Mulch A, Sarna-Wojcicki A M, Perkins M E, Chamberlain C P
Institut für Geologie, Leibniz Universität Hannover, Callinstrasse 30, 30167 Hannover, Germany.
Proc Natl Acad Sci U S A. 2008 May 13;105(19):6819-24. doi: 10.1073/pnas.0708811105. Epub 2008 Apr 25.
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 (deltaD(ppt)) and therefore deltaD(ppt) reflects a measure of the magnitude of the rain shadow. Hydrogen isotope compositions of volcanic glass (deltaD(glass)) 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 per thousand) 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.
源自太平洋的水汽形成的地形降水在加利福尼亚内华达山脉中部形成了雨影区，这与山脉南部形成了对比，在山脉南部，源自南方的季节性季风降水掩盖了这种雨影效应。地形降水的去除系统地降低了降水的氢同位素组成（δD(ppt)），因此δD(ppt)反映了雨影效应的程度。在地球表面水化的火山玻璃的氢同位素组成（δD(glass)）提供了一个独特的机会来追踪内华达山脉以及相邻的盆地与山脉省的海拔和降水历史。对来自从太平洋海岸到盆地与山脉省广泛分布的火山灰降落沉积物中的67个有精确年代测定的火山玻璃样品的分析表明，在0.6至12.1百万年前，大气降水的氢同位素组成从内华达山脉中部的迎风侧到背风侧呈现出大幅（>40‰）下降，这与那段时间存在现代规模的雨影效应一致。中新世至今持续存在规模恒定的雨影效应以及纵向气候和降水模式的系统性变化的证据有力地表明，内华达山脉现代的一级地形要素至少在过去1200万年中塑造了这片地貌。
