EAR-PF: Investigating Redox-Related Preservation of Organic-Walled Microfossils in Proterozoic Shales: a Multi-Scale Approach.

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Dr. Christina Woltz has been awarded an NSF EAR Postdoctoral Fellowship to carry out research and education plans at Stanford University. Before the widespread appearance of animals around 550 million years ago, life on Earth was almost entirely microscopic. Eukaryotes—the domain of life that includes animals, plants, and a great diversity of single-celled organisms—first appear in the fossil record around 1650 million years ago as microscopic organic structures compressed in fine-grained rock. Although their fossil record documents large-scale changes in diversity through time, it can also be influenced by changes in the environmental conditions that control fossilization. In particular, the presence of oxygen is thought to increase the degradation of organic matter, but the effects of oxygen on the quality of the early eukaryotic fossil record have not been examined comprehensively. The goal of this project is to explore how the presence of oxygen in the burial environment might have influenced the preservational quality of microfossil assemblages. By understanding the relationship between microfossil preservation and oxygen, this project will assess if certain time intervals of fluctuating oxygen concentrations have biased our current understanding of early eukaryotic evolution. This project will also develop teaching modules that connect undergraduate students to current research topics using a large open-access geochemistry database (available through the Sedimentary Geochemistry and Paleoenvironments Project). These teaching modules will expose students to applications of Earth science, which increases student engagement, and equips them with skills in data management to support their continued involvement in science. It is a long-held assumption that local ocean anoxia is necessary for the fossilization of organic remains. Although this relationship is well-documented in the Phanerozoic record of soft-tissue animal fossils, it is unclear if the same conditions apply to the record of Proterozoic organic-walled microfossils. Here, the preservational quality of microfossil assemblages will be compared to several redox proxies—including iron speciation, and uranium and molybdenum abundances—measured from a suite of shale samples that span mid to late Proterozoic time. Since redox conditions can vary significantly over short time intervals, redox proxies (including iron species) will be measured at ultra-high resolution on select samples using synchrotron-based x-ray spectroscopy. By quantifying the relationship between fossil preservation and paleoredox conditions, this project lays the groundwork for detecting largescale changes in preservation potential within the Proterozoic fossil record. For example, the first appearance of eukaryotes in the fossil record (around 1650 Ma) occurs much later than is predicted by molecular clock estimates and the complexity of the first eukaryotic fossils. It is possible that the appearance of eukaryotes at this time marks the onset of more favorable preservational conditions. Additionally, this project develops teaching modules aimed towards engaging a diverse undergraduate community in Earth science research. Laboratory modules will develop skills related to the management and analysis of large datasets, which are necessary yet underdeveloped skills in the undergraduate population.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.