【作者】 李冬玲；

【导师】 蒋辉；

【作者基本信息】 华东师范大学 ， 第四纪地质， 2012， 博士

【摘要】 冲绳海槽位于东海陆架与琉球群岛之间,作为我国东部边缘海和西太平洋之间的连接通道,冲绳海槽对海洋和陆地环境的变化反映敏感。冲绳海槽的沉积物同时保存着海洋和陆地环境的演化,是全球变化中海、陆记录的汇聚地,其保存的高分辨率沉积岩芯为区域古气候和古海洋环境研究提供了极佳的材料。对于冲绳海槽的古海洋学和微体古生物,前人已做了不少工作,但相关资料还是相当缺乏,有些方面尚未进行定量、系统的研究,因此,一个定量的高分辨率的古海洋记录显得意义重大。季风是全球中低纬地区的特有现象,对该区气候、环境、政治、经济、文化以及人们的日常生活等各个方面均存在非常重要的影响。有关东亚季风的演变,人们对陆上和深海都开展了大量研究工作,近年来越来越多的研究发现全新世以来东亚季风呈整体减弱的趋势,对季风区的气候变化产生了显著的影响,其中降雨量便是其中非常重要的一个表现方式。然而,东亚季风区的降雨量除了受到季风的影响以外,还受西太平洋副热带高压以及ITCZ位置的影响,甚至还与该区的地形有关。硅藻是一种生活在有水或潮湿环境中的微体生物,因物理、化学及水动力条件的差异,其种类、数量及组合特征都会有所不同。本文试图通过对冲绳海槽南部、台湾东北部MD05-2908孔7000cal. yr BP以来的沉积硅藻进行分析,利用硅藻-盐度转换函数,重建该区夏季海水表层盐度(SSSs)的变化；并与冲绳海槽其它区域以及中国其它地区已有的全新世气候和古海洋记录进行对比,综合讨论中全新世以来东亚季风的变迁以及冲绳海槽南部古海洋环境的演变。通过对MD05-2908孔7000cal. yr BP以来的沉积硅藻进行分析,共鉴定出硅藻种属109种,其中,菱形海线藻Thalassionema nitzschioides为该孔的优势种,且在整个7000cal. yr BP以来含量变化不大,为广布种。淡水种(Fragilaria spp., Amphora spp和Gomphonema spp.)、沿岸种(Cyclotella spp和Diploneis spp.)以及赤道热带浮游硅藻(A. marina, R. bergonii, A. neocrenulata/A.africana和A.nodulifera)的含量虽然相对较少,但因其对生长环境温度以及盐度均有一定的要求,因此,这些种的百分含量变化具有很好的环境指示意义,可以用来反映冲绳海槽南部水文条件的改变。利用硅藻-盐度转换函数重建的SSSs结果显示,冲绳海槽南部7000cal. yrBP以来SSSs,总体上呈升高的趋势.7000cal. yr BP以来SSSs与沿岸种的含量变化具有很好的一致性,表明中全新世以来冲绳海槽南部SSSs升高的趋势,主要与中全新世以来长江沿岸流的减弱和黑潮的加强有关。此外,冲绳海槽南部SSSs与董哥洞DA石笋δ180值变化的一致性验证了全新世以来东亚夏季风减弱的趋势。其中,7000～4000cal. yr BP期间,研究区海水偏淡,表明当时季风降雨丰富,沿岸流影响加强,对应“全新世大暖期”。此外,当时硅藻黑潮指示种的含量波动升高,表明黑潮暖流对研究区的影响一直存在且稳步增强。而淡水硅藻在该时期的低含量则表明7000～4000cal. yr BP期间台湾东北部的气候较为干旱。冲绳海槽南部SSSs在4000～3000cal. yr BP期间发生了一次大的转折,在这之前SSSs值均低于平均值,而之后则均高于平均值,表明随着东亚夏季风的减弱,冲绳海槽南部受沿岸流的影响减弱,表层海水盐度开始升高。此外,该时期内黑潮指示种的含量仍比较高,黑潮对冲绳海槽南部的影响显著,说明西太平洋边缘海的“PME”事件(4500～3000cal. yr BP)在本研究中并未反映。SSSs在4000～3000cal. yr BP期间的变化很可能暗示了当时冲绳海槽南部水文状况的一次重大转变,我们认为这与现代ENSO活动的形成与迅速发展有关。本论文还对MD05-2908孔上部近1000年来的沉积硅藻进行了高分辨率分析(平均分辨率为12年),并探讨了冲绳海槽南部近1000年来SSSs变化所反映的古海洋环境演变。结果表明,近1000年来SSSs的变化与淡水硅藻的分布特征有很好的关联,这表明该时段台湾东北部兰阳河的冲淡水对冲绳海槽南部海水盐度的变化影响显著。SSSs变化与模型模拟的台湾近1000年来的降水异常的比较亦揭示了二者的同步性。其中,中世纪暖期(AD930～1450)期间,黑潮指示种含量较高,SSSs普遍较高,反映台湾东北部降水较少,气候干旱；小冰期(AD1450～1900)期间,黑潮指示种含量明显降低,SSSs普遍下降,对应台湾东北部降水丰富或洪水事件频发,气候湿润。基于硅藻百分含量重建的冲绳海槽南部的SSSs反映整个7000cal. yr BP以来,SSSs的变化与沿岸种含量的关系密切,而1000cal. yr BP以来SSSs则与淡水硅藻含量的相关性较大,这可能暗示着近1000年来冲绳海槽南部环流水系的一次转变,即全新世以来不断增强的黑潮暖流阻止了沿岸流对研究区的入侵,使得沿岸流的冲淡作用被兰阳河所代替。具体原因,还有待进一步研究。冲绳海槽南部7000cal. yr BP以来夏季海水表层盐度变化存在着56年,70年,82年,110年,352年和1425年的周期,而近1000年来SSSs存在着27年和75年的周期变化。其中,1425年的长周期与北大西洋1500年的气候变化周期对应,说明冲绳海槽南部中全新世以来的古海洋环境变化主要受太阳辐射影响;而近1000年来27年和75年的周期与太平洋十年间涛动,即PDO事件有很大关联。更多还原

【Abstract】 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.更多还原