Date of Final Oral Examination (Defense)
Type of Culminating Activity
Doctor of Philosophy in Geosciences
Matthew J. Kohn, Ph.D.
Jennifer Pierce, Ph.D.
Marion L. Lytle, Ph.D.
The Cenozoic Era was a time period where dynamic shifts in climate created for both warm-wet greenhouse environments of the mid-Miocene Climatic Optimum (MMCO), and cool-dry, glacial periods of the late Pleistocene. The Cenozoic is close to our own time period, and although past climate reconstructions cannot be used as direct analogs for future climate change, understanding previous environmental responses can help inform policy surrounding future climate change. Presented here are climate reconstructions of the interior western United States, from two different geologic time periods. Each had a different climate, that differed greatly from modern day environments. The use of hydrogen isotopes in tooth enamel is also evaluated as a potential new approach for understanding climate. Expanding our isotopic toolbox for climate reconstructions allows for more certain interpretations, and the use of tooth enamel stable hydrogen (δD), oxygen (δ18O), and carbon (δ13C) compositions allow for more reliable climate reconstructions.
The MMCO, between ~17 and 14 Ma, represents the warmest period on Earth in the last 35 Ma, and is thought to reflect a high partial pressure of atmospheric CO2 (pCO2). Using tooth enamel δ13C values from the interior Pacific Northwest, mean annual precipitation (MAP) was estimated before, during, and following the MMCO, to test whether MAP tracks pCO2 levels. This work speculates high pCO2 contributed to higher MAP at ~ 28 and 15.1 Ma, and lower pCO2 contributed to lower MAP for other time periods. Terrestrial climates during the MMCO were likely more dynamic than originally considered, with wet-warm and cool-dry cycles reflecting 20-, 40-, and 100-ka Milankovitch cycles. Modern climate models predict that the Pacific Northwest will become wetter and warmer with increased CO2 levels, and this climate projection is consistent with MMCO climates associated with high pCO2 levels.
Tooth enamel and tufa (low-temperature CaCO3 precipitate) δ18O and δ13C values from well-dated late Pleistocene deposits in the Las Vegas Wash (LVW), Nevada, were used to reconstruct past precipitation seasonality, where enhanced net precipitation aided in the expansion of desert wetlands. Low late Pleistocene water δ18O values, inferred from tufa and tooth enamel, indicate that paleowetland expansion likely resulted from increased winter precipitation derived from high latitudes in the Pacific Ocean. Low tooth enamel δ13C and inferred %C4 grass values are again consistent with an increase in proportion of winter precipitation. Increased winter precipitation diverges from late Pleistocene climate reconstructions at lower latitudes in the American Southwest and modern-day climes that receive nearly equal proportions of winter and summer moisture.
Stable hydrogen and oxygen isotope compositions correlate between organic tissues and meteoric water. This correlation was tested for the first time in modern herbivore tooth enamel by measuring oxygen and hydrogen isotope compositions from localities where water compositions are well known. Against expectations, δD and δ18O values of modern tooth enamel do not align with the Global Meteoric Water Line (GMWL) and hydrogen isotope compositions display little isotopic variation (~35‰) between vastly different geographic locations. However, a strong correlation (R2 = 0.84) indicates a coupling between stable oxygen and hydrogen isotopes in tooth enamel. Tooth enamel δD values were compared to local water compositions, which generally correlate (R2 = 0.71), suggesting tooth enamel δD values at least partially reflect biogenic water compositions. However, when hydrogen amounts (H mg/sample mg) are compared to sample weights (mg), it is clear that additional, labile hydrogen is adsorbed onto bioapatite crystallites, and constitutes ~80% of measured hydrogen. The rate of exchange between adsorbed water and water vapor was determined by equilibrating powdered samples with enriched- and depleted-water for 48 hours, and then exposing samples to laboratory conditions for times ranging from a few minutes up to 8 hours. In both experiments, adsorbed water and laboratory water vapor equilibrate within 1 to 2 hours. Because adsorbed water (onto tooth enamel) and ambient water vapor equilibrate so quickly, it would be almost impossible to reconcile tooth enamel δD values for a single specimen across different laboratories, because of differences in local water compositions. Enamel heated at 70 °C in air for 48 hours shows lower δD values than samples equilibrated at room temperature, which likely reflects a different, temperature-dependent partition coefficient between adsorbed water (onto apatite) and water vapor.
Drewicz, Amanda E., "Stable Isotope Geochemistry of Bioapatite" (2018). Boise State University Theses and Dissertations. 1474.
Available for download on Saturday, December 19, 2020