Study Of Identification Of Sediment Source From The Yellow River In The Incised Yangtze Esturine Valley And Its Transport Mechanism During The Early To Mid-holocene

As an important interaction zone between land and sea,estuarine areas are not only the sink of the materials from the Yangtze River into the sea,but also they are channels for exchanging sediments from sea to land.During the period of Early to Mid-Holocene transgression when the sea level rose quickly,the contribution of the strong marine power carrying sediment to the Yangtze River Estuary needs to be considered.It has important scientific significance of this issue,which can help understand the historical characteristics of sediment discharge ed into sea from Yangtze River and revealed geomorphologic evolution during the Early to Mid-Holocene.Furthermore,the relevant assessment can also enhance better understanding of delta evolution in association to sea-level rise.The present study is based on construction of the Yangtze River and the Yellow River geochemistry and heavy mineral source indication system.The paper aims to difine how the sediment from the Yellow river.and was there sediment from Yellow River in the Yangtze incised valley in the Early to Mid-Holocene.In order to achieve our goals,analyzed geochemical elements and heavy mineral of samples from the 4 boreholes taken in the Yangtze River Estuary.Considering the grain size of sediment carried by marine dynamics,the paper analyses the geochemical elements of fine partical residual sediment(<4 ?m).we also propose that the Yangtze River Sediment has higher Ti,Ba,Zr,V,Cr,Ni content generally than that of the Yellow River.Among them,the content of Ti in the samples of the Yellow River is 4050-4650 mg/kg,the Ba is 442-486.5 mg/kg,the Zr is 131-162 mg/kg,the V is 114-121 mg/kg,the Cr is 92-94 mg/kg,the Ni is 18.5-22 mg/kg.The content of Ti element in the Yangtze River samples is 6620-7520 mg/kg,Ba is 610-670 mg/kg,Zr is 173-193 mg/kg,V is 148-157 mg/kg,Cr is 107-115 mg/kg,Ni is 27-32 mg/kg,the content of Ti in the Yellow River is about 60%-70%of the Yangtze River,and thecontent of Ba,Ni,V is about 70%-80%of the Yangtze River,and content of Zr,Cr is about 80%-90%of the Yangtze River.These provide key sediment criterion for sediment source identification between the two major rivers.Here,we should notice that the grain size and the pretreatment methods have obvious influence on the geochemical elements in the sediments of the Yellow River and Yangtze River,so the two factors are taken into accounted when we discriminate provenance.In addition,there are some differences in heavy minerals of coarse sediment from the Yellow River and the Yangtze River.Titanite is the diagnostic heavy minerals for the Yangtze River,and the maturity(ZTR)of heavy minerals also serves as the key indicator for the Yangtze River.In contrast,apatite and garnet are characteristic heavy minerals of the Yellow River,and the ATI and GZI index are higher in the Yellower River than that of the Yangtze River.The both geochemical elements of fine-sediment and heavy minerals of coarse grain sediment in the Yangtze River estuary show that the dominant material sources were mainly from the Yangtze in the early Holocene(12.0-10.0 cal.kyr BP).The reason is that sea level rising changed the paleo-incised valley to a tidal dominated river,which attracted high Yangtze sediment accumulation during this period.This was followed by the mixed sediment source in paleo-incised valley from the Yangtze and the Yellow during 10.0-7.5 cal.kyr BP,when the Yangtze River mouth became a typical tide-dominated estuary.The longshore and tidal currents can carry a massive suspended river sediments from the the South Yellow Sea into the paleo Yangtze incised valley.After this time,the Yangtze River sediment became the dominant source in the valley again,and the mixed source of theYangtze and the Yellow only appeared in the northern part of subaqueous Yangtze delta.The sediment contribution from the Yellow River weakened after the river flowed into the Bohai Sea.High sedimentation of the Yangtze sediment sources maked the rapid progradation of its delta coast.In summary,the sediment contribution from Yellow River in the Yangtze paleo incised valley can be 30-60%during the Early to Mid-Holocene.The influence on topographic formation of the paleo incise valley gradually increased landward over time.The contribution also closely relates to the sea level rising in the Yangtze River Estuary in the Mid-Holocene.With continuous retreat of the shoreline,the coastal currents and tidal currents carrying sediment in the Yellow River are also advancing towards the land.