南海現代深海事件層中顆粒性有機碳之組成與來源

Abstract

作為西太平洋最大的邊緣海，每年有大量的沉積物經由河流輸入至南海，加上其容易受颱風、地震與火山爆發等天然災害影響，使得南海深海沉積物具有保存古事件紀錄的潛力。本研究透過沉積物中顆粒性有機碳的組成與來源來分析南海現代深海事件層特性，並比較不同區域有機碳分布差異。 結果顯示南沙群島8支岩心多半只於表層發現事件層，整體TOC介於0.07-0.71%，d13C值介於-18.4~-22.4‰之間，TOC/TN則約介於5~10。有機碳主要由海洋藻類所貢獻，但事件層中d13C值有偏低之現象，推測可能為來自不同地區之海洋藻類，僅在鄭和群礁南側水道一支岩心(OR1-1068-4)中的事件層中有明顯陸源有機物的訊號。事件層中常含有大量碳酸鹽類導致TOC受到稀釋而呈現相對低值。中央海盆3支岩心則大部分受到事件擾動，整體TOC介於0.17~1.39%，d13C值介於-18.9~-22.8‰之間，TOC/TN則約介於3~9，有機碳來源為陸海源混合，顯示事件層沉積物應為陸源，且在細顆粒與粗顆粒沉積物上都有TOC隨著陸源訊號增加而升高之趨勢。其中TOC最高值出現在中央海盆正中央岩心(OR1-1133-A1)第三層事件層，其有機碳來源可能為近岸海草。 本研究進一步利用n-alkanes正烷烴分析檢驗禮樂灘北方岩心(OR1-1068-8)中之表層事件層，結果顯示TOC/TN隨著n-C26~35在整體正烷烴內的比例增加而升高，顯示TOC/TN與d13C在此區域確實可以指示有機碳之陸海源。另CPI顯示事件層中陸源有機碳分布類似於陸上土壤，代表事件層沉積物應為陸源，且帶來大量陸源有機碳，暗示極端氣候事件可能對區域性陸源有機碳之輸出與埋藏有重大影響。

As one of the largest marginal seas in the Western Pacific Ocean, the South China Sea(SCS) annually receives large amount of sediments from surrounding continental rivers. In addition, SCS is also frequently influenced by natural hazards like typhoon, earthquake and volcanic eruption. It makes SCS has potential to preserve the natural hazard records in the deep-sea sediments. In this study, we attempt to clarify the characteristics of the SCS modern deep-sea event-layer through the sediment composition and the provenances of particulate organic carbon(POC), moreover, to find out the features of POC distribution in different regions. In Spratly Islands, the natural hazards related event-layers were found in the core top of gravity cores. According to the total organic carbon(TOC)(0.07 to 0.71%), d13C-values(-18.4 to -22.4‰) and TOC/TN ratio(5 to 10) in the gravity cores, we suggest the organic carbon is mainly from marine algae, however, the lighter d13C-values in event-layers may influenced by marine algae of different species or regions. The terrestrial signal only can be seen in the event-layer which located at the southern channel of the Zhenghe Qunjiao(OR1-1068-4). The relatively lower TOC can also be found in event-layer which caused by dilution of carbonate. Three cores which were taken from the Central Basin are almost entirely affected by event-related activities, and its TOC is between 0.17 to 1.39%, d13C-values range from -18.9 to -22.8‰ and TOC/TN ratio ranges from 3 to 9. The composition of organic carbon indicates it may be formed by mixing of terrestrial and marine materials. The highest TOC is found in the third event-layer in the center of Central Basin(OR1-1133-A1), the result suggests it may be composed by nearshore sea grasses and marine algae. We further utilized n-alkanes analysis of the event-layer in core OR1-1068-8 which located at north of Reed Bank. It shows higher TOC/TN with increasing n-C26~35 of entire n-alkanes, which reveals TOC/TN and d13C can be used to identify the provenances of organic carbon in this region. Furthermore, the Carbon Preference Index(CPI) of the organic carbon in event-layer is similar to soil on land. It implies that large amount of terrestrial organic carbon might be delivered from land and formed thick event-layer in deep-sea and has great impact on carbon burial.