A high resolution method for 14C analysis of a coral from South China Sea: Implication for “AD 775” 14C event

Abstract A pre-heating method that improves the background and precision of 14 C dating significantly was applied for fossil coral dating with high resolution in our lab in Guangzhou Institute of Geochemistry, Chinese Academy of Sciences (GIGCAS). The reaction tube is heated under 300 °C in a vacuum line before it is used for graphitization. The method can reduce the contamination absorbed in TiH 2 , Zn and Fe power placed in the graphitization tube. With the pre-heating and average drilling method, bi-weekly resolution 14 C dating in a fossil coral is carried out to investigate the “AD 775 14 C spike event”. Different from the tree ring 14 C archives with the 14 C spike of ∼15‰ (Δ 14 C), the 14 C spike in the coral shows an abrupt peak of 45‰ and two smaller spikes of Δ 14 C > 20‰ in half a year in AD 776. And then, the 14 C content in coral decreases gradually in AD 777. The peak time of the 14 C spike event likely occurs in the summer of AD 776 according to the δ 18 O variation in coral. High-resolution dating of 14 C in coral provides not only a more detail process of the event than that from tree rings, but also the first report of the event from sea ecosystem. Both of them suggest an extraterrestrial origin of the event cause.