关键词: 铀同位素, 氧化还原, 早奥陶特世, 特马豆克期, 海洋氧化

Abstract: The oceanic redox state is one of the important environmental factors affecting evolution of animals in the ancient ocean. The Ordovician Tremadocian Stage was the‘eve’of the Great Ordovician Biodiversification Event (GOBE), during which the biodiversity was gradually increasing. However, there has been lack of researches on evolution of ocean redox state for this key interval. Studies have proved that the U isotope compositions recorded in well-preserved carbonates can effectively trace the redox state of the global ocean redox state. In this study, we conduct systematical analyses of carbon and oxygen isotope compositions and uranium isotope compositions of carbonate samples from the Xiangshuidong section in Songzi, Hubei Province. A significant positive carbon isotope excursion event—TSICE is identified at the base of the middle Tremadocian stage. The average value of  δ238U increased from -0.64‰ ±0.12‰ to -0.26‰ ±0.15‰ across the TSICE, which indicates that the redox state of the global ocean changed from significantly anoxic (mass balance model results indicating >2.56% of the seafloor covered by anoxic seawater) to much more oxygenated before and after the TSICE. This process of environmental change is consistent with the process of increase in metazoan proportion relative to microorganism proportion in reef composition. The positive drift of carbon and uranium isotope compositions may have been related to the regression and sea-surface temperature decrease, respectively, which may have been conducive to oceanic oxidation.