南海現代沈積作用與極端事件紀錄

Abstract

南海是西太平洋最大的邊緣海，地處歐亞板塊、菲律賓海板塊和印度板塊之間，構造活動十分頻繁。此外，因南海地處低緯度地區，每年六月到十月易受到颱風的影響，在此環境之下，天然災害引發之海床破壞、崩塌、濁流等事件可紀錄於沈積地層之中。南海每年來自周圍環繞國家的沈積物將近七億噸，是世界邊緣海之冠，因為邊緣海的封閉特性，造成大部分的沈積物堆積後就永久沈積於此。因此，此地區為建立古環境、古氣候的良好場址。本研究希望透過此地區岩心中現代災害性事件的沈積紀錄，分析其沈積特徵，並利用南海為建立古環境良好場址之特性，評估研究區海洋地質災害事件的歷史。本研究選取2014~2015 年海研一號於太平島周邊與南海海盆所採集的12 支重力岩心，利用岩心表面攝影、多重感應元岩心掃描儀(MSCL)、X 光螢光分析、粒徑分析和鉛210 等分析方法進行沈積事件評估。岩心表面攝影顯示除了位於海盆中央的岩心外，岩心表層都呈現棕黃色氧化層，且在表層以下的鉛210 活度都具有次表層高值，藉由採樣時間與當地災害性事件相近的關係，推測此事件層可能由2013 年強颱海燕颱風所引發的極端事件所造成。此外，X 光螢光分析顯示事件層內的鈣含量皆相對於其他元素高，且只有在現代事件層中能夠有高含量的錳元素，由於此地區的水深都在2000 公尺以上，因此可推斷現代工業化的汙染物可能藉由颱風等天然災害傳輸至深海埋藏。本研究也藉由鉛210 定年法計算出太平島周圍的岩心沈積速率，在穩定沈積時段，沈積速率約為每年沈積0.05 公分，但岩心所紀錄到的災害性事件紀錄厚度約為30公分，而在南海海盆岩心中，穩定沈積時段的沈積速率約為每年沈積0.2 公分，但災害性事件紀錄厚度約為87 公分，證明南海沈積物有成為西太平洋地區大型颱風紀錄站的潛力。而對比各岩心的沈積紀錄發現，海燕颱風對太平島南側影響較小，越往北邊事件沈積物堆積量越大，且所紀錄到的事件也較多。未來將進一步分析各事件層的沈積特徵並對比岩心中其他古事件沈積紀錄，探討其沈積機制與可能成因的差別。

The South China Sea is the largest marginal sea of the northwest Pacific. It is situated at the plate boundary of the Eurasian, Philippine Sea, and Indian plates and also on the North Western Pacific corridor of typhoons. The unique tectonic and climatic environment makesit has to face the potential of seafloor destructions, like submarine landslides and slumps, and high sediment discharges which induced by typhoon from Philippine. In this study, we analysis the sediment properties of modern extreme event records in cores and attempt to evaluate the history of extreme events in the South China Sea. Twelve gravity cores were collected in the central South China Sea basin and around Taiping island by using R/V OceanResearch 1 from 2014 to 2015 and a series of analysis including multi-sensor core logger, XRF core scanner (Itrax), core surface images, X-radiographs, bulk density, grain size, 210Pb chronology and X-ray diffractometer were conducted in this study. On core surface images, an obvious brownish oxidized layer exist in core top with higher 210Pb activity beneath this oxidized layer, and we speculate this layer is caused by nature hazard. According to the sampling time, we conjecture the oxidized layer might formed by typhoon Haiyan in 2013.In addition, the Itrax data shows high manganese content only exist in this layer which might related to the modern industrial pollution delivered by typhoon induced flooding from Philippine. The sedimentation rate of the non-event years in these cores which derived from 210Pb chronology method is about 0.02 ∼0.03 cm/yr. On contrary, the event layer caused by Haiyan with a recorded maximum 87cm deposits in the South China Sea. This study aims to characterize the typhoon induced deposits in the turbidite layer and use it to identify whether the other event layers recorded in these cores were related to typhoon activities and to reconstruct the strong tropical cyclone history in the western Pacific.