The Modern Sedimentation And Evolution Of Sedimentary Environment In The Zhejiang Nearshore Area During The Holocene

The nearshore area is the typical region of land and ocean interaction and there are frequent and intensive human activities.The Zhejiang nearshore area is located between the coast with the inner shelf of the East China Sea?ECS?,the input of massive terrigenous sediments from the Yangtze River is the major factor to control the nearshore sedimentary development.Under the combined action of the east Asian monsoon,sea level change and current system in the ECS,unique sedimentary characteristics and sedimentary records in nearshore area have been formed,which are of rich information of paleoclimate and paleoenvironment.Therefore,the study on the modern sedimentation and paleo-depositional environment have importance value in the scientific and practice value.In this paper,grain size,clay minerals,trace elements of 616 surface sediment samples,27 columnar samples,two cores,and about 7000 km shallow layer profilers were used to study the modern sedimentary processes and evolving law of sediment environments in the study area.The following achievements have been obtained through this research:?1?According with particle size analysis results,five primary sediment types occur in the Zhejiang nearshore area:clayey silt,sand-silt-clay,silty sand,silt,and fine sand.Clayey silt is the dominant sediment type?74%of all samples?and is primarily deposited in the center of nearshore area with a NE–SW banding distribution,which is also known as the mud area in the nearshore area.Silt is scattered in the western nearshore shallow water and mainly originates from the Yangtze River and some mountain rivers.From the center to the eastern area,sand-silt-clay,silty sand and fine sand gradually are presented,and sediments become gradually coarser in grain size.In the study area overall,there are coarse-fine-coarse gradations in the grain sizes of the sediments from the nearshore to the offshore.?2?The grain-size trend analysis was employed to ascertain the net surficial sediment transport pattern in the study area.In the northern part of the study area,sediments are transported southward under the action of the stronger southward Zhe-Fujian Coastal Current?ZMCC?in the winter.Simultaneously being affected by the obstruction of the Taiwan Warm Current?TWC?and the tidal current,surface sediment transport vectors display two grain-size trend convergences and indicate a high-speed sedimentation environment.Accordingly,there is a high modern sedimentation rate.In the southern part,however,the transport is mainly toward the north under the control of the prevailing northward ZMCC and the strong TWC in summer.?3?According to the dating result of ²¹⁰Pb and ¹³⁷Cs,the sedimentation rate gradually decreases from north to south.The highest rates are seen in the Zhoushan Archipelago area,dominantly varying from 2.36 to 3.88 cm/yr,and the lowest are seen at the southeastern outer shelf at less than 0.20 cm/yr.In the northern zhoushan islands,the deposition rate decreases with the increase of offshore distance,while the southern coastal waters show a low-high-low variation trend with the increase of water depth.The main reason for such a distribution of deposition rates is that the muddy sediments in the study area are mainly transported from the southern ZMCC,and deposition rates decline during the settlement process from north to south.In the southeast coastal area,the deposition rate decreased to the minimum or even close to zero due to the decrease of sediment source and the enhancement of tidal wave dynamics.?4?In the geochemical environment of the study area,the clay minerals of the surface sediment are mainly composed of Smectite,Illite,Chlorite,and Kaolinite.Illite group minerals are the dominant minerals in surface sediments with an average content of 60%,distributed in Marine sedimentary environments with alkaline medium affected by the TWC.The average content of chlorite was 20%,which was distributed in the area near the outer margin of the shore and the outer shelf.The distribution of high content of kaolinite and smectite was greatly influenced by land sources.The concentrations distribution of heavy metals?Zn,Cr,Ni,Cu,Pb,and Co?was higher near shore than far shore,and the highest value occurred in the central muddy area near shore,and the pollution load index?PLI?was also the highest.In the southeast sea area of the study area,there is a low value area,the content of which is far lower than the average value.Other sea areas have medium heavy metal content,and PLI value is slightly more than 1,indicating mild pollution.The distribution of heavy metal content also corresponded well with the change of deposition rate.?5?Seismic sequences revealed by a high-resolution shallow stratigraphic profile show four sedimentary units,such as DU1,DU2,DU3,and DU4,revealed by the two drilling cores in the study area,respectively.The formation time and sedimentary environment of each sedimentary unit are as follows:DU4 was formed from the middle and late stage of MIS 3 to the early stage of MIS 1.During this period,the eastern part of the research area experienced the transformation from river environment to estuary environment,while the nearshore area was dominated by river environment.DU3 was formed from the early MIS 1 to the eary Holocene.In the early MIS 1,transgression began from the far-shore to the near-shore.In the eastern far-shore area,the estuarine and shallow water subtidal environment were developed in the 14¹2 cal kyr BP.While in the near-shore area,tidal flat and littoral environment affected by tide were developed during 11¹0 cal kyr BP.The sedimentary unit developed during this period shows positive graded structure.DU2 was formed from the eary Holocene to middle.Tidal sand ridges or sand sheets were developed in the far-shore area during 12⁷ cal kyr BP.While tidal sand ridges or sand sheets developed in the near-shore area during 10⁷ cal kyr BP.The sedimentary unit developed during this period shows reverse grain size order.Fine and thin sand layers were deposited during the highest marine flooding surface of Holocene,which represents the condensed section.DU1 was formed from the Middle Holocene?about 7 cal kyr BP?to present,corresponding to the wedge-shaped argillaceous sedimentary body developed in Zhejiang nearshores area.It is roughly distributed in parallel along the coastline,and its near-shore thickness becomes thinner in the direction of the far shore.The sedimentary source of this wedge-shaped argillaceous sedimentary body is mainly from the Yangtze River,which can be regarded as the underwater delta of the Yangtze River far away from the Yangtze Estuary.