新疆玛纳斯湖沉积柱样的古气候古环境研究

A PALAEOCLIMATIC STUDY ON LACUSTRINE CORES FROM MANAS LAKE, XINJIANG, WESTERN CHINA

本文提供玛纳斯湖两个长约５ｍ柱样的研究结果，并据此讨论新疆西部晚第四纪古气候演化。研究内容包括沉积物的矿物组成、自生碳酸盐的稳定同位素组成、孢粉组合等；用加速器质谱（ＡＭＳ）测定碳酸盐或有机质的（１４）Ｃ年龄。讨论了δ（１８）Ｏ剖面与亚洲夏季季风的关系。柱样的研究结果表明：玛纳斯湖地区晚更新世的３７０００─３２０００ａＢ．Ｐ．气候潮湿，而后又回到末次冰期的极端干旱气候。１２０００─１００００ａＢ．Ｐ．一个短暂的暖湿事件以后是以干旱为主的晚更新世─全新世过渡期；约在１００００ａＢ．Ｐ．气候明显转为温暖，草原植被取代了荒漠植被，覆盖了整个玛纳斯湖流域，沉积物自生碳酸盐的δ（１８）Ｏ低值对应暖湿气候，为亚洲夏季风盛行期；早、中全新世气候比现时暖湿，但也有几次短时间干旱。６０００─５０００ａＢ．Ｐ．有一个湖泊收缩期，玛纳斯湖地区又回到沙漠化植被；第二潮湿事件发生在４５００─２５００ａＢ．Ｐ．，其间大约３８００─３５００ａＢ．Ｐ．也曾有短时间干旱。全新世最后的２５００ａ气候不稳定，并逐渐过渡到湖泊的盐化阶段。

This paper deals with the study of the lacustrine cores from the Manas Lake and the Late Quaternary palaeoclimatic evolution in western Xinjiang. Analyses were performed for the mineralogical compositions of the lake sediments, the stable isotopic contents of authigenic carbonates, and pollen influx and types and so on. The chronologic frame was extablished based on AMS 14C data on authigenic carbonates and organic matter. The palaeoclimatic significance of the δ18O profile of carbonates was enhanced in view of its close correlation with the strength of the Asian summer monsoon. Palaeoclimatic records from the Manas Lake suggest the presence of a Late Pleistocene humid episode during 37000 to 32000 a B.P., followed by a period of extreme aridity which was attributed to the Last Glacial Maximum. After a minor wet/ warm pulse occuringabout 12000 years ago, there was a Late Pleistocene-Holocene Transition period characterized by arid climate. The most important environmental change was recorded at ￣10000 a B. P. The climate apparently got warmer and wetter since then, finally leading to the development of a steppe vegetation cover around the lake catchment. The low δ18O values of the authigenic carbonates, corresponding to wet / warm climate, reveal a maximum strength period of the Asian summer monsoon. The Early-Middle Holocene records indicate that the climate during that period was warmer and wetter than today, but it was punctuated by several short terms of arid conditions.Around 6000-4500 a B.P., a contraction period affected the Manas Lake and coincided with the return to desert vegetation.The second wet episode took place from ￣4500 to 2500 a B.P., but it was interrupted by a dry spell from 3800 to 3500 a B.P.The record of last 2500 years shows climate unstability and the stepwise establishment of hypersaline conditions in the Manas Lake.