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的三个重要问题：在碰撞前的条件下，隆起加速铝硅酸盐风化的程度。这种加速最近被认为是造成近50年来气候恶化的反向温室效应的原因。2. 在一些喜马拉雅河流和恒河-布拉马普特拉河的初步取样中观察到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过程中的有孔虫方解石中U/Ca增加了50%。3. 迄今为止在采样的河流中观测到的独特的高放射性Sr通量的起源和原因。这一通量解释了近40年来海洋石灰石中87Sr/86Sr比值的急剧上升。如果这种通量增强是喜马拉雅山式大陆碰撞的特征，那么同位素曲线必须被视为特定地质事件的标志，而不是像目前的古二氧化碳分压模型模拟所假定的那样，是风化速率的一般代表。从巴基斯坦北部的印度河源头、西藏的印度河和布拉马普特拉河支流以及尼泊尔的恒河支流源头收集了一套非常广泛的样本。这个抽样程序正在进行中。这些将分析主要物种和U， Sr, 87Sr/86Sr, Rb， Cs和Ba。该数据集将限制铝硅酸盐与石灰石和蒸发岩对通量的反褶积，从而限制净CO2汇的大小。结合现有数据和地质信息，它将有助于确定铀和放射性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.