甘肃省临夏盆地更新世早期湖泊沉积孢粉记录的古气候演化

摘要

根据孢粉组合特征,将临夏盆地早更新统东山组(2.50-1.76Ma)的孢粉划分为3个孢粉带(含6个亚带),即3个植被、气候阶段,第一阶段(2.50-1.91Ma,带Ⅰ),植被类型是以柏科、榆属和禾本科为优势种的温带森林,气候表现为温凉稍干,并有逐渐干旱化的趋势.第二阶段(1.91-1.80Ma,带Ⅱ)是以云杉属、榆属和禾本科为主的温带森林,气候冷湿;尤其在1.89-1.87Ma期间,云杉林面积最大,气候最为湿润;第三阶段(1.80～1.76Ma,带Ⅲ),植被类型为以云杉属、梣属和禾本科为主的温带森林,云杉林面积逐渐减小,禾本科逐渐增加,气候逐渐干旱.%In most of Chinese Northern areas , the Early Pleistocene is one of the important transition period , before which the climate was relative warm and humid in the Tertiary and after which the extremely arid environment in the Later Quaternary developed. It is of great significant theoretical basis and scientific foundation to establish the ecological and climatic change sequence in this period , which can improve our understanding of the arid evolution trend in the northwest of China and predict its future climatic change and the rule of development.Linxia Basin, which located at the northeast rim of the Tibet Plateau, is the intersection zone of eastern monsoon area,northwest arid area and Tibet Plateau alpine-cold area. Continuous and complete Upper Cenozoic strata are well developed in this basin. So it was one of the ideal areas for extracting information of the ecological evolution and climatic change in the North West China since the Late Cenozoic Era. We present a continuous 78m thick Dongshanding Paleo-lake sediment section based mainly on clayey siltstone and siltstone, which is situated at Jinggou township Dongshanding village (35°35'N, 103°04'E,an elevation of 2429.5m). According to previous research results, the formation age of the section is 2. 50 ～ 1. 76Ma. 88 spore-pollen samples were collected systematically with the sampling interval of 1. 0m besides 0. 5m individually. The result of the identification showed that 91％ samples pollen amount attained statistical standard among them. Pollen types belong to 44 families and 57 genera , which includes Arbor trees pollen , such as Pinus , Picea , Cupressceae , Juniperus , Quercus , Betula , Fraxinus,and Shrub and Herb pollen , such as Poaceae ,Amnthemis type ,Aster type,Artemisia , Chenopodiaceae.In the whole section. Arbor trees pollen ( 37. 0％ ～ 98. 5％ , average of 66. 9％ ) absolute dominated , suggesting forest landscape in Linxia Basin during 2.50～ 1. 76Ma. According to variation characteristics of spore-pollen assemblage ,Dongshanding Formation ( Dongshanding Paleo-Iake sediment) into 3 spore-pollen assemblage zones ( consisting of 6 subzones) can be divided, representing 3 vegetational evolution periods or arid-wet-arid climate change sequence during Early Pleistocene.During the first period ( 2. 50 ～ 1. 9lMa, Zone Ⅰ ) , the dominate ecological landscape was the temperate forest mainly consisting of Cupressaceae , Ulmus and Poaceae ,indicating warm-cold and a little dry climate. From 2. 11Ma to 1. 91Ma,climate was drier than early with the characteristics of shrub and herb increasing. In the second period (1. 91 ～1. 80Ma, Zone Ⅱ ) , the ecological landscape was still the temperate forest, which can be inferred from extensive existence of majority species such as Picea , Ulmus and Poacene. It was significant different from Zone Ⅰ that Zone Ⅱ represented the most moist cold climate during Early Pleistocene. Especially, during 1. 89 ～ 1. 87Ma Picea woodland and Dongshanding Paleo-Iake expanded to the maximum extent. In the last period ( 1. 80 ～ 1. 76Ma.Zone Ⅲ ) , the vegetation type was also temperature forest , but was reconstructed with the Picea , Fraxinus , Poaceae as its majority species. The Picea woodland shrunk gradually. However,the climate was more humid than the one of Zone Ⅰ .