Sedimentary Records And Response Of Coastal Zone System To The Yellow River Southward Shifts In 1128-1855

The coastal zone is the transition zone between the ocean and the land,which is the most active area for material transport and energy conversion.The lower course of the Yellow River shifted,changing its outlet to the South Yellow Sea in AD1128,which was an important event in the study area,resulting in the formation of the abandoned Yellow River delta and the transformation from the estuary to land.In AD1855,the Yellow River shifted northward into the Bohai Sea to interrupt the sediment supply in the abandoned Yellow River delta.The twice shifts of the Yellow River lower course have caused twice system condition transitions in the abandoned Yellow River delta which also had a profound impact on both the Jiangsu coastal zone and the Yangtze River submarine delta.What kind of information and records would be left in the coastal zone by such a significant geological event?How did the sedimentary record change in both temporal and spatial terms?And how does the event relate to the evolutionary processes of coastal zone systems?By investigating these scientific questions,it is of great scientific importance to study the evolution of deltas,sedimentary records and environmental information extraction,coastal change,and to understand the construction of sedimentation model theory from the perspective of sea-land interaction.To address the above scientific questions,a total of five cores were collected from June to July in 2019 in three sedimentary systems:the abandoned Yellow River delta（X02,X03,and X04 in order from land to sea,and the length are 15.2 m,15.2 m,and20.6 m,respectively）,the coastal plain of Jiangsu Province（X01,the length is 15.2 m）,and the Yangtze River submerged delta（E01,the length is 6.7 m）,respectively.The samples were firstly selected for Optically Stimulated Luminescence（OSL）dating and the Bacon model was used to establish the dating framework;subsequently,magnetic susceptibility testing,geochemical element scanning,grain size testing,organic carbon&nitrogen,and stable isotopeδ¹³C testing were performed.The data were analyzed and systematically discussed,and the following conclusions were obtained.1.The dating framework found that the sedimentary sequences of the cores X02,X03,and X04 in the abandoned Yellow River delta are AD994～AD1573,AD983～AD1784,and AD114～AD1856,respectively.The sedimentary sequences of core X01 in the coastal plain of central Jiangsu province is AD632～AD1429.Sedimentary Sequence of four cores record the entire process of the Yellow River southward shifts.The time of land-formation from land to sea direction（X02,X03 and X04）is sequentially delayed.A series of depositional facies were formed during the transition from land to sea,which can well reflect the evolution of the delta.In the recent millennium,the sedimentary sequences of X02 and X03 are in the order of prodelta facies—delta-front facies—delta-plain facies;the sedimentary sequences of X04 are in the order of shallow marine facies—prodelta facies—delta-front facies—delta-plain facies;the sedimentary sequences of X01 are in the order of shallow marine facies—sea-land transition facies—plain facies.2.The abandoned Yellow River Delta is dominated by fine-grained sediments,with silt,mud,and sandy silt as its main sediment types.After the Yellow River joined the Huaihe River in AD1128,the grain size components in the sediment were significantly transformed,which showed that the sand component began to decrease at first.In the middle and late 14th century,the proportion of clay component in the sediments increased and remained high.In the middle of the 16th century,due to the further increase in sediment flux from the abandoned Yellow River to the sea,the clay content in the sediments increased sharply in the following 100 years.During the final stage of the transition from submerged delta to terrestrial delta,a slight increase in the proportion of sand component is a distinctive feature in the sedimentary record,with this change lasting from 10 to 90 years in different core records.In the coastal plain of central Jiangsu Province（X01）,the phase change of sediment components is less obvious than that of the abandoned Yellow River delta:the response to the Yellow River southward shift event lags almost about one hundred years,showing a decrease in the proportion of sand and an abrupt increase in clay component in AD1204,and the content of sand,silt,and clay components maintain relatively stable ratios until the land formation in AD1429.3.The main controlling factors affecting the evolution of the abandoned Yellow River delta are the sediment flux into the sea and initial submerged topography.After the interruption of sediment supply due to the northern return of the Yellow River in AD1855,hydrodynamic erosion by wave and tidal forces from the outer delta began to dominate,with sediment transported southwards along the contours.The development process of the abandoned Yellow River delta reflects the following law:when the sediment flux of river into the sea is certain,the land formation rate is inversely proportional to the vertical accretion rate,and the faster the horizontal land formation rate,the slower the vertical accretion rate.Before the 16th century,the sediments were deposited mainly in the estuary and nearshore,with rapid vertical accretion.The deposition rate of the estuary（X02）was the fastest,basically greater than 2 cm/yr,and the deposition rate of the outer side（X04）was slower,at 1.2～1.4 cm/yr.After the 16th century,the horizontal land formation was the main focus,and the seaward extension rate increased rapidly,reaching 1.5 km/yr at the fastest.4.After the Yellow River southward shift,the magnetic susceptibility,grain-size components,end-member decomposition,organic carbon and nitrogen,and geochemical elements in the sediment of the sink area were changed due to the transformation of sediments from land-based sources into the sea.Influenced by the lower magnetic susceptibility sediments of the Yellow River source,the magnetic susceptibility of the abandoned Yellow River delta sediments appears to decrease.As far as the results of grain size end-member decomposition are concerned,the sediment supply contribution from the abandoned Yellow River gradually increases,almost from30%to more than 90%.The rapid burial rate of organic carbon is the combined result of both the increased sedimentation rate and the high organic carbon content in the sediment after the sediment source change.The triangular diagram of Ca-Zr-Ti elements can clearly distinguish the Yangtze River source from the Yellow River source,which provides a new way to qualitatively judge the source of sediments.5.The Yangtze River submerged delta responds to both the Yellow River southward shift event and the Yellow River northward return event,and its sedimentary record signal（E01）appears with a significant time lag.The results of grain size end-member decomposition of the Yangtze River submerged delta sediments show that during the Yellow River southward shift,the contribution of the Yangtze River source ranges from 50%to 90%,the Yellow River source from 10%to 40%,and the contribution of the radial sand ridge from 0 to 32%.After the Yellow River returned northward,the contribution decreases gradually from 40%to about 15%.The magnetic susceptibility of the Yangtze River submerged delta during the Yellow River southward shift was low,with an average value of 53×10^-5 SI,while after the Yellow River returned to the north,the magnetic susceptibility increased to 72×10^-5 SI roughly.The deposition rate of core E01 was 0.8 cm/yr from AD1405 to AD1595,and the average deposition rate was 1.33 cm/yr from AD1595 to AD1877.In terms of deposition rate,the response of the Yangtze River submerged delta to the Yellow River southward shift event lagged by more than 500 years.