Carbon Isotopic Record In Ordovician Carbonates From Tarim Basin And Its Implication For Paleo-environment

The Great Ordovician Biodiversification Event?GOBE?began ⁴70 Ma ago in the Early Ordovician,and was terminated by a great mass extinction in the Late Ordovician.The Late Ordovician mass extinction was attributed to the environmental changes associated with the Late Ordovician glaciation.However,the cause of GOBE still remains unclear.Carbon isotopic compositions of carbonates from the Tarim Basin and Yangtze Platform,China were analyzed in order to investigate the Ordovician global carbon cycle and climate changes related to the GOBE.The evaluation of diagenetic alternation based on oxygen isotope and global correlation suggest that those C-isotopic data may record the primary carbon isotopic compositions in Ordovician.Four positive C-isotope excursions were observed in the high-resolution ?¹³Ccarb curves of Ordovician carbonate samples collected from the Tarim Basin as well as the Yangtze Block.The positive excursions occurred in the Early Floian,the Early Dapingian,the Early Darriwilian,and the Middle-Late Katian,respectively.The C-isotopic chemostratigraphy of the Tarim basin carbonate profiles can be correlated with C-isotopic records from contemporaneous sections globally,suggesting large perturbations of the global carbon cycle during the Ordovician.Those positive ?¹³Ccarb excursions observed in the Floian and the Dapingian indicated increased burial of organic carbon during the Early-Middle Ordovician.The episodic increased burial of organic matter could have drawn down atmospheric pCO₂ and led to a climatic cooling in the Early-Middle Ordovician.The climatic cooling after the greenhouse period could have played an important role in triggering the GOBE.Likewise,the ?¹³Ccarb increase in the Darriwilian to Katian suggests another episode of elevated organic carbon burial rates in the global carbon cycle during the late Middle and early Late Ordovician.The episodic increase of organic matter burial could have drawn down atmospheric CO₂,contributing to the climatic cooling.The temporal C-isotopic variations from the Tarim basin and Yangtze Platform may have recorded the onset of climate changes in a long-lived Late Ordovician glaciation that terminated the GOBE.