ITWire

The paper (“A Miocene to Pleistocene climate and elevation record of the Sierra Nevada (California )” by Mulch and his colleagues is published on the online Early Edition of the Proceedings of the National Academy of Sciences.

Mulch’s team includes A.M. Sarna-Wojcicki (United States Geologic Survey, Menlo Park, California, U.S.A.), M.E. Perkins (Geology and Geophysics, University of Utah, Salt Lake City, Utah, U.S.A.), and C.P. Chamberlain (Geological and Environmental Sciences, Stanford University, Stanford, California, U.S.A.).

Actually, Mulch performed the research while he was a postdoctoral student at Stanford University.

The abstract to the paper states, “Orographic precipitation of Pacific-sourced moisture creates a rain shadow across the central part of the Sierra Nevada (California) that contrasts with the southern part of the range, where seasonal monsoonal precipitation sourced to the south obscures this rain shadow effect. Orographic rainout systematically lowers the hydrogen isotope composition of precipitation ( Dppt) and therefore  Dppt reflects a measure of the magnitude of the rain shadow.”

“Hydrogen isotope compositions of volcanic glass ( Dglass) hydrated at the earth's surface provide a unique opportunity to track the elevation and precipitation history of the Sierra Nevada and adjacent Basin and Range Province. Analysis of 67 well dated volcanic glass samples from widespread volcanic ash-fall deposits located from the Pacific coast to the Basin and Range Province demonstrates that between 0.6 and 12.1 Ma the hydrogen isotope compositions of meteoric water displayed a large (>40 ) decrease from the windward to the leeward side of the central Sierra Nevada, consistent with the existence of a rain shadow of modern magnitude over that time.”

“Evidence for a Miocene-to-recent rain shadow of constant magnitude and systematic changes in the longitudinal climate and precipitation patterns strongly suggest that the modern first-order topographic elements of the Sierra Nevada characterized the landscape over at least the last 12 million years.”