Sedimentary Characteristics And Signiifcance Of Provenance And Sedimentary Environment Since The Late Pleistocene In The Western South Yellow Sea

Based on the grain size, clay minerals, detrital minerals and chemical elementsanalyses of surface sediments as well as the data of temperature, salinity, turbidityand suspended matter concentration in the western South Yellow Sea (SYS), theprovenance of surface sediments and the influence of circulation system on sedimentdistribution were discussed. The formation processes and reworking of the relictsediments were deciphered with the results of AMS14C dating. According to thegrain size and mineral distribution characteristics of two gravity cores, thesedimentary evolution of the western SYS since the late Pleistocene werereconstructed.The sediment distribution in the study area is mainly affected by the circulationsystem in the SYS and its seasonal variations. The hydrodynamic is weak in thecentral SYS because of the presence of the Yellow Sea Cold Water (YSCW) inspring. While in autumn, the emergence of the Yellow Sea Warm Current (YSWC)changes the structure of the circulation system. However, the flow path of the YellowSea Coastal Current (YSCC) is relatively stable in different seasons. The SubeiCoastal Current (SCC) flows northward in summer while southeastward in otherseasons. The seasonal variations of the circulation system lead to the different grainsize distribution characteristics of surface sediments in spring and autumn in theSYS.The clay minerals in the study area are mainly composed of illite (average67%),followed by smectite (average14%), chlorite (average11%) and kaolinite (average8%). The main sources of the sediments in the study area are: the modern YellowRiver (Huanghe River) substances carried by the YSCC, the sediments in the oldYellow River subaqueous delta, the relict sediments, part of the Yangtze River(Changjiang River) substances carried northward by the SCC, coastal riverdischarges and coastal erosion substances, which can be judged from the claymineral end-members figure. The impact of the Yellow River (including the oldYellow River substances) is greater than the Yangtze River. The northward spread of the old Yellow River substances and the Yangtze River substances carried by theSCC can mix with the relict sediments. The clay content distribution indicates thatthe branch of the YSCC along the south coasts of the Shandong Peninsula turnssoutheastward off the Qingdao coasts and disappears gradually.The detrital mineral analysis results show that the light minerals in the studyarea are mainly composed of plagioclase (average50.7%) and quartz (average28%).There are32kinds of heavy minerals in all, in which hornblende has the highestcontent (average37.7%), followed by epidote (average31.3%). The distributioncharacteristics of quartz/feldspar ratio in light minerals as well as the dolomitecontent in heavy minerals indicate the northward spread tendency of the YangtzeRiver substances. Controlled by the mineral differentiation, the chlorite distributioncharacteristics in clay minerals, light minerals and heavy minerals are different,which can indicate the spread and influence scopes of the modern Yellow River andthe old Yellow River substances.The provenance index value of PI was used to discriminate the scopes of theYellow River and Yangtze River substances. The results show that the Yangtze Riversubstances can reach the northern Jiangsu coasts and Haizhou Bay, which isconsistent with the results made by the clay minerals as well as the heavy minerals.Because of the seasonal variations of the circulation system, the scopes affected bythe Yellow River and Yangtze River substances are different. The Yangtze Riversubstances can reach the central SYS in autumn. The distribution characteristics ofChemical Index of Alteration (CIA) as well as the comparisons of REE normalizedpatterns between the Yellow River, Yangtze River provenance samples and theYellow River, Yangtze River substances further support the results made by the PIprovenance index.The silty sand distribution with the Folk classification can represent the area ofrelict sands. The age of the calcareous nodules is between15000and30000a B.P.with the judge of AMS14C dating results, during which the SYS was exposed as landand the calcareous nodules were formed after the loess deposition and/or duringpedogenesis. Moreover, the age of calcareous nodules becomes younger from shore to sea. Between about11-7.5ka B.P., the ancient Yellow River delta was formed andreworked during the sea level rise. The fine materials were taken away, while thecoarse remained in the place. Then the relict sands in the study area formed thereby.The modern substances are gradually mixing with the relict sediments with theinfluence of currents and waves.The sedimentary evolution of the western SYS since the late Pleistocene hasbeen discussed based on the vertical distribution characteristics of grain size andminerals as well as AMS14C dating results in two gravity cores. The results show thatthe sedimentary evolution of depositional unit DU2(10178-6057cal.a B.P.) wasmainly affected by sea-level changes. While after the establishment of the moderncirculation system in the SYS, depositional unit DU1(after6057cal.a B.P.) wasmainly affected by the material sources and circulation system. The threesub-depositional units of DU1reflect the impact of the Yellow River channeldiversions. The sensitive grain-size population was observed on the StandardDeviation-Grain Size Classes plot. There is a very good correlation between thesensitive fine fraction content changes in the depositional unit DU1-2(2049-499cal.a B.P.) and Chinese historical climate fluctuations. The channel diversion of theYellow River into the SYS in1494AD had greater impact on the depositionalenvironment of the study area.In general, the depositional environment of the study area was affected by thesea-level changes, the erosion and transformation of the ancient Yellow River delta,the channel diversions of the Yellow River as well as the climate changes prior to theestablishment of the modern circulation system. After the modern circulation systemformed, the study area was mainly affected by material supply and the impact of thecirculation system. During the modern deposition process, the sources in the studyarea are mainly from the Yellow River (including the old Yellow River subaqueousdelta), and the northward spread of the Yangtze River substances also have someimpact. The seasonal variation of the circulation system leads to seasonal differencesin the sediment distribution.