应用硅藻释读南海晚第四纪以来的古环境

【作者】 孙美琴；

【导师】 蓝东兆； 曹志敏；

【作者基本信息】 中国海洋大学 ， 海洋地质， 2009， 博士

【摘要】 本论文通过对取自南海的62个站位的表层样品和3个柱状岩芯进行了硅藻、沉积物粒度和地球化学分析,结合14C测年结果和已有的研究资料,查明了该海区硅藻的种类组成及分布特点,探讨了硅藻分布与现代海洋环境之间的关系,进而对南海西、南部晚第四纪以来古海洋环境演变进行了较为详细的研究。本次从南海表层沉积物和南海西、南部两个柱样沉积物中,共鉴定到硅藻272种和变种、变型,隶属57属,并附27个图版。其中记录2个新种和6个我国首次记录的种。新种是双角缝舟藻四角变型Rhaphoneis amphiceros f. tetragona Sun et Lan和珠网斑盘藻Stictodiscus arachne Sun et Lan;新记录种分别是Asterolampra grevillei, Dictyoneis marginata, Plagiogramma Papille, Rutilaria radiate, Triceratium contumax和Triceratium suboffieiosum。南海表层沉积硅藻种类丰富,以热带外洋种为优势,伴有一定量的热带—亚热带近岸种和广布种。优势种类为非洲圆筛藻、结节圆筛藻、柱状小环藻、楔形半盘藻、具槽直链藻、海洋菱形藻、方格罗氏藻、菱形海线藻、离心列海链藻以及长海毛藻。其中,热性种结节圆筛藻在南海分布最广,是主要的优势种类。由于各海区环境因素的差异,硅藻遗壳含量分布是不同的,总体上,从陆架至深海盆,其数量呈递增趋势。沉积硅藻的分布受海底地形地貌、水动力、沉积速率、水文气候等环境条件综合作用的影响。根据沉积硅藻中具有指示意义的硅藻种的分布和生态变化,可将该海区分成6个硅藻组合带。各组合分别反映不同的海洋环境,其分布主要受到海洋环流的影响,表现为黑潮暖流、印度洋暖水的入侵以及沿岸流对南海表层沉积硅藻分布的影响。其中,Coscinodiscus africanus、Coscinodiscus nodulifer、Hemidiscus cuneiformis和Nitzshia marina等热性硅藻种类可作为黑潮暖流及印度洋暖水入侵南海的指示种。而Cyclotella stylorum、Cyclotella striata、Melosira sulcata、Diploneis bombus、Diploneis crabro和Trachyneis antillarum等则可作为判断沿岸流对南海水域影响强度的指示种。沿岸种具槽直链藻在半深海一些区域的大量出现,可能是受到沿岸水的入侵,也或者是受到浊流沉积搬运的影响。长海毛藻Thalassiothrix longissoma在深海沉积物中大量出现可以作为南海水体高初级生产力的指示种。对南海西、南部的两个柱状样SA13-76和SA08-34沉积硅藻研究表明,两孔的硅藻均呈现氧同位素旋回变化特征。根剧表层沉积硅藻研究结果可以认为在上升流作用的高生产力区,历史时期的沉积硅藻相比钙质生物能更好地反映古季风演变。南海西、南部海区明显存在冰期时夏季风弱,全新世夏期风强的特点。季风是驱动上升流加强的主要因素。不论在冰期亦或全新世,南海夏季风均存在不稳定性和旋回性的特征。综合分析地球化学指标和微体古生物指标认为南海西、南部的古生产力演化趋势在氧同位素1、3、5期表现为高的特征,2、4期表现为相对较低的特征。对SA08-34孔各古生产力指标对比研究认为,硅藻和有孔虫、CaCO3在MIS3期显著的差异,可能是因为有孔虫和CaCO3均发生了较强的溶解作用。也可能与影响各个指标变化的主要因素不同有关,比如有孔虫受季风气候影响的程度可能比硅藻要小。古生产力演化过程的主要控制因素推测主要受季风影响。本次研究的三个柱状样中记录的一些气候突变事件,初步认为可能与首先在北半球高纬地区发现的YD事件及H事件(H1-H5)分别有关。根据SA08-34和SA09-90钻孔研究分析,推算南海南部末次冰盛期大约在13.1～13.4kaBP左右结束而进入冰消期。综合南海西、南部三个钻孔所揭示的沉积环境演变非常一致,三个钻孔反映的南海西、南部的古生产力、古季风、古气候演变趋势大致相同,初步认为晚第四纪以来南海西、南部气候均主要受东亚夏季风控制。更多还原

