Sr/Ca,Mg/Ca:珊瑚岛鸟粪沉积的物源指示计*

文章尝试用Sr/Ca,Mg/Ca比值作物源指示计,区分了珊瑚岛屿受到海鸟粪影响的湖泊沉积物中鸟粪、珊瑚砂、植物这几种主要的环境介质对沉积物的相对贡献,据此恢复1800年来西沙群岛东岛海鸟数量变化和植被演化的历史。结果表明,东岛海鸟数量的变化历史可划分为潜伏期、增长期、稳定期和衰退期4个阶段,历史时期东岛海鸟生态系统的建立和发展与岛屿植被的演变有着密切的联系。研究获得的方法和初步结论为进一步开展西沙岛屿海鸟数量变化、植被演化的广泛对比并探讨与气候变化、人类活动的相互联系提供了基础。

USING Sr/Ca AND Mg/Ca RATIOS AS SOURCE INDICATORS OF ORNITHOGENIC LACUSTRINE SEDIMENTS ON CORAL ISLAND

Sr/Ca and Mg/Ca ratios in tropical corals are commonly applied to reconstruct sea surface temperature (SST), in this study, however, they have been used as indicators of sedimentary material source in the lacustrine sediments influenced by seabird droppings on coral island. Using 11cm diameter PVC tube, a lacustrine sediment core with a length of 126cm was collected from a fresh water lake "Cattle Pond" on Dongdao Island(16°39'~16°41'N,112°43'~112°45'E)South China Sea. Lithology and chronology analyses indicated that the sediment core was well preserved without human activity disturbance and the calibrated ¹⁴C-age as a function of depth is described by the equation: Depth (cm) =0.0717 ¹⁴C-age (cal. aB.P.) (r=0.99, n=5). Linear extrapolation of the equation gives an age of 1750cal. aB.P. for the bottom sediments. Down-core variation profiles of grain-size textures, loss in ignition at 950℃ (LOI_950℃) and 550℃ (LOI_550℃), total organic carbon (TOC), Total nitrogen(TN), as well as some geochemical element concentrations such as Sr, Ca, Mg were analyzed. Furthermore, Sr, Ca and Mg concentrations in the sediment media around the Cattle Pond (e.g. coral sand, plant, cattle excrement and seabird dropping) were also examined. The results showed that the ratios of Sr/Ca and Mg/Ca in the plant, coral sand and seabird dropping on Dongdao Island are quite different from each other, thus these three major sediment-medium involved in the lacustrine sediments could be distinguished by Sr/Ca, Mg/Ca ratios. Using three end-member mixing model, we have calculated relative contribution of seabird dropping, coral sand and plant remains to the ornithogenic lake sediments. The change of seabird dropping input can be used to measure the relative size of historical seabird population. Based on above calculation, we have reconstructed the historical change of seabird populations on the Dongdao Island over the past 1800 years; the results are in good accordance with our previous study by the method of elemental geochemistry. On the whole, the development of seabird ecosystem over the past 1800 years can be divided into four stages. Stage Ⅰ : latent period from 1750aB.P. to 1300aB.P, this stage is characterized by great variation in seabird population, and the population is less than 10000 pairs; Stage Ⅱ : development period between 1300aB.P. and 900aB.P, in this stage the seabird population increased rapidly and reached a peak at about 900aB.P up to 30000 pairs; Stage Ⅲ: stable period (900~280aB.P.), this stage was characterized by slow declines in seabird population from 35000 to 25000 pairs, and its change showed about 200 years cycle; Stage Ⅳ : waning period (280aB.P.~present) , compared with the above three stages, the seabird population in this stage showed much quicker decline and greater variation amplitude, and the present population, about less than 25000 pairs, seemed to be a trough in the overall change curve of historical seabird population. Further research indicated that the establishment and development of the seabird ecosystem had a close association with the evolution of the vegetation on the Dongdao Island. The present results could lay the groundwork for further studies on the inter-relationships among seabird population, climate change and human activity on the Xisha Islands and could provide a effective geochemical method for palaeoecological reconstruction on coral islands.