The Chronology Of The Sedimentary Stratigraphy Of The Holocene Coastal Zone In Gangxi Area, Yancheng, Subei Basin And Its Environmental Significance

The coastal zone is the transition zone between sea and land.The coastal zone of the world has a large population,which is the main settlement for human.The coastal zone records the changes of sea level,the evolution of ecological environment and sedimentary environment,which is an important area to study the relationship between man and land.Holocene is a critical period for the formation and development of human society.It is of far-reaching scientific and practical significance to reinforce the research on the Holocene climate change and the reconstruction of sedimentary environment in the coastal area.The northern Jiangsu basin is located in the eastern part of china.The climate in this region is dominated by the East Asian monsoon variations.In the eastern part of the basin,there are thick interactive sedimentary strata that deposited during the land-sea interactions,which is an ideal place to study sea-level fluctuations,coastal geology and land changes.The reliable chronology is of crucial importance for the reconstruction of sedimentary environment in this area.Many researchers have carried out chronological studies in the northern Jiangsu basin,but the results of the transgression period are different from the actual chronological studies.In the study,the AMS¹⁴C dating and optically stimulated luminescence dating method were used to set up a robust chronological framework for the sedimentary sequence in the northern Jiangsu basin.The applicability and reliability of the two methods were discussed for different dating materials in the study area.The main conclusions are as follows:?1?A series of test measurements on the samples in Gangxi section indicated that the applied SAR protocol is suitable to date the quartz in the Gangxi section.The sedimentary environment in the study area is similar to the Yangtze River Delta,and the samples shall be well bleached before burial.In the calculation of external dose rate,the influence of water content,Uranium,Thorium,potassium and?efficiency is fully considered,and a certain error value is given.?2?By comparing OSL ages and the AMS¹⁴C results of the whole organic matter sample and the in-situ buried syngenetic shell samples,we conclude:The ¹⁴C ages of organic matter are reliable in lacustrine sediments;However,the age of transgressive sediments dated by the whole organic matter can not represent the age of deposition because of the sediment reworking;In the transgressive phase,the in-situ buried syngenetic shell was a living organism buried in the sediment when the stratum was deposited,which can better represent the age of strata.The fine-grained?4-11?m? quartz in the sediments of different sedimentary facies of GX profile is very suitable for OSL dating,which can reveal the age of deposition.?3?Based on the comparative test of the results of different materials and methods,a robust age framework of Gangxi section is established.Due to the significant variation in depositional environment at different depth,the whole section can be divided into three sections units:The depth between 500 cm and 426 cm is freshwater lacustrine deposit,the age is between 13-7.6 ka BP,and the average deposition rate is 0.012 cm/a;The depth between 426 cm and 150 cm is tidal flat shallow sea deposit,the age is between 7.6-6.5 ka BP,and the average deposition rate is 0.251 cm/a;The depth of 150-55 cm is fresh water lake facies sediment with ages between 6.5-1.4 ka BP,and the average deposition rate is 0.018 cm/a.?4?Based on the chronology,the evolution process of the sedimentary environment in the Gangxi section is discussed.Stage I:in the period of 13-7.6 ka BP?depth:500-426 cm?,the temperature warmed,the precipitation increased,and the lake surface expanded gradually.From the end of late Pleistocene to the early Holocene,the temperature was warming and the sea level was rising,but it had little influence on this area,and the deposition rate was low,which was freshwater lake facies deposition;Stage II:in the period of 7.6-6.5 ka BP?depth:426-150 cm?,temperature and precipitation continued to rise and increase,sea level was rising and reached the highest level at about 7 ka BP,and transgression occured.With the rise of sea level,the hydrodynamic conditions are enhanced,and there are burrows,fragments of sea shells and diatoms in the interval and the deposition rate was accelerated,which indicate that this segment is tidal flat shallow sea facies deposition;Stage III:in the period of 6.5-1.4 ka BP?depth:150-55 cm?,sea level reached the highest level,then gradually retreated from the study area,the sedimentation rate decreased,and the fresh water lake was developed again.