中国季风沙区晚更新世以来环境及其变化

Environment and Its Changes in the Monsoon Sandy Region of China during the Late Pleistocene and Holocene

本文以对毛乌素沙漠萨拉乌苏河流域米浪沟湾地层剖面晚更新世—全新世砂丘与河湖相、古土壤互为叠覆的高分辨率的气候地层记录的认识出发,同时基于对中国北方现代冬春季节风沙、尘霾活动发生的区域性气候—地质过程的理解,将今释古,反演了该时期中国季风区沙漠沙地曾经历的27个沙漠扩缩的周期性变化,并认为该剖面风砂沉积是过去地质时代季风沙区乃至整个中国沙区类似由现代这种冬春季西伯利亚—蒙古反气旋风系即冬季风向南伸展导致的若干风沙风尘动态过程的结果,而河湖相和古土壤的发育则主要与季风沙区受惠于东亚夏季风的影响有关。

Situated at the southeast magrin of the Mu Us Desert, the Milanggouwan stratigraphical section in the Salawusu River Valley records 27 depositional sequences of the aeolian dune layers alternating with the fluvio-la-custrine facies and palaeosols from the Late Pleistocene to Holocene. Grain-size analysis and measurements of the susceptibility of the section show that their changes in the section may be well correlated with the variations of the aeolian dune layers, fluvio-lacustrine facies and palaeosols. Usually, the particles of aeolian dunes are coarser and well sorted with lower susceptibility values (Sv), their MX (mean particle diameter),σ(standard deviation) and Sv being 2.33-3.48Φ, 0. 26-0. 90 and 2 - 10 (10-8 m3/kg SI) respectively; whereas the particles of the fluvio-lacustrine facies and palaeosols are finer, moderately to poorly sorted with higher susceptibili- ty values, their MX, σ and Sv falling within the range of 3. 04-5. 06Φ, 0. 61 - 1. 77 and 10-20 (10-8 m3/kg SI) respectively. Hereby it is considered with reference to the study results obtained by previous researchers that the above-mentioned alternately evolutionary sequences were, in fact, mainly induced by the climatic changes. The aeolian sand dune was deposited chiefly during the 'desert period' , formed under dry and cold climatic conditions; the fluvio-lacustrine facies and palaeosol are chiefly the product during the 'inter-desert period' , accumulated under warm humid climatic conditions. Thereby, the dune layers in the Milanggouwan section and its overlying fluvio-lacustrine facies or/and palaeosol may be regarded as a sedimentary cycle. A total of 27 cycles are recorded in the section. Each cycle represents a complete process from the cold dry to warm humid climate, and the region has experienced 27 climatic cycles of alternating cold dry and warm humid. Based on the understanding of the regional climate of the wind-drifting sands and the dust haze, induced by the southward stretch of the Sibrian-Mongolian anticyclones in the present winter and spring seasons in northern China and the space-time regularity of the summer monsoons, it is suggested that the 27 periods of aeolian sand dune accumulations in the Milanggouwan section should be a result of many times' dynamic processes of wind sands and dusts in the past geologic time, which were similar to those of the present winter and spring seasons in question , in the monsoon sandy region and even in the whole deserts of China; while the fluvio-lacustrine facies and palaeosoils sandwiched in between the dune layers were related to activities of the summer monsoon in the region. During the cold stages (glacial periods) in the Northern Hemisphere since the Late Pleistocene, the Sibrian-Mongolian anticyclones raced off towards the middle altitudes of China, the monsoon sandy region was in a desert period, the winter monsoons prevailed in the region, the climate became dry and cold, wind erosion and deposition were violent, the deserts were active intensely and expanded, and the sediments were mainly aeolian sand dunes. During the warm stages (interglacial periods) in the Northern Hemisphere, the frequencies of the winter monsoon occurring southwards decreased greatly; the monsoon sandy region was in an inter-desert period ; the summer monsoon prevailed; the climate became warm and humid; rivers, lakes and swamps were well developed; a series of soil-forming processes occurred on the ground. As a result, the fluvio-lacustrine facies accumulated in depressions between the dunes and palaeosols formed on mobile dunes, thereby making the dunes fixed. Multiple alternation of glacial and interglacial periods in the Northern Hemisphere from the Late Pleistocene to Holocene caused the confrontation and antipathetic changes of the winter monsoons and the summer monsoons and the alternating desert period and inter-desert period, and the activation of mobile sand dunes and fixation experienced a series of normal and reverse commutative processes.