Ediacaran phosphorites provide a principal record of the paleoenvironmental and paleoecological conditions of the oceans during Earth's second major oxygenation and the evolution of complex life. Although the fidelity of this record is high, diagenesis and metamorphism frequently alter or overprint primary minerals and structures, necessitating validation of results from mineralogical and geochemical analyses and determinations of fossil affinities. Therefore, it is imperative to unravel the complications of post-depositional alteration, and thus provide a strong foundation for environmental and biological interpretations, via an integrated sedimentological, stratigraphic, petrographic, and geochemical approach. Transmitted light, cathodoluminescence, and scanning electron microscope petrography in conjunction with Raman spectroscopic and X-ray diffraction analyses were employed to determine the origin of phosphorites from the early Ediacaran (632 – 614 Ma) Doushantuo Formation at the Wanjiagou section, near Zhangcunping, Hubei Province in South China. Results suggest granular phosphorites were deposited during one to two episodes of reworking of pristine phosphorite hardgrounds, which originated during redox-controlled and/or microbially-mediated phosphogenesis. Granular laminae were then cemented by a ferric iron-phosphate mineral, phosphosiderite. As a product of oxidative weathering and/or thermal stabilization of ferrous iron-phosphates (e.g., vivianite), this cement is suggestive of precipitation from ferruginous porewaters. This is the first direct evidence for iron-phosphate minerals in Ediacaran phosphorites and substantiates previous hypotheses of P burial beyond primary calcium phosphate. If accumulation and burial of phosphorites during this interval was rapid enough to have limited P availability and thus primary productivity, their formation may have governed oxygen production prior to the Neoproterozoic Oxygenation Event (NOE). Oxygenation of the oceans during the NOE and the appearance of complex, multicellular life are suspected to be causally linked within the Ediacaran. However, a fragmented fossil record with insufficient analogues and varied taphonomic modes leaves much of the Ediacaran fauna with uncertain taxonomic and phylogenetic affinities, leading to ambiguity regarding their life modes and environmental associations. Furthermore, demonstrating biogenicity is an often overlooked, yet fundamental component of Ediacaran fossil identification and interpretation, something that has particularly affected the morphologically simple discoidal group of fossils known colloquially as Aspidella. Petrographic observations supported by Raman and energy dispersive spectroscopy provide evidence that discoidal concretions from the Ediacaran Miaohe Member near Maxi, Hubei Province in South China are diagenetic in origin but superficially resemble Aspidella's morphology. Erosion of these syn-compactional concretions produced concentric rings on bedding planes caused by internally deformed laminae resulting in Aspidella pseudofossils. These results highlight the necessity for critical evaluation of the origin of discoidal structures observed in Ediacaran sedimentary successions. / Master of Science / Contemporary and ancient phosphorus-rich sedimentary deposits, known as phosphorites, precipitate within the oceans as a result of intricate chemical and biological interactions. The Ediacaran Period (635 – 539 million years ago) contains the first truly extensive occurrences of phosphorites in addition to a fossil record of the earliest animal organisms. Deposited after the end of Earth's last global glaciation, the origins of Ediacaran phosphorites are affiliated with these dramatic climatic and evolutionary transitions as well as the rise of atmospheric and oceanic oxygen concentrations to near modern levels. Deposition of phosphorites often occurs in low-oxygen oceanic environments and their formation constitutes the dominant mechanism by which phosphorus is removed from the phosphorus cycle on time scales greater than 1000 years. Therefore, phosphorite occurrences provide a record of phosphorus cycling, oxygen availability, and biological productivity. However, microscopic and chemical analyses of phosphorites from the Ediacaran Doushantuo Formation in South China demonstrate they are partially composed of phosphorus minerals that likely formed in non-oxygenated environments. The presence of these atypical phosphorus minerals has been previously hypothesized, with the implication that they further limited the availability of phosphorus for use by photosynthetic organisms. Such a limitation on photosynthesis would have resulted in decreased oxygen production and thus the formation of these phosphorites may explain the rate and trend of the change in oxygen concentrations observed during the Ediacaran. Ediacaran fossils also act as a proxy for environmental conditions of the ancient oceans through inferences about the preserved organisms' requirements for life. Although most fossils of this age are the first of their kind in terms of biological complexity, they are typically simple in terms of their morphology, making identification difficult. Furthermore, providing evidence that such simple structures actually represent a fossilized organism is often problematic due to the inability to compare them with modern organisms. Microscopic and chemical analyses of disc-shaped structures from the Ediacaran Miaohe Member in South China reveal that they are concretions that were not created by an organism, even though their morphology very closely resembles the Ediacaran fossil Aspidella. Identification of these concretions as pseudofossils suggests that close examination of fossils from Ediacaran rocks is necessary.

