毛乌素沙漠地处东亚季风边缘区，对季风环流变化响应敏感。以往在毛乌素地区开展的环境演变研究，多围绕地貌发育时间序列的建立来展开，关于典型时段地貌格局的重建工作则相对较少。末次盛冰期是末次冰期中全球冰储量最大的时期，此时期毛乌素地区气候干冷、多风，是风沙地貌发展的重要时段。认识末次盛冰期毛乌素沙漠的南部边界，对于理解毛乌素沙漠的演变过程具有重要意义。但目前，对于盛冰期毛乌素沙漠的南部边界位置仍存争议。本研究采用历史比较法，“将今论古”，对末次盛冰期毛乌素沙漠南界以及毛乌素沙漠地区风沙地貌分布南界进行了重建。其中，沙漠南界指示了风沙地貌集中、连续分布的范围。风沙地貌分布南界指示了风沙地貌断续、零散分布的范围。

       “将今论古”，首先要认识“今”。研究对现代毛乌素沙漠南界与风沙地貌分布南界进行了定量提取，基于南界区域的地貌变化与风成沙粒度变化，分析了现代南界的控制因素。在不同时段，风沙活动强度有所差别，但沙漠的形成均需要适宜的气候、物源与地形条件，且气候、物源与地形对于风沙地貌发育的控制方式相似。因此，虽然毛乌素沙漠的边界位置在不同时期会发生变化，但如果沙漠边界的控制因素相同，边界区域的地貌变化、风成沙粒度变化也会与现代相似。基于以上前提，研究选取了毛乌素沙漠南部地区26个典型风成沉积剖面，对44个年代样品进行了OSL年代测定，测试了剖面84个盛冰期古风成沙样品的粒度特征。对比已有剖面，分析了沙漠南部盛冰期古风成沙粒度的空间变化。对沙漠西南部31个钻孔的盛冰期沉积相进行了判断，分析了沉积相的空间变化。根据沙漠南界区域风成沙粒度变化、地貌变化特征与沙漠南界控制因素的对应关系，确定了末次盛冰期沙漠南界的控制因素。根据控制因素的作用位置，重建了末次盛冰期沙漠南界以及风沙地貌分布西南界。与此同时，对于控制因素不确定的风沙地貌分布东南界，研究则根据沟谷密度的空间变化以及剖面点的空间验证对其位置进行了重建。初步得到以下结论：

       （1）现代毛乌素沙漠南界位置主要受地形控制。在沙漠东南部（靖边以东至榆溪河入河口以西），无定河河谷以及高密度的黄土沟谷对于跃移质搬运具有很强的抑制作用，黄土丘陵地形也不利于风成沙的沉积保存。在沙漠西南部（定边以东至靖边以西），白于山山前洪积扇边缘地下水出露，形成了大面积的湖成地貌，阻碍了风成沙向下风向的搬运。在沙漠边界外围黄土区局地物源不充足的情况下，地形控制了现代沙漠南界的位置；现代毛乌素沙漠地区风沙地貌分布西南界受地形与物源控制，对应特定的地貌部位。风沙地貌分布东南界的控制因素则并不明确。

       （2）在现代沙漠南界区域，由于地形对于跃移质搬运的抑制，风沙地貌分布连续性与风成沙的粒度特征，出现了快速、同步的变化，形成了良好的对应关系。在沙漠边界内部，连续分布的风沙地貌以中沙、细沙为主。在边界外部，除无定河南部沿岸外，断续分布的风沙地貌以极细沙为主，中沙及以上组分趋近于零。在沙漠西南部，受控大尺度的地形变化，自北向南呈现风沙地貌-湖成/流水地貌-黄土地貌的地貌格局；毛乌素沙漠南部末次盛冰期古风成沙粒度的空间变化，以及西南部钻孔、剖面盛冰期沉积相空间变化所反映的地貌变化，均与现代相似。表明盛冰期毛乌素沙漠南部边界同受地形控制；在地形控制下，南界区域古风成沙的粒度特征对古风沙地貌的分布特征具有指代意义。以极细沙为主且中沙组分趋近于零的古风成沙，指示了小范围断续风沙地貌的发育，位于盛冰期沙漠边界外围。以中沙、细沙为主要组分的古风成沙，除河流沿岸物源为局地冲积物的古风成沙外，指示了大范围连续风沙地貌的发育，位于盛冰期沙漠边界内部。

