The dark colored organic soils represented by A horizon of Dark loessial soils, called “Heilutu”, are extensively distributed on the Loess Plateau, China. They show that the Plateau was formerly covered with dense vegetation. On the other hand, in the upper drainage basin of the Qingshui-he River, which flows into the Huang-he at the western part of the central Loess Plateau, Holocene river terraces are widely developed and their deposits consist of gravel bed, peat layers and loessial flood loam bed in ascending order. The peat layers intercalated in the terrace deposits indicate that a peatland was widely expanded along the Qingshui-he during a period of Holocene time. The authors examine the stratigraphical relationship between the peat layers of Holocene terraces and the dark colored organic soils distributed on mountain slopes, and discuss the environmental changes based on the changes in facies of terrace and slope deposits as well as ¹⁴C dates of peat and soils.
The upper drainage basin of the Qingshui-he, whose altitude ranges from 1, 500 to 2, 900 m above sea level, is situated in a semi-arid region with annual precipitation of 400 to 650 mm, and a part of the basin is occupied by a present peatland with small area. The peat layers of the present peatland are divided into the following four layers; the lower, the middle, the upper and the uppermost, by intervening clay layers. The four layers are different in color, thickness and in content of organic matter and clay. The middle peat layer is the most developed with dark color and rich in organic matter. Three peat layers except the uppermost layer are recognized in the Holocene terrace deposits. The dark colored soils on the mountain slopes and the Pleistocene terraces are stratigraphically correlated to the middle and upper layers of the peat deposits.
The ¹⁴C dates show 11, 800±200 B. P. (TK-826) for the lower part of the lower peat layer, 7, 980±80 B. P. (TK-827) for the lower part of the middle peat layer and 5, 040±70B. P. (TK-828) for the upper part of the upper peat layer at Kaicheng of 1, 930 m in altitude. The lowest part of dark colored organic horizon of Heilutu distributed on the mountain slope at Mt. Yunwu-shan (2, 148 m a.s.1.) shows 6, 280±70 B. P. (TK-829). The dark colored organic horizon are proved to have been formed mainly during the period from 8, 000 B. P. to 5, 000 B. P., meaning that the Loess Plateau was covered with dense vegetation during the Hypsithermal. The uppermost horizon of loessial flood loam covering over the Holocene terrace consists of thin and light colored humus, indicating that the formation of organic soils with weak activity was continued after 5, 000 B. P.
The changes in facies of terrace and mountain slope deposits, and in vegetation density suggest the environmental changes as follows;
The gravel bed of the lowest layer of Holocene terrace deposits, forming alluvial fans at mountain foot, indicates that the stream was powerful, suggesting that heavy rainfalls occurred frequently before 12, 000 B. P. During the period from 12, 000 B. P. to 8, 000 B. P., the periglacial debris layers were deposited on the mountain slopes, and the clayey deposits without gravel along the river courses. The former shows the cold and wet condition with sparse vegetation, and the latter the small precipitation. Therefore, the cold and dry climate but the wet soil condition are inferred.
During the period from 8, 000 B. P. to 5, 000 B. P., the dark colored organic soils were developed even in the present arid zone with annual precipitation less than 350 mm, indicating that it was more humid than the present. The periglacial action on high mountains ceased under dense vegetation cover, indicating that it was warmer.