Unraveling Paleoenvironmental Stress in the Late Devonian and Mississippian: Insights into Redox Dynamics and Photosynthetic Adaptations Elucidated Through Geochemical Proxies

Abstract

The Late Devonian and Mississippian Periods (382.7 to 358.9 million years ago) were key moments in the history of our planet. They contain one of the five major mass extinctions of the Phanerozoic which saw profound environmental and ecological disturbances and changed the trajectory of both marine and terrestrial ecosystems. The proliferation of vascular plants on land, major marine transgressions over the continents, and climatic changes also occurred during this time. The mechanisms behind the Late Devonian mass extinctions have been explored by many researchers. One proposed driver, marine deoxygenation, requires further investigation given our current state of understanding. The first two chapters of this dissertation explore the temporal and geographic extent of marine deoxygenation during the Devonian Lower and Upper Kellwasser Events as well as Frasnian-Famennian transition on both the local and global scales. Chapter 1 seeks to evaluate the regional distribution of marine deoxygenation within the Appalachian Basin by using a local redox proxy, Fe speciation, with a focus on the record across the Frasnian-Famennian stage boundary. Based on this data, deoxygenation and anoxia occurred during the Lower Kellwasser Event and persisted in the distal portions of the Appalachian Basin. Chapter 2 builds on the findings of Chapter 1 and applies the thallium isotope redox proxy to track marine deoxygenation on the global scale. Specifically, this proxy is dependent on the global distribution and deposition of Mn oxides and therefore can provide further insight into the role of marine deoxygenation in the Late Devonian mass extinctions. Here we find that global scale deoxygenation occurred across the Frasnian-Famennian transition, however, this deoxygenation post-dates the Lower Kellwasser Event and persists after the second, suggesting a longer time interval of global scale marine deoxygenation. Finally, Chapter 3 shifts the focus to the terrestrial biosphere and explores how environmental stresses such as aridity and low atmospheric CO2 in the Mississippian may have driven evolutionary changes in land plants. Specifically, this study looks for carbon isotope evidence for the emergence of an alternative photosynthetic pathway such as the Crassulacean Acid Metabolism (CAM) in Mississippian arborescent lycopsids. We find that all studied plant fossils had similar carbon isotope compositions. The lack of isotopic difference between the coeval C3 plants and that of lycopsids leaves open three possibilities for the photosynthetic pathway that they employed: 1) the C3 metabolism 2) the aquatic CAM pathway or 3) a facultative CAM pathway. These three chapters reveal the interconnectedness of environmental and evolutionary changes during the Late Devonian and Mississippian. These include the expansion of marine deoxygenation on both the local and global scales and its connection to Frasnian-Famennian extinctions. It also shows what photosynthetic pathways land plants utilized in the context of the environmental changes occurring during the time. More broadly, these findings highlight the connection between life and the environment not only during the Late Devonian and Mississippian, but more broadly over the history of our planet, and reveal how these changes shaped the Earth we see today.