毛乌素沙漠东南缘全新世剖面光释光年代及古气候意义

OSL DATING OF HOLOCENE SEQUENCE AND PALAEOCLIMATE CHANGE RECORD IN SOUTHEASTEARN MARGIN OF MU US DESERT,NORTH CHINA

在毛乌素沙漠东南缘锦界地区发现的具有3层深棕色至黑色古土壤的全新世剖面,记录了至少3次大型沙地固定与活化的交替演化。在锦界剖面厚约5m的全新世地层中采集了10个光释光样品,利用石英光释光测年单片再生法(SAR),建立了锦界剖面全新世(>7.5～0.2ka)年代格架。结合粒度、磁化率气候变化代用指标和光释光年龄序列,得到毛乌素沙漠全新世时期沙漠演化和气候变化记录:7.5ka之前,地表植被稀疏,为气候干旱时期;7.5～3.9ka时段毛乌素经历了一个湿润成壤期,地表植被发育厚层黑色古土壤;约3.9～2.9ka为沙丘活跃期;2.9～1.7ka时段为较弱成壤期,发育深棕色古土壤;1.7ka以来,气候波动频繁,气候环境整体较之前干旱,1.0～0.5ka经历短暂温暖期,200年左右气候再次趋于干旱,但近1.0ka气候记录可能受到了人类活动干扰。随着冬夏季风的相互对峙、占主导时期不同,毛乌素沙漠响应气候的变化也随之扩张收缩变迁。同时,锦界剖面记录的毛乌素沙漠气候变化与全球变化的干旱、寒冷事件以及温暖事件可以对应。

Dune fields with several palaeosoil layers are common in marginal desert in Northern China and potentially reflect drought-moist variability in the Late Quaternary. A Holocene profile of aeolian deposits with alternating units of dune sands and sandy palaeosoils record dune building and stabilization at least three times in the Jinjie area （southeastern margin of Mu Us desert in China） , which is specially located in loess-desert transitional region in response to East Asian monsoon alternation so as to likely archive millennial-scale patterns of palaeoenvironmental variability. Jinjie section （38.74°N,110.15°E;1181m a. s. 1）,30kin northwest of Shenmu County, is a ca. 5-mthick aeolian sandy profile （overlying layers of 2.5m modern dune and did not see bottom） with 3 layers of black or dark brown palaeosoils which correspond to three considerable dune fixations. In order to establish reliable timing of dune development, this study applied optically stimulated luminescence （OSL）dating techniques to the aeolian sediments features. 10 luminescence samples obtained from this profile were dated using the quartz single aliquot regeneration （SAR） protocol. Combining with palaeoclimate proxies （ e. g., grain sizes and magnetic susceptibilities）, the records revealed as many as seven periods of significant rapid climate changes during the time intervals the palaeosoil layers deposited. The spatial and temporal extent of the data have allowed for correlations to be made with evidence from global climate changes. The first arid episode occurred before 7.5ka in the early Holocene, with widespread aeolian sand mobilization and sparse vegetation. During 7.5 -3.9ka, climate turned to be warmer and more humid with the occurrence of thick palaeosoil layers in the lowest,at the same time with Holocene Optimum. When it comes to 3.9 -2.9ka,the dune rebounded to reactive with strong winter monsoon,and the climate became arid again. The timing of widespread dune reactivation coincided with well-known 4ka event. The climate became warm and humid during 2.9 - 1.7ka,with lower magnetic susceptibility than the upper palaeosoil layer. Although the summer monsoon is weakened, the winter monsoon failed to become predominant. Climate fluctuated more frequently after 1.7ka with a warm climate episode during 1.0 -0.5ka and an arid one around 0. 2ka. It also revealed that the human significance of Holocene climate variability since 1 ka ago. More importantly,the Holocene climate changes and evolution records from the Jinjie section could be correlated with the worldwide millennial-scale climate shifts,suggesting that the activity of East Asia monsoon in the Holocene may have directly been controlled or affected by global climate system. Several intervals human significance of Holocene climate variability at coincide with major disruptions of civilization, revealing the times.