| Summary: | This dissertation consists of two independent topics, both critical to understanding Earth history during the Proterozoic Eon.
(1) Paleoproterozoic Chuanlinggou Acritarchs: The eukaryotic affinity of acritarchs from the Chuanlinggou Formation has been questioned because of lack of indisputable morphological evidence. In this study, Chuanlinggou acritarchs were examined by jointly using optical microscopy, scanning electron microscopy, and transmission electron microscopy. In most cases, an ovoidal group of acritarchs represents a half-vesicle following a complete longitudinal rupture, which is a morphological model different from a whole envelope with medial splits, as proposed by earlier studies. This ovoidal group displays a bipolar morphology, longitudinal rupture, and occasionally striated wall structures that are consistent with a eukaryotic affinity. Thus, the Chuanlinggou ovoidal acritarchs probably extend the eukaryotic body fossil record into the Paleoproterozoic, circa 200 million years earlier than the morphologically more complex acritarchs from the Mesoproterozoic Roper Group.
(2) Neoproterozoic 17O-depleted Barite: Distinct, non-mass-dependent 17O depletion was reported in barite from Marinoan cap dolostones, and has been interpreted as an indication of an extremely high-pCO2 atmosphere. Understanding the origins of this barite and particularly the source of sulfate is critical to interpreting the anomalous 17O signature and its implications for sulfur and oxygen cycles after Marinoan glaciation. In this study, together with field, petrographic, and Sr isotope data, the expanded dataset (1) confirms large variability in ∆17O, δ18O, and δ34S of barite; (2) demonstrates a hyperbolic relationship between the ∆17O and δ34S; (3) reveals that individual barite crystal fans and fans of the same generation possess well-clustered sets of δ18O, Δ17O, and δ34S values; and (4) shows that barite crystal fans of different layers bear different sets of Δ17O, δ18O, and δ34S values. The study suggests that 17O-depleted barite crystals were formed under supersaturation when Ba2+ from sulfate-free deepwater came to mix with sulfate-bearing shallow water. The large variability in sulfur and triple-oxygen isotope composition and the high 87Sr/86Sr ratios indicate that the two sites were sufficiently close to continents so that the isotopic composition of sulfate was easily influenced by changes in riverine flux.
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