Re-Evaluating the Late Devonian Mass Extinction: A Geochemical Investigation of the Relationship between Carbon Isotope Fluctuations, Faunal Turnover, and Paleoenvironmental Change Recorded in Upper Devonian Carbonates of the Lennard Shelf, Western Australia
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The primary goal of this dissertation is to increase understanding of the so-called Late Devonian mass extinction through the use of stable isotope geochemistry. Despite decades of research, the timing, cause(s), and extent of the events surrounding this devastating interval in Earth history remain poorly understood. One of the best places to study the Devonian period is the Lennard Shelf in the Canning Basin of Western Australia. This region contains extensive, well-preserved exposures of Middle and Upper Devonian (Givetian, Frasnian, and Famennian) carbonate reefal platforms and slopes that are ideal for geochemical analyses. A significant amount of information could be learned about the Late Devonian mass extinction in the Canning Basin if we had a detailed chronostratigraphic framework of platform to basin strata and a better understanding of the paleoenvironmental conditions during the Upper Devonian. This research aims to use stable isotope chemostratigraphy to help build a high resolution chronostratigraphy and examine any environmental changes leading up to and following the F-F boundary that may have contributed to Devonian extinction events in the Canning Basin. This dissertation is divided into two parts; the first concerns my research on the Late Devonian mass extinction while the second is ancillary and documents my astrobiology research rotation. Part 1 contains introductory and concluding remarks as well as three chapters written as scientific manuscripts that have been, or will soon be, submitted for journal publication. The first of these three chapters examines the local and global controls on carbon isotope chemostratigraphy in the Lennard Shelf system to validate the use of secular variations as a chronostratigraphic tool for regional and global correlations. The next chapter provides a detailed, expanded view of the F-F boundary and constrains the pattern of carbon isotope perturbations across said boundary at the intra-zonal scale. The third scientific chapter broadens focus from the F-F to the entirely of the Upper Devonian and presents the first carbon isotope composite curve from Western Australia that is used to test the global nature (timing and extent) of Late Devonian events and examine the relationship between carbon isotope fluctuations, faunal turnover, and changes in the paleoenvironment (sea level, climate, ocean chemistry, paleogeography). Results from Part 1 not only have implications for re-evaluating one of the "Big Five" mass extinctions in the Phanerozoic, but are also relevant to studies in astrobiology and have additional applications in the oil and gas industry. The final chapter of my dissertation, which is in Part 2, concerns interdisciplinary, astrobiology work done during summer quarter, 2014, that was devoted to analyzing the relationship between astronomy and geology circa 1770-1810. This chapter is included in my dissertation as partial fulfillment of my dual-title astrobiology degree.