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(K)同位素组成不同。由于输送到海洋的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必然存在显著的钾同位素分异和/或在海水与海相沉积物/岩石相互作用过程中形成含钾矿物，从而使K在海洋中循环。研究人员建议通过一系列实验室实验和新的高精度地质样品分析，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。这项工作将为稳定K同位素在地表环境中的广泛潜在应用，特别是作为一种新的硅酸盐风化代理，提供重要的第一步。这些研究对于理解硅酸盐风化、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学生强调这一贡献，研究人员将与威斯康星大学地质博物馆（UWGM; www.geologymuseum.org）合作，该博物馆是校园中最受欢迎的科学推广场所，旨在教育和交流大陆风化和碳循环。UWGM的助理主任布鲁克·诺斯特德（Brooke Norsted）将通过创建内容和交付成果以及监督其实施来领导博物馆的参与。其次，钾是植物生长必需的营养物质，植物体内的钾同位素具有高度分馏性。研究人员将与西华大学土壤科学系和选修土壤科学326、植物营养管理的学生合作，开展一项本科生研究项目，探索钾营养在植物生长中的作用。在本课程中，学生在缺钾和充足条件下种植植物，以确定钾有效性如何影响植物生长；我们的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。拟开发稳定的钾同位素体系作为新的硅酸盐风化指标。钾作为主要元素广泛参与低温生物地球化学过程。最近的分析进展使研究人员首次在陆地样品中发现了δ 41/39K的微小但显著的变化。现有数据表明，与体积硅酸盐土相对均匀的delta41K值相比，河流和海水具有更高的delta41K值，因此提出了这项工作。现有的delta41K结果表明，阳离子交换和粘土形成是大陆和海洋中稳定的K同位素分馏最可能的机制。实验工作主要集中在通过一系列实验室实验，结合对天然样品的新分析，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。这些数据将为低温黏土K同位素分馏提供一个解释框架。此外，这项由美国国家科学基金会资助的研究将与美国国家航空航天局资助的有关粘土的铁、硅和镁同位素分选的实验研究相结合，为研究风化过程的多同位素方法铺平道路。该奖项反映了美国国家科学基金会的法定使命，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.