塔里木盆地南缘黄土粒度特征及其环境意义

Grain-size Characteristics and Its Environmental Significance of Loess from Southern Tarim Basin

黄土是地质时期沙尘天气形成的粉尘堆积物,其粒度特征记录了粉尘搬运过程中的古大气环流格局和古环境演化信息。本文对昆仑山北坡克里雅河支流的第5级阶地羊场黄土剖面进行了系统的粒度分析,结果表明,黄土粒度组成主要以粗粉砂和砂砾为主,分选较好;羊场黄土为塔克拉玛干沙漠的近源风成堆积,粒度指标主要指示沙漠范围和源区气候干旱程度;粒度结果的详细分析及其与全球其他记录的对比表明,塔里木盆地南缘气候在8.5~3.6 ka B.P.较为湿润稳定,3.6 ka B.P.以来气候急剧变干并出现大幅度波动的千年尺度突变事件,与其他地区古气候记录有较好的一致性。塔里木盆地南缘8.5 ka B.P.以来的气候变化具有季风模式和千年尺度气候振荡的双重特点,可能同时受低纬度亚洲季风和北半球高纬度地区气候的影响;太阳辐射减弱引起的亚洲夏季风衰退可能导致了3.6 ka B.P.以来盆地的干旱化,同时,受太阳辐射驱动的北半球高纬度地区的气候变化,可能通过增加西风环流的强度及其波动幅度,进一步加剧了3.6 ka B.P.以来塔里木盆地南缘气候的干旱化和不稳定性。

Loess deposit can be formed at geological periods during sandstorm weather and its grain size characteristics can record the atmospheric circulation patterns and paleoenvironmental evolution information during the dust removal process. The Yangchang（ YC） loess section in the village of Pulu is situated on the fifth terrace of a tributary of the Keriya River. YC loess is mainly composed of coarse silt and sand with high degree of sorting. The grain size analyses of YC loess indicate that the loess belongs to aeolian deposits from the Taklimakan Desert. Grain-size distribution of YC loess section is mainly controlled by the aridification of the source area,and thus,can indicate the drying process of the south edge of the Tarim Basin. Analysis of the grain-size distribution indicates that the southern margin of the Tarim Basin was relatively humid and stable during 8. 5 ~ 3. 6 ka,and during 3. 6 ~ 0 ka the climate became dramatically arid with several millennial-scale events,which is in accordance with records from other study areas. We think that the climate at the southern margin of the Tarim Basin has features of both the Monsoon mode and millennial-scale climate oscillation,and might be controlled by both the low latitude Asian Monsoon and high latitude climate from Northern Hemisphere. We speculate that decrease of the ASM intensity in relation to weakening of the solar activity might have caused the aridification of the Tarim Basin since 3. 6 ka. And the climate change in high latitude of the Northern Hemisphere might have further strengthened the drying trend and the climate instability since 3. 6 ka through intensifying the Westerlies and its amplitude.