Carbon isotope records of the Middle–Upper Ordovician transition in Yichang area, South China

The Ordovician was a critical period of the Phanerozoic with great biotic and environmental revolutions. During the transition from Middle to Late Ordovician, marine organisms including graptolites, brachiopods, and trilobites etc. experienced significant replacements and turnovers, along with the cooling of climate and eustatic fluctuations. Herein, we conduct a stable isotope analysis of the Middle–Upper Ordovician transition in Yichang area, South China. Some 69 samples of carbon isotope from three sections of the studied intervals in the area, the Zhenjin, Jieling, and Puxihe sections, were collected and analyzed. The results show a prominent positive excursion of δ13Ccarb slightly below the top of the Kuniutan Formation, which is correlated to the Middle Darriwilian δ13C Excursion (MDICE) recognized widely in the world. Our latest conodont biostratigraphy of the interval indicates that the topmost part of the Kuniutan Formation is referred to the Yangtzeplacognathus protoramosus Subbiozone of the Pygodus serra Biozone and suggests that the top part of the formation represents probably a condense sequence. However, the presence of a hiatus in the top part of the Kuniutan Formation cannot be excluded and requires further more intensive work. The lower part of the Pagoda Formation recorded a conspicuous positive δ13C excursion well correlated to the global Guttenberg δ13C Excursion (GICE). The higher peak value of the GICE in the area, which occurs in a lower horizon than previous records, indicates that the regional response in Yichang area to this global event is more prominent than previously thought.