Lake Dood is a fresh water lake,remnant of a large glacial lake in the Darkhad basin.Northern Mongolia is located at the coordinate of 51°20’ N,99°23’E,in 1538 m a.s.l..The length of the lake is 7.5 km with mean width2.4 km（maximum width is 5.6 km）.Mean depth of the lake Dood is 4.7 m,but deepest point is 17 m.The total area is about 41.3 km;.The climate of the Darkhad Basin is extremely continental with a mean annual temperature range of 40 ℃ and a mean annual precipitation of 300;00 mm.Thus,the lake Dood occupies a transition zone between mountain and forest steppe ecosystems to the south and the coniferous forests to the north.Because of its unique geographic location,this region is very sensitive to climate changes and ideal place for paleoclimate investigation.This paper makes an attempt to reconstruct the paleoclimate changes in Early Holocene and Late Pleistocene based on results of isotope geochemistry analysis of the core 1,obtained from the Lake Dood in North Mongolia,during the Mongolian-Russian joint expedition in summer of 2005.The core 1 with length of 645 cm was taken from northern part of the lake,at coordinates of 51°24’26.0 "N;99°19’ 30.7" E.Water depth is 3.5 m.The cores were cut lengthwise and after lithological description,it had been subsampled in 2 cm intervals.The water,organic matter and carbonate contents of sediments were determined by heating the selected 210 samples（approximately 5;0 cm intervals） at 105℃,550℃ and 1000℃,respectively.Stable isotope ratios of carbon and oxygen on bulk carbonate in 160 sediment samples were determined on Finnigan Mat 252 mass spectrometer at Key laboratory of Lanzhou Institute of Geology,Chinese Academy of Sciences.Isotope ratios are reported relative to the Pee Dee Belemnite（PDB） standard for carbon and oxygen isotope measurements on bulk CO,.Plant fragments,wood debris and shells from 11 samples from different horizons of core 1 were dated by using Accelerator Mass Spectrometry at the University of Arizona.Age model of core is under the discussion at present stage of investigation.Four age dates（8690±45;9590±55;9810±45 and 11270±45） from 449 cm,564 cm,584 cm and 644 cm depth intervals are apparently inconsistent;so far they have been excluded from age model.The preliminary age/depth interpolation based on the ages from shallow depth intervals and the inferred sedimentation rates(mean sedimentation rate is 0.13 cm·a;) suggest that core covers about 14400 a BR The δ;O values of bulk carbonate in the cores of Lake Dood varies between-18‰ and-12.8‰ and the δ;C values vary between- 8.2%o and +0.4%o.The general trends of variation of these isotopes parallel to each other.The covariance of δ;C and δ;O values of sediments for the investigated time intervals shows that the Lake Dood was closed basin.Contents of organic matter and carbonate are around 0.6%;3.6%and 1.42%;4.9%,respectively.Thus the Late Pleistocene and Early Holocene sediments of the core consists of 6 distinct sections based on down core variations of both δ;C and δ;O values and organic and carbonate contents,which in its turn are quite consistent with its lithological composition.Section 1 corresponding to the lower sandy and silty clay layers of 625;20 cm intervals can be divided into 3sub-units,1-1,1-2 and 1-3 based on the change of organic and carbonate contents and as well as isotope values.The sub-unit 1-1 characterized by relatively lower content of organic matter（0.8%） and moderate values of heavier δ;C;（-2.2‰） and 8 O;（- 14.7‰）.Gradual increase in content of organic matter and decrease of average values ofδ;C;（-5.5‰） and δ;O;（-15.2%o） have been observed in sub-unit 1-2.These observations suggest a dry and shallow water environment for accumulation of sediment in sub-unit 1-1 and while slightly increased precipitation and still dryer and cooler climate for sub-unit 1-2.Sub-init 1-3 at intervals of 550;20 cm is characterized by relatively high values of the δ;C;and δ;O;than lower sub-unit 1-2.Especially,samples from this unit shows highest value ofδ;C;,averaging-0.8‰.The δ;C;values are around-14.4‰.This silty clay layer contains about 5%of organic matter and 4.5%of carbonate in average.Thus,the high values of heavy isotopic ratios indicate the gradual exceed an evaporation over a precipitation leading to low stand due to a dry climate condition.In addition,lower content of organic matter suggests that primary productivity probably was relatively lower.This dry and cold period（section1） seems to be corresponds to the Oldest Dryas stadial occurred after the Weichselian giaciation in north Europe approximately in 18000;5000 BP（calibrated）.Section 2,at intervals of 520;60 cm（sandy silt） is characterized by relatively lighter δ;C;and δ;C;values,average of them are 5.4%o,and-15.6‰ respectively,indicating increased lake level due to drop in ratio of an evaporation to a precipitation for this time interval.The relatively higher contents of organic matter（to 7%;%in average）,suggesting increase in primary lake productivity,support this point.Consequently,it leads to conclude that the sediment of section 3 accumulated under the wet and warm climate condition coinciding with Boiling interstadial of the 15000;4000 a BP（14650;4000 a BP） in a result of sharp temperature rise.Section 3 from 460 to 290 cm,corresponds to the 170 cm thick clayey silt layer.In this section,both of carbon and oxygen stable isotope ratios increase from depth level of 450 cm.The average values of δ;C;and of δ;C;are-2.3‰ and-15.1‰,correspondingly.The contents of organic matter and carbonate are relatively constant along the section around and they are 5.4%and 4.7%.So far all above proxies seem to suggest that the silty clay of section3 accumulated under the considerably dryer and colder climate condition accompanied with lowered lake level and primary productivity relatively to the previous period.According to the age model,this abrupt increase in both of isotopic ratios may be resulted from the cooling coinciding with Older Dryas event in Europe.Section 4 is included sediments at intervals of 290;20 cm,where both carbon and oxygen stable isotope ratios drastically drop.Mean values of δ;C and δ;O are-7.0‰ and-16.9‰ respectively.Relatively lighter isotopic ratios(shifts of-5‰ for δ;C and-2‰ for δ;O) indicate abrupt changes toward wet and warm climate corresponding to the European Alleord interstadial.During this time interval lake level was high due to decrease of ratio of the evaporation to the precipitation and also due to ice melting.Section 5,at intervals of 220;60 cm,both carbon and oxygen stable isotope ratios are not much changed,but decrease in organic matter contents probably shows drop of primary lake productivity in a result of cold climate.Relatively higher content of carbonate is indicating increase in input of terrestrial carbonates to the lake.According to the proposed age model this time interval corresponds to the Younger Dryas stadial which is known as cold climate period between 12800 cal a BP and 11500 cal a BP.Section 6,corresponds to the upper 160 cm is separated into 2 sub-units,6-1 and 6-2.Section 6-1,from 160 cm to 42 cm consists of silty sediments（more clayey in 160;35 cm,sandy in 135;2 cm）.Values of isotopic ratios are slightly fluctuating,but much shifts（mean shift of 0.5%;‰） in values have not been observed.Nevertheless,quite sharp increases in of organic matter contents up to 8%;0%suggest high primary lake productivity.In accordance with age model the section 6 with above characteristics corresponds to early Holocene warming.Relatively the higher contents of carbonate may be result of increased input due to ice melting water during the early Holocene warm climate.Section 6-2,at intervals of
