Evolution of Carbon Isotope Composition in Potential Global Stratotype Section and Point at Luoyixi, South China, for the Base of the Unnamed Global Seventh Stage of Cambrian System

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• 作者：Zuo Jingxun ; Peng Shanchi ; Zhu Xuejian ; Qi Yuping ; Lin Huanling ; Yang Xianfeng
• 关键词：carbon isotope composition ; global seventh stage of Cambrian System ; Luoyixi Section ; northwestern Hunan
• 刊名：Journal of China University of Geosciences
• 出版年：2008
• 出版时间：February 2008
• 年：2008
• 卷：19
• 期：1
• 页码：9-22

文摘

This work deals with the evolution of carbon isotope composition in the Luoyixi Section, a candidate of the Global Standard Stratotype-section and Point (GSSP), defining the base of the as-yet-undefined seventh stage of Cambrian System at the first appearance of the cosmopolitan agnostoid Lejopyge laevigata. This level is favored in a vote of International Subcommission on Cambrian Stratigraphy (ISCS) as the biohorizon for defining the base of a global stage. Two hundred and sixty-four samples for carbon and oxygen isotope analysis have been collected from the carbonate successions at an interval of 0.25 to 0.5 m in this section. Results of the carbon isotope data exhibit a remarkable disciplinarian trend. The pattern of the carbon isotope evolution is gently undulant with a relatively long period during the underlying Drumian Stage, and then the values of δ¹³C fluctuate sharply with a short period in provisional seventh stage. The onset of sharp fluctuation in the δ¹³C values begins at the proposed GSSP level, defining the base of the global seventh stage, where δ¹³C values change from a gentle trend to a sharp trend. Distinct covariant-relationships among δ¹³C, δ¹⁸O, and sea level fluctuations suggest that a warming change in paleoclimate took place during the early global seventh stage, which led to a positive shift in δ¹³C values.