The records of gravity flow deposits in the down reach of Pearl River Canyon over the last 400 ky

• Main Authors: Zih-Hang Tang, 湯梓杭
• Other Authors: Chih-Chieh Su
• Format: Others
• Language: zh-TW
• Published: 2019
• Online Access: http://ndltd.ncl.edu.tw/handle/j94fp6

碩士 === 國立臺灣大學 === 海洋研究所 === 107 === 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.