中国北方晚第四纪气候变化的沙漠与黄土记录* ——以光释光年代为基础的直接对比

中国北方有约200×10⁴km²的面积被黄土、沙漠和荒漠戈壁沉积物覆盖,这些地区堆积物的形成变化与第四纪气候变化密切相关。关于中国北方沙漠与黄土记录晚第四纪古气候变化的研究不少,但是,由于受到方法和条件等限制,系统的(高密度的)和独立的年代学工作比较薄弱,从而使诸多已经发表的关于沙漠-黄土记录的亚轨道时间尺度气候变化的结论受到质疑。文章通过对黄土高原代表性的4个末次冰期以来的黄土剖面以及毛乌素、浑善达克和科尔沁沙地的20多个末次冰期-全新世的沙-砂质古土壤剖面的研究,获得了150多个独立的光释光年龄数据。在此基础上,结合200多个样品的粒度、有机质含量和磁化率数据,重建末次冰期以来中国北方气候变化过程,对比分析沙地与黄土古气候记录的联系。结果表明,中国北方古气候在57.5~52.5kaB.P.相对干旱,41.5~37.3kaB.P.相对湿润,26~22kaB.P.逐渐干旱,22~18kaB.P.极端干旱,15~10kaB.P.干旱,10~8kaB.P.为干旱减弱的时期,8.0~2.4kaB.P.相对湿润,而2.4kaB.P.以来气候为相对干旱。其间,在8.0~2.4kaB.P.的湿润时期有几次干旱事件,而2.4kaB.P.以来气候变化频繁。具有独立年代控制的黄土和沙漠记录表明,中国北方的气候变化对轨道时间尺度的太阳辐射变化有大约3~4ka的滞后。

LATE QUATERNARY CLIMATIC CHANGES IN NORTHERN CHINA——NEW EVIDENCES FROM SAND DUNE AND LOESS RECORDS BASED ON OPTICALLY STIMULATED LUMINESCENCE DATING

In Northern China, areas of desert, sand field, Gobi and loess account for nearly two million square kilometers of the landscape. Because of the relatively simple transport and unique deposition process, these aeolian deposits preserve good records of climatic changes during recent geological times such as the Late Tertiary and the Quaternary. There have been many investigations on the loess and the sand dune depositional sequences corresponding to climate and environmental changes. These studies have advanced our understanding of regional palaeoenvironment changes; however, previous studies on chronological framework, especially the absolute chronological controls with high-resolution on the loess and the sand dunes, are still very limited. Thus those conclusions on climatic and environmental changes based on investigations of these aeolian deposits, especially changes at millennial-centennial time scale, are still largely open to discussion. In this study, four typical loess sections of the Late Quaternary on Chinese Loess Plateau and more than 20 sand-paleosol sections in Mu Us, Otindag and Horqin sand fields are examined in order to reconstruct wet-dry climate variations in North China under absolute age controls. On the basis of optically stimulated luminescence ages from more than 150 samples and the proxy indexes of grain size distribution, magnetic susceptibility, and organic matter content obtained from more than 200 samples, which are directly related to the climatic variations, we refine the regional wet-dry climate changes in orbital time scale during the Late Quaternary. The loess and sand dune records reveal similar changes to closely reflect the climatic changes: It was relatively dry during 57.5~52.5kaB.P. , wet during 41.5~37.3kaB.P. , and gradually drying during 26~22kaB.P. , followed by an extremely dry episode during 22~18kaB.P. and then dry again during 15~10kaB.P. The climate gradually became humid in 10~8kaB.P. and was wet during 8.0~2.4kaB.P. The climate was dry again in the recent 2.4kaB.P. There were several desiccation episodes during 8.0~2.4kaB.P. and the climate had high-amplitude changes in the recent 2.4kaB.P. These climatic changes have lagged 3~4ka to the solar radiation variations which is believed driven by the Earths orbital trajectory changes.