TOPOGRAPHY AND PRECIPITATION ISOTOPES ACROSS CALIFORNIA AND NEVADA, WITH IMPLICATIONS FOR THE TOPOGRAPHIC EVOLUTION OF THE CORDILLERA OROGEN

Precipitation isotopes have become the preferred method for estimating the topography evolution of the Cordilleran orogen of California and Nevada. A stratigraphic record of ancient precipitation isotopes can be measured using soil carbonates, leaf wax, or hydrated volcanic glass from sedimentary samples. Modern studies of precipitation isotopes provide empirical support for the idea of an “isotopic lapse rate”, where the fractionation of precipitation isotopes is thought to be proportional to the height of the topography upwind of the sample site.

What is missing, however, is a more process-based understanding of the isotopic fractionation associated with orographic precipitation. Key issues in the “Cal-Vada” region are: 1) Is there an isotopic lapse rate, and how is it influenced by variations in global climate?, 2) What are the relative roles of the Coast Ranges and Sierra Nevada as distinct topographic features?, and 3) Do these topographic features cause lifting or blocking?

We use our Orographic Precipitation Isotope (OPI) program to model the precipitation-isotope climatology in the modern, using 159 samples of modern precipitation isotopes in the Cal-Vada region. The estimated sea-level temperature and mean wind speed and azimuth are consistent with reanalysis records of modern precipitation climate records. Our analysis indicates that: 1) there is a good correlation between isotopic fractionation and orographic lifting, but a 5 C increase in temperature will look like a 20% decrease in topography, 2) the controlling feature for orographic fractionation is the maximum elevation upstream of the sample location, 3) the moist stability during modern precipitation events is low, which means that blocking is not important. The overall conclusion is that precipitation isotopes appear to be a useful way to measure upwind topography. In addition, our analysis supports the view that the Sierra Nevada marks the western edge of a high-standing "Nevadaplano", which was established during the Late Cretaceous.