The Ca isotope geochemistry of chemical weathering in the New Zealand Southern Alps

在地质时间尺度上，大陆硅酸盐岩石的化学风化作用调节着大气中的二氧化碳水平，从而调节着全球气候。量化这一机制及其控制对于理解地球是如何成为并保持宜居的至关重要。钙（Ca）与长期碳循环相结合，因为随着岩石天气的变化，钙以溶解形式释放出来，由河流输送到海洋，并在海水中的碳酸盐沉淀过程中被隔离。海相碳酸盐岩的Ca同位素组成（δ 44Ca）反映了大陆风化历史和大气CO2浓度的相关变化。然而，海洋钙同位素记录的发展已经超过了对矿物风化和随后的水钙运输过程中引起钙同位素变化的过程的理解。通过分析流经新西兰南阿尔卑斯山的河流的钙同位素组成，本项目的目标是帮助解决有关构造和气候过程是否、如何以及在多大程度上影响海水钙同位素预算的问题。该山脉岩性一致，但构造和气候梯度较大。西侧气候潮湿，隆起快，物理侵蚀率高，而东侧气候干旱，隆起低，物理侵蚀率低。PI之前曾在新西兰南阿尔卑斯山进行过量化矿物风化反应、大气二氧化碳消耗率和河流Sr同位素通量的工作。利用这一框架，PI将测试有关控制河流钙同位素地球化学的具体假设。样本收集将包括水、悬浮沉积物、岩石和植被。PI将通过热电离质谱法测量δ 44Ca值，并使用质量平衡建模、碳酸盐平衡计算、水文分离以及元素和同位素混合方程综合数据。该项目的智力价值在于提高对风化过程中Ca同位素地球化学的认识，从而帮助利用海洋Ca同位素记录重建过去的大气二氧化碳水平。更广泛的影响包括教学和培养博士后研究人员。

Over geologic timescales, the chemical weathering of continental silicate rocks regulates atmospheric CO2 levels and thereby global climate. Quantifying this mechanism and its controls is crucial for understanding how Earth became and remains habitable. Calcium (Ca) is coupled to the long-term carbon cycle because as rocks weather, Ca is released in dissolved form, transported by rivers to the oceans, and sequestered during carbonate precipitation from seawater. The Ca isotope composition (delta 44Ca) of marine carbonate thus reflects the history of continental weathering and related changes in atmospheric CO2 concentrations. However, development of the marine Ca isotope record has outpaced understanding of processes eliciting Ca isotope variation during mineral weathering and subsequent transport of aqueous Ca. By analyzing the Ca isotope composition of rivers draining the New Zealand Southern Alps, the goal of this project is to help resolve questions concerning if, how, and the extent to which tectonic and climatic processes influence the Ca isotope budget of seawater. The mountain range has a uniform lithology but large gradients in tectonics and climate. The western side experiences a wet climate, rapid uplift, and high rates of physical erosion, whereas the eastern side experiences an arid climate, low uplift, and low rates of physical erosion. The PI has worked previously in the New Zealand Southern Alps to quantify mineral weathering reactions, atmospheric CO2 consumption rates, and riverine Sr isotope fluxes. Using this framework, the PI will test specific hypotheses concerning controls on the Ca isotope geochemistry of rivers. Sample collection will include waters, suspended sediments, rocks, and vegetation. The PI will measure delta 44Ca values by thermal ionization mass spectrometry and synthesize data using mass-balance modeling, carbonate equilibria calculations, hydrograph separations, and elemental and isotope mixing equations. The intellectual merit of the project is to improve knowledge about the Ca isotope geochemistry of weathering and thereby aid reconstructions of past atmospheric CO2 levels using the marine Ca isotope record. Broader impacts include the teaching and training of a post-doctoral research fellow.