The linked evolution of climate, topography and the biosphere in Cenozoic western North America [electronic resource]
Keywords: Environmental Earth System Science.

The response of the Earth system to anthropogenic climate change remains quite uncertain. This is particularly true with respect to the hydrologic cycle in the mid-latitudes, where general circulation models exhibit the greatest disagreement in the timing and amount of precipitation under future climate change scenarios. Given the growth in population, semi-arid to arid climate, and a water infrastructure reliant on mountain snowpack, climate change in the American West presents some of the greatest challenges we may face in the coming decades and centuries. Examining the state of the Earth system under different configurations provides one of the only ways in which we can gain insight into how the Earth has behaved in the past and how it may change in the future. In this dissertation, I contribute towards the broader objectives of paleoclimate by producing and interpreting records of the greatest changes in the hydrologic cycle from the past sixty-five million years in western North America. Given the importance in determining paleoclimatic and paleoenvironmental history, constraints on ancient temperature, precipitation and the timing of mountain uplifts are actively debated. Pre-Quaternary terrestrial paleoclimate records, however, remain underdeveloped. Continental records are essential, not only because we are mammals living in terrestrial environments, but also because terrestrial paleoclimate records can improve the mechanistic understanding of how different components of the Earth system interact. Cenozoic western North America presents a natural laboratory to study the interactions between processes such as topographic development, atmospheric circulation, and the role of the biosphere in vapor recycling. The history of these processes is preserved in sedimentary basins that are abundant in western North America. These basins, which lie in and around the Basin and Range, Rocky Mountains, Great Plains, Sierra Nevada and other geologic provinces, contain well-characterized paleosols, fluvial and lacustrine sediments, and abundant leaf and mammalian fossils. With countless prior studies by the structural geology, sedimentary geology, and paleobiology communities, sedimentary sections are well-described and have excellent radiometric age control. This dissertation is a collection of three studies that use the oxygen and hydrogen isotope composition of authigenic minerals to characterize and constrain information about different states of the hydrologic cycle in Cenozoic western North America. These minerals, such as pedogenic clays and carbonates, preserve the isotopic composition of the ancient water in which the mineral formed. By exploiting well-understood mineral-water isotope fractionation relationships, we use the isotope composition of minerals to infer paleoclimatic information such as ancient elevation, temperature, aridity, vapor recycling and seasonality of precipitation. The chapters of this thesis are arranged in chronological order: Chapter 1 examines the development of high topography and orographic rainout patterns prior to the Early Oligocene; Chapter 2 explores the rise of grassland ecosystems as a force driving climatic change in the Neogene; and Chapter 3 uses the isotope geochemistry of smectite to estimate paleotemperature and distinguish between multiple drivers of paleoenvironmental change primarily during the Neogene. Chapter 1 investigates the relationships between mantle, surface and atmospheric processes. The origin and evolution of high topography in the North American Cordillera is actively debated, yet popular tectonic models each have a testable topographic response. In this chapter, I examine Cenozoic surface uplift patterns using the oxygen and hydrogen isotope geochemistry of a variety of materials, including calcium carbonate, smectite, kaolinite, muscovite and hydrated volcanic glass. I compiled ~3000 of these mineral proxy data from western North America spanning the Late Cretaceous and Paleogene. Using independent temperature estimates from paleofloral assemblages and the appropriate fractionation factors, I produced a series of maps documenting how the oxygen isotope composition of ancient meteoric water evolved both spatially and temporally in western North America. This dataset revealed the progressive, north to south uplift of the North American Cordillera which culminated in the assembly of an Eocene-Oligocene highland 3-4 km in elevation. We interpret the Eocene-Oligocene development of rainout patterns similar to the modern as unlikely to be the result of Mesozoic crustal thickening and the development of an Altiplano-style plateau. Instead, our stable isotope paleoclimate data are consistent with tectonic models calling for the convective removal of mantle lithosphere or removal of the Farallon slab by buckling along an east-west axis. In Chapter 1, we demonstrated that the first-order rainout patterns of modern western North America were established by the Early Oligocene. How then did the climate of western North America change following the development of high topography? In Chapter 2, I explore the role of the biosphere in affecting the hydrologic cycle through evapotranspiration. First, I examined oxygen and hydrogen isotope records from Neogene western North America, and produced several new records from the Basin and Range, Rocky Mountains and Great Plains. Nearly all of these records, including those in regions that did not undergo significant topographic changes such as western Nebraska, exhibit large increases in δ18O, on the order of 2-6 ‰. My coauthors and I tested a hypothesis in which the dramatic rise of grass- dominated ecosystems at the expense of preexisting woodlands acts as a driver of Neogene climatic change in western North America. We developed an isotopic water vapor transport model that simulated air masses delivering moisture from the Gulf of Mexico to present day western-Nebraska with a variety of land cover scenarios. Our model results indicate that grasslands lead to δ18O of precipitation (δ18Op) that is up to 5 ‰ greater than broadleaf and needleleaf vegetation at inland study sites. We show that differences between δ18Op profiles are the result of: 1) Differences between plant types in the amount and partitioning of evaporation and transpiration; 2) Differences in the isotopic composition of transpired vapor due to differences in rooting depth and growing season. We argue that the expansion of grasslands in western North America could account for the bulk of the observed changes in Neogene stable isotope paleoclimate records, and that the observed isotopic signals are indicative of a mechanism wherein vegetation not only responds to prevailing regional and global climatic trends, but also acts as a driver of climatic change. We suggest that by enhancing seasonality and aridity downstream, grasslands may engineer the climatic conditions favorable for their expansion. While Chapters 1 and 2 examine interactions between tectonics, atmospheric circulation and the biosphere, they also highlight the challenges of telling a coherent story while using a single isotope system that responds to multiple paleoenvironmental influences. Often the difficulty in such studies involves eliminating confounding effects and incorporating corroborating information to arrive at an interpretation. In Chapter 3, I exploit the geochemistry of smectite to distinguish between multiple drivers of paleoenvironmental change. As these minerals incorporate and preserve both the oxygen and hydrogen isotope composition of parent water, phyllosilicates can provide insight into ancient meteoric water relationships and temperature. I collected and analyzed pedogenic smectite formed as a weathering product of silicic ashes. This study incorporated oxygen and hydrogen isotope data from over 200 smectite samples from 11 basins representing a range of paleoenvironments in the Cenozoic Basin and Range, Rocky Mountains and Great Plains. Our results indicate that the processes controlling smectite stable isotope composition vary both spatially and temporally. In some basins, such as Miocene Trapper Creek, ID and the Eocene to Miocene eastern flank of the Rocky Mountains, changes in ancient meteoric water composition are the dominant driver of change in the isotopic composition of smectite. In many other basins, such as those from the Miocene Basin and Range, changes in mineral formation temperature contribute to change in mineral isotopic composition, and in the case of the northern Basin and Range, are the dominant driver of isotopic change. My smectite geothermometry estimates indicate mineral formation temperatures of 30-40 °C in the Middle Miocene Rocky Mountains, Great Plains and Basin and Range, and a decrease of 10-15 °C since the Middle Miocene Climatic Optimum. These paleotemperature estimates are consistent with existing clumped isotope estimates, and the temperature trends are consistent with paleofloral and marine records to a first order.