Chronology And Sedimentary Environment Change In The Coastal-shelf Areas Of Eastern China Since 200 Ka

Sea level has fluctuated dramatically throughout the late Quaternary,and the coastal to shelf areas of eastern China have been subjected to alternations of marine and continental environments during the transgressive and regressive progresses.These sedimentary strata contain records of past environments that have the potential to inform our understanding of the evolutionary history of paleoenvironment.The stratigraphy sequence of the coastal to shelf areas of eastern China since the late Pleistocene have received widespread research focus.However,most of these studies focused on the Holocene.Weakness in chronological study and limited correlation in the coastal to shelf areas hinder us to understand the transgressive history since late Pleistocene and sedimentary evolution in response to the post-glacial sea level rise.In this study,nine cores were collected from the coastal to shelf areas of eastern China.In addition to the lithological investigations of these cores,the chronological framework for these cores were also established by combining OSL and AMS ¹⁴C methods.Moreover,23 vibrocores were collected from the muddy area along the Zhejiang coast to analyse their sediment accumulation rates over the past 100 years using ²¹⁰Pb geochronology.According to the lithological and chronological characteristics from all the cores(including some well-studied cores in previous studies),a detailed division and discussion on the sedimentary structure and evolution in the coastal-shelf areas of eastern China since 200 ka have been made.The dating results show that the ages of AMS ¹⁴C are consistent with that of OSL in the last 25 ka.A serious underestimate of ¹⁴C age is always found in old samples(>25 ka),while the OSL dating technique can acquire longer timescale chronological framework and have the potential to elucidate the sedimentary evolution since the Late Pleistocene.Moreover,¹⁴C age reversals are common in complex depositional environments because of the reworking.The OSL ages generally follow the chronological order.Combining chronological and lithological characteristics from all cores,the sedimentary evolution in the coastal to shelf areas of eastern China can be summarized as follow:(1)From MIS 7 to MIS 6,the sedimentary environments in the Zhejiang coastal areas were dominated by fluvial environments in the nearshore areas and tidal flat to littoral environments in the offshore areas.Fluvial and tidal flat environments prevailed in the western South Yellow Sea and Jiangsu coastal plain,and the environment of the Bohai Sea region had been mainly fluvial and tide-fluvial.(2)In MIS 5,the marine sedimentary beds occurred widespread and not appeared simultaneously,reflecting that fluctuations of global sea level.(3)The sea level fall and fluvial sedimentary environment developed in the study area in the period of MIS4.(4)A weak transgression occurred and tidal flat deposits were developed in the modern Yellow River delta in early MIS 3 stage.A large amount of delta strata occurred in the coastal area of Jiangsu and western Yellow Sea due to lots of river-derived sediments input during this period.(5)From middle MIS 3 to MIS 2,fluvial sedimentary environments prevailed and the sedimentary hiatus are common in the study area.(6)During the post-glacial rise in sea level,the sedimentary environments in the coastal-shelf region were transition from fluvial deposition to neritic deposition since MIS 1,forming a series of depositional systems such as tidal sand ridges and coastal muddy deposits.Finally,some major conclusions can be summarized as follows:(1)The MIS 3transgression in western Bohai Sea occurred in early MIS 3 stage and the sea level was estimated between-26.8 and-19.87 m.(2)The sedimentary environment in Yangtze Shoal changed from an estuary to a tidal shelf due to the abrupt sea level rise at?11.5ka,leading to the development of tidal sand ridges until 7.0 ka.(3)The sedimentary signals of the upwelling along the Zhejiang coast were revealed by the modern sediment accumulation rates,grain sizes of surface samples and distributions of Chlorophyll in surface water,and new evidence of the formation of the muddy deposit was presented.