内蒙古鄂尔多斯高原古冰缘遗迹科学考察研究进展

Periglacial phenomena are important for rebuilding the paleo-climate and -permafrost environments in the Late Quaternary. China is one of the countries with the most developed periglacial landforms during the late Quaternary and they are largely well preserved, among which the Ordos Plateau is one of the regions in northern China where periglacial phenomena have been extensively identified since 1980s. In order to accurately grasp the regional evolutionary history of periglacial and permafrost environments and their distributive features on the Ordos Plateau, the joint expedition from the Northwest Institute of Eco-Environment and Resources, Chinese Academy of Sciences and Free University Amsterdam, conducted several expeditions to the area. The one during May to June 2018 was focused on investigations for paleo-periglacial environments on the Ordos Plateau. The surveyed and investigated areas range from Jingbian County, Shaanxi Province to Chengchuan Town, Uxin Banner and Dongsheng District in the Ordos area of Inner Mongolia Autonomous Region, China, with an areal extent of about 12 000 km². It has been found through investigations and summaries of the paleo-periglacial phenomena on the Ordos Plateau that there are two major types of periglacial phenomena, namely, cryoturbations and cryogenic wedges. Based on the analysis of the climatic conditions required for the development of the recently identified cryogenic wedges and cryoturbations, the environmental conditions for the occurrences of periglacial relics were preliminarily studied. The following conclusions were drawn thereupon:The periods with remarkably low temperatures and well-developed permafrost favored for the formation of various kinds of wedges, such as paleo ice-wedge groups, ice-wedge casts, large primary sand wedges, and large thermal contraction polygons. Moreover, the periods with warming climate and degrading permafrost(not yet completely thawed)were conducive to the formation of cryoturbations. The regional and collective occurrences of cryoturbations in groups generally reliably indicate the environmental conditions and the regional degradation of the upper layer of permafrost to a certain extent under a warming or warmer climate. Based on the field observations and Optically Stimulated Luminescence(OSL)dating results, combined with the characteristics of periglacial remains, we preliminarily reconstructed the chronosequence in permafrost environment on the Ordos Plateau since 50 ka BP:A tundra-like or cold desert environment prevailed on the Ordos Plateau during the Last Permafrost Maximum(LPM, 25-19 ka BP), with an extensive occurrence of cold and thick permafrost. Under the next generally warming climate, permafrost gradually degraded from continuous and discontinuous to island, sporadic and isolated patchy permafrost and eventually thawed in to seasonal frost on the Ordos Plateau, forming the present landscapes.