Vegetation And Climate Changes Of Last 8660 Years In Central Mongolia, A Paleoecological Investigation Of Lake Sediment Of Ugii Nuur

The study of the temporal and spatial Holocene climate change on the Mongolia Plateau,which is influenced or controlled by the Asian Monsoon and westerlies related with the NorthAtlantic Oscillation,is vital for us to understand the interacting history of aforementionedclimate sub-systems and the interacting history of the air mass of the high and low latitude.However,the relatively poor level of the Holocene research in Mongolia,which ischaracterized by the uneven distribution,low resolution,poor chronology control,low proxysensitivity and contradictory results,greatly constrained us from understanding the Holocenetemporal and spatial climate changes and associated dynamics of the Mongolia Plateau.Therefore,we palynologically studied the 854-cm-long core of Ugii Nuur in centralMongolia.Based on 9 robust AMS ¹⁴C dates and under the reference of regional modernpollen investigations,detailed vegetation and climate changes over the last 8660 years werereconstructed.In addition,diatom,grain size and other proxy records were provided tosupport or multi-proxy reconstruct the climate and environmental change of the Ugii Nuurarea.Regional published Holocene climate data were reviewed and analyzed to discuss thepossible temporal and spatial laws of Holocene climate changes throughout the MongoliaPlateau and associated dynamics.The primary conclusions are as follows:1.The modern surface pollen survey on the transect from Gun Nuur to Ugii Nuur definesthe characters of pollen assemblages of conifer forest,forest steppe,montane steppe,steppeand desert steppe.Those include,1) the Pinus pollen is over-represented and the Poaceaepollen is normally under-represented; 2) the Chenopodiaceae pollen show higher percentagesin steppe than that in the forest steppe and montane steppe,and this could be used as a proxyto reflect the regional vegetation and climate moisture conditions; 3) the mesophytic forbs andCyperaceae pollen show higher percentages in the montane steppe and montane forest steppethan in the steppe,probably suggest that their percentages could be used as a auxiliary proxyto reflect climate conditions.The modern pollen survey on different type of modern sedimentsin the Ugii Nuur area suggests that,1) the Pinus pollen might have external origin and itsvariation could reflect the condition of regional montane forest steppe or large scale climateconditions; 2) the Poaceae pollen is under-represented with percentages less than 10% even inthe Poaceae plant dominated grass steppe; 3) the Chenopodiaceae pollen percentages could beviewed as a reliable proxy to reflect the regional moisture conditions. 2.The vegetation and climate changes of last 8660 cal.yr BP in central Mongolia werereconstructed based on the modern pollen survey and reliable chronology.Between 8660 and7800 cal.yr BP,Poaceae-steppe developed,accompanying wetland meadow presented in theriverbank,lakeshore and valley lowland,montane forest steppe grew in the nearby mountains,suggesting a mild and semi-humid climate prevailed with a noticeable cool and humidinterval 8350～8250 cal.yr BE During 7800～6860 cal.yr BP,xerophytic plant increased andthe climate became warm and dry gradually.From 6860 to 3170 cal.yr BP,semi-desert steppeexpanded,regional montane forest steppe retreated,suggesting a prolonged warm and dryclimate.Between 3170 and 2340 cal.yr BP,regional forest steppe expanded whereassemi-desert steppe retreated,indicating the climate became cool and wet gradually and thehumidity reached the maximum at the end of this stage.During 2340～1600 cal.yr BP,Poaceae steppe dominated whereas wetland meadow expanded and the montane forest stepperetreated,suggesting a cool and wet climate prevailed,wormwood grass steppe prevailed andthe climatic instability increased after 1600 cal.a BEThe climate and environmental reconstructions by pollen were supported by the diatomrecords.A relative high lake level and moister climate prevailed between 8660 and 7230 cal.yr BP as inferred by the dominance of planktonic diatom genera and high planktonic/benthicratio (〉40).low lake level and dry climate sustained during 7230～2330 cal.yr BP assuggested by the low abundance of planktonic diatom and low planktonic/benthic ratio (～20);and the lake returned to high level during 2330～930 cal.yr BP as inferred by there-dominance of the planktonic diatom and the high value of palnktonic/benthic ratio.The pollen-based climate reconstruction was also consolidated by the grain size andPediastrum concentration records.A relative low lake level between 8660～6010 cal.yr BPwas reflected by the clayey silt sediment with the median size less than 25μam and the sandcontent less than 4%,and the Pediastrum concentration was normally low.Between 6010 and2250 cal.yr BP,the median size and sand content increased greatly,and the Pediastrumconcentration raised sharply,suggesting a relative low lake level.After 2250 cal.yr BP,boththe median size and the Pediastrum abundance abruptly declined,suggesting the lake returnedto high level and the climate changed to humid.Remarkable differences of climate phaseswere also observed among the pollen records,diatom records,grain size records and thePediastrum records. 3.The pollen and diatom records from UG04 core suggest that a prolonged dry climatemight prevailed between 6860 and 3170 cal.yr BP in the central Mongolia.Reviews andcomparable analysis of the regional climate data show that a dry mid-Hoiocene climate mighthave prevailed in vast areas of the Mongolian Plateau,which centered at southern Mongoliaand extended northward to central-north Mongolia and southward to arid areas of west China.In contrast,the northernmost Mongolia and adjacent south Siberia might have experienced awarm and wet middle Holocene.Thus,the north boundary of the dry mid-Holocene might bedelimited at the transitional areas between forest steppe and Taiga forest in northern Mongolia.And the difference between the two might be attributed to the fact that the areas to the northof the boundary might be more influenced by the vapor input from Siberia lowland.Meanwhile,the south boundary of this dry phase might be in the southern boundary of desertin northwest China according to the climate records.4.The warm and dry climate in mid-Holocene in Ugii Nuur is well correlated to theclimate conditions simulated by GCM climate model by Bush.A reasonable mechanism tothis warm and dry climate was proposed by Bush,i.e.,the combination of still high insolationand the increased atmospheric CO₂ concentration resulted in the increase of atmospherictemperature,and the rising temperature led to the enhancement of transpiration and theincreasing of saturation vapor pressure,thus decreased the cloud cover and precipitationchances,and formed the dry climate in the end.5.The climate change reconstructed by the Ugii Nuur pollen records is characterized byasynchronous water and heat combinations,i.e.,warm/dry and cool/wet climate.Thisasynchronous water and heat combination was different to the climate pattern of southernSiberia and Chinese Monsoon areas,but it could be related to the decade to century-scaleclimate change pattern in Xinjiang and central Asia as revealed by Yang et al.6.An anti-correlation existed between the moisture variation revealed by theChenopodiacae percentages of UG04 core and the Asian Monsoon intensity suggested by thestalagmite oxygen isotope data both in the millennia-scale trend and decade to century-scaleabrupt climate event.