甘肃临夏黄土剖面:15万年环境演变的良好记录

THE LINXIA LOESS SECTION IN CHINA:AN EXCELLENT RECORD OF PALEOENVIRONMENTAL EVOLUTION FOR THE LAST 150000 YEARS

历经15万年的甘肃临夏北塬黄土剖面,沉积速率大、古土壤序列完整。特别是在距今14—8万年的末次间冰期形成的三层古土壤,末次冰期间冰段形成的数层弱成土型古土壤,其特征优于其它黄土剖面。对北塬剖面磁化率、孢粉、粒度、地球化学特征的综合研究表明,该剖面完整地记录了15万年以来的环境变化,为检验以往从大陆冰盖和深海沉积中得出的环境变化模式提供了途径。

The Beiyuan Linxia loess section at Linxia, Gansu, China records the informa-tion of the environmental changes over the past 15Ma. This section shows a highsedimentary rate and a complete paleosol sequence, In the Last Interglaciation(0.14--0.08Ma B. P.), three layers of paleosols and several layers of weak pedo-genic paleosols were formed in the Malan loess, which is rarely seen at other sec-tions. Sample analysis indicates that its environmental records are relatively comp-lete and have a relatively high resolution. The magnetic susceptibility curve canbe used to represant the features of paleoclimatic changes. The high resolution of this section is due to the following reasons: (1) Thisarea lies in the border zone of the Tibet Plateau, Loess plateau and inland deserts,where different bio-climatic zones intersect and climatic changes are sensitive. (2)A'narrow tube'effect of the atmosphere flow was formed in the Hexi Corridor, andLinxia is just situated at the exit of the'narrow tube'. The 'impediment' of theQinghai-Tibet plateau to the atmosphere flow and the anticyclone action of theflow of the atmosphere on the eastern margin of the Qinghai-Tibet plateau causethe rate of dust deposition in the Linxia area on the northeastern margin of theplateau to be 4-5 times higher than that in other areas, thus raising the resolu-tion of the information of the environmental changes of sedimentary strata. A comparison of the magnetic susceptibility carve of the Linxia section withthose of the Vostok ice core in the Antarctic and of the cores from deep-sea holeRC_(11-120) in the subpolar Indian Ocean shows a good consistency of the trendsof environmental changes reflected by the three above-mentioned curves on thetime scale of Ka. Besides, the loess records indicate that the features of environ-ments were similar in the early and late Full Glacial Stages of the Last Glaciation,which is consistent with the record of the temperature changes of the summersea surface from deep-sea hole RC_(11-120) of the Indian Ocean and the recordfrom the Vostok ice cores but is different from the record of δ^(18)O variation fromdeep-sea hole RC_(11-120) and also different from the idea that the climate wasmore severe in the late Last Glaciation than in the early Last Glaciation. Theloess records also reflect that there were two harsh climatic stages in the LastInterglaciation (14--8Ma B. P.), whose climatic conditions were close to those inthe early and late glacial stages of the last Glaciation.