A Core-recorded Environment Evolution And Response To Sea Level Changes On The Continental Shelf Off The Yangtze Estuary,East China Sea

Glacio-eustatic oscillation is an important based-level to control the sedimentary system in the continental shelf basin.The late Quaternary sea-level fluctuations have primarily controlled the sediment distribution and environmental evolution of the world's estuaries and continental shelves.The shelf of East China Sea(ECS)is one of the widest shelves and river-dominated ocean margin in the world,receiving a large amount of terrigenous sediments from the Yellow River and the Yangtze River.It is one of the major sediment sinks in the eastern China continental margin.The shelf of ECS has preserved detailed records of paleo-environment and geological data,which is one of best regions to study the sedimentary environment evolution response to the global sea level changes of the Quaternary.According to the global curve of eustatic sea level constructed by these previous studies,there are 3 transgressions since the late Pleistocene.The sea level approximates to or was even higher than the present level during MIS5,and it drops to about 30?70m during MIS3.There happened several transgressions and regressions cycles in the shelf of ECS since the late Quaternary and developed corresponding transgression and regression sedimentary sequences on the shelf.Researchers have achieved a lot of research results about sedimentary strata,sediment evolution paleo-river channels,incised valley-fillings and tidal sand ridge in relation to the sea level changes.Although many researchers have studied the late Quaternary sedimentary stratigraphy,sediment of palaeochannels,incised-valley and tidal sand ridge,et al.about the shelf of ECS.The study of stratigraphy sequence is more focused on the delta plain and the subaqueous Yangtze delta and the research of paleo-environment and sedimentary strata in middle-outer shelf is comparatively limited.During Late Quaternary,how sea level changes have influenced the sediment environment of the areas off the Yangtze estuary and what area's response to such changes are worthy of attention and in-depth study.In this thesis,we take the shelf off the Yangtze estuary as study area and we focus on the core YZ06(30°07.4932'N,123°33.5980'E)to research the environment evolution,sedimentary system and stratum structure of the study area and analyze how the sediment environment of the study area response to sea level changes.We take methods of grain-size,magnetic susceptibility,loss of ignition,identify of macrofossils and microfossils,combined the chronology framework of AMS 14C and OSL dating and comparison of related stratigraphy.The sediment units of YZ06 core can be divided into five parts from the borrom up(U1-U5):U5 deposition unit is at the bottom of the core,which consist of fluvial facies.It is the marker layer for the Quaternary stratigraphic correlation of the ECS shelf formed in the oxygen isotope stage 6.U4 unit was showed between 41.5?58.7m depths in the core YZ06,above the fluvial facies,developed shallow sea facies ?fluvial facies?littoral neritic facies,i.e.transgressive-regressivetransgressive sequence.Yangtze River Delta developed costal wetland and limnetic facies.According to the 3 AMS14C dating data which are all>43ka B.P.,U4 unit was may be formed in the oxygen isotope stage 5.U3 unit includes fluvial facies,thickness of 8.5m,the second hard clay was developed at around the same time in Yangtze River Delta region.U3 unit may be formed in the oxygen isotope stage 4.U2 consist of estuarine delta and neritic facies,while neritic facies,estuarine facies and sublittoral facies are developed in other cores.According to two OSL dating data in U2,U2 unit was may be developed in the oxygen isotope stage 3.U1 unit,at the top of the core with thickness of 8.3m,is neritic facies(middle shelf),According to AMS14C dating data in U1,U1 unit was formed after 3 ka B.P.in Holocene.YZ06 core's sediments consist of silt,fine sand,silty fine sand,clayed silt and coarse sand and so on.The main sediments are sand and silt,with an average content of 67.1%and 29.3%respectively and clay content in the least with an average content of 3.6%.The average particle size of the entire core deposits between 0.994?7.231?,an average of 3.416?,sorting coefficient between0.573?3.347,an average of 1.858,the skewness is between-0.20.6?0.687,an average of0.415,kurtosis between 0.640?2.761,an average of 1.303.Magnetic susceptibility between 0.09E-03?1.76 