MIDDLE HOLOCENE SOIL AND MIGRATION OF CLIMATIC ZONES IN THE GUANZHONG PLAIN OF CHINA.

This article discusses the findings from a microtexture determination and a chemical analysis of a field survey of the Holocene paleosol in the Guanzhong Plain part of the Loess Plateau in China. The paleoclimatic characteristics, the type of paleosol, and climatic change are discussed. The materials show that the Holocene paleosol is composed of a transitional horizon (AB) from an organic horizon (A) to a reddish-brown clay horizon (Bt), a weathered-leached parent horizon containing only eluvial soils, and a horizon of CaCO3 nodules. In the Guanzhong Plain, red ferri-argillans indicating chemical weathering and subtropical climate were well developed in the paleosol. The content of the ferri-argillans in this paleosol is higher than that in a luvic cinnamon soil and is close to that of a subtropical yellow -brown earth developed under forests. Clay minerals in the Bt horizon consist of mixed components of illite-askanite and kaolinite. The pollen assemblage shows that the vegetation was subtropical broadleaf forest, consisting of Quercus, Symplocos, Liquidambar, Pistacia, and so forth. Accordingly, it can be concluded that the soil-forming process was apparently stronger at that time than at the present, that the paleosol was yellow-brown forest earth developed in the northern subtropics, and that the climate was warm and wet. In the Middle Holocene, humid air masses traveling from the Pacific across southeastern China in the summer could frequently reach this area. At that time, a subtropical climate existed both south and north of the Qinling Mountains, and the mountains did not serve as a boundary between the temperate zone and subtropics of China. The mean annual temperature and rainfall were respectively about 15[degrees]C and 750 to 800 mm, which is 3[degrees]C higher and 150 to 200 mm wetter than present.