【作者】 刘明；

【导师】 范德江；

【作者基本信息】 中国海洋大学 ， 海洋地质， 2009， 硕士

【摘要】 长江是世界上第三大河流,受到自然过程和人类过程的双重影响,在其入海口处形成了规模巨大的现代三角洲沉积体系,高沉积速率及连续沉积使其成为进行高分辨沉积记录研究的理想区域。长江三角洲高分辨沉积记录的研究对深入了解河口三角洲沉积过程、三角洲沉积作用对自然过程和人类活动的响应具有重要的意义,并对世界上同类河口的研究具有普适性。基于取自长江三角洲前缘的柱状沉积物岩心(18站,岩心长225cm),进行了0.25cm(粒度样品)和0.5cm(其它分析)间隔的高密度取样,利用激光粒度分析仪、X射线荧光光谱仪、紫外分光光度计等仪器分析了粒度特征、元素组成和生物硅含量,结合X光成像技术和γ谱仪的210-Pb、137-Cs放射性活度测定,确定了该柱子的层序构成和形成年代。进而建立了该沉积物柱子的最近60a来的高分辨粒度、生物硅记录曲线,揭示了该记录的十年际、年际、季节性的周期性波动规律和伴随的非周期变化过程,探讨了长江三角洲沉积记录对太阳黑子活动、厄尔尼诺/南方涛动(ENSO)、季节性变化和沉积事件、人类活动的响应。研究表明:长江水下三角洲前缘18站沉积岩心较均一,连续性好。以粉砂质粘土为主,夹有粉砂薄层,发育水平层理,未见侵蚀间断面。该岩心沉积速率快,平均沉积速率为3.69cm/a,不同时段沉积速率略有差别,1986之前沉积速率为3.91cm/a,1986至今平均沉积速率有所减小,为3.21cm/a。建立了长江三角洲前缘最近60a来的高分辨沉积物粒度、生物硅含量曲线。平均沉积物粒径11.71μm,生物硅含量为0.831%。粒度记录、生物硅记录都显示了良好的周期性波动规律,存在11a,3-5a,1a等明显的周期。同时,它们的高分辨记录还伴随着非周期波动。此外,元素地球化学组成也显示了相类似的波动特征。周期性沉积记录与气候波动密切相关。1)沉积记录的11年周期对太阳黑子活动有良好的响应,显示了太阳活动对沉积作用的影响;与1月份北太平洋年代际涛动(PDO)的变化趋势相反,PDO的周期性变化对沉积作用的影响还需进一步验证。2)3~5年的周期对应于厄尔尼诺/南方涛动(ENSO)的周期,在整个周期的开始,厄尔尼诺刚发生时,沉积物粒度由较大值开始减小,之后拉尼娜的发生使其继续减小,到了周期的末期开始增大,呈现“U”型的周期性变化;生物硅含量则与粒度变化相反,由较低值开始增大,之后拉尼娜的发生使其继续增大,到整个周期的末期又逐渐减小。3)高分辨沉积层序的季节性沉积特征缘于长江输水输沙的季节性波动和东海水动力的季节性变化。研究发现夏秋季沉积物颗粒偏细、分选差,生物硅含量高;而冬春季沉积物颗粒偏粗、分选更差、各参数波动范围大,生物硅含量低。地球化学元素除了P、S、Cl的含量在两者之间基本相等,Si、Na、Ca、Ba、Sr、Zr的含量在冬春季节的沉积层中偏高外,其余元素在夏秋季沉积层中含量较高,主要受粒度变化控制。非周期变化则与事件过程和人类活动相关。1)该站沉积记录对较大的沉积事件有良好的响应。在大的洪水发生的年份,沉积物粒度有所减小,生物硅含量有所增加。在大的风暴潮发生的年份,平均粒径和生物硅含量变化不一,在平均粒径增大的年份,生物硅含量出现谷值;在平均粒径没有增大的年份,生物硅的含量出现峰值,这与风暴潮是否对底质沉积物造成扰动有关。2)人类活动对长江输水输沙及营养盐的输入造成重大影响。研究区沉积记录的变化在1986年之前主要受控于长江水沙的变化,1986年之后人类活动的影响更为显著,导致重金属和生物硅的含量呈现逐年增加的趋势。更多还原

