A Window into Terrestrial Paleoclimate: Soil Carbonate Formation Processes and Climate Proxy Applications Landon K. Burgener A dissertation submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy University of Washington 2018 Reading Committee: Katharine W. Huntington, Chair Gerard H. Roe Gregory D. Hoke Program Authorized to Offer Degree: Earth and Space Sciences © Copyright 2018 Landon K. Burgener University of Washington Abstract A Window into Terrestrial Paleoclimate: Soil Carbonate Formation Processes and Climate Proxy Applications Landon K. Burgener Chair of the Supervisory Committee: Katharine W. Huntington Department of Earth and Space Sciences The isotopic composition of pedogenic (formed in soil) carbonates provides a geologically abundant archive of terrestrial climate change and the interactions between Earth’s climate, geologic, and biologic systems. However, early paleoclimate reconstructions based on the carbon and oxygen isotope composition of soil carbonates were largely limited to qualitative estimates of change in key climate and environmental parameters such as surface temperature, precipitation, and soil biologic activity. The development of carbonate clumped isotope geothermometry has made it possible to make quantitative estimates of carbonate formation temperatures, and to relate those temperatures to changes in the Earth’s climate. The application of carbonate clumped isotope geothermometry to studies of soil carbonate systematics has shown that the seasonality of soil carbonate formation is closely linked to the timing of the local wet season, which has important implications for how the clumped isotope formation temperatures of soil carbonates relate to more meaningful climatologic parameters such as mean annual temperature. However, important questions remain regarding the impact of other climate and soil factors such as precipitation type (e.g., rain versus snow) and soil sediment grain size on the seasonality and mechanisms of soil carbonate formation. This dissertation places new constraints on the effects these soil and climate factors have on the seasonality of soil carbonate formation, sheds new light on non- equilibrium formation processes affecting soil carbonates in freezing environments, and provides new, quantitative terrestrial paleoclimate reconstructions of seasonal variability in surface temperature during the Late Cretaceous greenhouse period. Chapter 2 presents soil carbonate clumped and stable isotope data from an approximately 4000 m elevation transect on the western flank of the Chilean Andes. The results from this first chapter provide evidence that the presence or absence of a winter snowpack plays a critical role in modulating soil wetting and drying cycles, which in turn dictate the seasonality of soil carbonate formation. Additionally, this work provides the first evidence of soil carbonate formation under conditions of isotopic disequilibrium in freezing environments. Chapter 3 builds on the results of Chapter 2 by presenting the analyses of a suite of cold-climate soil carbonates from soils with both fine grained (the High Arctic and the Tibetan Plateau) and coarse grained (the Chilean and Argentinian Andes and the Dry Valleys, Antarctica) inter-cobble matrices. The findings of this chapter show that in freezing soils, matrix grain size is an important control on promoting or inhibiting kinetic isotope effects during soil carbonate formation. Soils with coarse-grained matrices experience rapid CO2 degassing associated with bicarbonate dehydration during soil freezing, which results in disequilibrium soil carbonate formation. In contrast, fine-grained matrices inhibit soil CO2 degassing and promote equilibrium carbonate formation, even in freezing environments. Chapter 4 represents and application of cutting-edge soil carbonate paleoclimate reconstruction techniques to the long-standing issue of terrestrial seasonal temperature variability during greenhouse climates. The clumped isotope composition of paleosol carbonates from Late Cretaceous sedimentary outcrops in south-central Utah and northwest Montana, USA are used to reconstruct summer soil temperatures along the mid-latitude, western margin of the Western Interior Seaway. These summer temperatures are then paired with previous reconstructions of local mean annual temperature to reconstruct mean annual range in temperature for these two sites. The results of this work show better agreement with model simulations of Late Cretaceous seasonal temperature changes than previous estimates, and add to a growing body of work that suggests that seasonal temperature variations in greenhouse environments did not differ significantly from the modern. These findings have important implications not only for our understanding of greenhouse climate, but also for the impact of climate on the paleogeographic distribution of ancient faunal communities. Table of Contents List of Figures ............................................................................................................................................... iii List of Tables ................................................................................................................................................. iv Acknowledgments .......................................................................................................................................... v Dedication ..................................................................................................................................................... vi Chapter 1. Introduction ................................................................................................................................... 1 1.1. Motivation: Reconstructing Terrestrial Paleoclimate from the Clumped Isotope Composition of Pedogenic Carbonates ................................................................................................................................ 1 1.2. Dissertation Organization and Chapter Overviews ............................................................................ 7 Chapter 2: Variations in soil carbonate formation and seasonal bias over >4 km of relief in the western Andes (30°S) revealed by clumped isotope thermometry .............................................................................11 2.1. Abstract ..............................................................................................................................................11 2.2. Introduction ........................................................................................................................................12 2.3. Background.........................................................................................................................................14 2.3.1. Soil carbonate formation and isotopic composition ....................................................................14 2.3.2. The Elqui and Turbio valleys, Chile ............................................................................................15 2.4. Methods ..............................................................................................................................................18 2.4.1. Field methods ..............................................................................................................................18 2.4.2. Laboratory methods .....................................................................................................................21 2.5. Results ................................................................................................................................................24 2.5.1. Radiocarbon ages ........................................................................................................................24 2.5.2. Meteorological station temperature, precipitation, and CO2 data, and vegetation survey .........26 18 13 2.5.3. δ O, δ C, and T(Δ47) results ......................................................................................................28 2.6. Discussion ..........................................................................................................................................31 18 13 2.6.1. Comparing soil carbonate T(Δ47), δ O and δ C to modern environmental parameters ............31 2.6.2. Systematic changes in soil carbonate T(Δ47) due to changes in soil moisture .............................34 2.6.3. High-elevation cryogenic carbonate formation ...........................................................................37 2.6.4. Biases in shallow soil carbonate T(Δ47)values ............................................................................40 2.7. Implications and conclusions .............................................................................................................40 2.8 Chapter Two Acknowledgements .........................................................................................................42 Chapter 3: Clumped isotope constraints on equilibrium carbonate formation and kinetic isotope effects in freezing soils..................................................................................................................................................43 3.1. Abstract ..............................................................................................................................................43 3.2. Introduction ........................................................................................................................................45