南海沈積物中現代極端事件紀錄特徵

Abstract

自然災害(地震、洪水和火山等)對人類社會不論是經濟和人身安全上都造成極大的威脅。但對於災害評估和重建災害歷史而言，現代的觀測紀錄難以涵蓋百年尺度以上大型自然災害。深海沈積物具有沈積環境穩定，並具有記錄大型自然災害的特性，為良好的災害紀錄器。不同災害事件所引發的重力流沈積物十分相似。近年來，多篇研究利用地震會造成大規模崩塌的特性和事件後的即時採樣建立大型地震事件層的特徵，將地震災害歷史延伸到千年以前。相比之下，颱風並不像地震會造成大規模的海底崩塌，難以於深海辨識颱風層位並建立大型颱風事件層的特徵。本研究於2013年海燕颱風侵襲菲律賓的四個月後在南海海盆採集重力岩心，在表層發現近百公分厚具有泥質濁積層層序的颱風層位，因此區缺乏大型峽谷系統的緣故，推測沈積物受到颱風懸浮傳輸至海盆堆積，顯示強烈颱風懸浮傳輸沈積物時，因水力淘選的緣故，在深海會同樣堆積出典型的濁積層層序，此發現拓寬傳統認知中粒級層為透過沈積物重力流沿海床傳輸形成的窠臼，對未來評估深海事件層提供更多討論。並且在颱風層的底部觀察到錳富集的特性，推測源自菲律賓中部富含錳的土壤與洪水引發沈積物孔隙中的錳釋放於水層後，水層中的顆粒與錳離子結合受懸浮傳輸至深海。此錳富集的特性可望為區分洪水和地震事件層的利器，因此本研究利用在高屏海底峽谷下部斜坡中前人已辨識出2009年莫拉克颱風和2006年屏東地震的岩心站位來驗證錳富集層於其他地區的適用程度。結果顯示，莫拉克颱風的層位同樣具有錳富集的特性，但其形成機制有所差異，莫拉克颱風的層位主要為洪水引發沈積物中的錳釋放於水層後，錳離子與水層中的顆粒結合後受異重流傳輸至深海。在颱風較為稀少，沈積物組成以生物源沈積物為主的南沙海域同樣有觀察到錳富集的特性，推測為南沙海域近年來頻繁的人為工程活動擾動表層的沈積物，降低海床的穩定性並導致表層沈積物孔隙水中的錳離子釋放並和颱風所懸浮的顆粒結合，傳輸至深海。根據颱風傳輸沈積物機制的不同，南海海盆適合建立強風型颱風紀錄，而高屏峽谷則適合重建強降雨型颱風歷史。本研究所發現的現代颱風層位特徵，可良好的區分地震和颱風事件層，為重建古颱風歷史的重要依據，為深海古颱風研究開啟新的篇章。

Natural disasters (earthquakes, floods, and volcanic eruptions) pose significant threats to human society in terms of economic and personal safety. While modern observational records are insufficient for assessing centennial-scale natural disasters, deep-sea sediments serve as excellent disaster archives due to their stable depositional environment. Although gravity flow deposits triggered by different disaster events are similar, recent studies have utilized post-earthquake immediate sampling and characteristics of large-scale submarine landslides to establish earthquake event layer features, extending earthquake history records to millennia. In contrast, typhoons rarely cause massive submarine landslides, making it challenging to identify typhoon layers and establish their characteristics in deep-sea sediments. In this study, we collected gravity cores from the South China Sea basin four months after Typhoon Haiyan struck the Philippines in 2013. We found meter-thick typhoon layer with typical mud turbidite sequences in the surface. Due to the absence of major submarine canyon systems in this area, we suggest that sediments were transported to the basin through suspension. This suggests that intense typhoons can produce typical turbidite sequences through hydrodynamic sorting during suspension transport, expanding the traditional understanding that graded beds are formed by sediment gravity flows along the seafloor. On the other hand, we found manganese enrichment at the base of the typhoon layer. We speculate sourced from manganese- rich soils in central Philippines and manganese released from the pore water after the sediment disturbance by flooding, combining with suspended particles before transport to the deep-sea. To validate the applicability of manganese enrichment as a tool for distinguishing flood and earthquake event layers, we examined previously identified cores from the 2009 Typhoon Morakot and 2006 Pingtung earthquake in the lower slope of the Kaoping submarine canyon. Results showed manganese enrichment in the Morakot layer, though a different mechanism involving flood-induced manganese release and gravity flow transport. Similar manganese enrichment was observed in the Nansha area, characterized by slow sedimentation rates and biogenic sediments, potentially due to recent anthropogenic activities disturbing surface sediments and releasing manganese from pore water. Based on these different typhoon sediment transport mechanisms, the South China Sea basin is suitable for reconstructing strong-wind typhoon records, while the Kaoping submarine canyon are better for heavy-rainfall typhoon history reconstruction. These modern typhoon layer characteristics effectively distinguish between earthquake and typhoon event layers, providing crucial evidence for paleotyphoon reconstruction and shed the new light for deep-sea paleotyphoon research.