       （3）末次盛冰期沙漠东南部边界位置与现代接近，受无定河河谷与黄土沟谷的分布控制。根据无定河中游不同河段的下切年代，以及不同河段南岸盛冰期古风成沙的古环境意义判断，至少在波罗下游段，至多在芦河入河口下游段，沙漠以无定河河谷为界。根据沟谷密度的空间分布推断，受黄土沟谷分布控制的边界段，部分位置可能比现代边界略偏东南，但在6 km以内；在沙漠东南界外围，黄土坡面沉积的盛冰期古风成沙抑制了沟谷侵蚀的发生，致使沟谷密度自西北向东南递增后趋于稳定。沟谷密度趋于稳定的位置，指示了盛冰期毛乌素地区风沙地貌分布东南界，也大致对应3条野外考察断面盛冰期古风成沙分布的末端。重建的末次盛冰期风沙地貌分布东南界位置与现代接近，这是因为盛冰期古风沙在出露之后为部分现代小型风沙地貌的发育提供了物源。

       （4）在毛乌素沙漠西南部，受大尺度地形变化控制，在白于山山前地带末次盛冰期同样发育了湖成地貌，抑制了风成沙向下风向搬运，决定了沙漠西南边界的位置。根据沙漠西南部钻孔、剖面盛冰期风成沙与冲湖积相、湖沼相沉积过渡转换的位置推断，末次盛冰期沙漠西南边界比现代偏南约1~6 km；末次盛冰期风沙地貌分布西南界的位置与现代基本一致，受地形与物源控制。在八里河以东至芦河以西，对应黄土涧地下风向的黄土丘陵。在八里河以西，对应黄土斜坡的背风侧。

The Mu Us Sand Sea is located on the margin of the area influenced by the East Asian monsoon, and is sensitive to variations in East Asian monsoon circulation. The previous studies on environmental evolution in the Mu Us were mostly focus on the time series of landform development, while the reconstruction of the geomorphological pattern in typical periods was relatively rare. The Last Glacial Maximum (LGM) was the period when the global ice volume were the maximum in the Last Glaciation. During this period, the climate in Mu Us area was dry, cold and windy, which was an important period for aeolian landform development. However, the southern boundary of the Mu Us during the LGM is still disputed. This study reconstructed the southern boundary of the Mu Us Sand Sea and the southern boundary of the aeolian landform distribution in the Mu Us area during LGM by using the historical comparison method. The southern boundary of the Sand Sea indicates the boundary of concentrated and continuous distribution of aeolian landform, and the southern boundary of aeolian landform indicates the intermittent and scattered distribution of aeolian landform.

This study quantitatively extracted the southern boundary of the modern Mu Us Sand Sea and the southern boundary of aeolian landform distribution in Mu Us area, and analyzed the controlling factors of the modern southern boundary based on the geomorphology change and aeolian sand grain size change in the southern boundary area. The intensity of aeolian activities is different in different periods, but the formation of sand sea requires suitable topography, provenance and climate conditions, and the control mode of climate, provenance and topography on the development of aeolian landform is similar. Therefore, although the boundary position of the Mu Us changed in different periods, if the controlling factors of the boundary are the same, the geomorphological changes and aeolian sand grain size changes in the boundary area are similar to those in modern. Based on the above premise, the OSL dating of 26 typical aeolian sections in the southern Mu us Desert was tested. And the grain size characteristics of the paleo-aeolian sand during the glacial maximum was tested. The spatial variation of grain size characteristics of aeolian sands in the southern Mu Us during the glacial maximum is analyzed by comparing the existing sections. The sedimentary facies of 31 boreholes in the southwest of the Mu Us Sand Sea were determined and the spatial variation of sedimentary facies was analyzed. The controlling factors of the southern boundary of the desert during LGM were determined according to the corresponding relationship between the variation of aeolian sand grain size and geomorphology and the controlling factors of the southern boundary. The southern boundary of Mu Us Sand Sea and the southwestern boundary of aeolian landform distribution during the LGM are reconstructed according to the position of controlling factors. The main conclusions are as follows:

