Late Pleistocene and Holocene environmental changes from sediment records of the Lake Dood(Darhad Basin)in northern Mongolia

The paper presents new results in geochemistry and diatoms analyses of the short sediment cores,obtained in 2005 from the Lake Dood(Darhad Basin).The names,length and locations of cores are as following;Core-1,613 cm,51°24’26.0 "N,99°19’30.7" E;Core-2,380 cm,50°23’43.4 "N,99°21’33.7" E;Core-3,150 cm,51°24’03.0 "N,99°21’17.07" E.The core samples have been subjected to lithology description,determinations of water contents(WC%),organic matter,total organic carbon(TOC),total nitrogen(TN),C/N and total sulfur(TS) contents with aims of recording paleo-environmental changes in northern Mongolia.Water and organic matter were determined using drying oven and furnace,while total carbon(TC),total nitrogen(TN) and total sulfur(TS) contents were analyzed by Elemental Analyzer(Flash EA112 series CHNS-O).The diatom study shows that the sediments of the Dood Lake contain diatoms belonging to the 24 genus and 50 species.Generally the lithology of the cores(from bottom to top) consists of alternations of sand,clayey silt,sandy silt and silty clayey units in accordance with visual lithology descriptions.The most full geochemistry analysis have been performed for Core-1 which has several age dating,and in accordance with those data the core covers about14400 a BP and upper 160 cm sediments belong to the Holocene,while lower part of the core sediments had been accumulated during the late Pleistocene.We here mainly concentrate on problems how the organic geochemistry contents and diatom abundances distribute along the cores,what information can be extracted from them,they are consistent or not with information got from other analysis etc.The TOC and TN contents in the Core-1 are very changeable from 1.32%to 6.59%and from 0.139%to 0.512%,respectively.Especially wide variation in contents of the TOC and TN revealed for upper 300 cm of sandy silt and silty clayey units in comparison to the lower 300 cm where their contents are relatively uniform and low.From this point of view,for core sediments could be suggested two completely different sedimentation environments,although there are some quite sharp oscillations in contents of the TOC and TN for upper 300 cm.If we assume that the TOC and TN contents indicate biological productivity linked with certain climate and paleo-environmental condition,than at least 4fluctuations could be recorded for upper 300 cm.They are(from top to bottom) 043 cm,43160 cm,160220 cm and220290 cm with average TN and TOC about 0.3%and 3%;0.4%and 4.8%;0.3%and 3%;0.5%and 5%respectively.For lower part such sharp oscillations have not been observed although at interval of 460-613 cm some increase and decrease in TOC and TN contents are visible.Attempt of the interpretation of these changes in TOC and TN contents with taking account into other proxies data such as TIC,TS and diatom analysis resulted in revealing at least two high stand and two lower stand conditions for the upper 300 cm section of the sediment.High stand condition revealed at intervals of 290220 cm and 16043 cm based on the highest concentrations of TOC and TN accompanies with high abundance of fresh water diatom species of Acbnanthes minutissima,Cocconeis placentula,Cyclotella ocellata and high of TIC,low of TS contents,while the lower stand conditions at intervals of 043 and 160220 associate with high concentration of TS and salinity indicator species Amphora veneta,Fragilaria construens,Fragilaria pinnata.However the available data in ages are quite complicated,main of them had been determined in reworked detritus organic materials,so far it is difficult to tie to the time.Nevertheless,in accordance with the age model suggested by Naran et al,16043 cm intervals of high stand probably belong to the early Holocene,while 290210 intervals are presenting the one of the transitional events between Holocene and late Pleistocene glacial.Also we have noted some increase in TOC and TN contents at intervals of 520-445 cm with high contents of TC and TIC.So far the results of geochemistry and diatom parameters are one of quite potential proxies for recording paleo-environmental changes.