Abstract It has been proposed that the riverine Mg isotopes are sensitive to primary mineral dissolution and the formation of secondary minerals and are served as an effective tracer for chemical weathering. Glacial comminution can produce fresh mineral surfaces that provide easily leached cations to the rivers and thus could potentially modify the riverine Mg isotopic behavior. However, the mechanism controlling riverine Mg isotopes under glacial environment is poorly constrained. Here we investigated Mg and Sr isotopic compositions of river and spring waters, bedrocks and sediments within the glacial-covered Muztag catchment in the northeastern Pamir Plateau. The δ26Mg and 87Sr/86Sr values of dissolved loads along the mainstream increased from −1.56‰ and 0.713050 at the glacier margin to −1.12‰ and 0.713855 at the downstream, respectively, both showing strong negative correlation with elevation. The lower δ26Mg but higher molar Ca/Mg and Ca/Na ratios of glacial river waters than those of non-glacial rivers indicate a preferential dissolution of carbonates under glacial environment, further supported by a negative liner correlation between δ26Mg and carbonate weathering proportion. In contrast, the high δ26Mg but low Ca/Mg and Ca/Na ratios at the downstream reflect an increased input of silicate weathering. Saturation state modelling suggests little chance of secondary mineral formation and limited impact on the Mg isotopic behavior in glacial rivers, characterized by low ionic concentrations and short water residence time. These observations, together with complied data from other glacial rivers, suggest preferential carbonate weathering under glacial environment could directly release low Mg isotopes to river water, different from the process of Mg isotopic fractionation by secondary mineral formation. Our findings would provide insight in carbonate weathering regulating riverine Mg isotopic evolution and its potential influence on global carbon cycle, in particular under present warming scenario.

中文翻译：

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帕米尔高原东部穆兹塔格流域内河流镁同位素对冰川风化的响应

摘要 河流镁同位素对原生矿物溶解和次生矿物形成敏感，可作为化学风化的有效示踪剂。冰川粉碎可以产生新鲜的矿物表面，为河流提供容易浸出的阳离子，因此有可能改变河流中的镁同位素行为。然而，冰川环境下控制河流镁同位素的机制受到的限制很少。在这里，我们研究了帕米尔高原东北部冰川覆盖的穆兹塔格流域内河流和泉水、基岩和沉积物的镁和锶同位素组成。沿主流溶解载荷的δ26Mg和87Sr/86Sr值分别从冰川边缘的-1.56‰和0.713050增加到下游的-1.12‰和0.713855，两者都与海拔高度呈负相关。与非冰川河流相比，冰川河水的 δ26Mg 较低，但 Ca/Mg 和 Ca/Na 摩尔比较高，表明在冰川环境下碳酸盐优先溶解，这进一步得到 δ26Mg 与碳酸盐风化比例之间的负线性相关性的支持。相比之下，下游的 δ26Mg 高但 Ca/Mg 和 Ca/Na 比值低，反映了硅酸盐风化的输入增加。饱和状态模型表明，次生矿物形成的可能性很小，对冰川河流中镁同位素行为的影响有限，其特征是离子浓度低和水停留时间短。这些观察结果，连同来自其他冰川河流的合规数据，表明在冰川环境下优先碳酸盐风化可以直接向河水中释放低镁同位素，不同于次生矿物形成的镁同位素分馏过程。我们的研究结果将为碳酸盐风化调节河流镁同位素演化及其对全球碳循环的潜在影响提供见解，特别是在目前的变暖情景下。