【作者】 李传顺；

【导师】 李安春；

【作者基本信息】 中国科学院研究生院（海洋研究所） ， 海洋地质学， 2009， 博士

【摘要】 作为大陆向大洋的过渡带,由于享有得天独厚的沉积环境,具有独特的构造特征以及与黑潮主流之间的密切关系,一直以来,冲绳海槽都是中外学者研究的重点靶区。2005年5月,由我国与法国联合主持的IMAGES航次在台湾东北海域获取MD05-2908柱状岩芯（24o48.04′N,122 o29.35′E,水深为1275米）,该柱状岩芯为一34.17米长高质量的连续沉积记录,岩性以深灰色粘土质粉砂为主,含水量较高,粘性、可塑性强,含有数层厚度不等的夹层。岩芯年龄模式依据17个AMS 14C定年数据建立,岩芯底部年代约6.8ka,为中全新世以来的沉积。在实验室对样品按照2cm的间隔进行分割后分别进行了粒度分析、粘土矿物提取与测试、碎屑矿物提取与鉴定、常微量元素和稀土元素分析等实验。粒度分析结果显示,MD05-2908岩芯沉积物粒度垂向上总体比较均一,以细颗粒的粘土与粉砂质为主,但不同层位也稍有差别,表现为底部层位粒度较粗,含砂量较高,说明底部沉积环境比较复杂。粘土粒级（<2μm）矿物主要由四种粘土矿物和少量石英、长石碎屑组成。其中,粘土矿物相对含量变化中,伊利石（～68%）与绿泥石（～17%）构成主要成分,含有蒙皂石（～10%）和高岭石（～5%）。结合台湾东北外海表层沉积物的研究,利用粘土矿物伊利石/蒙皂石和绿泥石/高岭石比值得出岩芯粘土矿物主要为陆源碎屑粘土矿物,其源岩主要为台湾中央山脉的变质岩与台湾东部的沉积岩。重矿物分析共选取了41个层位,对63～250μm粒级的样品在实体镜和偏光显微镜下进行鉴定,结果显示,岩芯重矿物主要由绿泥石（29%）、普通角闪石（22%）、白云石（10%）、黑云母（8%）、绿帘石（7%）、白云母（7%）、褐铁矿（5%）等组成。稳定矿物少,矿物成熟度低。碎屑矿物风化程度低,磨蚀不明显,分选较差,表明沉积物来自于近源,后期改造作用不明显。常量元素分析结果表明,SiO₂、Al₂O₃和Fe₂O₃是岩芯沉积物中的最主要组分,这三种组分占沉积物总量的82%左右。整个岩芯自下而上各常量组分变化不大,其平均值与东海陆架沉积物基本接近。微量元素变化比较明显, Ba、Cr、Cu、Zn元素的含量比东海陆架沉积物中的含量要高,而Sr的含量明显低于东海陆架。对常微量元素的R型因子分析表明,常量元素SiO₂、Al₂O₃、Fe₂O₃、MgO和K₂O,微量元素Cr、Cu、Ni、Zn、Pb、Rb和Mn可代表陆源物质;常量元素CaO和微量元素Sr、Ba可代表生物源物质。岩芯沉积物以陆源物质为主,生源物质的贡献起次要作用。岩芯沉积物中稀土元素总量平均为169.87×10^-6,并且轻稀土含量均高于重稀土,LREE/HREE平均值为10.14,表明了轻稀土对稀土总量的贡献远高于重稀土,沉积物富集轻稀土,反映了沉积物的陆源特征。岩芯MD05-2908中全新世以来平均5m/ka的高沉积速率主要源于丰富的物质供应和适宜的沉积环境。岩芯细粒级沉积物中,地球化学特征表明沉积物主要来源于陆源碎屑物质,粘土矿物特征与台湾东部陆源物质相同;粗粒级沉积物中,重矿物含量及矿物特征也表明岩芯沉积物粗颗粒组分主要来自于近源沉积。台湾宜兰境内的兰阳溪每年携带约一千万吨冲积物入海成为研究区重要的物质来源。由于受到黑潮的强烈影响,逆时针涡流及底层反向流的存在是岩芯高沉积速率重要控制因素。因此,利用动力分选的粉砂组分可以用来示踪古洋流强度,结果显示,6.8ka以来黑潮的强弱波动频繁,并表现出一定的旋回性变化,频谱分析表明,其具有的千年尺度周期（1500a）、百年尺度周期（604a、242a、192a、153a、133a）与十年尺度周期（22a）的周期性变化均与太阳辐射量变化有密切关系,因此,黑潮的强弱变化在大背景上是由太阳活动所控制的。根据测年资料可以识别出岩芯存在5期快速堆积事件,这与区域性降水增加有关,降雨量增加导致陆源物质输入的增加。另外,岩芯位于大陆斜坡区,附近存在有三支海底峡谷,并且地震活动频繁,沉积在宜兰陆架及东海陆架处的浅海沉积物由于受到地震、风暴等活动的影响而受扰动崩塌、因重力作用而向低处输送,产生二次侵蚀并经由海底峡谷搬运到冲绳海槽南段堆积,使得沉积环境更为复杂,但同时也为冲绳海槽提供了丰富的物质供应。更多还原

