An organic carbon isotope record of Late Ordovician to Early Silurian marine sedimentary rocks, Yangtze Sea, South China: Implications for CO2 changes during the Hirnantian glaciation

Studies of δ13C of marine sedimentary organic carbon in six clastic sections with sediments deposited from the latest Ordovician to earliest Silurian in the Yangtze Sea on the South China Plate, show a positive δ13Corg excursion of up to 4‰ in the latest Ordovician Hirnantian Stage, corresponding in time to a major continental glaciation on Gondwana. Various factors that might be responsible for the δ13Corg increase are considered, including input and contribution from exogenic carbon sources, secondary processes (such as heterotrophy and thermal maturation), and changes in Yangtze Sea surface-water temperature, productivity, primary producers, dissolved CO2 concentrations and the isotopic composition of dissolved inorganic carbon. Current data are consistent with the interpretation that the observed δ13Corg increase may have resulted from a decrease in the surface-water dissolved CO2 in the Yangtze Sea during the Hirnantian glaciation, probably reflecting a significant decrease in glacial ocean/atmosphere pCO2 levels at that time.