OCE-PRF: Development and Application of the Foraminiferal Iodine/Calcium Paleoredox Proxy

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Development and Application of the Foraminiferal I/Ca Paleoredox Proxy The amount of dissolved oxygen in seawater varies and is linked to the distribution of life in the oceans. Expansion of marine oxygen deficient zones (ODZs), where oxygen reaches levels low enough to limit habitability, are an expected result of climate change. As we work to predict how this will affect future oceans, studying marine oxygenation over past climate change events is increasingly valuable. The Miocene Climatic Optimum (MCO), which occurred ~15-17 million years ago, is such an event, during which the Earth’s temperature warmed up to 3C on average and atmospheric carbon dioxide levels rose to an estimated 400-500ppm. Multiple lines of evidence suggest that severe ODZs around the globe intensified during the MCO, but direct evidence is lacking. We will use experiments to validate and apply the foraminiferal iodine-to-calcium (I/Ca) proxy—which uses the iodine content of microscopic fossils to estimate marine oxygen concentrations—to sediments from the MCO in the Eastern Tropical Pacific to study the intensity of low oxygen in the region during the MCO and improve predictions of ODZ changes in future climate change scenarios. This project combines laboratory experiments, field work, and analytical procedures that will incorporate International Ocean Discovery Program archives as well as training and research experiences for graduate and undergraduate students (including specific plans for mentoring Latinx undergraduate STEM majors). This effort consists of three discrete but interrelated projects to test, ground-truth, and apply the foraminiferal I/Ca proxy. The foraminifera I/Ca proxy is a method for reconstructing oxygen concentrations in ancient oceans based on the iodine concentrations of microscopic shells of protists called foraminifera. This proxy works because the levels of the iodine species (iodate) which can be incorporated into foraminiferal shells is linked to the oxygen content of the water they live in. Although this proxy has already been described and utilized in studies to reconstruct marine oxygen concentrations over multiple periods of global change, it has not yet been experimentally verified using controlled laboratory conditions or systematically screened for an impact of diagenesis (alteration of the fossil material that occurs after it is deposited in the sediments). To this end, we will: (1) conduct experiments on benthic and planktonic foraminifera to document the uptake of iodate into foraminiferal calcite, testing for systematic offsets from predicted rates that exist in some proxies. These experiments include plans for a novel radiotracer approach using iodine-129, a rare isotope of iodine which can be used to track ambient iodine uptake into the test in culture. (2) Compare the I/Ca values of foraminiferal specimens from neighboring assemblages from the Pacific Ocean that have very different preservation states to test for an effect of diagenesis on foraminiferal I/Ca. (3) Apply the foraminiferal I/Ca proxy to reconstruct the redox dynamics of the Eastern Tropical Pacific region during the MCO by measuring planktonic and benthic foraminifera I/Ca along with stable carbon and oxygen isotopes in two key localities off the coasts of Mexico and Peru. Through this work we will strengthen an increasingly popular proxy system that can be applied to study many different regional and global paleoclimate events and constrain links between elevated atmospheric carbon dioxide and marine ODZs relevant to today during an analogous interval in Earth’s history.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.