南海古海啸重建与海啸沉积研究进展

中国东海、南海等近海临近琉球海沟、马尼拉海沟等俯冲带,地震频发。过去的海啸研究主要关注历史文献分析、海啸数值模拟等,据此评估中国近岸海啸灾害的历史和风险。历史时期是否引发了海啸,特别是具有特大致灾风险的大海啸记录,目前还不明确。近年来,本课题组通过对海岛、海洋沉积和海岸带及其岛屿的沉积过程、海啸遗迹和历史记录研究,阐述了确定古海啸的系列研究方法。首先通过对南海西沙群岛东岛湖泊沉积序列、大量砗磲和珊瑚块在海岛分布的特征分析,识别出距今千年的一次海啸事件。以此为标志,根据湖泊沉积结构作为识别海岛海啸沉积的特征。同时提出了确定海岛海啸发生时代的样品采集和定年方法,其中包括根据事件沉积层顶部和底部植物残体¹⁴C年龄定年和历史文献记录的印证。首次确定在过去1 300年中,南海发生过一次海啸,其发生时间为公元1076年。为了寻找更古老的海啸记录,结合对东海闽浙沿岸过去两千年海洋泥质沉积的分析,发现南海海啸在沉积序列中留下记录,但除此之外沉积记录中并无更强的扰动,因此东海在过去两千年中受到海啸的影响较小。1076年的海啸同时冲击了南海沿岸,通过对广东南澳岛考察发现,岛屿东南海岸保存着距今约1 000年的海啸沉积层,其中夹杂着宋代陶器瓷器残片。对遗迹数量变化的分析显示,岛上的文化受海啸破坏出现了长达500年的文化中断,直至明代中后期设镇之后才逐渐恢复。根据海啸层植物残体、贝壳¹⁴C测年、覆盖海啸层的海砂光释光定年以及瓷器碎片的年代鉴定了海啸的发生时代,并据此提出了海岸带古海啸沉积的定年方法。此外,不同环境下海啸沉积的特征也存在较大区别,需要结合地形、沉积物来源以及地球化学特征等多种指标进行识别。有迹象表明海南岛东侧海岸带有海啸破坏的明显证据,需要进行深入的研究。

Research progress in the reconstruction of paleotsunami in the South China Sea and the tsunami deposit characteristics

The East and South China Seas are adjacent to the Ryukyu and Manila Trenches, where earthquakes occur frequently. Tsunami researches in the past were focused on analyzing historical documents and numerical simulation to assess the risk and history of tsunamis striking the coastline, but the knowledge of historical tsunami events was uncertain. In the past decade, by investigating coastal, marine sediments and their depositional processes, tsunami relics, and historical documents, we have developed research approaches to reconstruct paleotsunami. We first recognized a 1000-year-old tsunami based on sedimentary evidences from Dongdao and Xisha Islands and the South China Sea, and proposed using lake sediment characteristics as a criterion for identifying paleotsunami. Sampling and dating methods, including validation from both the age of the event layer and historical literature, were also proposed. We suggest that a tsunami occurred in the South China Sea in the past 1300 years, and the year of this event is AD 1076.To expand the age span of the reconstruction, a 2200-year-long sediment record from the East China Sea was studied. Disturbance caused by the South China Sea tsunami could be seen in the record, whereas the absence of a stronger abrupt change indicated the East China Sea has not been hit by catastrophic tsunamis in the past 2200 years. The 1076 tsunami hit the coastal regions of South China. Tsunami deposits containing shards of Song ceramics were preserved on the coast of Nan’ao Island; the culture of Nan’ao experienced a 500-year decline after the South China Sea tsunami and did not recover until the late Ming Dynasty. We used radiocarbon ages of plant remains and shell carbonates, OSL analysis of sand, and ceramic dating to determine the age of the paleotsunami; based on the dating results, we put forth the methods for tsunami deposit dating. In addition to these dating methods, proxies of geomorphology, geochemistry, and sedimentary sources can also be used under different depositional environments. The east coast of Hainan Island is also vulnerable to tsunamis, and future studies are needed to reconstruct its paleotsunami records.