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.
| Identifer | oai:union.ndltd.org:VTETD/oai:vtechworks.lib.vt.edu:10919/106949 |
| Date | 19 June 2020 |
| Creators | Schwid, Maxwel Fredrick |
| Contributors | Geosciences, Xiao, Shuhai, Gill, Benjamin C., Hiatt, Eric E. |
| Publisher | Virginia Tech |
| Source Sets | Virginia Tech Theses and Dissertation |
| Detected Language | English |
| Type | Thesis |
| Format | ETD, application/pdf |
| Coverage | China |
| Rights | In Copyright, http://rightsstatements.org/vocab/InC/1.0/ |
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