北部湾表层黏土矿物分布特征及物源研究

Distribution and provenance of clay minerals in surface sediments of the Beibu Gulf, the South China Sea

利用 X 射线衍射(XRD)对北部湾表层沉积物中黏土矿物组成和结晶学特征进行了详细研究,揭示了源区和洋流对该区沉积物搬运和分布的控制机制。结果表明,北部湾表层沉积物中黏土矿物组成以伊利石、蒙脱石和高岭石为主,含有少量的绿泥石。在空间分布上,蒙脱石含量表现为中部高近岸低;伊利石含量表现为西南高东北低,绿泥石组分含量与之类似;高岭石含量表现为东北高,西南低。伊利石结晶度指数和化学指数平均分别为 0.25°(Δ2θ)和 0.5,表明伊利石结晶程度极好,为富 Al伊利石,来源于化学风化作用占主导的源区。黏土矿物物源分析表明,红河是北部湾伊利石、绿泥石和蒙脱石的主要物源,其中蒙脱石因受沉积分异作用影响而在研究区中部富集;高岭石受到了红河、珠江以及广西和海南岛沿岸河流物质输入的共同影响。本研究结果对理解北部湾沉积物“源-汇”过程和古环境有重要参考意义。

In this study, we analyzed the assemblages and crystallinity of four major clay minerals in the surface sediments of the Beibu Gulf, the South China Sea, based on X-ray diffraction (XRD), in order to constrain sediment provenance and transport mechanisms in this area. The results showed that the clay mineral assemblages are domi- nated by illite, smectite, and kaolinite, while chlorite is present in lesser amounts. On the spatial scale of the Beibu Gulf, the smectite contents are higher in the central part and lower near the shore. The illite contents are higher in the southwest and lower in the northeast, with chlorite contents being similar in distribution. To the contrary, the kaolinite contents are higher in the northeast and lower in the southwest. The average illite crystallinity index and illite chemical index are 0.25°(Δ2θ) and 0.5, respectively, indicating that illite has high crystallinity and Al content. This suggests that the source areas of illite are dominated by a chemical weathering environment. Clay mineral source studies suggest that illite, chlorite, and smectite originate mainly from the Red River, where smectite is en- riched in the central part of the study area due to sedimentary differentiation, while kaolinite is mainly derived from the Red River, the Pearl River, and the coastal rivers of Hainandao and Guangxi. The results in this study are helpful toward the understanding of “source-to-sink” processes and the paleoenvironment of the Beibu Gulf sediments.