青藏高原中、东部全新世以来多年冻土 演化及寒区环境变化*

古冻土存在的依据和判别标志主要是古冻土遗迹(深埋藏多年冻土层、古冻土上限、融化夹层、厚层地下冰)和古冰缘现象(古冻胀丘、古融冻褶皱、砂楔、土楔、冰楔假型、风成沙丘、黄土层、厚层泥炭和腐殖质层等)。文章结合大量的测年数据,利用古代和现代冻土以及冰缘现象的时空分布差异综合分析对比,将全新世以来青藏高原多年冻土演化过程和环境变化划分为6个较明显的时段:早全新世的气候剧变期(10800aB.P.至8500~7000aB.P.)、中全新世大暖期(8500~7000aB.P.至4000~3000aB.P.)、晚全新世寒冷期(4000~3000aB.P.至1000aB.P.)、晚全新世温暖期(1000aB.P.至500aB.P.)、全新世末小冰期(500aB.P.至100aB.P.)及近代升温期(100aB.P.至今);同时,概述了各时段高原冻土的发育条件、分布范围及总面积,和当时高原上的古气候、古地理环境。

EVOLUTION OF PERMAFROST AND ENVIRONMENTAL CHANGES OF COLD REGIONS IN EASTERN AND INTERIOR QINGHAI-TIBETAN PLATEAU SINCE THE HOLOCENE

The main body of the permafrost layers on the Qinghai-Tibetan Plateau is the remnant of the Last Glaciation in the end of the Late Pleistocene,which has been evolving under a changing climate since the Holocene,forming the present distribution patterns of permafrost.The principal criteria for identifying the presence of relict permafrost include the remains of ancient permafrost and periglacial phenomena.The former includes the deeply buried or double layered permafrost,relict permafrost tables,talik,and thick layered ground ice.The latter includes pingo scars,cryoturbation and solifluctions,primary sand and soil wedges,ice wedge casts,aeolian sand and loess,and thick peat and humus.Based on comprehensive and comparative analyses on the ¹⁴ C-dated soils,spatial and temporal distributions and their differentiations of relict and present permafrosts and periglacial phenomena,the evolution of permafrost along with the environmental changes of cold regions since the Holocene is divided into 6 discernible stages: 1) the period of dramatically changed climate in the Early Holocene (10800aB.P. to 8500~7000aB.P.),2) the megathermal period in the Middle Holocene (8500~7000aB.P. to 4000~3000aB.P.),3) the cold period in the Late Holocene (4000~3000aB.P. to 1000aB.P.),4) the warm period in the Late Holocene (1000aB.P. to 500aB.P.),5) the Little Ice Age (500aB.P. to 100aB.P.),and 6) the recent warming period during the last 100 years.The development,distribution and total areas of each stage are estimated based on paleo-climatic and paleo-geographical evidence.