ABSTRACT

The geologic record of past environmental change offers an illuminating glimpse into the behavior of Earth's natural systems. The deposits formed in ancient lakes are particularly useful in this regard for several reasons: they are highly sensitive to climate change; they often contain relatively complete geologic archives with relatively few gaps; and they can potentially be used to document changes that occurred over time scales meaningful to humans. The Green River Formation in the western U.S. offers an especially valuable record of the most geologically recent period of prolonged greenhouse conditions, which occurred approximately 50-53 million years ago during the Eocene Epoch. It is famous for its rich assemblages of fossil vertebrates, insects, and plants, and for containing the world's largest commercial deposits of soda ash and oil shale. This research project will focus on evidence for repeated rapid warming events during this time, by examining how river courses changed, the types of sediment that accumulated within the lake, and variations in ancient soil development and chemistry. Equally important, this study will build a time scale of unprecedented accuracy and precision for these important deposits, based on new radiometric dates of volcanic ash in the lake, the chronology of changes in the Earth's magnetic field, and sedimentation patterns that were driven by periodic oscillations in the Earth's rotation and orbit. Broader impacts of this proposal will include development of local 2-hour field trips for middle and high school students, enhanced through the use of innovative data visualization technology on portable devices.

The Early Eocene Climatic Optimum (EECO) was a time of persistent global warmth, of which the cause remains incompletely determined. Lake deposits of the Green River Formation preserve a rich and previously untapped record that complements that contained in marine sediments, and present an opportunity to advance the precision and accuracy of the Early Eocene timescale. This project is an integrated, multidisciplinary investigation focused on one of the world's richest and best studied systems of ancient lake strata. The research will investigate three principal questions: 1. How did the timing of terrestrial warming compare to that inferred from the marine record, and to predicted patterns of the Earth's orbital variations? Recent advances in radioisotopic dating can be used to directly test the relationship of warming events to Earth's orbital changes, and calibrate dynamic gravitational models of the solar system back to ca. 50 million years. 2. How did temperatures, seasonality, hydrologic cycling, and weathering on land evolve during the EECO? One hypothesis is that warming was amplified on continents relative to the oceans and was accompanied by more equable temperatures year-round, whereas precipitation seasonality and intensity increased. 3. How did the complex tectonic and magmatic evolution of western North America impact the preservation of EECO climate signals? The research will test the hypothesis that repeated, stepwise drainage reorganizations within the tectonically-active uplands modified the hydrologic balance and sedimentary deposits of the Eocene lake environment and its response to orbital changes.

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.

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