(ESH) The Development of the Asian Monsoon: Seasonal Isotopic Variation in the Paleorivers of Nepal, 12 Ma to 1 Ma

9510033德特曼亚洲季风是地球上最大、最重要的天气系统之一。强烈季风气候的发展被认为是巴基斯坦和尼泊尔C4草原扩张和土壤碳酸盐氧同位素转移的可能原因。这项研究将研究现代季风的发展，一种由青藏高原高地主导的季风，今天几乎排除了任何旱季降水。文石软体动物的同位素组成将被用来重建喜马拉雅山麓12-1 Ma(距今100万年)河流的季节性同位素变化。通过结合每周或每月微采样贝壳生长增量的18O与基于化石植物组合的温度估计相结合，可以计算出河水18O的季节变化。这些测量的目的是按季节检查河水的同位素组成，以便估计旱季的干旱程度和降雨量的季节性变化。旱季地表水的18O值为正，表明季节性干旱严重。季节降雨量可以粗略估计，因为降雨量的同位素组成是低纬地区降雨量的函数。壳牌将河流中13C的碳碳化。这使得18O记录的变化与该地区植物生物量的变化(C3与C4)直接相关。尼泊尔将是这项研究的主要重点，因为保存完好的软体动物在地层中的分布很有用，但巴基斯坦的一些贝壳层也将被研究，以便于比较受亚洲季风影响的不同地区。

9510033 Dettman The Asian Monsoon is one of the largest and most important weather systems on Earth. The development of a strong monsoon climate has been identified as a possible cause of the expansion of C4 grasslands and an oxygen isotope shift in soil carbonates in Pakistan and Nepal. This study will examine the development of the modern monsoon, one dominated by the high Tibetan Plateau which today virtually precludes any dry-season precipitation. The isotopic composition of aragonitic molluscs will be used to reconstruct the seasonal isotopic variation of rivers in the foothills of the Himalayas between 12 and 1 Ma (million years before present). By combining the 18O of micro-sampled weekly or monthly shell growth increments with temperature estimates based on fossil floral assemblages, the seasonal change in the 18O of river water can be calculated. The goal of these measurements is to examine the isotopic composition of river waters on a seasonal basis in order to estimate the aridity of the dry season and the seasonal variability of rainfall. Strong seasonal aridity is indicated by positive 18O values of surface water in the dry season. The seasonal amount of precipitation can be roughly estimated because the isotopic composition of rainfall is a function of the amount of rainfall at low latitudes. Shell carbonates 13C of carbon in the rivers. This allows changes in the 18O record to be directly related to changes in the plant biomass of the region (C3 versus C4). Nepal will be the primary focus of this study because of the useful stratigraphic distribution of well preserved molluscs, but some shell beds in Pakistan will also be studied to allow comparison of different regions under the influence of the Asian Monsoon.