A coupled geomorphic and geochemical model for testing the dominant controls on chemical weathering rates in eroding landscapes

一场关于化学风化速率的主要驱动力的激烈辩论已经出现。一种假说认为，由于风化反应是由孔隙水化学物质驱动的，气候，特别是降雨，控制着化学风化的速度。相反，另一种假说认为，化学风化是通过向风化带提供“新鲜”矿物来驱动的，而化学风化的主要驱动因素是物理侵蚀。评估这两个假说的相对重要性的研究成果有限。一方面，侧重于孔隙水化学的详细的地球化学研究没有考虑物理剥蚀和沉积物横向运移等地貌过程。这样的研究无法深入了解侵蚀景观导致化学风化速率增加的机制。这是因为化学风化速率在空间上是不均匀的(正如一个世纪的土壤科学所证明的那样)，侵蚀物质以不同的化学风化速率连续横向移动通过地貌的某些部分。另一方面，对侵蚀驱动的风化速率的研究主要是基于对流域范围风化速率的经验研究，并没有对风化反应做出规定。为了真正研究气候和物理侵蚀对化学风化速率的相对重要性，必须同时考虑风化反应和沉积物的产生和运输。在这项研究中，PI首次提出将最先进的地球化学模型与详细的地貌模型相结合。所提出的模型将能够使用两个端元化学风化假说来预测坡面土壤的地球化学和地貌演化。为了检验该模型以及化学风化的两个驱动因素的相对重要性，已经确定了一个现场，在那里，两个末端成员的假设预测了化学风化的对比空间分布。这一现场有一系列独特的综合地貌和地球化学测量：现场测量用于独立校准模型，并将模型结果与长期化学风化速率和固体化学进行比较。因此，通过使用最先进的数值模拟和详尽的地球化学和地貌数据集，该项目将测试气候(通过降雨和孔隙水化学)或物理侵蚀速率是否在控制侵蚀景观中的化学风化速率方面起主导作用。

A vigorous debate has emerged over the primary driver of chemical weathering rates. One hypothesis states that because weathering reactions are driven by pore water chemistry, climate, specifically rainfall, controls chemical weathering rates. In contrast, another hypothesis states that chemical weathering is driven by the supply of 'fresh' minerals to the weathering zone, and the dominant driver of chemical weathering is physical erosion. Studies evaluating the relative importance of these two hypotheses have had limited success. On the one hand, detailed geochemical studies that focus on pore water chemistry make no provision for geomorphic processes such as physical denudation and lateral sediment transport. Such studies cannot yield insights into the mechanisms that drive increased chemical weathering rates from eroding landscapes. This is because chemical weathering rates are spatially heterogeneous (as a century of soil science can attest) and eroding materials continuously move laterally through parts of the landscape with varying chemical weathering rates. On the other hand, studies focusing on weathering rates driven by erosion are largely based on empirical studies of basin wide weathering rates and make no provision for weathering reactions. To truly examine the relative importance of climate and physical erosion on chemical weathering rates, one must account for both weathering reactions and the generation and transport of sediment. In this study the PI proposes, for the first time, to combine a state of the art geochemical model with a detailed geomorphic model. The proposed model will be capable of predicting the coupled geochemical and geomorphic evolution of hillslope soils using both end member chemical weathering hypotheses. To test the model, and the relative importance of the two drivers of chemical weathering, a field site has been identified where the two end member hypothesis predict contrasting spatial distributions of chemical weathering. This field site has a uniquely comprehensive series of both geomorphic and geochemical measurements: at the site measurements exist to independently calibrate the model and compare model results with long term chemical weathering rates and solid state chemistry. Thus, by using a combination of state of the art numerical modelling and an exhaustive geochemical and geomorphic dataset, this project will test if climate (via rainfall and pore water chemistry) or physical erosion rates are dominant in controlling chemical weathering rates in an eroding landscape.