Middle And Late Ordovician Conodonts And High-resolution Integrated Stratigraphy In Yangtze Region,South China

The Great Ordovician Biodiversification Event is one of the major macro-evolutionary event in the Early Paleozoic,which built a complex marine ecosystems for the first time.Darriwilian is the key interval for the Ordovician conodont biodiversification.However,the case studies in South China focusing on Middle and Late Ordovician conodont biodiversity are rare.Another point is that the Ordovician carbon isotope studies in South China haven't established efficient correlations regionally and globally.Further comprehensive high-resolution integrated stratigraphic researches are required to understand the carbon isotope development on the Yangtze Platform thoroughly.On the other hand,conodont researches together with paleoenvironment index such as carbon isotope and apatite oxygen isotope would help us explore the co-evolution between organisms and environment during the Ordovician.In total,eight sections from different palaeogeographic areas have been selected for this study to investigate conodonts and carbon chemostratigraphy.Altogether,58 species belonging to 29 genera have been identified.Based on the occurrence of specific conodont species,the Lower Chiaochia Formation is restricted to the late Dapiangian to early Darriwilian age,the Upper Chiaochia Formation are restricted to the early Darriwilian to the early Katian age.The Shitzupu Formation in the Yongshan area in Yunnan Province is defined to the early Darriwilian to the early Katian age.At the studied sections,eight conodont biozones are recognized,including(in ascending order),the Lenodus antivariabilis,the Lenodus variabilis,the Yangtzeplacognathus crassus,the Dzikodus tablepointensis,the Yangtzeplacognathus foliaceus,the Yangtzeplacognathus jianyeensis,the Baltoniodus alobatus,and the Hamarodus brevirameus biozones.Based on the conodont data from the studied sections,the conodont biodiversity shows a prominent increase within the early to middle Darriwilian and reaches a peak in the middle Darriwilian.Further,the diversity declines during the late Darriwilian and rises slightly in the Sandbian,then keeps a relative low level in the late Sandbian and early Katian.Three positive carbon isotope excursions have been recognized at the studied sections,including the Middle Darriwilian Isotopic Carbon Excursion(MDICE),the Guttenberg Isotopic Carbon Excursion(GICE),and the positive excursion in the upper Pagoda Formation.These excursions can be correlated regionally and globally.The organic carbon isotope study reveals the coupling of the ?13C and ?13Ccarb in the Middle and Late Ordovician.The oxygen isotope data measured from the conodont apatite show an increasing trend in the middle and late Darriwilian and declining in the Sandbian,which implies a possible ice age in the late Darriwilian.The documentation of these carbon isotope shifts and oxygen isotope fluctuation in this study provides new evidence for changes in the global carbon cycle from Darriwilian to the early Katian that could have been influenced by changes of global climate and primary productivity in the ocean.The conodont diversity in the Ordovician of South China was primarily affected by the climate changes,while sea-level and sea-water chemical changes might be important factors.