(1) The position of the southern boundary of modern Mu Us Sand Sea is mainly controlled by topography. In the southeast of the desert, Wuding River valley and high-density loess gullies exerts a strong negative influence on saltating fraction transport, and the topography of the loess landform is also unsuitable for sand preservation. In the southwest of the desert, groundwater outcropping at the edge of the alluvial fan gives rise to a large lacustrine landform that efficiently impedes downwind transport of the saltating fraction; The southwestern boundary of aeolian landform distribution in the Mu Us area is controlled by topography and provenance, corresponding to specific geomorphic positions. The controlling factors of aeolian landform distribution southeastern boundary are not clear.

(2) In the southern boundary area of modern Mu Us, the distribution continuity of aeolian landform and the grain size characteristics of aeolian sand change rapidly and synchronously due to the restriction of terrain on saltation transport. And the distribution continuity of aeolian landform and the grain size characteristics of aeolian sand formed a good correspondence. Within the boundary of the desert, the continuous distribution of aeolian landform is mainly medium sand and fine sand. Outside the boundary, the aeolian landform of intermittent distribution is dominated by very fine sand, and the medium sand and above components tend to zero. In the southwest of the desert, the landform pattern of aeolian landform-lacustrine/fluvial landform-loess landform is forming from north to south controlled by large-scale topographic changes. The spatial variation of paleo-aeolian sand grain size during LGM in the south of Mu Us and the geomorphic change reflected by the spatial variation of sedimentary facies during the glacial maximum in boreholes and profiles in the southwest of Mu Us Desert are similar to the modern. The results show that the southern boundary of Mu Us Sand Sea was controlled by topography during glacial maximum. Under the control of topography, the grain size characteristics of the aeolian sand in the southern boundary have a referent meaning to the distribution characteristics of the aeolian geomorphology. The paleo-aeolian sand is dominated by very fine sand and its medium sand composition tends to zero, indicating the development of small area intermittent aeolian geomorphology, located at the periphery of the Sand Sea boundary during the maximum glaciation. The paleo-aeolian sands, mainly composed of medium and fine sands, indicate the development of a large range of continuous aeolian geomorphology, located within the boundary of the Sand Sea during the maximum glaciation.

      (3) The southeastern boundary of the Sand Sea during LGM was close to that of the modern, and was controlled by the Wuding River valley and loess gully. According to the cutting age of different reaches of the Wuding River and the paleo-environmental significance of the paleo-aeolian sand in the southern bank of different reaches during the glacial maximum, the Sand Sea is bounded by Wuding River Valley at least in the lower reaches of the Baltic River and at most in the lower reaches of the Luhe River estuary. According to the spatial distribution of gully density, the boundary segment controlled by loess gully distribution may be slightly southeast of the modern boundary, but within 6 km. In the periphery of the southeast boundary of the desert, the paleo-aeolian sand deposited on the loess slope during the glacial maximum limited the occurrence of gully erosion, and the gully density increased from northwest to southeast and tended to be stable. The steady position of gully density indicates the southeast boundary of aeolian landform distribution in Mu Us area during glacial maximum, and also roughly corresponds to the end of the distribution of paleo-aeolian sand in three field survey sections during glacial maximum. The reconstructed aeolian landform southeastern boundary of LGM is close to that of the modern, because the paleo-aeolian deposited during glacial maximum provided provenance for the development of some modern small aeolian landform after its outcropping.

(4) In the southwest of the Mu Us, controlled by large-scale topographic changes, the lacustrine landform also developed in the piedmont of Baiyu Mountain during LGM, which inhibited the downward transport of aeolian sand and determined the location of the southwest boundary of the Sand Sea. The southwest boundary of the Sand Sea during LGM was about 1~6 km farther south than that of the modern one, according to the location of the transition between aeolian sand and alluvial lacustrine facies and lacustrine facies in the borehole and section in the southwest. The distribution of aeolian landform in the southwest of LGM was basically the same as that in modern, and was controlled by topography and provenance. In the east of Bali River to the west of Lu River, corresponding to the loess hill downwind of loess valley plains. West of Bali River, corresponding to the lee side of loess slope.