黄土形成过程的实质与环境

ESSENCE AND ENVIRONMENT OF LOESS FORMATION

黄土地层的形成实质上是土壤化过程，不同特性的黄土层代表了不同的成壤作用。不同地区的黄土主要是草原、荒漠草原和森林草原地区发育的灰钙土、栗钙土、棕钙土、棕漠上和黑沙土。与红褐色古土壤相比，黄土是冷于气候条件下发育的灰黄色古土壤，黄土层能够作为冷干气候的指示。根据灰黄色古土壤与红色古土壤的交替，可将２５０万年来黄土区气候变化分为５１个旋口与亚旋回。

There are soil macrostructure in loess, such as similar prismy sturcture, lumpy structure, grangular structure, root holes, and microstructure, such as optically oriented clay film developed in stronger soil formation processes. Loess is rich in CaCO3, but it is mainly secondary and is the illuvial products formed by dissolving and depositing of CaCO3 and is a sign of illuvial layer of CaCO3. These characteristics show that loess experienced soil formation processes. The fossil animals, such as snail, and a lot of root holes also indicate that the loess formed in soil formation processes. The developmental process of loess was longer than of mellow soils, the reasons why it didn't change into red-brown paleosols were cold and arid climate and the features of the soil formation processes. The soil profile of loess is not clear, which results from continual dust deposition and CaCO3 illuviation. Loess developed mainly in steppe, desert-steppe and forest-steppe areas, the loess in different areas and of different characteristics is mainly Sierozems, Chestnut Soils, Brown Soils, Brown Desert Soils and Loessial Soils. Soil formation mainly indudes weak dissolving, migrating and illuvating of CaCO3 as well as weak biological action in desert-steppe. CaCO3 exists in the form of powder and spots and CaSO4 sometimes occurs in the areas where soil structure develps poorly and root holes and insect holes are a few. The obvious migrating and illuviating of CaCO3 and forming of biological structure are the dominant processes in which loess develops in steppe areas. CaCO3 exists in the form of film and soil structure, such ac granular structure and root holes, is obvious in the areas. Besides notable migrating and illuvating of CaCO3 as well as forming of biological of structure, clay grouting sometimes occurs in forest-steppe areas in the formation process of loess. CaCO3 nodules often occur, granular, lumpy, similar prismy sturcture and root holes developed well, and organic substance is high in content and optically oriented clay film sometimes can be seen in the areas. Grey-yellow loess that is the paleosol can be used as a mark in reconstructing paleoclimate. According to the alternation of loess and paleosol, the biological, soil and climatic changes since 2. SMa can be classified into 101 stages consistuting 51 cycles and sub-cycles.