The Distribution And Environmental Significance Of Clay Minerals In The Sediments Around Beibuwan Gulf

Clay minerals are the main components of the marine sediments. Due to the special structural characteristic and physical and chemical properties, clay minerals carry abundant information about the provenance, sediment environment, paleoclimate revolution, the eustacy and change of ocean circulation pattern etc. Clay minerals have been widely applied to the research and reconstruction of modern marine sediments, lithofacies paleogeography, paleoenvironment, stratigraphic correlation and the diagenic-metallogenic conditions etc.312 surface sediment samples were collected from Baibuwan Gulf. Through X-RAY Diffraction Analysis(XRD), the variety and relative content of the clay minerals were identified. The results showed that the surface sediments are predominated by illite/smectite mixed-layered clay minerals with average content of 49.95% in all area, followed by kaolinite with average content of 24.69% and lower content of chlorite and illite with average content of 14.07 % and 11.28% respectively. Illite/smectite mixed-layered clay minerals are the widest in distribution and highest in content. The changes of illite/smectite mixed-layered clay minerals play a distinctly negative relationship with that of kaolinite, illite and chlorite. It is rare to discover so high content illite/smectite mixed–layered clay minerals (nearly 50 percent) from modern ocean sediments. Besides the abundant supple of the terrigenous material, the rich basic volcanic substances in the studies ocean area as well as the volcanic rocks(basalt) along its east offshore (the Leizhou Peninsula and the south part of Hainan Island) contribute to the high content of illite/smectite mixed-layered clay minerals. Based on the assemblages of clay minerals and their distribution, together with the Scanning Electron Microscope(SEM) and Energy Dispersive Spectrum(EDS) analysis, as well as other influential factors, 4 procvinces of 1 to 4 were classified in Beibuwan gulf area. It is characteristic with very high content of kaolinite and low content of illite/smectite as well as illite in Province 1 in nearby offshore area; the content of illite in Province 2 is high and illite/smectite mixed-layered clay minerals lower. Illite/smectite is dominated in Province 3 while kaolinite is lowest and illite is lower. Province 4 is similar to Province 2 but with a higher chlorite in Province 4. The content of illite/smectite mixed-layed clay minerals is higher in deep water while kaolinite is higher nearshore. The composition and distribution of clay minerals are mainly controlled by sediment sources, hydrodynamic condition(especially the Beibuwan Gulf ocean current), the submarine topography, the depth and the salinity of the sea water etc.. The origin , genesis of the clay minerals in the study region are discussed in detail. The origin of the clay minerals in the studies area is terrigenous. With the dating of AMS14C in the surface sediments(0cm-20cm) from Baibuway gulf, it is found that the climate environment of the source region of clay minerals from Baibuway gulf has been in warm and wet since 2700a. And the climate there has been stable.325 samples in 35 sediment cores from Baibuwan gulf were analyzed by the X-ray diffraction and it showed that there were 4 different clay minerals including illite/smectite mixed-layer mineral, Kaolinite, Chlorite and illite. The differences in geologic condition, weathering environment and transport route in the lands around the studies area control the spatio-temporal variation of component distribution of clay minerals along the sedimentary core. Together with the Scanning Election Microscope, the Energy Dispersive Spectrum analysis and AMS14C dating, the material resource and depositional environment of the 9 sediment cores with AMS14C dating are discussed in particular. The environment of sedimentation of the sediment cores was studied by the distribution of different clay minerals, illite crystallinity, illite chemical index , sedimentation rate etc. SEM results showed that the clay minerals are mainly of terrigenous rather than anthigenic origin. It is interesting to discover that high concentration of illite-smectite mixed-layered clay minerals correspond to the warm or interglacial period, and its low concentration to the cold or glacial period. The behaviour and paleoceanographic indication of illite and chlorite are opposite to those of illite/smectite mixed-layered clay minerals in Baibuwan gulf. The vertical distribution characteristics and assemblage of the clay minerals can indicate the climate evolution and hydrodynamic condition of the source region. With AMS14C dating, the climate change events can be recognized through the information that the sediment core stored. The sediments in Baibuwan gulf is deposited since 24545aBP.Comprehensive analysis of sediment environment and climate events is carried basing the 4 sediments cores nearby shore and the 5 sediment cores in deep ocean area. The content ratio about (illite+chlorite)/illite/smectite mixed-layered clay mineral can reflect the vertical distribution of the clay minerals and indicate the paleoclimate environmental event significantly. The 4 offshore sediment cores reveal 4 paleoclimate events such as“New Glacial Period I”in about 7634±35aBP,“The Pacific High Temperature Period”in 6920±35aBP and 5749±35aBP,“The Cooling Events of South China”between 4600aBP and 4200aBP and“The Cooling Period of Northern and Southern Dynasty”from 1530aBP to 1310aBP. While the 5 deep oceanic sediment cores indicate the stable depositional and climatic environment since 13845aBP.