Sedimentation In The Offshore Area Of Changjiang River Estuary And Its Responses To The Environmental Changes During The Past Century

A great amount of terrigenous sediment coming from the Changjiang river basinhas formed a magnitude subaqueous delta. The researches on sedimentary evolutionof the subaqueous delta are important to explaining the land-ocean interaction. Forhigh sedimentation rate and its continuous and relative stable sedimentation, theresearches on the mud area of Changjiang delta has special advantages in revealingthe high resolution sedimentation and environment during the past century. Based onthe actual measurement data and samples from the offshore area, we discuss themodern sediment environment and changes in climate and environment during the last120a by the methods of grainsize analysis, clay mineral analysis and geochemicalanalysis.Based on in-situ measurements, we found that the current velocity has obvioustidal cycle variation and unconspicuous annual variation. The variation of suspendedsediment concentration relies on the hydrodynamic and there are2-4times peakvalues in concentration during a tidal period in general. For space distribution pattern,the hydrodynamics and suspended sediment concentration decrease from theChangjiang estuary to the north-east direction and sharply decease at123°E which isalmost the east boundary of the Changjiang diluted water. By the aid of DigitalElevation Model, the sediment discharge transported to the offshore area hasdecreased in the past20a and the geomorphology has been eroded. The influence ofhuman activities, dam construction and conservation of water and soil included, resultin the sharply decrease of sediment transport.There are3main types of surface sediment in the research area, including silt,silty sand and sandy silt. The data indicate that the provenance of sediment is mainly influenced by the Changjiang sediment. On the other hand, the Huanghe sediment hasa limited effect on the surface sediment distribution and its influence becomesincreasing from the estuary to the north-east.We have finished experiment tests about the core CJ0702. And the resultsindicate that the sedimentation environment within mud area is in stable conditionduring the last120a with average sedimentation rate3.11cm/a. The parametervariations, including grain-size, clay mineral and chemical elements, appear smallerchanges and fluctuating vertically. The provenance analysis proves that the sedimentconsisting of CJ0702core is mainly controlled by the Changjiang sediment andinfluenced by the Huanghe sediment jointly. According the vertical variation ofchemical elements, the sediment from the Huanghe River presents an increasing trendwithin the past century.A spectrum analysis was made for the variations of grain-size and chemicalwreathing index in core CJ0702. There are two types of periodic variation,2-8a and16-20a, observed in these two parameters within the core. These two periodicvariations have good correspondence with the ENSO and PDO respectively. Based onthe sensitive grain size combining with the Zr/Rb value and magnetic susceptibility,the flood events happening in the Changjiang river basin have great influence withsedimentation in the mud area and there are about20times flood events distinguishedfrom the sedimentation records. Despite of natural environment factors, the humanactivities, including major construction projects on the Changjiang river basin andsome other economic activities, have great impact on the sedimentation in researcharea equally.