Variability of clay mineral assemblages in the Western Pacific Warm Pool(WPWP) over the past 370 ka shows the prominent glacial-interglacial cyclicity.Smectite(62%-91%) is the dominant clay mineral,with decreased contents during interglacials while increased in glacials.In contrast,variations in chlorite(4%-21%),illite(4%-12%),and kaolinite(2%-10%) share a similar pattern with higher contents during interglacials than glacials,mirroring to that of smectite.The results indicate that the smectite-dominated clay minerals derive mainly from the river detrital inputs of New Guinea.The glacial-interglacial cycle of clay mineral assemblages well correspond to the fluctuation of sea level.When the sea level was low,the river materials can travel more easily across the narrow shelf off the island of New Guinea,inject directly into the subsurface currents flowing westwards,then merge into the Equatorial Undercurrent(EUC),and eventually deposit on the central part of WPWP.Precessional periods of the smectite content indicate the intensity of mechanical erosion in its provenance of New Guinea,responding to the river runoff and precipitation,and this could also be linked to the meridional migration of the Intertropical Convergence Zone(ITCZ).
High-resolution clay mineralogical analysis of Core MD05-2904 in the northern South China Sea(SCS) covering the period since the Last Glacial Maximum shows that illite(29%-48%),smectite(14%-45%),chlorite(17%-28%),and minor kaolinite(6%-14%) comprise the clay mineral assemblage,and that time series variation does not present glacial-interglacial cyclicity.Provenance analysis indicates three end-member sources:almost all smectite derives from Luzon,all kaolinite is sourced from the Pearl River,and illite and chlorite originate from both the Pearl River and Taiwan.By comparing clay mineral compositions in surface sediments from the three major source areas and of the SCS,we reconstructed a time series of clay mineral contribution from the major provenances to the northern slope of the SCS using the linear separation method for illite crystallinity.There were three stages of provenance change.(1) During 24.1-17.5 ka BP,contributions from Taiwan and Luzon were similar(30%-40%),while that from the Pearl River was only 25%.(2) During 17.5-14.0 ka BP,the contribution from Luzon decreased rapidly to 20%-25%,while that from Taiwan increased to 35% from an average of 25% at 18 ka BP,and that from the Pearl River increased largely to 40%.(3) During the Holocene,differences in contributions from the three major provenances increased:the contribution from Luzon increased slightly and then remained at 27%-35%,that from Taiwan increased rapidly and then remained at 55%-60%,and that from the Pearl River decreased to 15%.The change in clay mineral contributions from different provenances is influenced mainly by clay mineral production,monsoon rainfall denudation,oceanic current transport,and sea-level change.
High-resolution grain size and major element geochemical measurements were performed on the marine sediments of Core MD05-2895 to help understand the formation of turbidite sequences. Grain-size results show that these turbidite sediments contain more coarse sediment grains than normal marls. The coarse sediment grains are mostly derived from relict sediments on the Sunda Shelf. Relict sediments are composed mainly of quartz, feldspar, tephra and a few titaniferous or ferruginous heavy minerals. Corresponding to the concentration of these minerals, increases in Si/Al, K/Al, Ti/Al and Fe/Al ratios are observed in the turbidite layers. As all the observed turbidite sequences were deposited during the last glacial, the occurrence of these turbidity events is implied to be closely related to instabilities in sea-level-induced sediment supply. We suggest that deposition of sediment particles from a single turbidity current is usually controlled by a counterbalance between gravity and buoyancy, with the interaction of individual grains being of minor importance.
Weathering products of silicate rocks are particularly useful for evaluating the continental chemical weathering on the Earth's surface and its mechanism.Clay mineralogy and major-element geochemistry of surface sediment samples collected in major rivers of Malay Peninsula and North Borneo in the tropical Southeast Asian region are used to study the present chemical weathering process and its controlling factors of tropical regions.The results indicate that the clay mineral assemblage in Malay Peninsula consists dominantly of kaolinite(average 80%) and minor illite(average 17%),almost without chlorite and smectite,whereas in North Borneo it consists mainly of illite and chlorite,with minor amounts of kaolinite(average 14%) and no smectite.Total contents of illite and chlorite in both Northwest and Northeast Borneo are 84% and 87%,respectively.Major-element geochemical results of both bulk and clay-fraction sediments show intensive chemical weathering degree for both areas.Relatively,the chemical weathering degree is gradually strengthened from Northeast Borneo,Northwest Borneo,to Malay Peninsula,and it is extremely intensive in Malay Peninsula.Our results indicate that,in the tropical Southeast Asian region exampled by Malay Peninsula and Borneo,climatic condition of tropical warm temperature and East Asian monsoon rainfall is the first-order controlling factor on the chemical weathering,resulting in intensive chemical weathering throughout tropical areas,and tectonic activity and lithology of parent rocks are subordinate factors,which still have an important impact on the weathering products,forming completely different clay mineral assemblages between Malay Peninsula and Borneo.
WANG HaoLIU ZhiFeiEdlic SATHIAMURTHYChristophe COLINLI JianRuZHAO YuLong
