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

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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.