The Ocean Chemistry Changes Associated With The GOBE In Ordovician

The Ordovician is a profound period in Earth history,involving bio-events accompanied with significant perturbations of ecology and environment.The tremendous radiation at the family,genus and species levels,the rise of the 'Paleozoic and Modern Evolutionary Fauna,progressively replacing the 'Cambrian Evolutionary Fauna',together with the entire seawater column filled with marine organisms make the Great Ordovician Biodiversification Event(GOBE)a 'Milestone' interval in Earth history.An excellent Ordovician stratigraphy sequence is preserved in Yichang area of Upper Yangtze Platform,South China,the Ordovician succession of which is extensively exposed around Huangling Anticline,characterized by mixed facies with simple structures,complete sequences and abundant fossils.In this study,we measure inorganic C-isotopic data(?13Ccarb)to build up a standard chemostratigraphy for the study area,which is further utilized to global correlation.We also report the multiple S-isotopic records to investigate the redox structure of Ordovician ocean and the relationship between the ocean chemistry changes and the GOBE.Specially,Modem human activities have caused a sharp increase in the concentration of carbon dioxide in the atmosphere,and our study of the Ordovician radiation under extremely warm climate(14-18 times PAL)may help to provide important clues to the future environmental changes of the Earth and its impact on the evolution of life.1.High-resolution 513CCarb profiles from Yichang area,South ChinaBased on well-constrained biostratigraphy and lithostratigraphy,we firstly provide one of the most compete ?13Ccarnb records through the Ordovician in South China.The composite ?13Ccarb curve can be correlated globally and show great potential for further global correlation between different depositional settings and paleocontinents during Ordovician period.We suggest that the perturbation of global carbon cycle may result from periodic enhanced burial rates of organic matter and resulting in steady climate cooling in the Early Ordovician and suspension of cooling spanning the Middle Ordovician to the Early-late Ordovician.2.High-resolution multiple S-isotopic records profiles from Yichang area,South ChinaOur multiple S-isotopic study shed new light on the interplay between changes of redox conditions in seawater and biotic events.The A33S anomaly occurring in Darriwilian and Sandbian-early Katian directly refers to episodic shoaling of anoxic and/or euxinic water to shelf environments from South China.Combined with paleontological data,the two brachiopod diversity crises in Darriwilian and Sandbian-early Katian respectively likely resulted from episodic shoaling of anoxic and/or euxinic water to shelf environments.Fossil records indicated that the two brachiopod diversity crises in Dapingian and Sandbian-early Katian respectively were followed by recovery and peaks of Biodiversification separately in Darriwilian and late Katian.We suggest that episodic shoaling of anoxic and/or euxinic water to shelf environment may have affected and destabilized ecosystems or biotopes,which may produce more empty and new ecological niches.Highly adaptable species,which survived under such ecological stress together with invasive species,will quickly occupy and flourish in the new ecological niches,thus accelerating the process of the Great Ordovician Biodiversification Event.