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

Abstract

Studies of δ 13 C 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 δ 13 C org 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 δ 13 C org 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 CO 2 concentrations and the isotopic composition of dissolved inorganic carbon. Current data are consistent with the interpretation that the observed δ 13 C org increase may have resulted from a decrease in the surface-water dissolved CO 2 in the Yangtze Sea during the Hirnantian glaciation, probably reflecting a significant decrease in glacial ocean/atmosphere p CO 2 levels at that time.