Veracity of Neoproterozoic negative C-isotope values: The termination of the Shuram negative excursion |
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| Institution: | 1. Department of Geoscience and NASA Astrobiology Institute, University of Wisconsin-Madison, Madison, WI 53706, USA;2. Department of Geology, University of Maryland, College Park, MD 20742, USA;3. Department of Geosciences, Virginia Tech, Blacksburg, VA 24061, USA;4. Key Laboratory of Economic Stratigraphy and Palaeogeography, Nanjing Institute of Geology and Palaeontology, Chinese Academy of Science, Nanjing 210008, China;5. Department of Geology and Geophysics, Louisiana State University, Baton Rouge, LA 70803, USA;6. Earth System Science Interdisciplinary Center, University of Maryland, College Park, MD 20742, USA |
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| Abstract: | There is widespread interest in the Neoproterozoic period of the Earth's history (1000 to 542 Ma) because of unprecedented δ13C fluctuations to < ? 10‰ PDB through thick (> 1000 m) succession of stratigraphically complex sedimentary rocks deposited during tens of millions of years. In contrast, Phanerozoic large negative C-isotope excursions have been interpreted as the result of diagenetic fluid mixing during carbonate stabilization and burial and are less enigmatic due to the excellent biostratigraphic control on their timing and duration.The Ediacaran Nafun Group of Oman (part of the Huqf Supergroup spanning the Cryogenian–Early Cambrian) contains a large δ13C negative excursion (the Shuram excursion) reaching values as negative as ? 12‰ at the base of the Shuram Formation. A steady recovery to positive values occurs over the entire Shuram and half through the overlying Buah Formation, suggesting a duration on the order of tens of My. Based on trace metal, chemostratigraphic and sedimentological analyses, the carbon isotope record obtained from the Buah Formation of northern Oman indicates a systematic and reproducible shift of δ13C values from ? 6‰ to + 1‰ in 1 — a demonstrably diagenetic altered carbonate-cemented siliciclastic facies, and 2 — a least diagenetically altered stromatolitic facies. The identical reproducible isotopic pattern in these time-equivalent sections combined to the presence of exceptionally preserved δ18O values around ? 2 to + 1‰ associated with the most negative δ13C values rules out isotopic resetting by diagenetic fluids as a mechanism to explain these values.It is concluded that it is possible to retain depositional δ13C values in demonstrably diagenetically altered carbonates. This raises the issue of the ability to recognize diagenetic alteration of C-isotopic values in Neoproterozoic rocks where a robust time frame to support reproducibility is not available. The results of this study provide strong support to a non diagenetic origin of the negative Shuram C-isotope excursion, believed to be the most profound (in terms of amplitude and duration) in the Earth's history. |
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