Collaborative Research: Spatially Varying Topographic and/or Climatic History of the Sierra Nevada Mountains

关于内华达山脉何时发展成为一个重要的地形特征，以及它如何影响该地区过去的气候，存在相当大的争论。一种观点认为，内华达山脉是一个年轻的山脉，在过去的1000万年里，它的净表面隆起了约2公里。相比之下，最近的研究表明，内华达山脉是一个长期存在的地形特征。先前对内华达山脉东侧部分火山灰风化产生的蒙脱石的氢和氧同位素组成的分析表明，在盆地和山脉的中央，蒙脱石的氧同位素在过去的1600万年中略有增加。这一结果与盆地和山脉南部的一个剖面形成对比，该剖面显示，从1400万年到600万年，氧同位素大量增加。这些结果表明，内华达山脉至少在过去的1600万年里一直是一个长期存在的地形特征。此外，如果这些同位素变化仅仅是由地形变化引起的，那么西拉山脉的地形历史在空间上是变化的，晚新生代北西拉山脉的地形损失约700 m，南西拉山脉的地形下降约2000m。这些数据也可以用晚新生代夏季风的加强来解释，这可能导致同位素较重的降水，特别是在盆地和山脉的南部地区。本研究通过以下方法验证了内华达山脉具有空间变化地形历史的假设：1)收集了南部盆地和山脉的同位素数据，以便将地形影响与气候影响分开；2)构建盆地和山脉区域同位素图，增强对该区域地形和气候历史的认识；3)在更大的时间范围内取样，以确定年龄小于200万年和年龄大于1600万年的岩石中发生了哪些同位素变化；4)收集来自其他底物的同位素数据，特别是哺乳动物化石，以减少与任何一种古降水同位素代用品相关的不确定性。

There is considerable debate as to when the Sierra Nevada developed as a significant topographic feature and how it affected the past climate of this region. One view holds that the Sierra Nevada is a young mountain range, with net surface uplift of some 2km in the last 10 million years. In contrast, recent work suggests that the Sierra Nevada has been a long-standing topographic feature. Previous analysis of the hydrogen and oxygen isotope composition of smectite produced by the weathering of volcanic ashes from sections along the east side of the Sierra Nevada shows that, in the central Basin and Range, the oxygen isotopes of smectite increased slightly in the last 16 million years. This result contrasts with that of one section in the southern Basin and Range, which shows a large increase in oxygen isotopes from 14 to 6 million years. These results indicate that the Sierra Nevada has been a long-standing topographic feature at least for the past 16 million years. Moreover, if these isotopic shifts result solely from topographic change, then the Sierra has a spatially varying topographic history, with some 700 m of topographic loss in the northern Sierra and ~2000m of lowering of the southern Sierra in the Late Cenozoic. The data can also be explained by a strengthening of the summer monsoon in the Late Cenozoic that could result in isotopically heavier precipitation, particularly in the southern areas of the Basin and Range.This study is testing the hypothesis that the Sierra Nevada had a spatial varying topographic history by: 1) collecting isotopic data from the southern Basin and Range in order to separate topographic from climatic effects; 2) constructing a regional isotopic map of the Basin and Range to enhance understanding of the regional topographic and climactic history of this region; 3) sampling over a larger time range to determine what isotopic changes have occurred both in rocks younger than 2 million years and older than 16 million years; and 4) collecting isotopic data from other substrates, particularly fossil mammals, to reduce the uncertainty associated with any one isotopic proxy for paleoprecipitation.