【Abstract】 Yangtze River is the third largest river of the world. A large-scale modern delta is developing in its mouth, which is resulted from the impact of natural and human processes. The high deposition rate and the continuous deposition in the sedimentary squence make this area be a ideal region for the research of high-resolution sedimentary record. And the research has a great significance to understanding the response to natural processes and human activities by deposition and sedimentation of the delta. And it is a universal study for this kind estuaries in the world .Based on a sediment core (18 stations, core length 225cm) collected from the Yangtze River delta front, with high– resolution subsampling by sampling intervals of 0.25cm (for grain size samples) and 0.5cm (other samples), the characteristics of grain size, elemental composition and biogenic silica content were analysed with laser particle size analyzer, X-ray fluorescence spectrometer and ultraviolet spectrophotometer. Combined with X-photograph method andγspectrometer of 210-Pb, 137-Cs radioactivity meaurement, the sequence structure and age of the core were determined. After that, the high-resolution records of grain size, biogenic silica were established. They shew periodic changs of decade, annual and seasonallity and non-periodic changes. The relationship of sedimentary records and clamite changes, sedimneatry environment changes and human activities was discussed. The following conclutions were obtained:The lithology of 18 station on subaqueous Yangtze River delta front is homogeneous, the sequence is continous. The sediments are mainly silty clay, mixed with many silt interlayers, the stratification is horizontal, and have no erosion trace. The deposition rate of the core is fast, with average deposition rate of 3.69cm /a. It is differences between different times, the deposition rate is 3.91cm /a before 1986, and after1986a, it id reduced to 3.21cm /a.The high-resolution grain size and biogenic silica records curves of latest 60 years in Yangtze River Delta front is established. The average sediment size is 11.71μm, and the biogenic silica average content is 0.831%. Both of grain size and biogenic silica records show good cyclical fluctuations, having the cycles of 11a, 3-5a, 1a, especially. At the same time, the high-resolution record was also accompanied by non-periodic fluctuations. The elements geochemical composition also shows similar fluctuation characteristics.The cyclical fluctuations of sedimentary record are related to climate changes closely. 1) The 11-year cycle of sedimentary record is consistent well with the sunspot activity, demonstrating the impact on the deposition of solar activity; but has a opposite trend with the North Pacific decadal oscillation (PDO) in January, so the impact of PDO to sedimentation needs further authentication. 2) 3 ~ 5-year cycle is corresponds with the El Nino / Southern Oscillation (ENSO). The grain size has a "U"-shaped cyclical change, in the beginning of a cycle, just at the time of El Nino, sediment grain size decreases from a larger value, and continue to reduce followed by the occurrence of La Nina, but increases in the end of the cycle; the change of biogenic silica content is against to grian size. 3) The seasonal characteristics of high-resolution sedimentary sequence due to seasonal fluctuations of flow and sediment flux in the Yangtze River and the East China Sea hydrodynamic. In summer and autumn, the sediment is of find grain size, bad sorting and high BSi contents; in winter and spring, it is of coarse grain size, worse sorting, large fluctuation and low BSi contents. The contents of P, S, Cl are basically equal between the two kinds of sediment, Si, Na, Ca, Ba, Sr, Zr have higher level in the sediment layer of winter and spring, and the other geochemical elements contents are higher in the summer and autumn deposition layer, which is mainly due to changes of particle size.Non-periodic changes related to sedimentary events and human activities. 1) The sedimentary records have a good response of the deposition events on this station. In flood years, sediment particle size has been reduced, biogenic silica content increased. In storm years, the average grain size and biogenic silica content varied regularly. When the average particle size increases, the BSi content decreases. Constrary, when the average grain size did not increase, BSi content has a peak value. These flactuations were mainly caused by whether the sediment is disturbed by storm or not. 2) The human activities have a significant impact on the Yangtze River water sediment discharge and the delivery of nutrients. There is a little change in the Yangtze River runoff in recent years, but a sharp reduction in the flux of sediment. In the latest 50a, nutrient contents of N, P have been increasing, but DSi decreasing. The changes of BSi content and grain size in the study area are mainly controlled by the water and sediment discharge in the Yangtze River before 1986, and the impact of human activity is more prominent after 1986. As results, Bsi and heavy metals contents are increasing from 1986 year by year.更多还原