南海东岛湖泊沉积物中的生态环境记录

ECO-ENVIRONMENTAL INFORMATION RECORDED IN THE LAKE SEDIMENTS OF THE DONGDAO ISLAND, SOUTH CHINA SEA

东岛牛塘为目前在西沙群岛少见的淡水湖泊,发育保存完好的沉积物。文章通过对一根长126cm的沉积柱开展多方面的沉积学分析,包括沉积物的岩性特征,反映碳酸盐含量的白度、CaO和LOI950℃以及反映有机质含量的TOC,TN,TS和LOI550℃等地球化学指标随深度的变化特征,沉积物和环境介质的碳、氮同位素的对比分析,沉积物的稀土元素和Sr,F,P,Se等元素地球化学特征。结果认为:该湖泊沉积剖面自上而下可分成3个具有明显不同沉积学特征的沉积单元,其中25～96cm的沉积层明显地受到了海鸟粪的影响。识别出的海鸟粪土沉积层,为进一步研究历史时期岛屿鸟类生态系统和气候演化提供了良好的信息载体。

The Cattle Pond on the Dongdao Island is a rarely observed freshwater lake in Xisha Islands of South China Sea. From the Cattle Pond, we collected a 126cm long and well-preserved sediment core that contains seabird feces. In this paper, we have carried out multiple sedimentological analyses including the lithological characteristics of sediments, white degree reflecting carbonate content, geochemical indices such as LOI950℃, TOC, TN, TS and LOI550℃, the carbon and nitrogen isotopic compositions and chemical element contents （REE, Sr, F, P, Se, CaO etc.） in the lacustrine sediments and environmental mediums. According to the in situ field observational data, as well as the sample-splitting documents in the laboratory, the sediment layer below 96cm is entirely composed of white coral, shell sand and gravel, and a large number of 2-10cm long coral rubbles are present in this layer. In contrast, the sediments above 96cm are of variable colors and evident lamination, and they contain a large amount of plant remains such as leaves, fruits and roots, and animal remains such as bones of seabirds and fishes, typical products deposited in a lacustrine sedimentary environment. The TOC, TN, TS and LOI55℃ values in the sediments below 96cm were much lower than those in the top 96cm sediments, indicating relatively high organic matter content present in the upper 96cm sediment layer. The depth of 96cm is a critical point, likely representing a sharp change in the sedimentary environment from lagoonal facies into lacustrine deposition. At the depth of 25cm, the notable decrease in the LOI950℃ and CaO contents contrasts the remarkable increase in the abundances of TOC, TN, TS and LOI550℃, implying significant contribution of greatly enriched organic matter materials such as cattle excreta, plant remains, and humus soils. The moderate content of organic matter in sediments from the 25cm to 96cm interval is most likely derived from guano input. This is supported by the geochemical characteristics of rare earth elements （REE） and some inorganic elements such as Sr, F, P, Se, CaO etc. and carbon and nitrogen isotopic compositions. These geochemical analyses suggested that the sediments between 25cm and 96cm are notably influenced by the seabird droppings, for the upper 25cm sediments, however, the influence of seabird guano, likely still present, is diluted by the large amount of organic matter input from cattle excreta and humus soil, and thus becomes insignificant. These results could lay groundwork for future and large temporal scale studies, such as the reconstruction of palaeoclimate and the seabird occupation history on the islands of South China Sea.