Sedimentary Environment Evolution In The Qidong Area Of The Yangtze River Delta During The Last 1,000 Years And The Influence Of The Old Yellow River

Delta,located at the site of intensive land-sea interaction,is an economic center and a region sensitive to global change.The study of Late Holocene sedimentary process and sediment source is critical for predicting future delta evolution and for managing delta sustainability,particularly for those deltas facing significant fluvial sediment discharge decreasing in recent years.Yangtze River delta,a typical tidal influenced delta system,abundant research has been carried out regarding its late Holocene evolution process.However,the chronological framework and provenance of late Holocene deposits remain poorly studied.In this paper,three late Holocene cores(~25 m long)were collected from Beixin(BX),Miaoqiao(MQ)and Wangbao(WB)in the Qidong city of the Yangtze River delta.Optically stimulated luminescence(OSL)dating,particle size,environmental magnetic and geochemical element analysis were conducted on bulk samples and size-specific samples of three cores.Historical documents combined with analysis of sediments form major tributaries of Yangtze River(i.e.,the Jinsha,Jialing,and Han Rivers)and old Yellow River delta,were used to constrain the chronology and sediment source.Based on these analyses,chronological framework of the late Holocene deposits were established.Sediment source was discussed,especially the influence of the Yellow River sediment discharging into the Yellow Sea in the last ~1000 years.The main results obtained are as follows:1.The OSL ages range from ~190 to 3490 a for the ~25 m core sediments.It reveals that the ages of the delta front and delta plain facies are younger than 500 a(deposition rate is averaged 8-13 cm/a),while the sediments in the underlying prodelta facies are older than 2000 a(deposition rate is <0.5 cm/a).The central core MQ has older and coarser delta front sandy deposits than the neighbouring cores,suggesting that core MQ was located on a sandy mouth bar,while core WB and BX are located within distributary channels of the estuary.The OSL ages of delta plain facies(the top ~8 m layer)becomes younger from the north to the south,which are ~300 a(core WB),~250 a(core MQ)and ~200 a(core BX).This spatial pattern of surface sediment ages is consistent with the historical documents records of coastline change and land forming process.2.Synthesizing the results of magnetic characteristics,redness and FORC-PCA on bulk samples and size-specific samples,it is found that magnetic properties of core BX,MQ and WB are dominated by ferrimagnetic minerals,which are mainly concentrated in the <16 ?m fraction.Magnetic properties of the top ~2 m layer are significantly affected by the pedogenesis,while those of sediments older than ~400 years ago were significantly affected by the early diagenesis.3.There still exist magnetic properties of sediments in three cores for the last 400 years,when the effects of particle size and diagenesis effects are excluded.In the finegrained sediments(<63 ?m),especially in the <32 ?m fraction,magnetic properties of WB sediments at the northern site are similar to the old Yellow River sediments,compared to those of southern core BX sediments,which are more similar to the Han River sediments of the YangtzeRiver.The central core MQ data have intermediate values.4.The geochemical results of bulk samples show that the element content in the sediments is strongly influenced by the particle size.The geochemical characteristics of the <16 ?m fractions for sediments of the recent 400 years show similar source ascription results with those of magnetic results.Quantitative analysis indicates that contribution of old Yellow River sediments to core BX,MQ and WB is 12±9%,44±12% and 64±14% in the <16 ?m fractions,respectively.5.According to the OSL dating results and provenance analysis,we infer that,the old Yellow River sediments contributed to the formation of Yangtze River delta,when it discharged into the Yellow Sea during AD 1128-1855,especially in the recent ~400 years.Once sediments of the Yellow River reached the Yellow Sea,fine-grained sediment can be transported southward by the Yellow Sea Coastal Current.Thus,for a tidal-dominated delta,marine contributions,including longshore transportation of neighbouring delta/estuarine sediment,should not be neglected.