Evaluating Stable Isotope and Geochronologic Techniques for Paleoclimate Reconstruction: Case Study of the Santa Cruz Formation, Argentina
Date of Final Oral Examination (Defense)
Type of Culminating Activity
Doctor of Philosophy in Geosciences
Matthew J. Kohn, Ph.D.
Mark Schmitz, Ph.D.
Richard F. Kay, Ph.D.
Christopher L. Hill, Ph.D.
Creative Commons License
This work is licensed under a Creative Commons Attribution 4.0 International License.
Stable isotope analysis has become the method of choice for many studies investigating the paleoecology and paleoclimate of fossil mammal faunas. While organic tissues (collagen, keratins, proteins) persist for < 105 years highly mineralized tooth enamel is resistant to alteration and degradation and faithfully preserves its isotopic composition for millions (> 106) years. Reconstructing past climates from these records relies on both understanding both micro-scale mechanisms of isotope incorporation into individual teeth, and macro-scale changes in isotope compositions over hundreds of thousands or millions of years. In this dissertation I address three questions.
First, how does the geometry and timing of tooth enamel mineralization affect isotope compositions? Tooth enamel mineralization occurs in two stages with different geometries and proportions of chemical components. In chapter one I present oxygen isotope data and high-resolution major element compositional maps of forming tooth enamel to clarify the process of enamel mineralization. Oxygen isotope results show that enamel records the isotopic composition of only second stage mineralization. Given this, ideal enamel sampling strategies should follow the geometry of second stage mineralization. High resolution maps of calcium density for several teeth reveal that enamel mineralizes at a high angle relative to the underlying dentine, suggesting that existing techniques are appropriate.
Second, how can geochronology and stratigraphic information be convolved to strengthen interpretations of paleoclimate proxy records? Many reconstructions of past climates rely on proxy records (e.g. pollen, isotope samples) distributed through a stratigraphic section. Interpreting these data in a global context relies on both the ability to determine the absolute age of some stratigraphic positions, and a model that describes the relationship between stratigraphic position and age, allowing the age of unknown positions to be determined. In chapter two I report new U-Pb ages for several volcanic tuffs from the Santa Cruz Formation of southern Argentina. U-Pb data commonly exhibit complexities which make existing age-depth models unsuitable. I modified an existing age-depth model to better account for variations in absolute age, relative age, and the complex age uncertainties that often arise from magmatic crystal populations.
Finally, I present a case study from the Santa Cruz Formation, Argentina. Fossil rich sediments span the initiation of the mid-Miocene climatic optimum, the most recent period of greenhouse conditions in the Cenozoic. carbon isotope-based estimates of mean annual precipitation reveal a progressive aridification of Patagonia with precipitation decreasing prior to the mid-Miocene climatic optimum. This trend is interrupted by the onset of greenhouse conditions which drove a rebound in precipitation and increase in global temperatures.
Trayler, Robin B., "Evaluating Stable Isotope and Geochronologic Techniques for Paleoclimate Reconstruction: Case Study of the Santa Cruz Formation, Argentina" (2018). Boise State University Theses and Dissertations. 1499.
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Erratum 4/9/2019: The original document has been updated to fully display the colors that were in some of the graphics.