南海北部表层沉积物碳酸钙含量及主要钙质微体化石丰度分布

DISTRIBUTION OF CARBONATE CONTENTS AND THE ABUNDANCES OF MAJOR CARBONATE COMPONENTS IN SURFACE SEDIMENT FROM THE NORTHERN SOUTH CHINA SEA

通过研究南海北部陆坡至深海平原表层沉积物碳酸钙含量、浮游有孔虫壳体丰度、钙质超微化石丰度空间分布,认为研究区碳酸钙溶跃面位于水深约3000m处,碳酸钙补偿深度（CCD）位于水深约3500m或者更小水深.浮游有孔虫壳体、钙质超微化石是表层沉积物碳酸钙的主要组分,浮游有孔虫壳体对数值、钙质超微化石丰度均与碳酸钙含量对数值呈较好线性关系.水深3000m以浅的表层沉积物中,碳酸钙含量对数值、浮游有孔虫壳体丰度对数值随水深增大趋于线性减小,而钙质超微化石丰度随水深增大而趋于线性增大.浮游有孔虫壳体丰度和钙质超微化石丰度分布差异可能与这两类碳酸钙颗粒沉积学行为不同有关：浮游有孔虫壳体丰度随水深的变化可能主要受到陆源粘性颗粒稀释作用;钙质超微化石和陆源粘性颗粒沉积学行为相似,可能主要在海底边界层参与絮凝作用和反絮凝作用,随水深增大皆趋于富集.随着水深从3000m逐渐增大,碳酸钙溶解作用成为海底碳酸钙分布的主要影响因素,碳酸钙含量对数值与浮游有孔虫壳体丰度对数值随水深增加呈加速减小趋势,而许多表层沉积物不含或贫钙质超微化石.然而,所有表层沉积物碳酸钙都没有被溶解殆尽.通过碳酸钙严重不饱和环境下表层沉积物碳酸钙含量的分布,识别出一些底流活动的影响.特别地,在中沙北海岭周缘深海平原存在浮游有孔虫壳体与钙质超微化石高丰度带,该高丰度带可能是底流作用下快速沉积的结果.碳酸钙含量小于约2％的许多表层沉积物具有浮游有孔虫壳体贫乏、钙质超微化石相对富集的特点,该现象很可能与这两类碳酸钙颗粒粒径悬殊有关,体积微小的钙质超微化石更有机会成为永久性沉积物.

A total of 278 surface sediment samples, which were obtained from region of 16° -20°N, 114°- 120°E with water depth range of 285 - 4270m and distribute in the pattern of a near dot-matrix, are used to analyze the distribution of carbonate contents, the abundances of planktonic foraminiferal shells, and the abundances of calcareous nannofossils. Calcite lysocline in surface sediment from northern slope to the abyssal plain in the South China Sea is recognized to be located at about.3000m water depth, while calcite compensation depth located at about 3500m water depth or shallower. Planktonic foraminiferal shells and calcareous nannofossils are generally the major components of the carbonate in the surface sediment, and both the logarithm of the abundance of planktonic foraminiferal shells and the abundance of calcareous nannofossils have a linear relationship with the logarithm of the abundance of carbonate content. For the surface sediment with water depth less than 3000m, both the logarithm of the abundance of carbonate content and the logarithm of the abundance of planktonic foraminiferal shells generally decreases with increasing water depth, while the abundance of calcareous nannofossils increase with increasing water depth. The changes of the abundance of planktonic foraminiferal shells with water depth is probably effected by the dilution of terrigenous cohesive particles, but the calcareous nannofossils have a similar sedimentological behavior with terrigenous cohesive particles, and they often take part in flocculation and anti-flocculation within the benthic boundary layer, and tend to be richer in deeper water. With water depth increasing from about 3000m, carbonate disslution becomes a strong factor affecting the carbonate distribution, and in general, the decrease of the logarithm of both carbonate content and the abundance of planktonic foraminiferal shells accelerates with increasing water depth, and calcareous nannofossils in much sediment disappear completely or only a few calcareous nannofossils has survived. However, not all carbonates in the surface sediment are dissolved completely, and some impact of bottom currents are recognized through the carbonate distribution in the environment seriously unsaturated respect to calcite. Especially, the high abundances of both planktonic foraminiferal shells and calcareous nannofossils in the abyssal plain around ZhongShaBei ridge likely results from quick sedimentation of bottom currents. Much surface sediment with carbonate content less than about 2% is poor in planktonic foraminiferal shells and relative rich in calcareous nannofossils, and these features are probably related to huge size difference of these two calcite materials, the much smaller calcareous nannofossils have far more chances to become permanent sediment.