Constraining iodine cycling and redox transformations in the Eastern Tropical Pacific

This project focuses on the chemical cycling of iodine in the ocean. Ocean iodine chemistry is important to biological cycles and to atmospheric levels of iodine which in turn are important to ozone levels and other air quality indicators. In addition, changes in ocean iodine chemistry recorded in ocean sediments are used to study past changes in ocean oxygen levels. The project will generate a large, high-quality dataset of iodine chemistry in the Pacific Ocean and use ocean modeling to test hypotheses about the roles of ocean circulation, chemical reactions, and biology in controlling the concentration and chemical forms of iodine. The project will train a postdoctoral researcher in mechanistic ocean modeling and an undergraduate student in redox chemistry and advanced analytical methods, fostering skills applicable to both academic and applied careers. This project will test whether striking iodine enrichments observed in the Eastern Tropical Pacific can be explained entirely by in situ transformations -- without invoking external inputs from reducing shelf sediments. In seawater, most dissolved iodine occurs as one of two anions -- iodate or iodide -- but recent high-precision measurements have revealed a third, previously unrecognized pool referred to as “dark iodine.” This pool, invisible to conventional methods, may increase total dissolved iodine concentrations by up to 40 percent above the canonical value, fundamentally altering the marine iodine budget. The project addresses three integrated questions: (1) What is the distribution of inorganic iodine species and dark iodine across the Eastern Tropical Pacific? (2) What is the proportion of each distribution attributable to conservative transport versus local transformation? (3) What are the rates and environmental sensitivities of the key redox transformations between iodine species? These questions will be answered using (1) high-precision, artifact-free measurements of iodine speciation via liquid chromatography-inductively coupled plasma-mass spectrometry, (2) multivariate water-mass decomposition to separate mixing from transformation signals, and (3) mechanistic modeling to quantify transformation rates and oxygen sensitivities. The resulting synthesis will test targeted hypotheses on the role of dark iodine in redox cycling, reshaping our understanding of iodine geochemistry and its interactions with other biogeochemical cycles in low-oxygen ocean regions. By clarifying the controls on marine iodine speciation, this work will improve interpretation of paleoceanographic records and sharpen predictions of air–sea iodine exchange under changing ocean oxygen levels. 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. NSF Award ID: 2546523 | Program: 01002627DB NSF RESEARCH & RELATED ACTIVIT | Principal Investigator: Tristan Horner | Institution: Woods Hole Oceanographic Institution, WOODS HOLE, MA | Award Amount: $679,773 View on NSF Award Search: https://www.nsf.gov/awardsearch/show-award/?AWD_ID=2546523 View on Research.gov: https://www.research.gov/awardapi-service/v1/awards/2546523.html