Development Of The Saltmarsh In South Flank Of The Yangtze Delta In Response To The Variation Of Sediment And Runoff From Yangtze During The Past Century

The saltmarsh in south flank of the Yangtze Delta is an important sedimentation zone for the Yangtze sediment load and runoff entering the estuary,where the land–ocean interactions is significant.The Yangtze sediment load and runoff entering the estuary have changed greatly during the past century.The saltmarsh which is deeply affected by human interference is sensitive for the change of sedimentation environment.In this paper,I focus the research on the shoreline progradation and the deposition sequence to analyze the development of the saltmarsh in south flank of the Yangtze Delta in response to the variation of sediment and runoff from Yangtze.The saltmarsh in south flank of the Yangtze Delta is the study area.The seawalls are regarded as the coastline to study the south flank of the Yangtze Delta progradation with collected materials of seawall construction and reclamation.The sediment cores were collected in the south flank of the Yangtze Delta in March 2017.Grain size,environmental magnetism,organic matter(POC,TN,?¹³C)were measured.The intertidal mudflats are subdivided into upper,middle,lower section.Comparing the sequence with the distribution and source of sediments,combining the Yangtze sediment load and reclamation,the influence factors of saltmarsh are studied.Over the past century,the progradation of the South flank of the Yangtze Delta experienced 3 stages:slow progradation,stable progradation,fast progradation.The stage of slow progradation is dominated by the process of enrosion-depostion changing.The stage of stable progradation is divided into 2 phase:1949¹979,1980¹994.The high load of Yangtze sediment is the dominator for shoreline progradation in1949¹979.The reclamation keeps the progradation in 1980¹994.The stage of fast progradation is dominated by excessive reclamation.The sediment of the saltmarsh in south flank of the Yangtze Delta is silt mainly,the content of clay and sand is low.Comparing the intertidal section of core NH-1 and NH-2,there is a decreased trend with the content of clay and silt in the intertidal section of core NH-3 and NH-4.The trend is unanimous with the suspended sediment concentration?SSC?entering the estuary in the period of intertidal formation.To observe the grain size parameters in intertidal section of core NH-1^NH-6,we conclude:the value of mean???is lower than the median???,which means the skewness is more positive values.The kurtosis values increase vertically,and the size classification is poor.The sediment of the saltmarsh in south flank of the Yangtze Delta have the high content of ferrimagnetic minerals,and the ferrimagnetic minerals are mostly pseudo-single domain?PSD?/multi-domain?MD?grains in intertidal section of core NH-1,NH-5,NH-6,while the intertidal of core NH-2^NH-4 are mostly single-domain?SD?.The magnetic minerals have the positive correlation with the fine grains.The properties of magnetic minerals show that the magnetic minerals are controlled by the Yangtze sediment load entering the estuary in the period of intertidal formation.The construction of seawalls and reclamation affect the sedimentation of intertidal section.The intertidal section of core NH-1^NH-3 are controlled by siltation,and core NH-4^NH-6 are affected by reclamation with incomplete sequence and the deposit process ending in advance.The content of POC and TN have the unanimous trend in distribution and good correlation.The distribution of organic matter in the saltmarsh is dominated by fine grains?<16?m?.The?¹³C value for POC in saltmarsh is-26.41‰^-22.43‰.The?¹³C values of core NH-5^NH-6 increase in comparing with core NH-1^NH-3.The proportion of terrestrial organic matter decreases and marine organic matter increases,from landward to seaward.Although the declining influence of Yangtze river,the development process of saltmarsh is still dominated by Yangtze River.The correlation between C/N ratios and?¹³C value show that C/N ratio is not a good indicator for tracking the sediment source in south flank of the Yangtze Delta.