| It is essential to study the processes of the earth system associated with sea level changes and intensive human activities for reconstructing the history of regional environment evolution and understanding the regional response to global change.Such studies can also provide useful information on,and improve our predicting ability for,the environmental events in the future,e.g.,storms.The coastal deposits on the central Jiangsu coast are characterized by a high degree of temporal continuity and resolution resulting from a tidally-dominated environment with abundant sediment supply,which are suitable sedimentary records for past climate,environment and eeosystem studies.An understanding of late Quaternary eoastal sedimentary systerms is of great importance in providing evidence on sediment source to sink patterns.In the present study,three long cores?i.e.,YC01,YC02 and YC03?were collected from the northern Jiangsu plain in 2014,which are analysed in combination with 65 surficial sediment samples collected from the modem Yellow River?MYR?,abandoned Yellow River?AYR?,Yangtze River?YR?and offshore areas?RSR?off the Jiangsu coast in 2011-2013.The late Quatermary sediment provenance changes of the central Jiangsu coast are identified.Furthermore,the formation of the sedimentary systerms and the future evolution of the central Jiangsu coast since the late Quaternary are discussed,taking into account the factors of the late Quatermary sea-level,tidal regime,sediment supply,the estuarine morphology of the Yellow and Yangtze rivers,and the present-day shelf circulation in the Yellow Sea.The major results are summarized below.1.The grain size data of the three cores suggest that the late Quatermary deposits of the study area are dominated by silt and sand fractions,being 55%and 40%on average,respectively,with an average clay content of around 5%.In cores the magnetic minerals are mainly ferrimagnetic minerals,and the grain size reveals pseudo single domain?PSD?and multi-domain?MD?patterns.The magnetic mineral content is relatively low,with the magnetic susceptibility being associated mainly with ferrimagnetic minerals?the contributions of the superparamagnetic and anti-ferromagnetic minerals are less significant?.The three cores have a similar geochemical composition,with the major elements consisting mainly of Si02,A12O3,CaO,Fe2O3,K2O,MgO and Na2O and the trace elements being composed of Rb,Sr,Cr and V.Meanwhile,for most of the geochemical elements,their contents are higher in muddy sediment than in sandy sediment.According to the age-depth relationships,the deposits of the three cores are late Quaternary sequences.In combination with grain size,magnetic parameters and geochemical elements of the cores,three depositional units,i.e.,DU3,DU2 and DU1,are identified.DU3 represents the deposits before 40 ka B.P.,DU2 40-12.5 ka B.P.,and DU1 the Holocene deposits(DU 1-3 is early Holocene deposits,and DU 1-2 and DUI-1 are middle-late Holocene deposits).2.Seabed erosion over the offshore areas off the Jiangsu coast generates an independent,“new”source.Thus,three end members representing the contributions from the Yellow River,the Yangtze River and the offshore eroding areas are established.An analytical procedure is proposed to determine the best tracer fingerprints for the bulk,coarse and fine sediments,respectively.The results suggest that mean grain size and magnetic susceptibility are the suitable tracers for bulk sediment,SIRM vs SIRM/y and Sr vs Zn are suitable for muddy sediment,and SIRM vs HIRM-?0o and Nb vs Cu are suitable for sandy sediment.3.The contributions of the two rivers and the offshore areas to the central Jiangsu coast are evaluated using the mixing model.The Yellow River and the offshore areas off the Jiangsu coast have a great influence on the development of the northern Jiangsu coastal plain in the late Quaternary period,and the contribution of the Yellow River is slightly higher than that associated with the seabed erosion.The sediment provenance pattern of the central Jiangsu coast since the late Quaternary can be divided into four stages:???before the Holocene(40-12.5 ka B.P.),the contributions of the Yellow River,the Yangtze River and the offshore areas were 64.7%,3.9%and 31.3%,respectively;???in the early Holocene?12.5-6 ka B.P.?,these became 53.4%,4.5%and 42.2%,respectively;???6-1.7 ka B.P.,these became 54.8%,3.4%and 36.6%,respectively;and?iv?1.7 ka B.P.-present,the values were further changed into 58.6%,5.0%and 36.5%,respectively.Thus,the contribution from the Yangtze River was generally minor during the late Quaternary,i.e.,around 4.2%.4.In the sediment source tracing analysis,the mean deviation of the estimated contributions of the various sources is relative small,i.e.,15.2%on average.Meanwhile,the vertical trends of maximum?or minimum?and mean values are similar.In addition,based on the sediment provenance changes of this study,in combination with the information of other cores on the central Jiangsu coast,the time uncertainty is also relative small.5.The characteristics of grain size,magnetic parameters and geochemical elements in the different grain size groups from MYR and AYR indicate a large difference between the MYR and AYR sediments,probably due to the differences in chemical weathering.Further,the AYR sediment may have been significantly influenced by the Huai River.Therefore,AYR represents an independent source for the period after 1128 AD on the central Jiangsu coast.6.The offshore radial sand ridges off Jiangsu coast are characterized by strong tidal current,high suspended sediment concentrations,and active sediment resuspension-transport proeesses.There exists a significant time-velocity asymmetry in the offshore areas,resulting a net landward suspended sediment transport,with a transport rate being of an order of 100 kg m'1 s'1.In addition,numerical simulation indicates that the Holocene paleo-tidal current field is almost consistent with the present-day current field.The current field can provide necessary hydrodynamic conditions for offshore areas transporting sediment to the central Jiangsu coast.7.The sedimentary environment evolution of the central Jiangsu coast since the late Quatermary is closely linked to sea-level fluctuations,the changes in tidal regime,sediment supply,the estuarine morphology of the Yellow and Yangtze rivers,and the present-day shelf circulation in the Yellow Sea.The three cores reveal that the sedimentary environments of the study area are indicated by terrestrial faces?floodplain?,tidal fiat-neritic facies,tidal flat facies,transitional facies and terrestrial faces,respectively,from the bottom towards the top parts of the cores.The future shoreline evolution on the central Jiangsu coast will be constrained by the changes in sediment provenance and hydrodynamics.The shoreline may prograde for another 24 km based on the morphologic characteristics of tidal flat and sediment supply from the AYR and the offshore areas off the Jiangsu coast in 300 years.However,sea level rise is an uncertain factor,which should be taken into account for a more accurate estimate.8.There are some significant research challenges:?1?grain-size and material component sorting associated with sediment transport processes?for grain-size sorting,some grain size fractions are needed to be separated,and mixing models should be established for every grain size fraction;for material component sorting,variation function should be established to define the content variation of tracer before and after sediment transport,based on sediment dynamics and morphodynamics?;and?2?process-based modeling is an important research field,as a supplementary approach to core analysis,such as modeling the formation of event beds. |
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