EAR-PF: Investigating the utility of cherts for constraining the long-term evolution of oxygen isotopes in the oceans

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Dr. Justin Hayles has been awarded a postdoctoral fellowship to work with at Rice University and Pennsylvania State University to study oceanic oxygen isotope compositions of seawater throughout Earth history. The ocean is the most important reservoir of oxygen on Earth's surface. Its isotopic composition varies with ice volume and is influenced by interactions with the solid Earth on million-year timescales. Marine minerals record these variations: they inherit the oceans' oxygen-isotope composition. Marine mineral systems, however, show long-term secular variations that are not easily explained by changes in temperature or ocean isotopic composition exclusively. The ocean is thought to be buffered by high- and low-temperature interactions between seawater and seafloor rocks. If these interactions do not vary on billion-year timescales, the oxygen-isotope compositions of marine sedimentary rocks mostly relate to changes in seawater temperature and global ice volume. However, if these interactions do vary there could be a secular change in the oxygen isotope composition of seawater toward heavier compositions. By measuring the triple-oxygen isotope composition of ancient chert samples, it will be possible to constrain oceanic oxygen isotope compositions and ultimately advance our understanding of the surface environment throughout Earth history. The PI will work with undergraduates on research projects and provide outreach to local schools through career day events.Recent experimental results have demonstrated that silica, precipitated under most common Earth surface conditions, forms with a predictable temperature dependent offset in oxygen isotope composition from the water which its oxygen was ultimately derived. In practice, the oxygen fractionation based geothermometer system depends on the assumption of the oxygen of the water. By constraining this relationship, it is possible to fully constrain the oxygen isotopes and temperature which the silica formed from. However, because marine cherts are nearly ubiquitously diagenetically altered it is necessary to determine what effects, if any, there will be on the determination of seawater oxygen using this method. The effects of diagenetic alteration of marine cherts will be done through the characterization and triple oxygen isotope analysis of silica phases from the Miocene to Pliocene Monterey and Sisquoc formations of Southern California where three phases of silica (Opal-A, Opal-CT and microquartz) are known to exist.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.