【Abstract】 The South China Sea (SCS) is one of the largest marginal sea of the West Pacific Ocean, which plays an important role in the monsoon climate variation of the East Asia. Diatom is one of the dominant groups found in marine. The ecological distributions of diatoms are closely related to physical, chemical and hydrological properties of the water masses and controlled mainly by ocean circulation. The diatoms in sediments can provide lots of information on paleoceanographical environments and paleoclimate variations, which contain invaluable data for the global environmental changes and climatic processes.The present study is based on a quantitative analysis of diatom from 72 water samples and 71 surface sediments samples collected in the SCS, and integrated with grain size as well as other relevant information. This paper present the influence of the coastal currents in SCS and the intrusion of Kuroshio Current and Indian oceans warm water in the paleoenvironment, discussed the preservative mechanism of the diatom remains under the ocean environment. The results are shown as follows:The diatom species composition and cell density were investigated from water sample collected in summer of 2007 in South China Sea. In total,35 diatom species belonging to 28 genera were identified. The dominant planktonic diatoms in South China Sea were Thalassionema nitzschioides, Pseudo-nitzschia pseudodelitissima, Pseudo-nitzschia pungens, Melosira sulcata, Pleurosigma naviculaceum, Chaetoceros spp. etc. The species composition and cell abundance of phytonic diatoms were influenced by water temperature, salinity and nutrient etc.Diatom analysis of 71 surface sediments samples shows rich diversity of 256 species belonging to 56 genera. The dominant groups were:Chaetoceros messanensis, Coscinodiscus africanus, Coscinodiscus nodulifer, Cyclotella stylorum, Ethmodiscus rex, Hemidiscus cuneiformis, Melosira sulcata, Nitzschia marine, Roperia tesselata, Thalassionema nitzschioides, Thalassiosira excentrica and Thalassiothrix longissoma etc. The hot water species Coscinodiscus nodulifer was the dominant species in the surface sediment. A diatom genus-Rutilaria, was recorded for the first time in China. And 13 new recorded species were observed:Asterolampra grevillei, Biddulphia turrigera, Cocconeis citronella, Cocconeis ocellata, Cocconeis cyclophora, Dictyoneis marginata, Entogonia davyana, Nitzschia campechiana, Plagiogramma Kinkeri, Plagiogramma Papille, Rutilaria radiate, Triceratium castelliferum, Triceratium contumax. The distribution pattern of diatoms reflected the difference of environmental conditions. The concentration of diatom in surface sediments varied between 71 and 623,438 valves pergram of dry sediment, the abundances of diatom frustule increased sharply from littoral to deep sea basin area. Diatoms distribution in surface sediments was influenced by submarine geomorphology, hydrodynamics and hydroclimate etc.Seven diatom assemblages were distinguished and the distribution of these assemblages is correlated with oceanic current patterns in the region. AssemblageⅠ: Melosira sulcata-Coscinodiscus nodulifer-Pyxidicula weyprechtii-Diploneis spp.;Ⅱ: Cyclotella stylorum-Melosira sulcata-Coscinodiscus nodulifer;Ⅲ:Coscinodiscus nodulifer-Thalassiothrix longissoma-Roperia tesselata;Ⅳ:Coscinodiscus nodulifer-Roperia tesselata-Thalassiothrix longissoma;Ⅴ:Coscinodiscus nodulifer-Thalassiosira excentrica-Roperia tesselata;Ⅵ:Coscinodiscus nodulifer-Melosira sulcata-Cyclotella stylorum;Ⅶ:Coscinodiscus nodulifer-Trachyneis antillarum-Hemidiscus cuneiformis. These assemblages may be of great importance for future palaeoceanographic studies.After dissolution, transfer, sediment and resediments, The diatom from sediments can reflect the composition of plantonic diatom groups partially.In paleoceanographic reconstruction of the area, Coscinodiscus nodulifer, Coscinodiscus africanus, Nitzshia marina, Hemidiscus cuneiformis may be used as indicators of the warm Pacific Ocean water, perhaps including the Kuroshio Current flowing into the SCS. Whereas Cyclotella striata, Cyclotella stylorum, Diploneis bombus, Diploneis crabro, Melosira sulcata, Trachyneis antillarum could be used to identify the influence of the coastal currents in SCS. And the occurrences of Thalassiothrix longissoma might be used as a signal of high primary productivity conditions.更多还原