南澳宋城:被海啸毁灭的古文明遗址

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

一直以来,在中国大陆海岸线上未发现遭受海啸破坏的地质遗迹和可靠的历史文献记录.2013年,在西沙群岛的东岛发现了距今一千年前后南海古海啸存在的地质证据.模拟研究表明,海啸的地震震源位于马尼拉海沟,其影响的范围可能波及广东、海南和越南海岸带.本研究选择广东省南澳岛作为海啸波及区进行研究,结果证明南澳岛东南沿岸受到了海啸袭击,海啸沉积层内陆源动物骨骼样品的^(14)C校正年代范围是公元894～1011年;在海啸层中还保存有大量的宋代陶瓷器残片.这次海啸的破坏性是巨大的,以至于在海啸后的500年间,南澳岛的文化出现了衰退.位于不同地形的海啸剖面展示了海啸撞击海岸的结果,它们从地质学的角度证明了南海古海啸对南澳岛海岸的影响.本文表明要高度重视南海发生海啸的危险,这对于正确评估中国南海海啸风险、现代海上丝绸之路沿岸的海上交通以及沿海核电站等工程的安全和建设具有重要的科学和现实意义.同时它将推动南澳宋城的考古发掘,从而为南海沿岸的古海啸及其影响的研究提供更多证据.

It has remained unknown whether the coastline of the mainland of China was ever hit by tsunamis due to the lack of geological investigations and reliable historical records. In 2013, we found sedimentary and biological evidence 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(14C 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.