The Direction Of Changjiang Diluted Water And Its Impact On The Sedimentation Off The Estuary Area In Spring

The runoff of Changjiang(Yangtze)River inputs a large number of diluted water into the sea.The suspended sediments carried by the runoff migrate and settle in the estuary and the continental shelf area,and become the main source of seabed sediment.Previous studies mainly focused on the area from the innner Changjiang River Estuary(CRE)to sandbar area on the sedimentary environment,and the reasearch of modern sedimentary environment was quite a few.The study of the Changjiang Diluted Water(CDW)expansion on the sedimentary environment off the CRE is of great significance for revealing the source-sink process and environmental effects of the terrigenous sediments carried by the runoff in the East China Sea continental shelfSpring is the transitional season for the diversion of the Changjiang River Dilute water.In this study,based on the field survey data of hydrology and suspended particulate matter(SPM)off the(CRE)in spring(April,May and June)of 2015,combining with multi-source hydrometeorological data observed during the same period,the section 12250 E and 12300E(122.5°E and 123°E)were studied.To explore the CDW expansion process and its influence factors off the CRE in spring,and analyze the distribution of SPM,so as to better understand the material composition change and its mechanism in the process of SPM diffusion to the sea,and discuss the influence of SPM transport on the sedimentary environment.The results show that:The expansion range of CDW increased monthly in spring off the CRE,its direction gradually changed from south to north.In April,the CDW quickly moved south off the estuary and only distributed in the southern area of section 12250 E.In May,the CDW has been extended to section 12300 E,the main axis turned to the northeast.In June,the salinity was further decreased,and the range of CDW was obviously increased,showing a bidirectional branch,and the main axis was still to the northeast.The monthly change of CDW was mainly controlled by the hydrometeorological environmentoff the CRE.The direction was influenced by the moonsoon,and the runoff determined the range of CDW and the surface salinity.The Taiwan Warm Current(TWC)has been reached the CRE in spring,but not directly influenced on the CDW.However,with the upwelling of TWC month by month,the thermocline between TWC and CDW became more and more strong.The thermocline inhibited the downward expansion of CDW,while the rising of upwelling also reduced the thickness of CDW.And the tidal mixing process in the bottom off the CRE in spring had no significant effect on the vertical mixing of surface CDW.The monthly variation of SPM off the CRE in spring was mainly controlled by the strength of tidal mixing.During the spring tide,the tidal mixing was stronger and the SPM is the highest overall.Due to the barrier of sediment front off the CRE,the sediment carried by the runoff was blocked,resulting the majority of SPM transported and settled in the coastal area,not expand with the CDW to the northeast.As a result,the SPM in section 12250 E near the shore was about 10～100 times than that in section12300 E off the shore.At the same time,the upwelled TWC with monthly enhancement off the CRE slowed down the settlement of SPM.It strengthened the thermocline between the TWC and the upper CDW,and restricted the upward diffusion of SPM near the bottom,resulting in a high concentration of SPM in the central area of the upwelling current.During the investigation period,the source of SPM in the two sections was different,which led to the obvious difference in the composition of SPM.According to the grain size,the SPM could be divided into fine component(<128 μm)and coarse component(>128 μm).The surface SPM of section 12250 E was mainly affected by the riverine sediment of the Changjiang River,while the bottom was strongly affected by the resuspension of submarine sediments.According to the correlation analysis between turbidity,chlorophylla(Chla)and cumulative volume concentration(VC),the suspended particles in this section were mainly terrigenous and resuspended fine sediment particles and its flocs.Section 12300 E was layered.The fine component in lower layer was suspended sediment particles and its flocs,while the fine component in CDW layer was phytoplankton particles containing Chla and its flocs.In the study area,the correlations between the large particles and Chla,turbidity were very low,and there was an apparent negative correlation between the effective density((35)r)and average grain size.It was inferred that the large particle component was organisms with larger particle size or bioflocs with biogenic particles.The diversion of CDW and its sedimentary environmental effects off the CRE are the main factors affecting the distribution of seabed sediments in this area.Because the terrigenous fine sediment carried by the CDW was mainly transported and settled in the nearshore area,the seafloor sediment from the CRE to the inner continental shelf was mainly composed of fine particles such as silt and clay from the Changjiang River.However,due to the long-term lack of modern sediments,the sediment of outer continental shelf was mixed with residual sand.