Thermal-contraction-crack networks as evidence for late-pleistocene permafrost in inner Mongolia, China

Numerous wedges on the Ordos Plateau show typical characteristics of periglacial sand wedges that enable them to be distinguished from desiccation cracks in clayey illuviation soil horizons. The sand wedges are organized in two generations of polygonal networks. The older generation is a large-scale network with a diameter of 8 to 9 m and wedge depths up to more than 2 m. The younger generation has a mean diameter of 3 to 4m and is formed within the large networks. It consists of shallow (0.6 to 1 m deep) but relatively wide wedges. In contrast to the typical sand wedges that form within continuous permafrost, the shallow wedges formed probably as 'ground wedges' by seasonal freezing. Ice wedges and cryoturbations developed only rarely, and exclusively in the most humid areas. Thermal-contraction cracking occurred mainly between 26 and 20ka BP, indicating mean annual temperatures at least 13degreesC lower than present. Before and after that period mean annual temperatures were at least 7 to 8.5degrees lower than today. The presence of these periglacial phenomena show that the southern limit of continuous permafrost in Inner Mongolia occurred south of 38degreesN during the Last Glacial Maximum. Copyright (C) 2004 John Wiley Sons, Ltd.