Developing a new proxy for silicate weathering: an investigation of Potassium isotope fractionation during clay formation

新的分析方法使研究人员能够证明大陆和海洋的钾同位素组成是不同的。 由于钾的最终来源是大陆岩石的风化作用，因此在化学风化过程中和/或在海水与海洋沉积物/岩石相互作用过程中通过含钾矿物的形成而在海洋中进行的钾循环过程中，必然存在显著的钾同位素分馏。研究人员建议通过一系列实验室实验和对地质样品的新的高精度分析，研究和量化阳离子交换和粘土形成过程中K同位素的交换动力学和分馏。这项工作将提供一个重要的第一步走向广泛的潜在应用的稳定钾同位素在表面环境中，特别是作为一个新的硅酸盐风化代理。 这些研究对于了解硅酸盐风化、大气二氧化碳水平和过去气候变化之间的相互作用至关重要。为了向年轻的K-12学生突出这一贡献，研究人员将与威斯康星州地质博物馆大学（UWGM; www.geologymuseum.org）合作，这是校园内最受欢迎的科学宣传场所，以教育和交流大陆风化和碳循环。UWGM助理主任布鲁克诺斯特德将率先通过创建内容和交付成果，以及监督其实施博物馆的参与。 其次，钾是植物生长的必需营养元素，植物中的钾同位素可以高度分馏。研究人员将通过与UW土壤科学系和参加土壤科学326，植物营养管理的学生合作进行本科研究班项目，探索植物生长中的钾营养。 在这门课中，学生将在缺钾和充足钾的条件下种植植物，以确定钾的有效性如何影响植物生长;目标是评估这些营养条件是否会导致钾同位素分馏，并向全班展示同位素分析在追踪营养利用方面的优势。拟议的研究是开发稳定的钾同位素系统作为一种新的硅酸盐风化指标。钾作为一种主要元素广泛参与了低温地球化学过程。最近的分析进展使研究人员能够首次在陆地样品中确定Δ 41/39 K的微小但重要的变化。拟议的工作的动机是现有的数据显示，河流沃茨和海水具有较高的delta 41 K值相比，一个相对均匀的增量41 K值的散装硅酸盐地球。现有的delta 41 K结果表明，阳离子交换和粘土形成是最有可能的机制，负责稳定的钾同位素分馏的大陆和海洋。建议的实验工作中心通过一系列实验室实验，结合天然样品的新分析，在阳离子交换和粘土形成过程中定量交换动力学和K同位素分馏。 这些数据将提供一个解释框架的低温K同位素分馏由于粘土。此外，NSF赞助的这项研究将与NASA资助的有关粘土中Fe、Si和Mg同位素分馏的实验研究相结合，为风化过程的多同位素方法铺平道路。该奖项反映了NSF的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

New analytical methods have allowed the investigators to demonstrate that the potassium (K) isotope composition of the continents and the oceans are different. Because the ultimate source of K delivered to the oceans is the weathering of continental rocks, there must be significant K isotope fractionation during chemical weathering and/or the cycling of K in the oceans by formation of K bearing minerals during the interaction of seawater with marine sediments/rocks. Researchers propose to study and quantify the exchange kinetics and fractionations of K isotopes during cation exchange and clay formation processes through a series of laboratory experiments and new high precision analyses of geologic samples. This work will provide an essential first step toward a wide range of potential applications of stable K isotopes in surficial environments, particularly, as a new silicate weathering proxy. Such studies are critical for understanding the interplay between silicate weathering, atmospheric carbon dioxide levels, and past climatic changes. To highlight this contribution to young K-12 students, investigators will partner with the University of Wisconsin Geology Museum (UWGM; www.geologymuseum.org), the most popular science-outreach venue on campus, to educate and communicate about continental weathering and the carbon cycle. UWGM Assistant Director Brooke Norsted will spearhead the Museum's involvement by creating the content and deliverables as well as overseeing their implementation. Second, K is an essential nutrient for plant growth, and K isotopes in plants can be highly fractionated. The investigators will explore K nutrition in plant growth by conducting an undergraduate research class project by partnering with the UW Soil Science Department and students that take Soil Science 326, Plant Nutrition Management. In this class students grow plants under K deficient and sufficient conditions to identify how K availability effects plant growth; the goal will be to evaluate if these nutrient conditions cause K isotopic fractionation and to demonstrate to the class the advantages of isotope analyses to tracing nutrient utilization.The proposed research is to develop the stable K isotope system as a new silicate weathering proxy. Potassium as a major element participates widely in low temperature biogeochemical processes. Recent analytical advances have allowed the investigators to identify small but significant variations of delta 41/39K in terrestrial samples for the first time. The proposed work is motivated by existing data that show river waters and seawater have higher delta41K values as compared to a relatively homogeneous delta41K value of the bulk silicate earth. Available delta41K results indicate that cation-exchange and clay formation are the most likely mechanisms responsible for stable K isotope fractionations on the continents and in the ocean. The proposed experimental work centers on quantifying the exchange kinetics and fractionation of K isotopes during cation exchange and clay formation processes through a series of laboratory experiments, combined with new analyses of natural samples. These data will provide an interpretive framework for low-temperature K isotope fractionations due to clays. Furthermore, this NSF-sponsored research will be integrated with NASA-funded experimental studies on Fe, Si, and Mg isotope fractionations associated with clays, paving the way for a multi-isotope approach for weathering processes.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.