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Title: A nitrogen and organic carbon isotope study of Ediacaran-Cambrian sedimentary rocks of South China (Yangtze Platform) : palaeoenvironmental implications
Author: Cremonese, L.
Awarding Body: University College London (University of London)
Current Institution: University College London (University of London)
Date of Award: 2012
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The Precambrian-Cambrian boundary is claimed to represent one of the most crucial biological breakthroughs on our planet when changes in chemical and physical conditions contributed to a biodiversity “explosion”. The geological record of the Yangtze Platform (South China) offers a wonderful cross-section of this interval, characterized by continuous and relatively unaltered successions which are ideally suited to high-resolution investigations of palaeoseawater. The present study is focussed on δ15Ntot and δ13Corg isotopic compositions of sedimentary rocks during the Pc-C transition, providing insight into local and global geological and ecological developments. For a better understanding of the nitrogen cycle, a broad description of studies focusing on nitrogen isotopes in ancient sediments is reported, with critical discussions of isotope data and related geological interpretation including appraisal of syn- and post-diagenetic effects. The main body of the thesis consists of N and Corg isotope results and discussion focusing on biogeochemical cycling variations and implications for biological evolution, palaeogeography, stratigraphic correlation and data reliability. The Xiaotan and a composite section from the Yangtze Gorges area show meaningful similarities in their nitrogen isotope trends, interpreted as chemocline fluctuations in the water column also testifying to rapid water mixing within the shallow platform. A negative nitrogen isotope excursion throughout the Yangtze platform was found at the Pc-C boundary in shallow domains as well as in the basin, probably in response to photic zone anoxia and intense nitrogen fixation/assimilation by Green and Purple sulfur bacteria. A more complex biological kingdom with increased bioturbation and food-chain complexity can be envisaged across the Pc-C transition, characterized by more rapid chemical reactions which facilitated ocean pool and isotope perturbations in a more productive marine environment. The renewed nitrogen cycle established in response to profound biogeological changes reflects the beginning of a new geological era biologically controlled.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available