Evolution Of Seabird Eco-environment On Xisha Islands Of South China Sea

Using the basic principles and methods of Eco-geology to focus on the special bio-geochemical circulation of "ocean→seabird→coral island", in this thesis we have studied the development and evolution of the ecosystems on three coral islands of Xisha archipelago, South China Sea, over the past ～2000 years, and have reconstructed the historical record of seabird population change on these islands. Furthermore, some influencing factors on the seabird ecosystem in the sea area of Xisha Islands such as seabird food supply, climate change and human activity were also discussed. Through these studies, we have obtained some new finding on the changes of ecosystem and seabird ecology on coral islands presently or previously occupied by numerous seabirds. Main contents and research results are given as follows:â…  According to the study on the interrelation among soils, plants and birds, we found that elemental concentrations in surface soils on Dongdao Island were mainly controlled by guano abundance contained in the surface soil, and their spatial distribution was characterized by circular pattern around the island, in consistent with the distribution pattern of plants on this small island. The elemental contents in the plants were closely associated with these in the soils, indicating that the plant distribution was likely to be mainly controlled by elemental contents in the soils, and it provided optimal habitat for the seabirds on Dongdao Island.â…¡ Based on the analyses of chemical element concentrations and C, N stable isotope compositions in the lacustrine sediments of Cattle Pond, we found new scientific evidence for the time when the wild cattle species were firstly brought to Dongdao Island and the history of island development. The main impact of wild cattle on seabird ecosystem was also discussed. Additionally, we performed detailed analyses of sedimentology and elemental geochemistry on the sediments of Cattle Pond. The results showed that the upper 96cm sediments in the DY4 core from Cattle Pond were notably influenced by seabird droppings. Using the characteristic assemblage of bio-elements and the element ratios in the ornithogenic lacustrine sediments as indicator of guano content, an indirect proxy for the relative seabird population, together with the accurate AMS¹⁴C dating on organic matters, we have reconstructed the historical record of seabird population change over the past 1800 years. On this base, we have discussed the inter-relationships among seabird population, plant development, climate change and human activity.â…¢ During the field investigation on Dongdao Island, we observed cemented coral calcarenite not only in the central areas, but also on the northeastern shore. The coral calcarenite outcropped on the shore is cemented by either brown materials or white carbonate, its steep-sided outlier is present seaward of the cliff, and its thickness is up to ～5m. The brown cementation layers are covered by subsequent and relatively thin veneers of bioclastic sediments, which are cemented mostly by carbonate. In the laboratory, we collected typical samples from a seaward sediment sequence and performed petrological and geochemical studies including thin-section observation, powder X-ray diffractometry (XRD), major and trace element contents, scanning electron microscopy (SEM)/energy dispersive spectroscopy...