. will also be presented.