Carbon Isotopic Evolution Characteristics and the Geological Significance of the Permian Carbonate Stratotype Section in the Northern Upper-Yangtze Region, Southern China

The Permian global mass extinction events and the eruption of the Emeishan flood basalts in the Upper Yangtze region should display certain responses during the evolution of carbon isotope. In this paper, the Permian carbon isotopic evolution in the Upper Yangtze region is examined through systematic stratotype section sampling and determination of 13 C in the northern Upper-Yangtze regions and Southern China. Additionally, the carbon isotopic evolution response characteristics of the geological events in the region are evaluated, comparing the sea-level changes in the Upper Yangtze region and the global sea-level change curves. Results of this study indicated that the carbon isotopic curves of the Permian in the Upper Yangtze region are characterized by higher background carbonisotope baseline values, with three distinct negative excursions, which are located at the Middle–Late Permian boundary and the late period and end of the Late Permian. The three distinct negative excursions provide an insightful record of the global Permian mass extinction events and the eruption of the Emeishan flood basalts in the Upper Yangtze region. The first negative excursion at the Middle–Late Permian boundary reflected the eruption of the Emeishan flood basalts, a decrease in sea level, and biological extinction events of different genera in varying degrees. The second negative excursion in the Late Permian included a decrease in sea level and large-scale biological replacement events. The third negative excursion of the carbon isotope at the end of the Permian corresponded unusually to a rise rather than a decrease in sea level, and it revealed the largest biological mass extinction event in history.

Application of Strontium Isotopic Stratigraphy to Dating Marine Sedimentary Units:A Case Study from the Permian Stratotype Section in Southern China

The calibration of sedimentary rock absolute dates is one of the difficulties in sedimentological and stratigraphic research.Since strontium(Sr)resides in seawater much longer(≈10^(6) a)than the seawater intermixing time(≈10^(3) a),the Sr isotopic composition of global seawater is uniform at any time and results in a stable system throughout geological history,based on which a global Sr isotope composition dating database has been established for age-calibration of marine strata.The Permian stratigraphic sections in the northern part of the Upper Yangtze block,southern China,record continuous marine sediments with clear stratigraphic boundaries and is suitable for stratigraphic dating of Sr isotopes.Based on sampling and Sr isotopic compositions of Permian carbonate strata in the northern part of the Upper Yangtze,a Permian Sr isotope evolution curve was established.According to the basic principles of Sr isotope stratigraphy,the global Strontium isotope age database can be used to calibrate the Permian stratigraphic dates in the northern Upper Yangtze.The results show that the Sr isotope evolution curves for the marine carbonate rocks in the Permian stratigraphic section of the Upper Yangtze present a decreasing trend from the mid-Qixia stage(P_(2))to the mid-Wujiaping stage(P_(3)),and then rise from the middle Wujiaping stage to the end of Changxing stage(P_(3)).When the Permian Sr-isotope evolution curve is compared with the global Sr isotope evolution curve in the northern Upper Yangtze,the two are consistent in their long-term evolutionary trend,indicating that Permian global geological events are important controlling factors for the composition and evolution of Sr isotopes.The 87 Sr/86 Sr value decreased gradually in the background of large-scale regressions at the turn of middle to late Permian period,revealing that the Emeishan basalt eruption occurred near the Maokou/Wujiaping boundary(GLB).Srisotope stratigraphy dating was performed on the boundaries of the Qixia Formation/Maokou Formation,Maokou Formation/Wujiaping Formation(GLB),Wujiaping Formation/Changxing Formation(WCB)and the Permian/Triassic(PTB)using the Global Strontium Isotope Age Database.The results are 270.4 Ma,261.2 Ma,254.5 Ma and 249.7 Ma,respectively.Based on this,the eruption age of the Emeishan basalts is defined at about 261.2 Ma.,which is more coincident with that acquired from other previous dating methods on the eruption age of the Emeishan basalts,and therefore proves that the application of Sr isotopic stratigraphy to dating marine sedimentary units is an effective method.