The timing of oxygenation events throughout Earth’s history remains controversial. Oceans and the atmosphere were essentially devoid of oxygen until the ‘great oxygenation event’ around 2.4 billion years ago that raised oxygen levels to 1-10% of todays present atmospheric levels (Holland, 2006). Earth’s second canonical oxygenation event, that raised atmospheric levels to near present, is thought to have occurred in the Neoproterozoic prior to the diversification of animal life. Using geochemical proxies, two frameworks of oxygenation have been developed for the timing of the Neoproterozoic event. The first suggests oxygenation of the deep oceans early in the Ediacaran succeeding the Marinoan Glaciation (~635Ma) (Shen et al., 2008; Sahoo et al., 2012), whereas the second places the event roughly coincident with the first large multicellular eukaryotes following the Gaskiers Glaciation (~580 Ma) (Canfield et al., 2007; Fike et al., 2006). Understanding the timing of oxygenation is critical in understanding the mechanisms and feedbacks associated with the diversification of animal life. A multi-proxy geochemical approach on black shale is useful to investigate the redox conditions in the Early Ediacaran following the Marinoan glaciation. Black shale samples are from well-exposed sections of the Sheepbed Formation, of known early Ediacaran age, at its type locality in the Mackenzie Mountains, Northwest Territories, Canada. Shen et al. (2008), Canfield et al. (2008), and Johnston et al. (2013) all conducted geochemical redox studies on the formation in different localities with mixed results. The current study extends the geologic coverage of redox data in the complex and laterally variable Northwest Canada Neoproterozoic strata. High-resolution iron speciation data in this study suggests a transition from ferruginous to oxygenated ocean waters with episodic ferruginous conditions. These data support the conclusions of Shen et al. (2008) that oxygenation is recorded within the Sheepbed. They do not suggest a “great event” occurred due to fluctuations in the redox stat following the oxygenation signal. Furthermore, in light of the studies at other localities, these data suggest heterogeneity in redox conditions across the early Ediacaran basin strata of Northwest Canada. Redox sensitive trace element data in ferruginous bottom waters of the basal Sheepbed Formation do not support the conclusion of Sahoo et al. (2012) that global ocean oxygenation directly followed the Marinoan Glaciation. Rather, moderate enrichments later in the section could suggest a possible global ocean oxygenation signal. More redox sensitive geochemical studies in post- Marinoan transgressive units are necessary to elucidate a clear understanding of global ocean redox conditions in the early Ediacaran and overcome the noise associated with localized effects on redox proxies.
This project is focused on the study of climate, hydrology, and surface processes of western North America during the late Cretaceous (~75 million years ago) in what is now southern Utah. The goal of this project is to describe in detail the hydrology of fluvial systems associated with the deposition of the Kaiparowits Formation. Differences in stable isotope ratios of gar ganoine, pedogenic carbonate, and enamel from hadrosaur and crocodile teeth, in conjunction with previously published bivalve data, indicate there are three main parts to the fluvial system: 1) FS1—large anastomosing rivers draining upland areas 2) FS2—lakes subject to episodic flooding and 3) FS3—smaller streams draining the foreland basin. Furthermore, it is possible to infer mixing of water between these sources, in particular the mixing of FS1 and FS3 waters to form FS2 water, presumably during seasonal flooding events that were analogous to processes taking place in modern-day Tonle Sap Lake in central Cambodia. The organic content of sediment, carbon isotope ratios of paleosol carbonate, and the carbon isotope ratios of hadrosaur dentine and enamel from different sites all indicate that soils along the margin of the FS2 lakes were characterized by episodic flooding and saturation, with those closer to the margin being saturated for a longer period of time, compared to more distal localities. Furthermore, hadrosaurs that ate vegetation located closer to the lake margin have teeth with high carbon isotope ratios, consistent with the existence of closed canopy forests in these localities. Thus, variations in the hydrology of these fluvial systems appears to have played an important role in determining the distribution of plants over Kaiparowits landscapes, with closed canopy forests perhaps accounting for the large diversity in herbivorous dinosaurs observed in southern Utah during the late Cretaceous.