| 摘要: | 本研究在南海東北部大陸斜坡的近海床地層中發現了一個廣泛分布的層序邊界，且此區域性層序邊界可作為大陸斜坡環境中區分末次冰盛期前後的一個重要指標界面。為了釐清區域性層序邊界的形成機制，並重建相關的沉積環境變化歷史，本研究結合多種海洋地球物理及地質分析方法，包括底拖式聲納系統、沉積物岩芯分析、放射性定年等。根據底質剖面影像，本研究綜整了區域性層序邊界在好景海脊、手掌海脊以及指標海脊的分布情形，發現區域性層序邊界主要座落於地形相對平坦之高區，例如好景海脊兩翼的階地、海脊之間的槽溝，與手掌海脊、指標海脊的上方等區域。從沉積物岩芯MD18-3545的粒徑分析結果、放射性定年及岩芯影像所顯示的沉積構造特徵進行討論，南海東北部大陸斜坡在過去三萬五千年可分為四個獨立的分期：第一期為侵蝕事件前，相對海平面約為-60公尺，此時期的沉積環境以穩定堆積的半遠洋沉積物為主，並且有較弱的底流；第二期位於末次冰盛期的低水位時期，並結束於約11,900年前，此時期的強烈底流使得海床轉變為侵蝕環境；第三期起始於約11,900年前，此時海平面已回升至-60公尺，沉積環境轉為以堆積為主，並形成區域性層序邊界。第三期的底流強度隨著海平面上升的趨勢由弱轉強並持續上升，沉積機制也從半遠洋沉積型態轉為受到底流控制的等深流岩系統；第四期起始於約6,500年前，此時海平面高度已趨於穩定，同時底流強度也隨之減弱，海床的環境仍以沉積型態為主，並同時受到底流的影響而持續發展等深流岩系統。在結論部份，本研究認為南海東北部大陸斜坡近三萬五千年以來的沉積環境很大程度地受到全球氣候及海平面高度的影響，此影響同時改變底流強度的變化，進而控制了大陸斜坡的沉積環境。;In this study, we identified a near surface regional sequence boundary in the continental slope of the northeastern South China Sea. This regional sequence boundary can be used as an important indicator for distinguishing the sequences which formed before or after the Last Glacial Maximum in the continental slope environment. We employed multiple marine geophysical and geological methods, including deep-towed sonar system, sediment core analysis, and radiocarbon dating, to investigate the formation mechanism of the regional sequence boundary and reconstruct the sedimentary environment change history in the study area.
According to the sub-bottom profiler interpretations, we found the regional sequence boundary predominantly occurs in topographically flat areas, such as terraces on the flanks of the Good-Weather Ridge, bathymetric troughs between ridges, and atop the Palm Ridge and the Pointer Ridge. Based on grain size analysis, radiocarbon dating and core images of the core MD18-3545, we propose four sedimentary periods in the last 35,000 years in the study area. The Period 1 was in the stage before the regional seafloor erosive process. The relative sea-level was approximately -60 meters, and the sediment facies present a stable depositional hemipelagite facies with weak bottom current. The Period 2 started in the sea-level lowstand stage of the Last Glacial Maximum (LGM), and ended in about 11,900 year BP. The strong bottom current in this period transformed the sedimentary environment from depositional to erosive. Following the Period 2, the relative sea-level in the Period 3 has rose to -60 m and the sedimentary environment returned to depositional, forming the regional sequence boundary. Bottom current intensity increased with rising sea-levels. The sediment facies also shifted from hemipelagite to contourite, which controlled by strengthened bottom current. The Period 4 started in around 6,500 year BP. The relative sea-level is in stable highstand. Although bottom current intensity decreased compared to the previous period, it still played a significant role in shaping the seafloor environment and developing the contourite system. We propose that global climate changes and sea-level fluctuations have impacted bottom current intensities, sedimentary environments, and seafloor morphologies in the northeastern slope of South China Sea over the past 35,000 years. |
