Carbon and Water Dynamics in Modern and Ancient Plants and Soils

Abstract

During modern times of unprecedented climate change, historical and geological records of past climate allow us to understand and project ancient patterns in the carbon and water cycle. Plants and soils are excellent records for climate because of the direct interaction between the terrestrial biosphere and the atmosphere and their preservation potential. Organic carbon isotope records in plants and soils have been linked to several environmental, evolutionary, and edaphic drivers, in addition to being used to measure ecosystem stress. Most of the works use organic carbon isotope values in ancient plant fossils with unconstrained and uncertain environments or in modern plant growth experiments with highly controlled environments that are not directly comparable to the natural world. The period of Industrialization provides a natural experiment with measurable, but not lab-controlled climate change, ideal for investigating the relationship between climate variability (particularly the carbon and water cycles) and C isotope values. This dissertation includes several high-resolution spatiotemporal studies of several plant species through the pre-Industrial era. In Chapter II, leaf carbon isotope values from a single Great Lakes region species are compared to environmental variables involved in the water and carbon cycles (e.g. atmospheric CO2 concentration and isotopic signature, temperature, precipitation, etc.) over the span of 200 years. Leaf isotope biogeochemistry of this species is unresponsive to every environmental variable except the isotopic composition of atmospheric CO2. Chapter III expands on results from previous experiments, and investigates the relationship between the carbon dioxide of the atmosphere and plant carbon isotope geochemistry with eight Northern Hemisphere focal species. These specimens are complemented by C3 plant isotope analyses of aggregated from literature. Results from this work demonstrate that plants do not change biochemically as expected in response to CO2 rise. In Chapter IV, findings from previous chapters are re-examined from the context of aboveground carbon integrator: soils. The role of weathering in the development of soils provides incentive for investigating soils as potential water cycle records. As in Chapter III, this study includes soils collected across a climosequence, supplemented by aggregated carbon isotope values from prior publications. This work also tests the relationship between soil carbon isotopes and precipitation in ancient soils (paleosols). In both modern and ancient studies, carbon isotope values of soil correlate to precipitation. Chapter V integrates the results and conclusions from early dissertation chapters with pre-established geochemical and physiognomic proxies to provide a comprehensive analysis of early Eocene hothouse sediments. Depositional basins from the Eocene are well-studied and contain opportunities for multi-proxy-based environmental reconstruction. Using organic and inorganic tools to constrain provenance, parent material, and other features of the depositional environment, an early Eocene fluvial-lacustrine subtropical forest deposit shows constant sedimentological and hydrological inputs and consistent climate during a time of expected cooling and tectonic activity. In summary, this dissertation contains case studies relating soils and plants to the carbon and water cycle in modern, historical and deep time and demonstrates the recording power of the terrestrial biosphere. These findings can be used to contextualize and guide applications of organic isotope biogeochemistry in paleoclimate reconstructions. Related to modern climate problems, results from this dissertation provide integral information about plants and soils as carbon and water distributors and mitigators of anthropogenic climate change.