The coastal waters adjacent to the Changjiang estuary is characterized by abundant supply of fine-grained sediments and active hydrodynamic effects. The growth of the Changjiang delta is constrained by a number of factors such as sea-level rise, the original topography, sediment supply, sediment retention in the estuary, ground subsidence and large water conservancy and hydropower projects ( e. g., the Three Gorges Dam ) . Hence, the sedimentation sequence of the Changjiang subaqueous delta in response to physical processes(i. e.,changes in material source, migration of tidal channels,storm events,etc. ) and anthropogenic activities( i. e.,dredging of navigational channels) have become important scientific problems.rnSediment cores were obtained from 31 stations over the Changjiang subaqueous delta,during April 13 to April 27, 2009,by utilizing a gravity corer. We took sub-samples from the cores in 2cm intervals. Grain size distribution was derived for 2468 sub-samples. The grain size parameters indicate that the material of the Changjiang subaqueous delta is mainly from modern river input, with a few stations where sandy deposits that are indicative of submarine erosion were present. The vertical variations of grain size parameters show 4 patterns: (Ⅰ) fining upwards; (Ⅱ) uniform;(Ⅲ) sectional or cyclical vertical variations; and (Ⅳ) coarsening upwards. Core samples with different patterns of sediments grain size profiles located at different topographical region with various erosion or siltation dynamics.rn210Pb activities of sediments were determined for 511 sub-samples, in 4 ~ 10cm intervals, taken from the 30 sediment cores of the Changjiang sub-aqueous delta. The vertical variation patterns of 210Pb activities can be classified into:(1) normal 210Pbex decay profiles;(2 ) uniform total 210Pb activity profiles;( 3 ) sectional 210Pbex decay profile; and (4) sectional 210Pbex decay and inverse 210Pbex decay profiles(topsy-turvy 210Pb activity profiles). The second pattern outlined above might imply a contradictory condition; if the total 210Pb activity is close to the supported 210Pb activity of its adjacent stations, then the uniform total 210Pb activity profile should indicate a sedimentation rate of zero; in contrast, if the total 210Pb activity is much higher than the supported rb activity of its adjacent stations, such a profile might represent an extremely high sedimentation rate, too high to be calculated from the present short sediment core samples. In this study, the sediment samples with uniform total 210Pb activities all belonged to the former condition. As shown by the spatial distribution of the 4 types of rb activity profiles, at majority of the stations the sediment accumulation was not continuous. The various sedimentary records represent complicated responses of the Changjiang subaqueous delta to a number of factors such as sediment dynamic processes and dam constructions in the catchment. The spatial distributions of grain size and rb characteristics of the Changjiang subaqueous delta indicate the influences including changes in material source and hydrodynamics, erosion and reaccumulation of sediment over the adjacent areas, spatial variations in hydrodynamics, and event deposits.%长江口细颗粒物质来源丰富、水动力作用活跃,近年来又受到三峡大坝等流域大型工程的影响,因此其水下三角洲沉积层序对自然过程(物源变化、潮汐水道地形调整、风暴潮事件等)和人类活动(航道疏浚等)的响应成为一个重要的科学问题.对覆盖长江口水下三角洲的31个站位的沉积物柱样进行了粒度分析,并对其中30个站位的柱样进行了210Pb测定,结果表明,长江口物质主要是来源于现代流域供给,少数站位的砂质沉积则代表海底侵蚀.沉积物柱状样的平均粒径有4种类型的垂向分布趋势:Ⅰ.向上细化,Ⅱ.垂向稳定,Ⅲ.垂向波动式变化,Ⅳ.向上粗化.分别对应于不同的地貌部位和冲淤动态.210Pb比活度垂向变化有4种类型:1)标准衰变剖面,2)均一210Pbtotal活度剖面,3)分段式衰变剖面以及4)分段式衰变与倒置衰变复合剖面.4种类型的空间分布格局显示,多数站位不是连续堆积的.不同站位的柱状样沉积记录代表了长江口水下三角洲沉积对各种因素(沉积动力过程、流域水库建设等)的复杂响应模式.长江口水下三角洲粒度与210Pb的空间分布特征指示了研究区的多种沉积动力特征,如物源变化、水动力变化、相邻区域的侵蚀搬运与二次堆积、水动力强弱的空间差异以及事件沉积的影响.
展开▼
