柴达木盆地东部36～18kaB．P．期间的孢粉记录及其气候环境

POLLEN RECORD REFLECTS CLIMATE CHANGES IN EASTERN QAIDAM BASIN DURING 36～18kaB.P.

对柴达木盆地察尔汗古贝壳堤剖面的沉积物进行孢粉分析,结果显示这个地区植被与气候在晚更新世中晚期经历以下几个阶段的变化:36.2～31.2kaB.P.(未校正14C年代,下同)期间,以禾本科、藜科、蒿属、莎草科为主,发育草原-草甸植被,气候温和湿润,盘星藻出现较多,反映淡水湖泊,水深在10m左右;31.2～27.6kaB.P.期间,松属、云杉属、桦属等为主的木本植物的含量增加,周围山地森林发育,表明气候温暖,降水量增多。但由于蒸发量大,有效湿度下降,荒漠成分柽柳属等增加,盘星藻在30kaB.P.以后消失,反映湖泊盐度增大;27.6～23.3kaB.P.期间,植被中荒漠成分显著增加,周围山地森林萎缩,气候趋向相对寒冷干旱,湖面积缩小;23.3～18.0kaB.P.,孢粉浓度很低,蒺藜科、藜科等荒漠成分明显增加,植被稀疏,已趋向荒漠化草原,反映气候寒冷干旱。从整个剖面来看,主要的陆生植物孢粉类型为禾本科、柽柳属、蒺藜科、麻黄属、松属、云杉属、柏科、胡桃属和桦属等,藜科和蒿属含量很少,这与柴达木盆地东部地区的表土分析结果完全不同,也与其他草原以及荒漠草原的表土花粉结果相异。这说明晚更新世中晚期柴达木盆地东部地区的植被和现在无法进行比较,气候环境与现在显著不同。

A continuous pollen record from the Shell Bar at the Qarhan Lake in Qaidam Basin provides information on the vegetation and climate changes in the Late Pleistocene. According to pollen percentage and concentrations, four palynological zones were identified. From 36.2aB. P. to 31.2kaB. P., Poaceae （Graminae） , Chenopodiaceae, Artemisia, and Cyperaceae dominated the pollen assemblage and grassy steppe-meadow plant communalities developed,corresponding to a mild and humid climate. The lake was fresh watered and deep. From 31.2kaB. P. to 27.6kaB. P., the percentages of Pinus, Picea, and Betula increased, indicating the expanding of forest to the area surrounding mountainous and the increasing of temperature and the annual precipitation. However, as the evaporation might also increased, effective humidity droped, desert vegetation types such as Tamarix increased, and lake water became salified which caused the disappearing of Pediastrum after 30kaB. P. From 27.6kaB. P. to 23.3kaB. P. ,the surrounding forest shrank and desert vegetation components increased significantly which suggest a dry and cold climate condition. The lake area also shrank. From 23.3kaB. P. to 18.0kaB. P. ,pollen concentrations were very low, and the percentage of Zygophyllaceae and Chenopodiaceae increased, indicating a sparse desert steppe vegetation and an even colder and drier climate. Through the entire profile, the terrestrial pollen types are mainly Tamarix , Zygophyllaceae, Ephedra , Picea , Pinus , Cupressaceae, Juglans , and Betule , Chenopodiaceae and Artemisia which dominate modern pollen spectra in steppe and desert area were rare. The results are different not only quite from surface pollen analysis in the eastern part of the Qaidam Basin, but also from modern pollen records from other steppe and desert vegetation zones in NW China. They show that pollen record from the Shell Bar at Qaidam Basin can not be linked to modern pollen assemblages and the climate reflected by pollen record was significantly different from that of the present in the eastern part of the Qaidam Basin during the Late Pleistocene.