南海北部近海陆架表层沉积物类型及其稀土元素特征

Sediment Types and Their Characteristics of Rare-Earth Elements in Surface Sediments from Shelf of Northern South China Sea

对采自南海北部近海陆架区273个表层沉积物样品进行了系统的粒度及常微量和稀土元素分析。依据粒度分析结果,采用福克沉积物分类法,判断本区沉积物类型主要为砂质粉砂和粉砂,两者约占总样品数的60%,其次为泥、砂质泥、泥质砂和粉砂质砂,约占总样品数的27%。稀土元素的分析结果表明,研究区沉积物的稀土总量变化范围为21.34~244.15μg/g,平均值为155.26μg/g,与中国黄土的稀土总量(平均值171μg/g)相近,而与深海粘土沉积物的稀土总量(平均值411μg/g)差异较大,具有明显的亲陆性。稀土元素富集和分布主要受沉积物类型及水动力条件的影响,本区不同类型沉积物的稀土元素含量差异较大,稀土元素主要富集在粉砂、砂质粉砂、泥和砂质泥等几种类型的沉积物中;ΣREE等值线呈近似平行海岸的条带状分布,总体上,随着离岸距离的增加稀土元素含量逐渐降低,其中最高含量位于珠江口西侧、上下川岛、海陵湾等附近海域,这一特征可能与南海北部珠江口自东向西发育的沿岸流有关。但本区不同类型沉积物的稀土元素配分模式很相似,且与周边几条主要河流及上地壳稀土配分模式基本一致,表现为轻稀土强烈富集,具有明显的Eu负异常,而Ce无异常,表明其为典型的陆源沉积且源区具有一致性。进一步的分析表明,本区的沉积环境相对比较稳定,沉积物主要来源于华南大陆的花岗质母岩,并且大部分物质是通过珠江而带入。

In this paper, grain size as well as some major and trace elements, including rare-earth elements(REE), were systematically analysed for the 273 surface samples collected from the shelf of the northern South China Sea. The sediments were classified based on grain-size according to Folk's classification system. It showed that the main types of sediments in the study are mainly sandy silt and silt, which make up 60% of the whole sample, followed by mud, sandy mud, muddy sand and silty sand, which make up 27%. The analytical results of REEs showed that the total content of rare-earth elements(ΣREE) in this study range from 21.34 to 244.15 μg/g with an average of 155.26 μg/g. This is similar to the average ΣREE content in the Chinese loess, but different from that in deep-sea clay. It is evident that the REE has an 'inherited' or 'philo-continental' property. The distribution and enrichment of REE is generally controlled by sediment types and hydrodynamic characteristics. REE content in different sediment types vary greatly, mainly enriched in silt, sandy silt, mud and sandy mud. The contours of REE distribution parallel the coast, like strips, and the content of REE declines gradually with increasing distance from the coast. The highest content is found in the western part of the Pearl River estuary, the Shang/Xiachuan islands and Hailing Bay. These may be related to the coastal current that goes from east to west along to the Pearl River estuary in the northern part of the South China Sea. But chondrite-normalized REE patterns in various sediment types show no significant difference and are basically the same as those of coastal rivers and the upper continental crust. They all show relative enrichment in light REEs, a noticeable negative Eu anomaly and no Ce anomaly, indicating that these sediments are terrigenous sediments and are from the same source region. Further analysis shows that the sedimentary environment in the study area is relatively stable, and that granite widely distributed throughout the southern Chinese mainland is the main source of REE, which are mainly transported by the Pearl River.