Li Anchun

Characteristics of Clay Minerals in the Northern South China Sea and Its Implications for Evolution of East Asian Monsoon since Miocene

Clay mineral assemblages, crystallinity, chemistry, and micromorphology of clay particles in sediments from ODP Site 1146 in the northern South China Sea (SCS) were analyzed, and used to trace sediment sources and obtain proxy records of the past changes in the East Asian monsoon climate since the Miocene, based on a multi-approach, including X-ray diffraction (XRD) and scanning electron microscopy combined with energy dispersive X-ray spectrometry (SEM-EDS). Clay minerals consist mainly of illite and smectite, with associated chlorite and kaolinite. The illite at ODP Site 1146 has very well-to-well crystallinity, and smectite has moderate-to-poor crystallinity. In SEM the smectite particles at ODP Site 1146 often appear cauliflower-like, a typical micromorphology of volcanic smecites. The smectite at ODP Site 1146 is relatively rich in Si element, but poor in Fe, very similar to the smectite from the West Philippine Sea. In contrast, the chemical composition of illite at ODP Site 1146 has no obvious differences from those of the Loess plateau, Yellow River, Yangtze River, and Pearl River. A further study on sediment source indicates that smectite originates mainly from Luzon, kaolinite from the Pearl River, and illite and chlorite from the Pearl River, Taiwan and/or the Yangtze River. The clay mineral assemblages at ODP Site 1146 were not only controlled by continental eathering regimes surrounding the SCS, but also by the changing strength of the transport processes. The ratios of (illite+chlorite)/smectite at ODP Site 1146 were adopted as proxies for the East Asian monsoon evolution. Relatively higher ratios reflect strongly intensified winter monsoon relative to summer monsoon, in contrast, lower ratios indicate a strengthened summer monsoon relative to winter monsoon. The consistent variation of this clay proxy from those of Loess plateau, eolian deposition in the North Pacific, planktonic, benthic foraminifera, and black carbon in the SCS since 20 Ma shows that three profound shifts of the East Asian winter monsoon intensity, and aridity in the Asian inland and the intensity of winter monsoon relative to summer monsoon, occurred at about 15 Ma, 8 Ma, and the younger at about 3 Ma. The phased uplift of the Himalaya-Tibetan plateau may have played a significant role in strengthening the Asian monsoon at 15 Ma, 8 Ma, and 3 Ma.

Manganese Abnormity in Holocene Sediments of the Bohai Sea

Manganese abnormity has been observed in the Holocene sediments of the mud area of Bohai Sea. On the basis of grain size, chemical composition, heavy mineral content and accelerator mass spectrometry (AMS) C-14 dating of foraminifer, relationships between manganese abnormity and sedimentation rates, material source, hydrodynamic conditions are probed. Manganese abnormity occurred during the Middle Holocene when sea level and sedimentation rates were higher than those at present. Sedimentary hiatus was not observed when material sources and hydrodynamic conditions were quite similar. Compared with the former period, the latter period showed a decrease in reduction environment and an inclination toward oxidation environment with high manganese content, whereas provenance and hydrodynamic conditions showed only a slight change. From the above observations, it can be concluded that correlation among manganese abnormity, material source, and hydrodynamic conditions is not obvious. Redox environment seems to be the key factor for manganese enrichment, which is mainly related to marine authigenic process.

Grain-size records at ODP site 1146 from the northern South China Sea : Implications on the East Asian monsoon evolution since 20 Ma

Abstract273 samples from Ocean Drilling Program (ODP) Site 1146 in the northern South China Sea (SCS) were analyzed for grain-size distributions using grain-size class vs. standard deviation method and end-member modeling algorithm (EMMA) in order to investigate the evolution of the East Asian monsoon since about 20 Ma. 10–19 μm/1.3–2.4 μm, the ratio of two grain-size populations with the highest variability through time was used to indicate East Asian winter monsoon intensity relative to summer monsoon. The mass accumulation rate of the coarsest end member EM1 (eolian), resulting from EMMA, can be used as a proxy of winter monsoon strength and Asian inland aridity, and the ratio of EM1/(EM2+EM3) as a proxy of winter monsoon intensity relative to summer monsoon. The combined proxies show that a profound enhancement of East Asian winter monsoon strength and winter monsoon intensity relative to summer monsoon occurred at about 8 Ma, and it is possible that the summer monsoon simultaneously intensified with winter monsoon at 3 Ma. Our results are well consistent with the previous studies in loess, eolian deposion in the Pacifc, radiolarians and planktonic foraminifera in the SCS. The phased uplift of the Himalaya-Tibetan Plateau may have played a significant role in strengthening the Asian monsoon at 8 Ma and 3 Ma.

