Doctor of Philosophy (PhD)
First Committee Member
Stephen M. Rowland
Number of Pages
This study presents new data on the sedimentology of the northern Yangtze Platform, the delta13C stratigraphy of South China and Nevada, and the decline and virtual extinction of archaeocyaths. Paleoecological, sedimentological, and chemostratigraphic data were collected from multiple localities that span from the mid-Early Cambrian of Nevada (Poleta Formation) and China (Xiannudong Formation) to the late-Early Cambrian of Nevada (Harkless Formation) and China (Tianheban Formation); Facies represented in the Xiannudong Formation describe a Bahamas-type platform for the Yangtze Platform with a static and negative delta 13C record. All four formations analyzed show a static delta 13C record that varies from slightly negative to slightly positive. This is unexpected because stasis is not observed in the composite delta 13C record of the Siberian Platform, suggesting that our data may represent intrabasinal delta13C records and not global; Another important feature I observed in the Xiannudong Formation is a faunal changeover from predominantly regular-type archaeocyaths in the lower Xiannudong Formation to predominantly irregular-type archaeocyaths in the upper part. This changeover is also seen between the Poleta Formation and the Harkless Formation in Nevada; Finally, I analyzed the physical changes in archaeocyathan skeletons over time. A progressive thinning of skeletal thickness was expected, however a distinct trend of skeletal thickening was observed. Skeletal thinning was expected due to a lowered carbonate saturation state driven by rising atmospheric CO2. To explain thickening, I propose that irregular archaeocyaths contained photosymbionts, which are documented to counteract a lower carbonate saturation state. Irregular archaeocyaths, which inherently contain more elements in their intervallum, and hence, more soft tissue, could house more endosymbionts than regulars. Therefore, regulars declined due to the lack of abundant photosymbionts, while irregulars flourished; Currently, there is no definitive evidence to support our hypothesis that the ultimate extinction of archaeocyaths was due to increased sea surface temperatures due to the greenhouse warming effect of increased pCO2. Prolonged temperatures would have increased thermal stress on the archaeocyaths until some threshold was reached in which even irregulars could not survive. This scenario may have implications for modern reef reactions to anthropogenic CO2 and greenhouse warming.
Archaeocythan; Archaeocyathan Reef; Cambrian; Characterizing; China; Decline; Early; Evidence; Global; Harkless Formation; Nevada; Poleta Formation; Reef; Tianheban Formation; Xiannudong Formation
University of Nevada, Las Vegas
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Hicks, Melissa, "Characterizing global archaeocyathan reef decline in the Early Cambrian: Evidence from Nevada and China" (2005). UNLV Retrospective Theses & Dissertations. 2658.