蒙古高原东南缘现代灌丛沙丘的发育过程及其对沙漠化的指示意义

NEBKHA FORMATION AND ITS SIGNIFICANCE TO DESERTIFICATION RECONSTRUCTIONS IN THE SOUTHEASTERN MONGOLIA PLATEAU

在蒙古高原东南缘，灌丛沙丘是主要的风沙地貌类型，也是与风蚀和沙漠化密切相关的风沙堆积，一般在土地开垦后大规模发育，因而记录了区域沙漠化的发展过程。本文对发育在这一地区的灌丛沙丘沉积物地球化学元素的变化进行了详细的解析，结果表明，沙丘沉积物的Zr／Rb比值良好地记录了区域风沙活动的演变情况：当沙丘高度发育至10-30cm，39-59cm，67-77cm，95-105cm，118-130cm和138-140cm时，区域处于风沙活动的活跃期；沙丘高度发育至78—94cm时，区域处于近80年以来风沙活动的低谷期。结合区域土地开垦史、现代器测气象资料、风沙活动与沙漠化等情况，可以确定，自20世纪30年代以来，蒙古高原东南缘经历了30年代末期至50年代初期沙漠化的初步发展时期，50年代后期至80年代中后期、90年代中后期和本世纪初的沙漠化快速发展时期，以及80年代末期至90年代中前期的沙漠化逆转期。鉴于无航测资料时期沙漠化正逆过程演变研究的缺失，这一区域所发育的灌丛沙丘是延长区域环境变化记录特别是沙漠化演变研究的良好载体。

The nebkhas（coppice dunes）have extensively been developed in the south eastern Mongolia Plateau,the agro- pastoral transitional zones of semi-arid Northern China. Due to its accumulation characteristics the nebkha formation processes can be taken as highly reliable indicators of regional wind erosion and land degradation. The nebkha that we selected for sampling was located in the Huade County, Inner Mongolia （42°11. 325＇N, 114°26. 740＇E ; 1360m a. s. 1. ）, and the underlying landforms are denuded hills with gentle slopes. From the crest towards the bottom of the dune,we vertically excavated and collected the sediments at 1-cm interval throughout the profle. In total 140 nebkha and 20 underlying sediment samples were acquired. Compared the nebkha sediments with the underlying sediments, the latter were richer in clay and silt and even mixed with some gravels. After the samples were air dried we sieved them to acquire fractions of〈2mm in diameter for geochemical analyses. The macroelements （mostly represented by their oxide forms） we measured included SiO=, A12 03 , Fe203 , MgO, CaO, Na2 O, K2 O, and Ti ; and the microelements included P, V, Cr, Mn, Co, Ni, Cu,Zn, Ga, As, Rb, Sr, Y, Zr, Nb, Ba, Ce and Pb were measured. For most elements we measured the analytic uncertainty was less than ＋5%, except for Cr, Co, V and Pb. In addition, using the seed and Caragana microphyUa litter buried in the nebkha sediments at the dune height of 10cm and 60cm（from the bottom of dune towards the crest）we also performed accelerator mass spectrometry （AMS） 14C dating. Based on these analyzed results, the desertification and wind activity fluctuation in the south eastern Mongolia Plateau have been reconstructed. For instance, dating results showed that although there were nearly five centuries for the origination of the sampling nebkha,it only rapidly developed after this region was reclaimed in the late 1930s. The Zr/Rb ratio trends of the sediments in different layers showed that during the dune heights reached at 10 to 30cm,39 to 59cm,67 to 77cm,95 to 105cm,118 to 130cm,and 138 to 140cm,which were also from the late-1930s to the early 1950s,the late-1950s to the early 1960s,the mid-1960s to the mid- to late-1980s,the mid- to late- 1990s, and the early-2000s to the mid-2000s this region was under controls of high wind activity. And during the dune heights increased from the 78 to 9acm,which also from the late-1980s to the early- to mid-1990s, this region experienced periods with low wind activity. In addition, the other geochemical characteristics of nebkha sediments such as the CIA（ Chemical Index of Alteration） , CPA（ Chemical Proxy of Alteration）and Al2O3 contents, and the KEO/Na20, and Fe/Sr ratios showed that the periods with highly moisture conditions were from the late 1930s to the 1980s,the mid- to late-1990s,and the early-2000s to the mid-2000s,and the moisture trends over the past 80 years were not always consistent with that of the desertification monitored by field investigations and by the remote sensing methods. In this region, over the past nearly 80 years the desertification mainly occurred from the late-1930s to the early-1950s, and rapidly occurred from the late 1950s to the mid- to late-1980s, the mid- to late-1990s, and the early-2000s to the mid-2000s, and the rehabilitation mainly occurred from the late-1980s to the early- to mid-1990s. In this region the desertification trends are always closely related with the wind activity fluctuations. Because of absences of the proxies for environmental reconstructions in most of arid and semiarid regions,our analyzed results show that the nebkha sediments may provide some distinct clues for the wind activity and desertification reconstructions, especially, during the periods with the remote sensing images or the other surveving data are unreliable.