CHARACTERISTICS OF EOLIAN DUST OVER THE EASTERN CHINA SEAS

Eolian dust is an important material source of marine sediment and heavy metals influencing marine environment and primary productivity. Although the eastern China Seas and their adjacent regions are downwind of East Asian dust sources and have potentially high atmospheric input, study on the eolian dus in these regions is limited. This study on the compositions of eolian dust indicated that dust concentration and particle sizes varied with seasons and meteorologic conditions, that mineral and chemical compositions have significant regional variation, and that high Fe content is one of the characteristics of eolian dust which was found to be a mixture of natural loess and/or soil and anthropogenic matter that contained fairly high S and heavy metals.

PROVENANCE OF SURFICIAL SEDIMENTS OF THE EAST PHILIPPINE SEA:EVIDENCE FROM RARE EARTH ELEMENTS: PROVENANCE OF SURFICIAL SEDIMENTS OF THE EAST PHILIPPINE SEA:EVIDENCE FROM RARE EARTH ELEMENTS

To further understand the provenance of the surficial sediments in the East Philippine Sea,30 samples have been analyzed for rare earth elements(REE) compositions and clay mineral assemblages.Based on the REE data and a simplified two end-member mixing model,we quantitatively calculated the contributions of the eolian dust and the local volcanic debris to the study area.It shows that the contribution of the eolian dust to the study area is decreasing with the distance from the source area of the dust(i.e.,the Asian continent),while the contribution of the local volcanic matter shows a converse trend,with high values near the Parece Vela Trench.The results are very consistent with the conclusion from clay mineral assemblages and 87Sr/86Sr data of the studied samples and the conclusions from the two end-member mixing equation provided by published literatures.It suggests that the simplified calculation model is valid for provenance study in this region.

QUATERNARY ASSEMBLAGE CHARACTERISTIC AND PROVENANCE OF CLAY MINERALS IN THE PARECEVELA BASIN OF THE EAST PHILIPPINE SEA

Using XRD method,the clay minerals of two Quaternary cores F090815 and F100609 located in Parece Vela Basin,East Philippine Sea were studied in this article.The results show that there are some differences in mineral assemblage between the north and the south.In the southern core F090815 the most abundant content is smectite(50%),then illite(37%),and the lowest is kaolinite and chlorite(13%);however,in the northern core F100609 the most abundant content is illite(47%),then smectite(39%),and the lowest is kaolinite and chlorite(14%).According to contents and characteristic of clay minerals,both the cores F090815 and F100609 can be divided into four parts.According to assemblage characteristic,chemical index and crystallinity of clay minerals,we think that smectites were originated from submarine basalt alteration in the Mariana Trough;while illites were derived mainly from the west continent under strongly physical weathering probabeg associated with the tormation of loess in the East Asia continent,so were kaolinite and chlorite.

Episodes of volcanic activity and their environmental effects in the Okinawa Trough during the last 150 ka

A piston core Z14-6 was used in this study. The core, 896 cm long, was collected from eastern slope of the Okinawa Trough (27°07′N, 127°27′E, water depth 739m). The δ18O and δ13C values of the sediment bearing planktonic foraminiferaG. sacculifer andN. dutertrei were determined; and the abundance of volcanic glass was analyzed. The volcanic glass content high occurred in early stage of polar ice-sheet growth period, or the beginning of cold climate periods corresponding to Milankovitch cycles (Peak I, II and V are corresponding to the beginnings of oxygen isotopic stages 2, 4 and 6, and Peak III and IV are matching oxygen isotopic stage 5b–5d.). It might be possible that volcanic episodes and climate changes were responding to orbital forcing in the Okinawa Trough in late Quaternary. The δ18O difference betweenN. dutertrei andG. sacculifer shows no clear correlation to the volcanic glass content high, which suggests that the volcanic eruptions did not influence the structure of upper water column. However, the low δ13C difference betweenG. sacculifer andN. dutertrei is coeval with the volcanic glass high or sub-high content. This fact suggests that volcanic eruptions might influence the reduction in vertical nutritional gradient and carbon cycle process in upper water column. A possible mechanism is that huge quantity of ash and dust had weakened the light intensity, resulting in photosynthesis reduction, productivity decrease, and biological pumping.