南海南端30 萬年來沉積物源區變化及古環境變遷研究

Abstract

東亞季風在全世界的氣候系統中扮演著重要的角色，整個東亞地區的降 雨、植被、風力深深受其控制。南海是東亞最大的邊緣海盆地，擁有高解析度的 海洋沉積紀錄，提供東亞季風的證據。 本研究利用國際海洋古全球變遷計畫IMAGES(International Marine Past Global Changes Study)於2001 年在南海所採取的岩芯MD012390(岩芯位置 8°32.57’N, 113°24.56’E，水深1545m，總長度43.5m)及MD012391(岩芯位置 6°38.112’N, 110°20.94’E，水深1312m，總長度42.48m)作為研究材料。採樣間距 為20 公分，共計217 和212 個標本。利用X 光螢光分析儀(XRF)和X 光粉末繞 射儀(XRD)來分析沉積物中主要元素和黏土礦物含量，並結合有孔蟲氧同位素地 層以及古地磁紀錄建立年代模式(岩芯分別涵蓋22.4 和30.5 萬年)，研究南海南 部過去30 萬年以來的沉積紀錄，以及可能的環境變遷。 黏土礦物指示MD012391 的沉積物主要來源以湄公河為主，而婆羅洲則是 MD012390 主要的沉積物供應源區，而在海水面下降的時期加入了來自巽他陸棚 的沉積物，兩口岩芯中的黏土礦物以伊萊石為主，其次是綠泥石以及高嶺石，含 量最少的是膨潤石。根據前人研究推測MD0132391 中伊萊石和綠泥石主要來自 西藏高原東部，而高嶺石在海水面低的時期表現出較高的含量，可能是原本高水 位時期堆積在中部湄公盆地的高嶺石，由於海水面下降再次被侵蝕，然後經由河 流向南海海盆搬運輸送，因此婆羅洲可能為MD012390 中伊萊石和綠泥石主要來 源，而膨潤石及高嶺石在海水面下降的時期表現出增加的趨勢指示著這兩種礦物 可能因為海水面下降，將原本高水位時期堆積在巽他陸棚上的膨潤石以及高嶺石 再次被帶到盆地中堆積。 主要元素方面，為了降低生物源沉積物的稀釋效應，先減去來自碳酸殼體貢 獻的CaO 後，再重新計算其他元素的含量，並利用TiO2 對其他元素標準化，經 過標準化後的主要元素變化可以看出在兩根岩芯中，陸源元素的指標SiO2 和 Al2O3 在間冰期時(MIS1.3.5.7)增加，但在冰期(MIS 2.4.6)時減少，ㄧ般認為夏季 季風會在間冰期的時候增強並在冰期減弱，而夏季季風的增強會增加夏季的降雨 量，因降雨量的增加會強化陸域的化學風化，其變化也反映在岩芯中陸源的主要 IV 元素上面，這些陸源元素的變化在南海的其他岩芯中也有相同的變化趨勢可茲印 證。

The East Asian Monsoon (EAM) is an importment force of climate system in the world and controls the precipitation and wind in the east Asia. The South China Sea (SCS) is the largest marginal basin in the western Pacific which has high sedimentation rate and well-preserved carbonate fraction, and is located in the main route of EAM. Thus, it records very important climatic changes of the EAM. The materials in this study were from the sediments of the Cores MD012390 (8°32.57’N, 113°24.56’E) and MD012391 (6°38.112’N, 110°20.94’E), collected near the eastern slope of the Sunda shelf in the southern South China Sea by the giant piston cores of IMAGES. Sampling intervals for analyses were about 20 centermeters in the Cores MD012390 and MD012391 which had the total 217 and 212 samples, respectively. The clay mineral (<2μm) analysis was carried out by the X-ray powder diffraction (XRD) and the major element composition of bulk sediments was done by the X-ray fluorescence (XRF). The aims are to reconstruct the weathering condition, to explore the climatic changes and to understand the sediment provenance of the last 300 ka in this region. Clay mineral analyses of both cores indicate that the major components are illite with subordinate chlorite, kaolinite and minor smectite. The smectite content increases during the MIS 2, 4 and 6 in the Core MD012390, but didn’t in the Core VI MD012391. The clay mineral distributions in surface sediment of sea bottom of the southern SCS suggest that the sediments from the Indonesia arc provided high content of smectite. However, it was trapped in the Sunda shelf during the interglacial periods. Therefore, the clay mineral assemblages of the Core MD012390 suggest that the Borneo may be the major source for the sediments during the interglacial period, but had kaolinite and smectite from Sunda shelf during the glacial period. On the other hand, the Mekong River may be the major sediment sources for the Core MD012391. For eliminating the biogenic effect on the bulk compositions of sediment, we subtract the CaO which contributed from carbonate firstly and recalculate the major element contents. Then normalize them with TiO2 to examine their variations during glacial-interglacial cycles. The normalized rations of SiO2 and Al2O3 increase during the MIS1, 3, 5, and 7, but decrease during the MIS 2, 4 and 6. These two elements are predominantly derived from the continent and their abundance could be controlled by chemical weathering on land. Therefore, the higher ratios of them suggest that the MIS 1, 3, 5 and 7 may have higher precipitation in summer to supply the higher weathered sediments to the cores.