四十萬年來珠江峽谷下游重力流沈積紀錄

Abstract

國際海洋探測計畫367航次(IODP Expedition 367)在南海北部珠江峽谷下游水深3760.2公尺處的站位點U1499取得沈積物岩芯，其中表層50公尺處在船上以生物地層方法得知年代約為四十萬年，並以367航次現有觀測資料如岩芯攝影、岩芯描述與多重感應元岩芯紀錄儀資料辨識出七十餘個重力流沈積物層位，而重力流沈積物再依其厚度區分成大於5公分厚事件層與小於5公分薄事件層。其中厚事件層出現於特定層位，應為古海洋環境影響南海北部沈積物傳輸有關。為瞭解厚事件層分布與古海洋環境之關係，本研究分析U1499岩芯表層50公尺的碳氮含量與有機指標變化來重建站位U1499四十萬年來的古海洋環境，並分析重力流沈積物的有機碳特徵，進而追蹤重力流所夾帶的有機碳來源。從事件層的分布顯示，事件層由50公尺處至表層變得越來越少且薄，這暗示著南海北部四十萬年來的沈積環境可能由較動態的環境轉趨穩定，或是能傳輸至站位U1499的重力流變得愈來越少。而厚事件層僅出現在岩芯中較高碳酸鈣濃度的層位，說明產生厚事件層之重力流幾乎都發生在南海北部深海可以較好保存碳酸鈣的時期。而這時期可能為冰期，因海平面下降的關係導致珠江河口銜接至珠江峽谷，使陸源物質更有機會藉由重力流的機制與珠江峽谷的通道傳輸至深海。此結果顯示古海洋環境與厚事件層在岩芯中分布的關係。至於有機碳特徵上，岩芯中有兩種不同有機碳特徵之層位，分別以陸源有機碳為主的0-45 m層位與海源有機碳為主的45-48.85 m層位。而薄事件的有機碳特徵較接近0-45 m層位；厚事件的有機碳特徵則較接近45-48.85 m層位，說明兩種事件層所夾帶的有機碳來源不同。形成厚事件層之重力流夾帶較多陸源有機碳，其沈積物來自陸地；而形成薄事件層之重力流含有較多的海洋有機碳，因此可能為海床沈積物再搬運的結果。而正烷烴指標應用上，四十萬年的沈積物仍能表現出明顯的新鮮陸源植物訊號，說明正烷烴指標仍能適用於南海北部深海沈積物四十萬年以內的時間尺度。

In 2017, International Ocean Discovery Program (IODP) retrieved sediment cores from site U1499 which located at the down reach of Pearl River Canyon with 3760.2 m water depth at the northern South China Sea during expeditions 367. The age from upper 50 meters is approximately 400 ky by biostratigraphy and more than 70 gravity flow deposits from upper 50 meters were identified by core images, core description and multi-sensor core logger data which measured on board. Gravity flow deposits were divided into thick event layers (>=5 cm) and thin event layers (<5 cm). Thick event layers deposited in specific depths and may be closely related to paleo climate condition that affected sediment transport. Carbon, nitrogen concentrations and organic proxies were analyzed to investigate the relationship between gravity flows and paleo deep water environment from 400 kyr BP and trace organic carbon source of gravity flows. From distributions of event layers, event layers are thinner and fewer from 50 m to surface. This observation implies that environment of sedimentation is from dynamic to stable in the northern South China Sea or fewer gravity flows were transported to Site U1499. Thick event layers only appear in high carbonate concentration layers indicated that gravity flows which caused thick event layers occurred in specific periods when carbonates had well preservation. Specific periods are supposed to glacial periods. During glacial periods, lower sea level made the mouth of Pearl River closer to Pearl River Canyon and lead to frequent gravity flows transported terrestrial materials through the pathway of Pearl River Canyon into deep-sea. This result illustrates the relationship between gravity flows and paleo deep water environment. For characteristics of organic matters, two layers from upper 50 meters have different types of organic matters. 0-45 m layers are dominated by marine source organic matter and 45-48.85 m layers are predominated by terrigenous source organic matter. For gravity flow deposits, the type of organic matter about thin event layers is close to 0-45 m layers and the organic characteristic of thick event layers is close to 45-48.85 m layers. Thin event layers and thick event layers have different source of organic matter. This result demonstrates that gravity flows caused thick event layers were originated from lands and transported terrestrial materials into deep-sea and gravity flows caused thin event layers were originated from sea floor and redeposited sediments to Site U1499. For application of n-alkanes, deep-sea sediments with 400 ky still have fresh higher plants signals which indicate that n-alkanes as biomarker can apply to the South China deep-sea sediment within the age of 400 ky.