Fluvial Geochemistry of the Himalaya and Tibet

喜马拉雅造山带和青藏高原的河流地球化学将制约古环境地球化学中的三个重要问题：1.碰撞前条件下隆升加速铝硅酸盐风化的程度。这样的加速最近被认为是反向温室效应的原因，而反向温室效应是过去50年气候恶化的原因。2.在几个喜马拉雅小溪和恒河-雅鲁藏布江的初步采样中观察到的U通量增强的普遍性。这一通量似乎占全球总量的10%至15%，可能有助于解释目前正在开发的有孔虫方解石中U/Ca比增加约50%的原因。3.迄今在采样河流中观测到的独特的高放射性成因锶通量的来源和原因。这一通量解释了海相石灰岩中记录的海水87Sr/86Sr比值在过去40My期间急剧上升的原因。如果这种通量增强是喜马拉雅式大陆碰撞的特征，那么同位素曲线必须被视为特定地质事件的标志，而不是像目前古PCO2模型模拟所假定的那样，总体上代表风化速率。从巴基斯坦北部的印度河源头、西藏的印度河和雅鲁藏布江支流以及尼泊尔恒河支流的源头收集了一套非常广泛的样本。该抽样计划正在进行中。将对主要物种以及U、Sr、87Sr/86Sr、Rb、Cs和Ba进行分析。这套数据将限制铝硅酸盐相对于石灰岩和蒸发岩对通量的贡献的去卷积，从而限制净二氧化碳汇的大小。结合现有数据和地质信息，将有助于查明铀和放射性成因锶通量升高的来源和原因。

Fluvial geochemistry of the Himalayan orogen and the Tibetan Plateau will constrain three important issues in paleoenvironmental geochemistry: 1. The extent to which the uplift has accelerated aluminosilicate weathering over pre-collision conditions. such an acceleration has recently been postulated to be the cause of a reverse greenhouse effect responsible for the climatic deterioration of the last ~50 my. 2. The generality of the enhanced flux of U observed in the preliminary sampling of a few of the Himalayan streams and the Ganges-Brahmaputra. This flux appears to be between 10 and 15% of the global total and could help account for the ~50% increase in the U/Ca in foraminiferal calcite now in the process of being developed. 3. The origin and cause of the uniquely high fluxes of radiogenic Sr observed in the rivers sampled to date. This flux accounts for the dramatic rise over the last ~40 my in the seawater 87Sr/86Sr ratio as recorded in marine limestones. If such a flux enhancement is characteristic of Himalayan-style continental collisions then the isotope curve must be regarded as a marker of specific geologic events rather than a proxy for weathering rates in general as assumed in current model simulations of paleo-pCO2. A very extensive suite of samples has been collected from the headwaters of the Indus in Northern Pakistan, the Indus and Brahmaputra tributaries in Tibet and the headwaters of Ganges tributaries in Nepal. This sampling program is on-going. These will be analysed for the major species and for U, Sr, 87Sr/86Sr, Rb, Cs and Ba. This data suite will constrain the deconvolution of the aluminosilicate versus limestone and evaporite contributions to the flux and hence the magnitude of the net CO2 sink. Coupled with existing data and geological information it will help identify the sources and causes of the elevated fluxes of U and radiogenic Sr.