Clay minerals have played a significant role in the study of the East Asian monsoon evolution in the South China Sea by being able to track oceanic current variations and to reveal contemporaneous pa- leoclimatic changes prevailing in continental source areas.As one of the most important rivers input- ting terrigenous matters to the northern South China Sea,the Pearl River was not previously paid at- tention to from the viewpoint of clay mineralogy.This paper presents a detailed study on clay minerals in surface sediments collected from the Pearl River drainage basin(including all three main channels, various branches,and the Lingdingyang in the estuary)by using the X-ray diffraction(XRD)method. The results indicate that the clay mineral assemblage consists dominantly of kaolinite(35%-65%), lesser abundance of chlorite(20%-35%)and illite(12%-42%),and very scare smectite occurrences (generally<5%).Their respective distribution does not present any obvious difference throughout the Pearl River drainage basin.However,downstream the Pearl River to the northern South China Sea,the clay mineral assemblage varies significantly:kaolinite decreases gradually,smectite and illite increase gradually.Additionally,illite chemistry index steps down and illite crystallinity steps up.These varia- tions indicate the contribution of major kaolinite,lesser illite and chlorite,and very scarce smectite to the northern South China Sea from the Pearl River drainage basin.The maximum contribution of clay minerals from the Pearl River is 72%to the northern margin and only 15%to the northern slope of the South China Sea.In both glacials and interglacials,kaolinite indicates that the ability of mechanical erosion occurred in the Pearl River drainage basin.
LIU ZhiFeiChristophe COLINHUANG WeiCHEN ZhongAlain TRENTESAUXCHEN JianFang
High-resolution clay mineral records combined with oxygen isotopic stratigraphy over the past 450 ka during late Quaternary from Core MD05-2901 off Middle Vietnam in the western South China Sea are re-ported to reconstruct a history of East Asian monsoon evolution. Variations in Illite, chlorite, and kaolinite contents indicate a strong glacial-interglacial cyclicity, while changes in smectite content present a higher frequency cyclicity. The provenance analysis indicates a mixture of individual clay minerals from various sources surrounding the South China Sea. Smectite derived mainly from the Sunda shelf and its major source area of the Indonesian islands. Illite and chlorite originated mainly from the Mekong and Red rivers. Kaolinite was provided mainly by the Pearl River. Spectral analysis of the kaolin-ite/(illite+chlorite) ratio displays a strong eccentricity period of 100 ka, implying the ice sheet-forced win-ter monsoon evolution; whereas higher frequency changes in the smectite content show an ice sheet-forced obliquity period of 41 ka, and precession periods of 23 and 19 ka and a semi-precession period of 13 ka as well, implying the tropical-forced summer monsoon evolution. The winter monsoon evolution is generally in coherence with the glacial-interglacial cyclicity, with intensified winter monsoon winds during glacials and weakened winter monsoon winds during interglacials; whereas the summer monsoon evolution provides an almost linear response to the summer insolation of low latitude in the Northern Hemisphere, with strengthened summer monsoon during higher insolation and weakened summer monsoon during lower insolation. The result suggests that the high-latitude ice sheet and low-latitude tropical factor could drive the late Quaternary evolution of East Asian winter and summer monsoons, respectively, implying their diplex and self-contained forcing mechanism.
