| With abundant environmental information, the diatom in sediment has been widely used to reflect the environment of the upper water. The distribution of abundance and species of the diatom assemblage in the surface sediment is synthetically produced by all the environmental factors in the upper water. Therefore, the relationship between diatoms and environmental factors could be built up, which is an important premise for using fossil diatoms to study paleoclimate and paleoenvironment.In the study area of the Yellow Sea and adjacent sea of Yangtze River, the species and abundances of fossil diatoms and silicoflagellates in surface sediment were analysed, and the environmental factors including temperature, salinity, nutrients condition and depositional environment were surveyed and measured. On this basis, the dissertation systematically showed the spatial distribution of diatoms and silicoflagellates in the surface sediment in the Yellow Sea and adjacent sea of Yangtze River, analyzed the main factors promoting the characteristics of fossil assemblage during the deposition process, screened out the environmental indicator species, and discussed the roles of these species in indicating the environmental factors as to reveal the environmental significance of the fossil assemblage.In the Yellow Sea and adjacent sea of Yangtze River, there were 97 out of 113 surveyed sites were identified with rich fossil siliceous valves, including 269 taxa-267 diatom taxa and 2 silicoflagellate species. In all the 97 samples, the absolute abundances of fossil are 0.6×103-646.9×103 valves/g DW, averaging at 30.1×103 valves/g DW, the Shannon-Weaver indices are 1.7-6.8, and the species richness indices are 1.9-5.4. According to Q mode cluster analysis with 29 taxa of higher proportions involved in, the fossil assemblage was classified into 3 biofacies and 5 sub-biofacies, namely, I(1), I(2), II, III(1), III(2), and III(3). Meanwhile, 10 taxa were screened out, which are Paralia sulcata, Podosira stelliger, Thalassiosira eccentrica, Coscinodiscus spp., Actinocyclus ehrenbergii, Cyclotella stylorum, Actinocyclus octonarius var. ralfsii, Actinoptychus senarius, Trachyneis spp., and Pleurosigma spp.. Amongst, Paralia sulcata distributed most widely with the highest proportion, followed by Podosira stelliger as the second.The spatial distribution of fossil assemblage mainly displayed an apparent “coastal-offshore-large river” pattern in the Yellow Sea and adjacent sea of Yangtze River, and in the sea areas of northern Yellow Sea, southern Yellow Sea and adjacent sea of Yangtze River, the fossil assemblages showed their distinct local characteristics. The absolute abundances of fossil diatom and silicoflagellate were generally higher in the offshore zone than that in the coastal zone, with the high values appeared in the northern Yellow Sea and the transection of 32°N-33°N, and the low values at the transections of 37.4°N and 34°N. The trends of Shannon-Weaver and species richness indices were opposite, as their high values were mainly in the coastal zone near to Yangtze River mouth, and their low values in the central area of southern Yellow Sea. Moreover, the distribution of biofacies also showed the “coastal-offshore-large river” pattern with more species in the coastal biofacies.The fossil assemblage in the Yellow Sea and adjacent sea of Yangtze River was able to reflect the biomass of planktonic assemblage to some extent. The high abundances of diatoms were always at the coast of Liaodong and Shandong peninsulas and the transection of 32°N-33°N, while the low values of both assemblages appeared at the Bohai Strait, the central area of southern Yellow Sea and the southern coast of Yangtze River mouth. However, the low abundances fossil assemblage at the south could not correspond to the high production in the upper water. Even more, the dominant species in the two assemblages were not the same either. This is because the light-siliceous species seldom deposit to be preserved in the sediment, and the benthic species hardly live in the planktonic assemblage to be dominant.The sedimentation rate and hydrodynamic effect were the key factors to influence the fossil absolute abundance in the Yellow Sea and adjacent sea of Yangtze River. At the south to Yangtze River mouth, the extremely high sedimentation rate, Yangtze plumes and coarse sediment together generated a very low abundance siliceous fossil in the sediment. At the coasts of Chengshan Cape of 37°N and Subei Shore, the extremely low sedimentation rates(nearly 0) resulted the less abundant siliceous fossil in the local sediments since there were few siliceous valves depositing. Besides, the relatively shallow depth also went against the deposition and preservation of siliceous valves.There were 6 species being screened out as the environmental indicator, namely, Coscinodiscus oculatus, Dictyocha fibula, Actinocyclus ehrenbergii, Cyclotella stylorum, Paralia sulcata and Podosira stelliger. The relative abundance of Coscinodiscus oculatus increased gradually from north to south, indicating the temperate difference between the north and the south and its latitudinal variation. The two exceptional high value of this taxon in the northern Yellow Sea may imply the influence of Yellow Sea Warm Current. All the proportions of Dictyocha fibula, Actinocyclus ehrenbergii and Cyclotella stylorum displayed the decreasing trends on the direction away from the coast. This indicated the ascent salinity gradient from coast to offshore, and showed the influence of Yangtze dilute water on the sea salinity. However, the inconsistency in the spatial distribution of the three species showed their growths and distributions were also control by other factors at the same time. The appearance of abundant Paralia sulcata in the northern Yellow Sea and transection of 32°N-33°N indicated that the concentration of nutrients is relatively richer in these areas, and that the nutrients level has been went up in the last 30 years through comparison with previous study.Through a wide survey of the siliceous fossil assemblage in the surface sediment in the Yellow Sea and adjacent sea of Yangtze River, the study analysed the spatial distribution of siliceous fossil assemblage, which fills the research blank of fossil diatom in surface sediment of this sea-area in recent 30 years. The study also supplied a reference for the research in using fossil diatom to reflect environmental changes, and gained data and experience for the further study on paleoclimate and paleoenvironment. |
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