The Ediacaran Period (635-542 Ma) just preceded the radiation of animals, yet witnessed profound changes in biological innovation, including the first appearance of large spiny acritarchs called the acanthomorphic acritarch, followed by the radiation of the Ediacara biota (575-542 Ma), and earliest recognizable bilaterally symmetrical animals (~550 Ma). It has been proposed that key environmental events, such as the termination of the Cryogenian glaciations, the Neoproterozoic Acraman impact event, and oxygenation of the deep oceans may have played an integral role in the evolution of Ediacara organisms and early animals. However, the extent to which these events shaped biological evolution remains elusive. The Doushantuo Formation in South China, radiometrically constrained between 635.2±0.6 and 551.1±0.7 Ma, is ideal for high-resolution interdisciplinary research, and has the potential to clarify the relationship between environmental and biological events. Research in this dissertation aims to address the following questions: (1) was the Doushantuo Formation deposited in an open marine or a (partially) restricted basin; (2) are Doushantuo paleoenvironmental and biostratigraphic proxies consistent with an Ediacaran oxidation event; and (3) can the Doushantuo acanthomorphic acritarchs be useful biostratigraphic tools for the Ediacaran Period? Detailed (sub-meter) sampling of six sections in the Doushantuo Formation in the Yangtze Gorges area of South China reveal a complex depositional history. Eight broad lithostratigraphic facies and 6 cycles packaged into 3 sequences can be identified and potentially traced into basinal sections. It is likely that the deposition of the Doushantuo Formation occurred under open marine conditions and became increasingly restricted with the development of thick carbonate accumulations at the platform margin. Geochemical analysis shows extreme isotopic variability in the Doushantuo Formation that may be the result of pulsed oxidation of a deep oceanic organic carbon reservoir. Oxidation events may have had further implications on the radiation of early animals. Distinct assemblage biozonation of the Doushantuo acanthomorphic acritarchs is concurrent with isotopic variability, suggesting an ecological and/or evolutionary response during the early Ediacaran. Further efforts in refining the internal geochronology of the Doushantuo Formation is needed in order to test competing hypotheses on the radiation of important taxonomic groups. / Ph. D.

The three projects described herein focus on two instances of exceptional preservation and on potential source of bias in the fossil record. The occurrence of exceptionally preserved fossil assemblages and the existence of systematic bias in the fossil record from a variety of sources represent opposing forces acting on the information quality of the fossil record. Exceptionally preserved assemblages capture features of anatomy and components of assemblages not normally recorded in the fossil record. Systematic biases affecting the fossil record do the opposite, skewing our perception of patterns of diversity, the relative dominance of clades and changes in ecosystems through time. Chapter one presents the results of an analog modeling analysis to assess whether and how a newly proposed potential mechanism, the preferential sampling of larger specimens during fossil sampling due to the greater likelihood of larger specimens being intersected by a fracture surface, contributes to the lithification bias. Chapters two and three focus on the exceptionally preserved vermiform fossils from the Winneshiek Lagerstätte in northeastern Iowa and microfossils from the Doushantuo Formation of South China, respectively. Chapter two aims at resolving the identity of the Winneshiek vermiform fossils and presents evidence that these structures represent phosphatized bromalites, an ichnologic category that includes coprolites and cololites. Chapter three presents a biostratigraphy study of exceptionally preserved microfossils at three sections of the Doushantuo Formation in South China. Acanthomorphic acritarchs represent a promising tool for subdivision and correlation of the Doushantuo Formation of South China and Ediacaran strata around the world. However, the occurrence of acanthomorphic acritarchs within the Doushantuo Formation is controlled by the availability of early diagenetic chert nodules that host microfossils such as acanthomorphic acritarchs. One of these sections contains the rare occurrence of early diagenetic cherts in an upper slope section. This new biostratigraphic data adds to the growing body of integrated chemostratigraphic and biostratigraphic data from the Doushantuo Formation. By understanding both exceptional preservation and sources of bias in the fossil record it is possible to separate artifact and noise from the true signal of the history of life. / PHD