E-03,an average of 0.63E-03;organic matter content of 0.74?6.4%,with an average of 2.58%;the carbonate content of between 0.34?5.37%,with an average of 2.26%.By analyzing the experimental results of magnetic susceptibility,the trend line between the magnetic susceptibility value and grain size,we found magnetic susceptibility with higher values are usually showed around 3? intervals.The relationship between magnetic susceptibility and grain size shows that magnetic susceptibility in silt and clay sediments at a lower value and in sand sediments at a higher value.The relationship between magnetic susceptibility and sediment environment shows that magnetic susceptibility in the fluvial environment(U4-2)at the highest value and in neritic environment(U1&U4-3)t at the lowest value.It indicates that lower energy environment with lower magnetic susceptibility and higher energy environment with higher magnetic susceptibility.In the core YZ06,sediments with more fine material contain more organic matter,e.g.,neritic deposition(0?8.3m&58.7?66.5m)while coarse deposits contain less,e.g.fluvial deposition(66.5?80.6m).In high energy environment,such as river and strong tidal tide environment,it is not good for organic matter to deposit,especially in coarse sand with gravel deposition with lowest content.Carbonate content's change trend is similar to organic matter.In neritic silt-clay deposition with higher content,which owing to foraminifer fossil.In sand and terrestrial deposition with lower content,but fluctuate more strongly,calcium carbonate(calcium tuberculosis)developed in exposed environment is a key reason to the increase of carbonate content.Sedimentary environment evolution showed during the low sea level in MIS6,the shelf of ECS was completely exposed to the terrestrial environment and developed the fluvial,lacustrine and paludal sediments.Study area developed fluvial environment.There is a thin layer(about 10cm)of fine depositions between the rivers layer,rich of foraminifera,which may be related to a short warmer period during MIS6.In MIS5(about 128?75 ka BP),period of high sea level.During MIS5.5,period of sea levels rising,incised valley formed in MS6 inundated by seawater and filled with neritic deposition.After sea level rose to the highstand,MIS5.4 regression occurred,the study area development short fluvial environment.During the period of MIS5.3?MIS5.1,the sea level rose again and filled incised valleys formed in MIS5.4.Study area developed near shore-shallow sea deposition.Yangtze River Delta developed near shore and wetland environment.MIS4 stage(75?60 kaB.P.),sea level dropped,the study area developed fluvial environment,and inner shelf exposed with the development of terrestrial sediments,such as fluvial lacustrine and paludal facies.Yangtze River Delta coastal plain uaually developed the 'second hard clay' on the top of MIS5 deposits.From MIS3.3 to MIS2,sea level fluctuate declined,study area developed estuarine delta environment.During MIS2(approximately 25?15 ka B.P.),sea level fluctuate decreased,the Yangtze estuary continued to extend to the shelf.At that time ECS coastline may be at 140 to 160m depth contour(without regard to seabed subsidence and sediment compaction and other factors).The whole ECS exposed and developed flat plain.In the low-areas,lakes were developed and numerous rivers across the plain with form of rivers and estuaries deposition.In the period of MIS 1(about 15kaBP?so far),post-glacial global climate got warm again and sea level began to rise rapidly.Before 12 ka B.P.the sea level has rebounded to today's 60m depth contour.Around 7 ka B.P.,sea level has approached today's.Yangtze River Delta has began to develop.Since 3 ka B.P.,development and study area has developed neritic deposit environment with deposition of clay and silty clay.During the period of MIS6,the shelf of ECS experienced significant regression with the cooler climate.In the exposed shelf,terrestrial deposits,such as rivers,lakes and swamps,were developed,with a wide distribution and even thickness,and taken as the marker layer for the Quaternary stratigraphic correlation of the East China Sea shelf.Subsequently in the period of MIS5,with the rising of sea levels,the incised valleys formed MIS6 were filled and sea level was similar today.At that time today's plain and Yangtze River delta plain region,inner and outer shelf of ESC began to develop different environments from former terrestrial deposits,showing significant different marine and continental environments.