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

This work deals with the evolution of carbon isotope composition in the Luoyixi （罗依溪） 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 iaevigata. 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.

Palynoflora and climatic dynamics of the Laizhou Bay of Bohai Sea,North China Plain,since the late middle Pleistocene

The coastal plain of Laizhou Bay in Bohai Sea is a transitional area of land-sea interaction.Sediments in this area can bear significant information of sea-level fluctuation,climate change,as well as regional geological setting.Here,in this study,new sporopollen data from three boreholes(GK138,GK111 and GK95)in the coastal plain of Laizhou Bay,Bohai Sea were investigated,and the pollen spectrum since the late middle Pleistocene was established as six sporopollen assemblage zones(Ⅰ-Ⅵ),i.e.,Pinus-Quercus-Artemisia,Gramineae-Artemisia-Chenopodiaceae,Picea-Pinus-QuercusArtemisia,Picea-Pinus-Betula-Gramineae-Artemisia,Picea-Pinus-Cupressaceae-Chenopodiaceae and Pinus-Quercus-Gramineae-Artemisia-Chenopodiaceae.Combining with existing sedimentary successions and detailed radiocarbon dating results of the sediments from the three boreholes,paleovegetation and climatic evolution since the late middle Pleistocene were reconstructed.The findings revealed that climatic changes in this area since the late middle Pleistocene were characterized by alternating cold-arid and warmhumid conditions,which were well correlated with marine isotopic stages(MIS).The present study offers specific insight into the climatic dynamics in the North China Plain since the late middle Pleistocene and provides evidence of a clear link among the palynoflora in the area,the glacial-interglacial period climatedriven sea-level changes,and the marineδ^(18)O records.

Sedimentary features of the Permian-Triassic boundary sequence of the Meishan section in Changxing County,Zhejiang Province

The Global Stratotype Section and Point(GSSP) for the Permian-Triassic boundary was placed in Bed 27 of Meishan section D where the conodont species Hindeodus parvus first occurs.Bed 27 was usually considered comprising continuouslydeposited,homogeneous silty limestone,with no depositional hiatus near the boundary.Detailed study on the boundary sequence revealed that a typical firmground characterized by Glossifungites ichnofacies developed about 2 cm below the Permian-Triassic boundary in Bed 27.Fossil content and lithology show apparent differences across the firmground crust.The abundance of the Permian bioclasts decreases significantly across the firmground,and is accompanied by a shift of dominating carbonate precipitation from calcite to dolomite.The firmground marked a rapid transgression at the very end of the Late Permian and significant shifts of sedimentary environment and paleoclimate.This transgressive submerging surface is also observed at the Huangzhishan section of the shallow-water carbonate platform facies in Zhejiang Province,the Jiangya section of the lower-slope to basinal-margin facies in Hunan Province,the Pingdingshan section of the basinal facies in Anhui Province of South China,as well as the Selong section in Tibet of the northern peri-Gondwana.The transgressive submerging surface marks the onset of a rapid global transgression at the latest Permian.