【Abstract】 Owing to its location, geomorphology and hydrodynamic conditions, the southernmost part of the Southern Okinawa Trough （SOT） acts like an efficient receptacle for sediments from Taiwan and the East China Sea shelf. The region is also known for the Kuroshio Current, which turns northwestards and brances off from its mainstream before continuing to flow to the North Pacific. Between 29 May and 29 May 2005, one core was drilled at Site MD05-2908 （24o48.04′N, 122 o29.35′E） off northeast Taiwan during IMAGES XII-MD147 Marco Polo1. The site is located on a gentle topographic high on the southern slope of the southern Okinawa trough at 1275 m water depth. Site MD05-2908 was designed to obtain a high resolution record of paleoceanographic changes of the Kuroshio Current during the latest Quaternary. The core is 34.17m long with high water content; the sediments in core MD05-2908 consist of homogenous dark gray hemipelagic clay and fine silt-sized sediments with three insignificant thin layers of very fine sand and silt. The depth-age relationship of core MD05-2908 is well constrained by the seventeen AMS14C dating points and the 34.17m core has a continuous record of the past 6,800 years. The so high-resolution record is a good material for research the millennial to decade scales of the historic climate change.Grain size analysis shows that, the sediment of core MD05-2908 is mainly made of clayey silt, generally uniform but small changes in different depths.Subsection comparision indicates that the grain size coursing downwards gradually, especially in the bottom where sand in unignored. It probably reveals the enhancement of terrigenous clastic material. XRD diagrams display different minerals including 4 clay minerals and few quartz and feldspars in <2μm fraction of sediment. In descending order of abundance, the clay mineral assemblage consists of illite （～68%）, chlorite （～17%）, smectites （～10%） and kaolinite （～5%）. In eastern Taiwan, illite and chlorite are abundant in widespread slates and schists as well as in soils, whereas kaolinite and smectites only occur locally. The dominance of illite, chlorite, and unaltered feldspars as inferred from geochemical data suggests that the immature nature of sediments and sedimentary rocks is probably a result of low residence times in the source region or river basin and quick removal of materials from the soil profile by steep, mountainous rivers （physical weathering dominates）. The well-known earthquake and storm triggered extremely high physical weathering rate in Taiwan is consistent with these results.We selected 41 layers for heavy minerals analysis, and the heavy minerals consist mainly of Chlorite（29%）、Hornblende（22%）、Dolomite（10%）、Biotite（8%）、Muscovite（7%）、Epidote（7%）. The sediments in the core are weakly weathered, lightly abraded and bad sorted, which is derived from near source and did not experience seriously transformation by ocean. The content of stable mineral is small and mineral maturity index is low. Based on studies of detrital minerals it is unraveled that the coarse-grained sediments are of single and stable source and have the same trait as intermediate-acidic granite and metamorphites.Major elements analysis shows that the main components on the sediment core are SiO₂, Al₂O₃ and Fe₂O₃, which account for as much as 82%. There is no obverse change in the core, which is quite similar with the value of sediments from the East China Sea shelf. According to the distribution of elements and the results of the R-model factors analyses, the sediments of the South western Okinawa Trough are mainly composed of terrigenous composition, such as SiO₂、Al₂O₃、Fe₂O₃、MgO、K₂O、Cr、Cu、Ni、Zn、Pb、Rb and Mn, and biogenic compositions, such as CaO、Sr and Ba. The elements are devided into dirrerent groups, combined with the minerals distributions, every elements group are given a different source indication.The sediments span across the mid-Holocene （6.8 ka B.P.） and have remarkablely high sedimentation rates between 1.8 and 21.2 m/ka, which is well consistent with the modern observations from sediment traps. We identified five 70―200 a periods of abnormally rapid sedimentation events at 6790―6600 a B.P., 5690―5600 a B.P., 4820―4720 a B.P., 1090―880 a B.P., and 260―190 a B.P., during which the highest sedimentation rate is up to 21.2 m/ka. As the most significant sediment source, the Lanyang River in northeastern Taiwan annually deliver about 10Mt materials to the coastal and offshore region of northeast Taiwan, a portion of which could be carried northward by currents toward the study area. Therefore, we concluded that the 5 abnormally rapid sedimentation events may be related to intensified rainfall in Taiwan and thus increased materials to our study area at that time. However, a few extreme-rapid sedimentation events cannot be explained by normal river runoff alone. The large earthquakes or typhoons induced hyperpycnal discharge of fluvial sediment to the ocean may also act as a potential source supply to the Okinawa Trough. Mass wasting processes, seafloor morphology and grain sizes play important roles in transporting sediment from near shore to deep water. Frequent earthquakes, typhoon-induced floods and ocean currents add to the complexities of sedimentary processes on the southwestern Okinawa Trough.更多还原