Middle Ordovician-Llandovery Carbon Isotope Chemostratigraphy In Upper Yangtze Platform,South China

Middle Ordovician to Llandovery(Silurian)was an important time period,during which the marine biodiversity and the environmental conditions changed significantly.With the popularity of carbon isotope chemostratigraphy,the carbon cycle and its relationship with the paleoclimate conditions during Middle Ordovician to Llandovery have received great interests.The varied lithofacies and continuous stratigraphic sequence in South China preserve abundant and diverse rock and fossil records.During the past half century,Chinese stratigraphers have established a relatively complete biostratigraphical framework for the Middle Ordovician-Landovery rocks in South China.However,due to the unique biofacies and the lack of reliable index fossils,the Pagoda and Linhsiang formations of Upper Ordovician in Upper Yangtze Platform are still hardly correlated with the simultaneous strata in the world.Meanwhile,comparing with the abundant carbon isotope records in Baltic and Laurentia,the carbon isotope chemostratigraphical sequence has not yet been established,and the relationship between the carbon isotope records in South China and the global carbon cycle are not well studied.Two scientific drillings,i.e.,Yihuang-ldrillcore and Yijie-1 drillcore in the Upper Yangtze Platform,were conducted by the author's research team recently.A total of 1037 whole rock powder samples from the Darriwilian(Middle Ordovician)to Rhuddanian(Llandovery)strata were collected.Then a total of 1219 carbon isotope results of inorganic and organic carbon isotope analyses were obtained for the present research.Based on the results,two high-resolution carbon isotope curves from Darriwilian to Rhuddanian in Upper Yangtze Platform can be presented here and correlated globally.The globally well-correlated MDICE,GICE and HICE events are recognized in the two carbon curves.Five more carbon isotope events which were recognized in Baltic and Laurentia,namely LDNICE,Upper Kukruse Low,Kope,Fairview and Waynesville in ascending order,are also recognized in South China for the first time.The recognition of these events creates a reliable,high-resolution multi-stratigraphical framework of the Middle Ordovician-Llandovery strata in South China,especially the Pagoda Formation and Linhsiang Formation.Meanwhile,the asynchronous Upper Ordovician HICE records in Upper Yangtze Platform was analyzed herein by using carbon cycle model,which explains the global carbon cycle perturbation during the late Darriwilian to middle Katian,based on the combined data of the strontium isotope perturbation,biodiversity change and orogeny in the Middle and Late Ordovician.It is considered that enhanced igneous activities like mid-ocean range activity indicated by large marine 87Sr/86Sr drop in upper Darriwilian to lower Katian caused enhanced 12C flux to the marine,and triggered the negative carbon excursion during early Sandbian.And the 87Sr/86Sr records showed that the igneous activities calmed down somehow during the late Sandbian to Katian,but the enhanced global orogeny activities might cause the enhanced 13C flux to the marine,and result in the end of the negative carbon excursion.