Guo Dongxin, Li Zuofu
This paper discusses alternations of cold- and warm-climate happened in Northeast China following the world climatic fluctuation since the beginning of Quaternary on the basis of palaeo-vegetation evolution, palaeo-periglacial remains and palaeontological facies.It was just under the control of palaeo-climatic changes that the formation, development and disappearance of palaeo-permafrost in this region had been in progress since the beginning of Quaternary.There had been several times of appearance and disappearance of palaeo -pemafrost happened in this area with the alternations of cold- and warm-palaeoclimate. Based on the palaeo-periglacial remains, palaeo-air temperature determined by the spore-pollen analysis, and palaeoenvironment recovered by the mineral composition analysis, the author divides the history of palaeo-permafrost in Northeast China into following three stages, and infers the southern boundary of the palaeo-permafrost of each stage and the general situation of their development.a) Late Pleistocene StageAssuming that permafrost could be conserved and developed under the condition of annual mean ground-surface temperature at 0℃, on the basis of the inferred palaeo-air temperature,the author infers that the isotherm of 0℃ of annual mean ground-surface temperature roughly stretched along Ouhan-qi,Ganqika,Changtu within the Xialiaohe Plain area during Guxiang--tun periglacial epoch of late Pleistocene, and it approximately agrees with the isotherm of 7-8℃ of annual mean air temperature today,and also agrees with the southern boundary of Palaeo-permafrost in this area determined by the palaeo-periglacial remains of cold climate period. With this in view, according to the isotherm of 7-8℃ of annual mean air temperature, the author has worked out the southern boundary of palaeo-permafrost during the cold climate period of late Pleistocene at Liaodong Peninsula where the palaeo-periglacial remains have not yet been discovered till now.b) Stage from Post-Glacial Period to Hypsithermal Interval of Holocene At this stage, most of the permafrost in the mentioned area developedat the former stage had been thawed with the global air temperature rising, resulting in that the southern boundary of the permafrost widely retreated north-ward. Based on the inferred palaeo-air temperature and environment, the author considers that there was a palaeo-periglacial environment in the northern part of the existing continuous permafrost of this area, i. e., there was a part of permafrost left there during late Pleistocene.c) Stage from Hypsithermal Interval to Late HoloceneAfter the Hypsithermal interval, the disappeared palaeo-permafrost developed again there with the air temperature lowering. As a result, the ground temperature of the old permafrost (developed at late pleistocene) had been lowered down and its thickness had been enlarged in the existing continuous permafrost area, and consequently the permafrost gradually stretched south-forward, and finally to the farthest southern boundary in the 17-18th century. According to the palaeo-periglacial remains discovered in the black earth layer of 7500-2500 B.P. at places beyond the southern boundary of the existing permafrost, the author considers the permafrost had extended beyond the southern boundary of existing permafrost at the time when the old permafrost most widely developed.Considering the prediction that the air temperature in the discussed area may rise up during the coming 20-30 years (excepting it may fall down in the coming 10 years), and the fact that the increasing human activities must have an influence on permafrost, the author infers that the permafrost in this area most probably tends to retrograde in the coming years of this century.
