AN ORGANIC CARBON ISOTOPIC RECORD FROM SEDIMENTS OF THE HUANGQIHAI LAKE,INNER MONGOLIA: IMPLICATIONS OF ENVIRONMENTAL EVOLUTION

Section(H3) at 1.64 m depth from the south bank of Huangqihai Lake,Inner Mongolia recorded paleoclimatic and paleoenvironmental changes during 8.3 and 2.2 kaBP.Bulk organic carbon isotopic composition((δ~(13)C_(org))) of H3 section is analyzed and the relation between(δ~(13)C_(org)) record from the shallow closed-drainage lake and environmental evolution is discussed.The positive values of(δ~(13)C_(org))indicate that the climate in the period of time was cold and dry,while the negative values of(δ~(13)C_(org)) indicate that the climate in the period of time was warm and wet.Results suggest that the climate at Huangqihai Lake was warm and wet during 8.3 and 7.4 kaBP,and(δ~(13)C_(org)) values of the sediments are 0.46‰ lower than the average in the whole period of time of study.There is about 2.66‰ shift in(δ~(13)C_(org)) values in the whole period of time,and it was mostly related to the rapid increase in the organic productivity of the paleo-lake.The substantial fall in(δ~(13)C_(org)) values of the sediments from Huangqihai Lake of 8.2 kaBP provides evidence for an abrupt warming of the regional climate at that time.The period(8.2～4.0 kaBP) is Megathermal Maximum in Holocene and is regarded as the period of high stand of Huangqihai Lake.Thereinto,it was the warmest at about 7.8 kaBP.During 4.0 and 2.2 kaBP,the δ~(13)Corg average value is-22.8‰(obviously positive),which indicates that the climate was quite cold and dry after 4.0 kaBP.The Megathermal Maximum period ended and the climate drastically deteriorated after 4.0 kaBP.Variation in(δ~(13)C_(org))during the whole period of time is characterized by both negative and positive excursions from the line of the average(δ~(13)C_(org)) value of(-23.5‰.) These excursions may principally reflect the change in productivity and preserving condition of organic matter in the lake,which in some way link to the climate,especially to the lake-level fluctuations.