Late Quaternary clay minerals off Middle Vietnam in the western South China Sea: Implications for source analysis and East Asian monsoon evolution

Abstract

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 reported 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 kaolinite/(illite+chlorite) ratio displays a strong eccentricity period of 100 ka, implying the ice sheet-forced winter 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. clay minerals, provenance, East Asian monsoon, late Quaternary, Middle Vietnam, South China SeaThe East Asian monsoon is a significant component of the global climate. It dominates seasonal patterns of winds, precipitation and runoff, and determines the character of land vegetation over East Asia [1] . The South China Sea, the largest marginal sea in the western Pacific, is characterized by anticlockwise surface currents forced by northeasterly winds in winter, and in contrast, by southwesterly surface currents forced by southwesterly winds in summer. The South China Sea received products of chemical weathering and physical erosion occurred in adjacent continents [2] . Among them, as the most important detrital materials of terrigenous origin in the abyssal South China Sea, clay minerals can be used to track oceanic current variations and to reveal contemporaneous paleoclimatic changes prevailing in con-

A high-resolution clay mineralogical record in the northern South China Sea since the Last Glacial Maximum, and its time series provenance analysis

“…Studies in the southern SCS considered chemical weathering driven by contemporaneous climatic changes of source areas to be the major mechanism, with a case study off the Mekong River mouth showing that the Mekong River basin serves as the major provenance of the southwestern SCS [14,19]. A study in the summer upwelling region of the western SCS, however, considered surface current transport controls, i.e., north-southerly dominant surface currents during glacials transported kaolinite originating mainly from the Pearl River to the south, whereas south-northerly dominant surface currents during interglacials carry smectite and illite from mainly southern sources to the north [20]. These studies are based upon long-term trends at glacial-interglacial scales during the late Quaternary.…”

Dynamic control on grain-size distribution of terrigenous sediments in the western South China Sea: Implication for East Asian monsoon evolution

“…The western South China Sea is adjacent to the very narrow continental shelf in Middle Vietnam and far from the northern continental shelf. The obvious contribution of dust to the sediments in this area has not been previously reported [11] . The grain-size distribution of most sediment samples from Core MD05-2901 exhibits single mode, which indicates that the influence of eolian dust was little, if existed.…”

“…Studies on clay minerals of Core MD05-2901 confirmed the sedimentation of terrigenous materials originated from Red River and Zhujiang River on the northern shelf through the transport of anti-clockwise current forced by winter monsoon [11] . The content of kaolinite mainly from Zhujiang River exhibits strong glacial-interglacial cyclicity [11] .…”

“…The content of kaolinite mainly from Zhujiang River exhibits strong glacial-interglacial cyclicity [11] . The southwestern continental shelf, Sunda shelf near the South China Sea, had different climate environments from the northern continental shelf.…”

“…And the continental materials contain lots of information about tectonic sedimentary history, chemical weathering and physical erosion, and East Asian monsoon evolution [1] . However, the studies on terrigenous sediments have not been paid enough attention relative to biogenic materials [10,11] .…”