Reconstruction Of The Paleoenvironmental Changes In The Southern Okinawa Trough Since The Mid-Holocene

The Okinawa Trough is located between the East China Sea and Ryukyu Islands, an area heavily influenced by variations in Pacific ocean circulation. As the major transport link of the West Pacific and the East China Sea, the Okinawa Trough is very sensitive to climate changes in both oceans and land. Under the control of the Pacific circulation and the East Asia monsoon, sediments from oceans and land are transported and deposited in this deep sea basin. And high-resolution records in sediment cores retrieved from the Okinawa Trgouh provide much more good materials for palaeoclimatic and palaeoceanographic research. The principal aim of this thesis is to reconstruct the palaoceanography change in the southern Okinawa Trough during the mid-late Holocene, based on a high-resolution diatom record.Monsoon is a typical phenomenon in low to mid-latitudes which plays a very important role in the reginal climate change, environment, culture, politics, economy as well as the way of people's living. Many studies have been done to reconstruct the East Asian monsoon variability during the Holocene, with samples from both inland and deep seas. An acknowlaged point is that the East Asian monsoon have declined gradually during the Holocene, which affects the palaeoenviromental change in the monsoon area. Besides the East Asian monsoon, however, precipitation in the monsoon area is also controlled both by the migration of the west Pacific Subtropical High(WPSH) and intertropical convergence zone (ITCZ), as well as the topography of the study region.Diatoms are microscopic algae found virtually everywhere that water is present, and each habitat develops its own characteristic diatom flora depending on the chemical and physical environment. This thesis attempts to reconstruct the summer sea-surface salinity (SSSs) in the southern Okinawa Trough since7000cal. yr BP, based on diatom-environment transfer functions and in combination with the fossil diatoms in sediment core MD05-2908from the southern Okinawa Trgouh. Additionally, an extensive comparison was made between the monsoonal records from core MD05-2908and other sites from inland or deep seas. Primary conclutions are specified as follows:A total of109taxa had been identified from248sediment samples of core MD05-2908. Subtropical planktonic diatom taxa Thalassionema nitzschioides were the dominant diatoms. Species with low percentage, including freshwater species (Fragilaria spp., Amphora spp. and Gomphonema spp.), coastal species(Cyclotella spp. and Diploneis spp.) and tropic planktonic species(A. marina, R. bergonii, A. neocrenulata/A.africana and A. nodulifera), were much more sensitive to temperature and salinity of sea water. Thus, changes in their percentage can be used to indicate the hydrographic change in the southern Okinawa Trough.A gradual increase trend has been shown in the reconstructed SSSs since7000cal. yr BP, based on the diatom-salinity transfer function. Good correlation between changes in abundance of coastal species and the SSSs indicates that Changjiang Coastal Current plays a significant role in the hrdrographical conditions in the southern Okinawa Trgouh through the last7000years. It also corresponded well with the declining East Aisan summer monsoon over the Holocene, reflected by a δ18O record from Dongge Cave, in Southwest China.During7000～4000cal. yr BP, low SSSs suggested less-saline conditions in the southern Okinawa Trough than present, corresponding to the Holocene Climatic Optimum (HCO). Compared to high abundance of coastal species at this period, freshwater species were rarely found in the study area. This implies a low precipitation in Taiwan at the interval7000～4000cal. yr BP. In addition, the abundance of the Kuroshio indicator species increased with some fluctuations, indicating that the Kuroshio have a strong impact on the study area since7000cal. yr BP. A siginificant hydrographical anomaly in the southern Okinawa Trough was suggested by an obvious increase in SSSs during4000～3000cal. yr BP. Hereafter, the reconstructed SSSs values were above33.6%o, possibly resulted from the weakended Changjiang Coastal Current, corresponding to decrease in abundance of coastal species. A still positive impact of the Kuroshio on the study area was indicated by a relative high abundance of the Kuroshio indicator species between4000-3000cal. yr BP. Thus, the PME event (4500-3000cal. yr BP) in the northwest Pacific marginal seas was abscent in this study. The hydrographical anomaly during4000-3500cal. yr BP could be attributed to the formation and development of modern ENSO system.The character of SSSs variability over the past1000years were different from the variability through the whole mid-late Holocene. Instead of coastal species, there is a good correlation between changes in the SSSs and abundance of freshwater species. This suggests that freshwater discharge from Lanyang River, in northeastern Taiwan, was the major factor to make the study area less-saline. Thus, significant decrease of SSSs could be explained by high precipitation in northeastern Taiwan. During AD930-1450, the abundance of the Kuroshio indicator species increased fluctuantly, while the freshwater species decreased, showing that the influence of the Kuroshio Current was intensified at that interval and the precipitation of the study area was relatively low. The Kuroshio indicator species decreased remarkably and was maintained at a low abundance during the interval of AD1450-1900, which suggests that the impact of the Kuroshio Current on the southern Okinawa Trgouh weakened during the period corresponding to the Little Ice Aage (LIA). Moreover, the high abundance of the freshwater species at the same interval indicates a distinct increase in precipitation in northeastern Taiwan, which may be correlated to the south-detention of the rainfall belt in China caused by the southward migration of the western Pacific subtropical high. Spectral and wavelet analyses of the reconstructed SSSs indicated that the time series of last7000years sequence is dominated by the1425-year periodicity, corresponding well with the "1500-year" cycle recored in North Atlantic. Thus, the reconstructed SSSs in this thesis also inducate the significant influence of solar output. During the last1000years, two dominant periodicities of SSSs centred around～27and ～75years, correspond reasonably well with the dominant periodicities associated with the Pacific Decadal Oscillation (PDO). This suggests that hydrographic changes in the southern Okinawa Trough could be related to the PDO.