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

关于内华达州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，内华达州山脉是一个年轻的山脉，在过去的1000万年里，它的表面净抬升了大约2公里。 相反，最近的研究表明，内华达州山脉是一个长期存在的地形特征。 以前对内华达州东侧沿着剖面火山灰风化产生的蒙皂石的氢和氧同位素组成的分析表明，在中央盆地和山脉，蒙皂石的氧同位素在过去的1600万年中略有增加。 这一结果与南部盆地和山脉的一个剖面形成对比，该剖面显示氧同位素从1400万年到600万年大幅增加。 这些结果表明，至少在过去的1600万年里，内华达州山脉一直是一个长期存在的地形特征。 此外，如果这些同位素的变化仅仅是地形变化的结果，那么塞拉利昂有一个空间变化的地形历史，与一些700米的地形损失在北方塞拉利昂和~ 2000米的降低南部塞拉利昂在晚新生代。 这些数据也可以解释为晚新生代夏季风的加强，这可能导致同位素更重的降水，特别是在盆地和山脉的南部地区。这项研究是检验假设，即内华达州的地形历史的空间变化：1）收集同位素数据，从南部盆地和山脉，以分离地形和气候的影响;（2）绘制海盆和山脉的区域同位素图，以加深对该地区区域地形和气候历史的了解;（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什么同位素变化; 4）从其他基质，特别是哺乳动物化石中收集同位素数据，以减少与任何一种同位素替代物有关的不确定性。

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.