长江三角洲南汇潮滩沉积速率及其影响因素

Sedimentation rates of Nanhui tidal flats in Changjiang River Delta and analysis of their influence factors

近100余年来,南汇潮滩尤其是中、高潮滩,以淤涨为主,使岸线迅速向海推进。但潮滩沉积并非连续,而是以冲、淤相间,长期净淤积的方式进行。潮滩沉积剖面主要由风暴成因的小型层序组成,粗、细粒交替分别代表风暴与平静天气的产物。潮滩沉积环境的不稳定性使其难以满足两种常用来计算210Pb沉积速率的方法——CIC(稳定的初始比度)模型和CRS(稳定的沉降通量)模型的前提条件。在实验时,如果只选择代表平静天气沉积的细颗粒层进行210Pb活度分析,则可能得到一组相对符合CIC模型的数据,利用此方法计算出南汇潮间带环境的平均沉积速率为6.11～6.23,cm/yr。对历史海图数字化建立的数字高程模型(DEM)分析表明,中、高潮滩近50余年的平均沉积速率为1.91～2.05,cm/yr,不及210Pb沉积速率的一半。造成这种差异的原因有:(1)DEM法在高程推算过程中使用简单的潮滩纵剖面模型,导致计算的沉积速率偏低;(2)210Pb法受到沉积过程中强烈的物理混合作用和沉积后的生物扰动、化学迁移作用等影响,使计算的沉积速率偏高。推断近50余年来,南汇中、高潮滩的沉积速率为4～5,cm/yr。

The Nanhui tidal flats are characterized by multi-decadal cycles of erosion and deposition,and net accretion over a longer time.Rapid deposition occurs on the upper and middle intertidal flats,resulting in shoreline progradation seaward quickly.The regressive tidal flat sequence is principally composed of storm-generated small successions with the coarse-grained layers deposited during storms and the fine-grained layers during calm-weather conditions.Highly active surface layer generally disables the premise of CIC（constant initial concentration）or CRS（constant rate of supply）model to calculate 210Pb sedimentation rates.In this study,210Pb dating samples are strict from the fine-grained layers,and the resulting 210Pb data almost satisfy the CIC model,illustrating mean sedimentation rates of 6.11～6.23,cm/yr on the intertidal flats of Nanhui.The sedimentation rates are calculated by using Digital Elevation Model（DEM）on the basis of nautical maps to be 1.91～2.05,cm/yr,less than half of those of 210Pb inventory.The significant difference between two deduced sedimentation rates is due to not only the deflated rates based on DEM that the elevation data are generated by simple assumption of uniform slope across the intertidal flats,but also the inflated rates based on 210Pb inventory which is greatly influenced by physical mixing during the deposition and post-deposition bioturbation and elemental migration.It is therefore reasonable to extrapolate the real sedimentation rate of recent upper-and-middle-intertidal deposits around 4～5,cm/yr in the late 50 years.