钱塘江河口区晚第四纪古环境演化及其元素地球化学特征

摘要

In 2009, a 51. 5-m-long core SE2 was drilled at 120°21'57" E, 30°12'38" N, from the Kanshan town, Hangzhou city, in the south bank of the Qiantang River. The ground surface elevation of the core is 6. Om above mean sea level. Core SE2,10. 8cm in diameter,was obtained by rotary drilling with a 96% recovery. A total of 294 samples were collected at 10 ~20cm intervals along the cored succession for grain size analysis,93 samples at 20 ~ 80cm intervals for foraminifera identification, and 27 samples at 200cm intervals for rare earth elements analysis. The lithology, grain size, sedimentary structure, foraminifera, and rare earth elements of Late Quaternary sediments in the borehole SE2 were analyzed to research the sedimentary environment evolution, and the associated distribution variation of rare earth elements in the Qiantang River mouth area. The results indicate that five sedimentary facies, including fluvial channel,floodplain,tidal flat,shallow marine,and estuarine sand bar,can be distinguished,and the rare earth elements distribution is closely related to sedimentary environment. Fluvial channel facies consists of sandy gravel,gravelly sand, and fine sand, with high content of rolling and saltation population. Foraminifera are absent. The sediments have the lowest total rare earth elements content in all sedimentary facies,and high light rare earth elements concentration. The fractionation degree of light and heavy rare earth elements, and fractionation degree of heavy rare earth elements are positively related with the total rare earth elements content. The fractionation of light rare earth elements shows negative correlation with the total rare earth elements content, heavy rare earth elements concentration, the fractionation of light and heavy rare earth elements, and fractionation in heavy rare earth elements. Floodplain facies are dominated by clay, with foraminifera and rolling population absent. Total rare earth elements content is the highest. The fractionation degree of light and heavy rare earth elements, and fractionation of heavy rare earth elements are positively correlated with the total rare earth elements content. The fractionation of light rare earth elements displays positive relationship with the total rare earth elements content, and negative correlation with the fractionation degree of light and heavy rare earth elements. Tidal flat sediments are composed of rhythmic couplets between silt and clay, which have lower total rare earth elements concentration, and display positive correlation between all parameters of rare earth elements. Shallow marine sediments are mainly composed of muddy clay with abundant foraminifera. Only suspension population is found. The sediments generally exhibit enrichment of rare earth elements. The fractionation degree of light and heavy rare earth elements, fractionation of heavy rare earth elements,and fractionation in light rare earth elements are negatively correlated with the total rare earth elements content. The fractionation of light rare earth elements displays positive relationship with the degrees of fractionation of light and heavy rare earth elements,and fractionation of heavy rare earth elements. Estuarine sand bar sediments are composed mainly of silt and fine sand with high amount of saltation population. Sediments of this facies have moderate rare earth elements content. The fractionation degree of light and heavy rare earth elements, and fractionation of heavy rare earth elements are negatively correlated with the total rare earth elements content, and fractionation of light rare earth elements. The fractionation of light rare earth elements shows positive relationship with the total rare earth elements content.%对钱塘江南岸SE2钻孔沉积物的岩性、粒度、沉积构造、有孔虫和稀土元素进行分析,结果表明,钱塘江河口区晚第四纪以来依次发育河床相、河漫滩相、潮坪相、近岸浅海相和河口湾砂坝相,各沉积相稀土元素分布差异明显.河床相由砂砾、砾质砂和细砂组成,滚动和跳跃组分含量高,缺失有孔虫；稀土元素总量最低,轻稀土富集,轻重稀土分异程度、重稀土元素分异程度与稀土元素总量呈正相关,轻稀土元素分异程度与稀土元素总量、重稀土含量、轻重稀土分异程度、重稀十元素分异程度呈负相关.河漫滩相主要为粘土,缺乏滚动组分,不含有孔虫；稀上元素总量最高,轻重稀土分异程度、重稀土元素分异程度与稀土元素总量呈正相关,轻稀土元素分异程度与重稀土含量呈正相关、与轻重稀土分异程度呈负相关.潮坪相南互层的粉砂和粘土组成,偶见有孔虫；稀土元素总量较低,各稀上元素参数均呈正相关.近岸浅海相主要为淤泥质粘上,均属悬浮组分,有孔虫丰富；稀土元素总量较高,轻重稀十分异程度、轻稀土元素分异程度、重稀土元素分异程度与稀土元素总量呈负相关,轻稀十元素分异程度与轻重稀土分异程度、重稀土元素分异程度呈正相关.河口湾砂坝相由粉砂和细砂组成,跳跃组分为主,稀土元素总量中等,轻重稀土分异程度、重稀上元素分异程度与稀上元素总量、轻稀上元素分异程度呈负相关,轻稀土元素分异程度与稀十元素总量呈正相关.