Grain Size,Nuclide Distribution And Its Environmental Significance In The Tidal Flat Of The Modern Yellow River Delta

Since the Yellow River flowed into Bohai through the North China Plain in 1855,it has developed themodern Yellow River Delta,while the diversion of lower river course in alluvial fan is frequently.The delta front is not only influenced by the runoff and sediment yield of the Yellow River but also the marine dynamic action,so it has a strong evolution of erosion and deposition.The tidal flat of Yellow River delta is located at the intersection of land and sea.Because of its special location and complex dynamic action,it has sensitive repose to coastal environment.The sediment of tidal flat also records the change of environment and human activity.In this paper,the columnar cores and surface sediment of the tidal flat near the Qingshuigou were collected,they are Y1,Y2,Y3 and Y4 respectively.Then field survey,grain size,²¹⁰Pb,¹³⁷Cs and ⁷Be,runoff and sedimentation field of the Yellow River were analyzed,changes of shoreline and the discharge regime of the Yellow River were also be studied.Based on the above information,this paper discussed the distribution of nuclide,sedimentation characteristics and the evolution of tidal flat under the human and nature function.The grain size analysis results of four cores sediments show that the tidal flat of study area is mainly composed of silt and clayey silt,indicating the typical sedimentary feature of silt-muddy under long-term accumulation.Different depositional facies of tidal flat have different sediment components,sediments are coarsening from intertidal zone to subaerial delta,reflecting that study area is under the action of marine and river dynamics respectively.The columnar samples Y2,Y3 and Y4 all exist thinner or coarser cline obviously,these mutations were usually associated with transition of estuarine sedimentary environment,storm-surges,floods and human activities.After combining the water content and the grain-size,we found that water content is proportional to clay content,showing that fine particulate matter mainly originates from weak marine dynamics.A red clay layer in columnar samples Y4 has at 48?50cm was found,combining with the rainfall of the Loess Plateau and the sediment field of the Yellow River,we infer this red clay layer corresponded to 1998.In 1998,the Yellow River basin encounter a flood,the paleosol of the Loess Plateau were carried to the study area,then the red or brown clayey silt were formed due to the strong change of hydrodynamics.Due to the complex dynamics and volatile sedimentary environment in the tidal flat of the Yellow River delta,the ²¹⁰Pb activity distribution curves of the four columnar samples do not show an ideal exponential decay.In this paper,these four distribution curves were divided into two types,one is stage distribution and another is event-influenced--mixed distribution.In the tidal flat of the Yellow River delta columnar samples,the ¹³⁷Cs signal of Y2?supratidal zone?,Y3?high tide beach?and Y4 delta?land?were measured,all the ¹³⁷Cs activity present a discontinuous distribution with a accumulation peak,corresponding to 18cm,22cm and24cm.Based on the area activity,we speculated that the sediments in the study area often experience alternation of deposition and erosion.According to the distribution rule,these three activity curves can be divided into two types,surface detected and undetected,indicates the difference of surface sedimentary environment.The nuclides in the estuarine delta area are mainly from river sediment and marine suspended matter deposition,so the distribution of ¹³⁷Cs is different from the law of atmospheric deposition.In this paper,the maximum accumulation peaks of the three columnar samples are set as 2003 based on the data of seawater sediment inflow from the Yellow River over the years.This flood also resulted in the ²¹⁰Pb_ex fault at 20cm at Y1.According to the time scale of ¹³⁷Cs,the dating of ²¹⁰Pb within 10cm of the surface of Y3 and Y4,and the red clay layer in Y4,the deposition rate of the study area was calculated.The results show that the average deposition rate from 2003 to 2019 was 1.25cm/am at Y1,1.125cm/a at Y2,1.375 cm/a at Y3,1.5cm/a at Y4.The average deposition rate from 1998?2003 at Y4 was 5.8cm/a.The results of ⁷Be analysis of the surface samples of the tidal flat on the north side of the Yellow River delta show that the maximum measurable depth of ⁷Be in the study area is 6mm,and reaches the maximum within 0?2mm.The measurable depth of ⁷Be gradually increases from the intertidal zone to the supratidal zone to the continental delta.⁷Be activity of sample surface Y4 shows exponential decay,three different depth of ⁷Be correspondence,so surface sample Y2 and Y3 may be absent.The ⁷Be atmospheric subsidence of the Yellow River delta area was estimated by using rainfall-⁷Be atmospheric precipitation formula,the result is almost the same as the total area activity value of Y4,which is far greater than that of Y2 and Y3.It reflects that the surface sedimentary environment of the land Delta area is stable,and the average annual deposition rate of the surface is 1.31cm/a.Combined with field observation and analysis of historical data,the surface sediments in the upper tidal zone and the high tide beach area experiences wind erosion and seawater erosion during the storm surges respectively,and the tidal flat of the Yellow River delta shows overall net erosion in nearly half a year.The development of tidal flat in the Yellow River delta is affected by both natural and human factors.This paper extracted the changes of shoreline of Qingshuigou lobes from 1984 to 2019.It can be seen that after the Yellow River changed its channel to Qing8 branches,the overall shoreline migration rate to the sea decreased obviously.Especially in 2004-2019,the shoreline changes are very little,and most areas in tidal flat basically reached the balance of erosion and deposition.This is closely related to the water-sediment regulation scheme in Yellow River.Since the implementation of the project in 2002,the runoff and sediment of the Yellow River basically concentrated into the sea in one month,the flood peak and the sand peak were greatly reduced,the riverbed has changed from deposited to eroded.The grain size of sediment in the study area also responds to it.The median grain size of columnar Y2 and Y3 increases since the water-sediment regulation scheme,and the sediment change of columnar Y4 becomes more stable.Generally speaking,the topography and landform of the tidal flat area of the Yellow River delta has been stable in recent years,and it will remain stable in the future when the water-sediment regulation scheme is carried out.