Nan’ao, an archaeological site of Song dynasty destroyed by tsunami

It has remained unknown whether the coastline of mainland China was ever hit by tsunamis due to the lack of geological investigations and reliable historical records. In 2013, we found sedimentary and biological evidences on Dongdao Island of Xisha Archipelago, indicating that a tsunami occurred in the South China Sea and struck the island about a thousand years ago. Results of tsunami modelling suggest that the possible trigger of the tsunami is an earthquake in the Manila Trench, and that the coastlines of Guangdong, Hainan Provinces of China and those of Thailand are under the impact of the tsunami. In this study, a geological investigation was conducted on Nan’ao Island of Guangdong Province to study the influence of the South China Sea tsunami. Tsunami deposits were identified in the coastal sedimentary profiles, and the radiocarbon ages of the samples are about 1000 a BP, identical to the age of the tsunami hitting Dongdao Island. The deposit layer contains small marine shells. The sedimentary samples have the same grain size distributions with the coastal sands, suggesting that the deposits have a coastal or nearshore sand origin. The sea level of the study area for the past 1500 years was 0.5 m lower than the present level; therefore the deposit layers was not sandy deposits formed by high sea level. There are no large rivers on the island, so the deposits were not formed by river, either. The absence of planar stratification and numerous individual laminations, typical textures of sandy storm deposits, excludes the influence of storm deposits. The single-layer, homogeneous and structureless textures presented by the deposit layers are indicative of rapid deposition. Giant boulders, typically requiring strong waves to transport, were distributed near the sampling sites. All these evidences strongly support that the southeastern coast of the island was struck by the paleotsunami. The calendar age of a bone sample collected from the event layer is 1090±20 AD ( 14 C age: 860±20 a BP). The dating results of the tsunami deposits of Xisha Archipelago ranges from 1017 to 1034 AD. Besides, the coastal regions of Chaozhou were struck by destructive sea wave in 1076 AD as recorded by historical archive. Taking all the chronological data into account, we conclude that the time of South China Sea tsunami is 1076 AD. The deposit layer also contains shards of ceramics, which were identified as Song dynasty ceramics. Ceramics remains produced before and after the Song dynasty were absent in the deposits. We analyzed the temporal distribution of the cultural relics of Nan’ao Island and found that the amount of the relics significantly decreased after the tsunami and remained low until the late Ming dynasty. Beside, more than 15000 ancient coins were retrieved in Nan’ao One, a shipwreck 2 km south off Nan’ao Island, and the characters on the coins indicated that they belonged to the 69-a period before the tsunami. These cultural evidences indicate a drastic cultural decline caused by the tsunami. This study confirms the risk of tsunamis in the South China Sea. Such risk should be considered in future planning and construction of nuclear power plant, harbor and petroleum reserve structure in the coastlines of China. This study also shows the cultural consequence of the tsunami and suggests archaeological excavation of the Song-dynasty relics of Nan’ao.