【作者】 庞仁松；

【导师】 潘少明；

【作者基本信息】 南京大学 ， 海洋地质学， 2011， 硕士

【摘要】 本文对长江口泥质区沉积中心18#站位的柱状样进行了粒度测试、210Pb、137Cs同位素放射性比度测试。通过分析210Pb和137Cs的剖面特征,以及它们在长江口地区的不同地球化学性质,揭示了区域的沉积侵蚀变化过程,并探讨了区域沉积速率减缓的主要原因。通过对柱样的岩性、粒度分析得知,沉积物柱样以黄灰色、灰黑色粘土质粉砂为主,局部有薄层细沙夹层。柱状样沉积物粒度指标表明,沉积物分选较差,且表层为极正偏,同时上部沉积物峰态较下部宽。沉积物组分以粉砂为主,其次是粘土,砂的含量极少,并且不含砾石。样品平均粒径在6.158～7.775φ之间,粉砂含量在58.71～79.71%之间,粘土含量在16.75～36.86%之间。137Cs、210Pb放射性活度与粘土含量的相关性表明,137Cs最大比度与相应层位粘土含量存在较好的统计相关性(R2=0.717),而210Pb与相应层位粘土含量之间没有明显的相关性,这可能说明了137Cs相对于210Pb更依赖于粘土吸附,210Pb在沉积物中的再迁移和重新分布作用较弱。由137Cs和210Pb同位素测年分析得到的站点现代沉积速率表明：近50多年来泥质区沉积速率较大,且呈现阶段性差异：由137Cs时标计年法得到柱样1954～1964年的沉积速率为5.9cm/yr,1964～2006减小为3.36cm/yr;沉积柱样的210Pb剖面呈两段分布,由此得到沉积速率120～225cm为5.47cm/yr,对应于18cm～100cm减小为4.58 cm/yr。对比两种研究方法得出沉积速率开始减小的时间为1968～1972年,这与长江入海泥沙通量变化的趋势基本对应,这有利地支持了长江入海泥沙量的减少可能是导致泥质区沉积速率减小的主要影响因素的观点。由210Pbex剖面计算的沉积速率变化趋势与含沙量的减小趋势并不完全相同,这说明长江来沙量并不是影响长江口泥质区沉积速率的唯一因素,也可能与港槽分水分沙比改变引起的沉积动力环境变化有关,需做进一步分析探讨。通过计算柱样核素的蓄积总量发现,其值远大于区域背景值,这说明水下三角洲泥质区沉积物中的核素主要由陆源侵蚀带来的核素输入贡献。结论与前人研究结果相一致,这也反映了虽然近二十多年来长江入海泥沙明显减少,但水下三角洲泥质区的核素仍然存在着净输入、幅聚作用。更多还原

【Abstract】 The 18" core collected in Yangtze River Estuary was analyzed for 137Cs、210Pb radioisotope measurement and particle size measurement. Regional changes in the deposition process of erosion can be revealed by analyzing the profile characteristics and different geochemical properties of 210Pb and 137Cs.And also,the main reasons for regional deposition rate decrease were discussed.The core sediments consist mainly of yellow-gray and gray-black clayer silt by visual inspection. Based on grain-size analysis, silt is dominant, with the next being caly,and sand content is minimal, whilst no gravel has been found. Moreover,Median diameter have ranged from 6.158-7.775cp,and mean contents of clay have ranged from 16.75～36.86%,while clayer silt ranged from 58.71～79.71% in profile.There was a higher correlation (R2=0.717)between 137Cs activities and clay content than 210Pb and clay content,which showed that 137Cs was more particle reactive than 210Pb. Sedimentation rate in recent 50 years was large and could be divided into some distinct stages:sedimentation rate calculated from 137Cs method was 5.9 cm/yr during 1954-1964,while reduced to 3.36 cm/yr during 1964-2006.Two parts of the graph showed a good exponential decay in the 210Pb profile of the 18# core.Sedimentation rate is 5.47 cm/yr during 120～225 cm,and corresponds to 4.58cm/yr during 18～100 cm.It can be found that erosion occurs in this area through analyzing the results of two methods maybe,and sedimentation rate decreases from 1968～1972. The time when sedimentation rate changed corresponds to the variation of sediment fluxes from changjiang river. Sedimentation rate trend calculated from 210Pbex profile isn’t exactly the same as the variation sediment fluxes,which shows that sediment fluxes isn’t the only factor for the variation of sedimentation rate in the Yangtze Estuary mud area.Comparing radionuclide inventories in sediment cores with these references of East China Sea,these radionuclides in sediment not only contain direct fallout from atmosphere,as well as mainly derive from the Changjiang drainage basin due to erosion,to a great degree.The conclusion is consistent with previous study.It also illustrates that although sediment fluxes have distinctly decreased in the past two decades,radionuclides in subaqueous delta still epresent the process of net input and accumulation.更多还原