北部湾北部中全新世硅藻及气候变化研究

156 sea surface diatom sediments will be analyzed from the north part of the Gulf of Tonkin(Beibu Gulf) in order to obtain the distribution of diatom assemblage which might be used to correlate with the mainly environmental factors, as sea surface temperature, sea surface salinity and oceanic productivity, etc. So the diatom index to certain environmental factors will be defined and the data base of diatom-environmental factors and a suitable diatom-sea surface temperature/salinity trasform function will be also set up for the north part of the Guf of Tonkin. Based on the variety of diatom assemblage in 150 core sediments from 2 high-resolution sea cores located in the north part of the Gulf of Tonkin, this program will discuss the variety of mainly environmental factors as paleo-temperature, paleo-salinity, marine productivity, runoffs, tides, offshore sea circumfluence, etc. From the study of sea surface diatom assemblage and the suitable diatom-sea surface temperature/salinity trasform function for the north part of the Guf of Tonkin, the quantitative sea surface paleo-temperature and paleo-salinity will be acquired which may be used to reconstruct the climate and environmental changes from the Mid-Holocene and to discuss the relationships with global change by comparing with the results of previous studies.

本项目将系统分析北部湾北部海域156个表层样硅藻种类的分布规律，据此探讨本区域主要海洋环境因子对硅藻组合面貌的影响及相互之间的关系，以确定对主要海洋环境因子（海水表层温度、盐度、海洋生产力等）有指示意义的硅藻指标，建立北部湾北部海域现代硅藻-环境变量数据库，获取适合对本区域硅藻数据进行古温度、古盐度计算的转换函数。 本项目还将分析本区域2个高分辨率钻孔（计150个样品）中全新世以来硅藻组合特征，根据硅藻组合中各主要成分的变化，探讨本区域如海水古温度、古盐度、古海洋生产力、入海河流、潮汐及近岸洋流等主要海洋环境因子的变化规律。在表层硅藻组合数据和建立起的适宜本区域的硅藻转换函数基础上，定量分析本区域海水古温度、古盐度信息，重建本区域中全新世以来古气候与古海洋环境，对比前人研究成果，探讨本区域中全新世以来海洋环境变化与全球变化的关系。

本项目分析了136个北部湾北部表层沉积硅藻样品，共鉴定出硅藻226种，分属59属。其中Thalassionema nitzschioides含量最为丰富。结合海水表层盐度、海水深度和距岸线距离3个环境因子，利用典范对应分析（Canonical Correspondence Analysis，CCA）方法，得出以下结论：1）影响表层沉积硅藻分布的最主要因素是海水表层盐度；2）Achnanthes delicatula、A.fugei、A.hauckiana、A.levanderi、Amphora exigua、A. eximia、A. holsatica、A. normanii、Nitzschia ovalis、Navicula minima和N. minuscula可以作为低盐度海水环境的指示种，Cocconeis disculus可指示半咸水环境，Cyclotella striata、C. stylorum、P. sulcata、Thalassiosira eccentrica、T. leptopus、T. oestrupii、T. frauenfeldii和T. nitzschioides则是很好的高盐度指示种；3）钦州湾外湾海水表层盐度较高，珍珠湾由于更加的封闭，海水表层盐度最低。.由于Q24与Q43钻孔过于近岸，过高的沉积速率导致沉积样品中硅藻数量稀少，对2个钻孔沉积硅藻样品均做了额外的重液浮选工作，Q24钻孔浮选后硅藻依然不可见，Q43钻孔硅藻较稀疏。项目完成了对Q43钻孔65个沉积硅藻样品的鉴定工作，共鉴定出硅藻69种，分属32属。相对表层样品，硅藻种类稀少，且大多数是抗溶性较强的硅藻属种。其中C. striata、C. stylorum和P. sulcata这3种硅藻，数量就占据了2/3以上，且种属间比例差异不大。项目组认为，Q43柱状样沉积物硅藻组合面貌受沉积速率过高的影响，不能体现5500 yr BP以来的区域环境变化。