关键词: 黄土地层, 古土壤, 环境变化, 气候旋回

Abstract: According to the field survey and laboratory analyses of microstructure and susceptibility, the soil character and the climate cycles indicated by paleosol were researched in the paper. Loess has clear soil structure such as granular structure, lumpy structure, similar prismy structure and many holes for plant roots or animal worms. In some layers, there is optically oriented clay film. Secondary CaCO3 is rich in loess and there is some secondary CaSO₄ in the northwestern loess. They are all illuvial products of soils after being leached. In general, there is organic matter in loess. Sometimes, it appears in the form of black spots, which suggests that it formed in the soil-forming processes. All of them show that loess has the typical soil character in arid areas or semiarid areas and the formation of the loess is soil-forming processes. The development of the loess is very slow and time is long enough for it to become mature paleosol. Because of the cold and arid climate condition then, it didn't become the red-brown paleosol. Grey-yellow mature soils only could be formed at that time. Loess had been in a stable state and it can be regarded as a reliable indicator for the cold and arid climate. The research material about the climate for soil development and the soil character show that loess is mainly made up of Brown Soils, Sierozems, Chestnut Soils, and Black Loessial Soils which developed in the steppe and woods-steppe areas. On the basis of the replacement of the loess and the red-brown paleosols, we can divide the creature, climate and soil change cycles. There are 50 layer red-brown paleosols indicating warm and wet climate and 51 layer grey-yellow paleosols indicating cold and arid climate in loess strata since 2.50 Ma BP. They are all macrostropic appearance of the creature, climate and soil change for a large range. According to the replacement of these two types of soils, we can divide the environment change since 2.50 Ma BP into 51 cycles and sub-cycles. The difference between cycle and sub-cycle can be judged by the degree of the soil development. The paleosols representing climate cycles developed strongly, while the paleosol representing sub-cycles did weakly. The climate changes more frequently in the warm stageswhich the red paeosol indicates in the past 500 ka. Comparing the 51 cycles of environment change with the 53 cycles of oxygen isotope in deep-sea deposit, we can conclude that loess strata can be regarded as the standard of the global climate change. What's more, the oxygen isotope in deep-sea deposit only reflects the temperature and ice volume on the land while loess strata has more environment significance. It can reflect not only temperature and precipitation but also the alteration of the plants and soils.

Key words: loess strata, paleosol, environmental change, climatic cycle