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

A pre-heating method that improves the background and precision of C-14 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 degrees C in a vacuum line before it is used for graphitization. The method can reduce the contamination absorbed in TiH2, Zn and Fe power placed in the graphitization tube. With the pre-heating and average drilling method, bi-weekly resolution C-14 dating in a fossil coral is carried out to investigate the "AD 775 C-14 spike event". Different from the tree ring C-14 archives with the C-14 spike of similar to 15 parts per thousand (Delta C-14), the C-14 spike in the coral shows an abrupt peak of 45 parts per thousand and two smaller spikes of Delta C-14 > 20 parts per thousand in half a year in AD 776. And then, the C-14 content in coral decreases gradually in AD 777. The peak time of the C-14 spike event likely occurs in the summer of AD 776 according to the delta O-18 variation in coral. High-resolution dating of C-14 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. (C) 2015 Elsevier B.V. All rights reserved.