末次冰期毛乌素沙地湖泊消涨过程:沉积证据

The processes of lacustrine evolution during last glacial period inferred from sedimentary records in Mu Us dune field, North China

毛乌素沙地位于东亚季风边缘区,对于气候变化响应敏感.基于沙地的侵蚀-堆积关系,在侵蚀作用为主的沙地西北部和中部很难找到沉积相对连续且分辨率较高的地层剖面,因此已有的末次冰期古气候环境研究多集中于堆积作用为主的沙地东南缘的萨拉乌苏河河谷地区.本文选取位于沙地腹地,厚度为3.5 m的风成砂-河湖相沉积序列——神水台(SSTG)剖面为研究对象,基于AMS 14C和OSL年代数据,推测得出剖面中湖相层的上界和下界分别为约24.2 ka B.P .和42.7ka B.P.,依据沉积地层和环境代用指标分析结果,针对末次冰期毛乌素沙地湖泊消涨的一般过程进行了探讨.结果表明,自约5万年以来,由神水台(SSTG)剖面指示了两次明显的湖泊主要发育期,时间分别约为42.7~34.3 ka B.P.和31.1~24.2 ka B.P.,期间粘土含量、有机质、红度均表现为低值,为含淡水螺壳化石的典型湖泊相沉积,且与毛乌素沙地已有湖泊发育的年代结果基本一致.在两期湖相层之间的砂层,平均粒径、砂含量表现为峰值,指示了一次明显的湖泊消退的过程,其时间约为34.3~31.1 ka B.P.;在湖相层上下可能均为风力作用为主导的风沙沉积.通过区域对比并初步探讨其可能的驱动机制,结果表明SSTG剖面记录的湖泊消涨过程主要受控于东亚季风强度的变化.

The Mu Us dune field is situated at the fringe area of East Asian monsoon and therefore is a climatically sensitive region that is well located for recording palaeoclimatic and palaeovironmental information. Over the years, many geological sections during the Last Glacial period were established, which provided the possibilities for the reconstruction of lacustrine evolution and climatic change. However, based on the erosion and accumulation relation in Mu Us region, it is difficult to find a relatively continuous section with high-resolution in the erosion-dominated northwestern and middle part in Mu Us dune field, thus previous researches focused mostly on the Salawusu River valley in the accumulation-dominated southeastern Mu Us dune field. In this paper, we selected SSTG section in the hinterland of the Mu Us region, which is mainly composed of lacustrine sediments and aeolian sand with 3.5 m thickness. Based on the stratigraphic evidence and the environmental proxies results from 130 sediments of the SSTG section, the processes of lacustrine evolution during Last Glacial period were explored. Results showed that since about 50 ka B. P. there existed two obvious periods ca. 42.7~34.3 ka B. P. and 31.1~24.2 ka B. P. approximately when lacustrine sediments were dominant and the time spans basically agreed with the previous studies. The sedimentary facies are typical lacustrine layers with freshwater shells fossils. The content of clay particle, the content of organic matter exhibited high levels, and mean grain size, the content of sand particle, redness value showed low levels. Actually, an obvious layer of sand was intercalated in the two lacustrine layers at the interval of 34.3~31.1 ka B. P. During this time, the mean grain size and the content of sand particle presented remarkable peak levels, accompanied by the low value of the content of organic matter and redness value, indicating the retreatment of lacustrine sediments. Additionally, except the lacustrine layers, the upper and lower parts were all aeolian sediments possibly. Based on the results of regional comparisons, we believed that the lacustrine evolution during the Last Glacial period is controlled by the strength of East Asian monsoon.