| As a unique tectonic unit,the Tibetan Plateau plays an "amplifier" role in climate change,making it an important record of environmental evolution.In paticular,the Zoige Basin,which located on the east margin of the Tibetan Plateau,is sensitive to the climate changes.The previous research revealed that there is a lake in the Zoige Basin for along time.Then the basin dissected,and the lake disappered,which were replaced by the Yellow River,resulting a great change in environment.However,the details of the procedure are still unresolved,such as the age of these events and so on.Based on the sedimentology and chronology studies of the sediments,the age of the lake-river evolution and the climate changes were discussed.These results are of great significance in understanding the formation of the Yellow River's source area and the environmental evolution since the late Pleistocene of the Tibetan Plateau.Field investigations were carried out in the Zoige Basin and the valley of the Yellow River.Two typical riverbank profiles were selected as research objects.Sedimentology stratigraphic characteristics,particle size distribution,magnetic susceptibility,mineralogical analysis,geochemical elements,and OSL dating were studied.A comprehensive analysis and demonstration of the environmental evolution after the disappearance of the palaeo-lake were conducted.The main conclusions are as follows:(1)The OSL dating of different sediments was studied by SAR.260? was chosen as the preheat temperature and 220? was chosen as the cut heat temperature,which were determined by preheat plateau and dose recovery test.The CAM model or MAM model was used to calculate the equivalent dose for different samples,depend on the comprehensive analysis of quartz OSL signal,De versus sensitivity-corrected natural OSL signal,histogram characteristics,radiation map and over dispersion(OD)of equivalent dose.As a result,the age of JYM profile is during 17.87±1.67 ka to 1.66±0.11ka,and that of DEQ profile is during 13.27±1.16kato 3.68 ± 0.24ka.(2)Based on the OSL datings,the stratigraphic sequences of the study area and the ages of significant event were deduced:Palaeo-lake(>15.3ka)?peat and aeolian sand(15.3ka?13.8ka)?Overbank flood deposit or Slackwater deposit(13.8ka)? flood plain-aeolian sand layer(15.3ka?13.8ka)? Overbank flood deposit or Slackwaterdeposit(13.8ka)?Aeolian Loess Layer(13.8ka?9.2ka)? Holocene paleosol(8ka?2ka)?modern soil(2ka?0ka).(3)Based on the stratigraphic sequences,the date of extinction of the palaeo-lake and the Yellow River occurred over the Zoige Basin was deduced.OFD1 or SWD1,which are at the bottom of the profiles,are undoubtedly the sediment left by the palaeo flood,so they are the direct evidence of the apperance of the Yellow River.The OSL ages of the palaeo flood sediments were during 13.59 ± 1.2 ka to 12.98 ± 1.0 ka(TWR),13.27 ± 1.2 ka(DEQ),and 13.66 ±1.20 ka to 14.61 ± 1.17 ka(JYM).In summary,the overall ages range from 12.98 ka to 14.61 ka.Therefore,it was concluded that the transiting time of the Yellow River was no later than 13.8 ka BP.This age corresponds to the B(?)lling/Aellrod warm stage of the last deglaciation,suggesting that the flood event may be related to the large-scale melting of mountain glaciers in the upper reaches of the Zoige Basin.(4)The environment evolution since the extinction of palaeo-lake was reconstructed.It can be roughly divided into the following stages:Before 15.3ka BP,the palaeo-lake was the end of the internal water system at that time.In the environment of still water deposition,the silt-clayey silt sediments with high magnetic susceptibility were formed.During 15.4ka?13.8ka:the palaeo-lake was discharged and its bottom was exposed.In some low-lying areas of the exposedlake bottom,the peat was developed.At the same time,aeolian sand was deposited crosswise because of strong wind action.Around 13.8ka BP,the Yellow River flood occurred,indicating that the Yellow River already flowed through the Zoige Basin.During 13.8ka BP?9.2 ka BP:corresponding to the transition stage from last glacial decay period of B(?)lling/Aellrod to YD period,the climate was dry and cold.Affected by wind force,loose sediments,such as river sand and lake sand,which distributed over a large area on the surface,were moved and stacked,forming sand dunes of various size.Around 9.2ka BP,the Yellow River flood occurred again.During 9.2ka BP?8ka BP:without flood,the deposit environment became stable again,and the eolian loess and eolian sand of spot-like distribution formed,which were transformed from floodplain and river flood sediments.During 8ka BP?2ka BP:with stronger East Asian monsoon,precipitation increased,so the sand activities weakened,and the Holocene palaeosol formed.During 2ka BP?0 ka BP:along with the end of the Holocene climate optimum,the modern meadow soil formed. |
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