中文摘要：

马衔山是目前黄土高原地区唯一证实有多年冻土发育的山脉.残存的多年冻土被誉为黄土高原地区多年冻土的＂活化石＂.近20a来马衔山多年冻土发生了明显的退化,目前仅在小湖滩有岛状多年冻土残存,属于典型的高温多年冻土,1990年代初在其它区域发现的零星多年冻土已经基本消失.马衔山岛状多年冻土地温从10～16m的-0.2℃向上和向下升高,地温梯度±0.01℃·m-1左右,相比1990年代初,多年冻土地温上升了0.1～0.2℃,年升温率为0.006～0.012℃·a-1,小于青藏高原高温多年冻土平均升温速率.马衔山多年冻土最大厚度约40m,正在发生着上引式和下引式退化,而岛状冻土边缘区域侧引式退化起主导作用.马衔山多年冻土发育有丰富的地下分凝冰,根据地下冰发育特征和埋藏有机质层14C测试资料分析,马衔山多年冻土在新冰期形成后发生过多次地表重复堆积,共生共长作用是地下冰形成的重要原因.丰富的地下冰和厚层有机质层的保护作用,以及区域寒冷的微气候环境,应该是马衔山多年冻土残存的主要原因.

英文摘要：

The Mahan Mountain is the only region in the loess plateau where permafrost still exists,and the residual permafrost has been called＂live fossil＂ of permafrost.In the past 20years,permafrost in the Mahan Mountain has accordingly degenerated badly.At present,island permafrost still remains only in Xiaohutan,which is characterized by typical high-temperature permafrost.Scattered permafrost,found in other places,has generally disappeared since the beginning of the 1990s.The lowest ground temperature of island permafrost in the Mahan Mountain is -0.2℃ at the depths between 10 mand 16 m,increasing upwards and downwards with a gradient about ±0.01℃·m-1.The annual temperature increasing rate is 0.006～0.012 ℃·a-1,which is less than that in the Tibetan Plateau.The maximum depth of permafrost in the Mahan Mountain is about 40m,which is undergoing upward and downward degradation.The lateral degradation in the edge regions of the island permafrost is dominant.Abundant segregated ice exists in the permafrost region.From the analysis on development characteristics of ground ice and the determining of 14C from the buried organic matter,it is believed that after the Neoglaciation many permafrost-accumulations took place in the Mahan Mountain.Symbiosis is the principal reason for the formation of underground ice.In this paper,it is suggested that the main causes for permafrost remnant in the Mahan Mountain are the protective effects from the abundant ground ice and organic matter,as well as the local colder micro-climate environment.