MODELING NOVEL STABLE ISOTOPE RATIOS IN THE WEATHERING ZONE
MODELING NOVEL STABLE ISOTOPE RATIOS IN THE WEATHERING ZONE
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DOI：
10.2475/04.2013.01
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发表时间：
2013-04-01
影响因子：
2.9
通讯作者：
Schuessler, Jan A.
中科院分区：
文献类型：
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作者：
Bouchez, Julien;Von Blanckenburg, Friedhelm;Schuessler, Jan A.
当岩石转化为风化产物时，可以使用金属（例如Li， Mg, Ca, Fe, Sr）和类金属（B, Si）的稳定同位素比率来识别所涉及的过程。在这里，我们构建了一个框架，用于在地貌背景下定量解释风化带隔室中的这些“新型”稳定同位素比率。该方法适用于任何新的稳定同位素系统，并基于一个简单的稳态质量平衡模型，该模型代表了从土壤柱尺度到整个大陆尺度的风化带。我们的模型是基于与同位素分馏相关的两个主要过程的假设，即次生降水（如粘土）的形成和植物对养分的吸收。模型结果表明，风化带室室中某一元素的同位素组成取决于(1)原生矿物溶蚀释放通量与次生沉淀和有机质组成的同位素分选固体物质的侵蚀通量之比；(2)与次生矿物降水和植物吸收相关的同位素分馏因子。通过简单的元素浓度测量，建立了同位素比率、同位素分馏因子和化学风化指标（如化学损耗分数（CDF）和元素传质系数（tau））之间的关系。根据这种关系，同位素分馏因子可以从野外材料的化学和同位素测量数据进行校准。此外，我们展示了如何通过比较基岩、水和沉积物之间的同位素组成来估计风化系统中给定元素的固体输出与溶解输出的比例。这种计算可以应用于土壤、河流和沉积记录的样品，并且不需要知道反应中涉及的同位素分馏因子。最后，我们将该模型应用于海相碳酸盐岩沉积物中重建的海洋Li同位素记录，以讨论新生代全球地貌格局的变化。
When rock is converted to weathering products, the involved processes can be fingerprinted using the stable isotope ratios of metals (for example Li, Mg, Ca, Fe, Sr) and metalloids (B, Si). Here we construct a framework for interpreting these "novel" stable isotope ratios quantitatively in the compartments of the weathering zone in a geomorphic context. The approach is applicable to any novel stable isotope system and is based on a simple steady-state mass balance model that represents the weathering zone from the scale of a soil column to that of entire continents. Our model is based on the assumption that the two main processes associated with isotope fractionation are formation of secondary precipitates such as clays, and uptake of nutrients by plants.The model results show that the isotope composition of a given element in the weathering zone compartments depends on (1) the ratio between the release flux to water through primary mineral dissolution and the erosion flux of isotopically fractionated solid material, consisting of secondary precipitates and organic matter; (2) the isotope fractionation factors associated with secondary mineral precipitation and uptake by plants. A relationship is established between isotope ratios, isotope fractionation factors, and indexes for chemical weathering [such as chemical depletion fractions (CDF) and elemental mass transfer coefficients (tau)] derived from simple elemental concentration measurements. From this relationship, isotope fractionation factors can be calibrated from chemical and isotope data measured on field material. Furthermore, we show how the ratio of solid export to dissolved export of a given element from the weathering system can be estimated from the comparison of the isotope composition between bedrock, water, and sediment. This calculation can be applied to samples from soils, from rivers, and from the sedimentary record, and does not require knowing the isotope fractionation factors involved in the reactions. Finally, we apply the model to the oceanic Li isotope record reconstructed from marine carbonate sediments in order to discuss changes in global geomorphic regimes through the Cenozoic.