The transition from the Neoproterozoic to the early Cambrian world was a critical time in the co-evolution of life and the environment, when dramatic changes in the environment, including global glaciations and rising atmospheric oxygen levels precipitated the conditions in which variety of complex life emerged, diversified, and proliferated. However, questions persist regarding the timing and duration of those environmental events, and thus the extent of their direct effect on the evolution of life. These changing environmental conditions also produced unique taphonomic conditions that preserved a wide variety of organisms, and also produced unusual early diagenetic features that can be difficult to distinguish from fossilized organisms. In order to better resolve the link between life and the environment during the Neoproterozoic to Cambrian transition I present three chapters. The first chapter of this dissertation examines the carbonate clasts from a glacial diamictite deposited at the end of the Cryogenian Period in South China. By determining the source locality of those carbonate clasts using stable carbon and oxygen isotope compositions, we can better understand the extent of the Marinoan Glaciation and the intensity of its impact on the Earth's surface. In the second chapter of this dissertation, I investigate the changing chemical conditions of ocean waters following both the Marinoan glaciation and potentially one of the most intense disruptions to the carbon cycle in Earth History, the Ediacaran Shuram carbon isotope anomaly. I use Hg concentrations and stable isotope compositions to determine changes in sediment sourcing along with changes in marine redox, such as the development of photic zone euxinia. In the third chapter, I investigate the identity of Brooksella alternata, a purportedly cnidarian fossil that was later suggested to be a hexactinellid sponge fossil. My morphological and compositional analysis reveals B. alternata to be a concretion and thus a pseudofossil. / Doctor of Philosophy / The Earth experienced major changes between 635 and ~514 million years ago; the last global scale glaciation in the history of Earth ended, early animals arose, primary producers such as algae grew larger and more complex, and the first animals that were mobile and with hard skeletal parts evolved and diversified tremendously. Concomitantly, the environment on the Earth changed dramatically: the carbon cycle experienced one of the greatest disruptions in all of Earth history and the oceans and atmosphere gradually became more oxygen-rich, though areas with low levels of dissolved molecular oxygen and high concentrations of dissolved iron or hydrogen sulfide persisted, which may have impeded the diversification of complex animals. The exact timing and intensity of these changes are not fully resolved, and by investigating this time interval in the geologic record, we can better understand how the world changed and how life at the time responded. These changing environments also produced unique conditions which led to fossils being preserved in unique ways, though these conditions also produced non-biological structures that superficially resemble fossils. The first chapter of this dissertation investigates the source of clasts made of carbonate rock from a glacial diamictite (the poorly sorted mixture of the fine-grained and coarse-grained deposits left behind as glaciers recede) in South China deposited at the end of the last global scale glaciation about 635 million years ago. By determining the source, we can evaluate how far glacial activity transported sediments. I found that the previously suggested source of these clasts does not match the geochemical fingerprint of the carbonate clasts. I propose that these carbonate clasts may have come from a source that has since become "extinct" because of glacial erosion, or they may have come from another continent (for example, India). These scenarios highlight the magnitude of the global glaciation. In the second chapter, I investigated the mercury concentrations and stable isotope compositions from organic rich shales deposited ~550 million years ago. Mercury is an emerging tool for evaluating the level of dissolved molecular oxygen in ancient oceans. In this study, I found evidence of locally complex marine oxygen levels, including evidence for photic zone euxinia, meaning waters where oxygen was absent and hydrogen sulfide was present. Such conditions are toxic to animals and may have delayed the diversification of complex animal life. In the final chapter, I analyzed the ~503 million-year-old fossil Brooksella alternata. It was first described more than 100 years ago as a jelly-fish fossil. More recently it was suggested to be a sponge fossil (though it has been ascribed many other identities). Based on morphological analysis of a large collection, I determined that Brooksella alternata is not a fossil but rather a concretion with unusual shape.