Holocene aeolian mantles and inter-bedded paleosols on the southern Tibetan Plateau

Abstract

On the southern Tibetan Plateau (TP) a sediment cover of aeolian sand and silt is widely distributed beyond the basins in an elevation belt above 4000 m asl. In this geomorphological environment a continuous aeolian sedimentation occurred during the Holocene and erosion processes kept subordinate until late Holocene. A dark humic paleosol, which is dated to between 4100 and 2000 years cal BP, divides the aeolian sediment cover into two stratigraphic units. The lower unit started with the beginning of the aeolian sedimentation since late Glacial and early Holocene and extended until mid-Holocene when the paleosol developed. The sediments show the bright color of fresh loess-like material and no paleosol representing the early to mid-Holocene climate optimum is detected. The upper sediment unit started after the end of paleosol development and lasted until the late Holocene with an increased deposition of re-transported loess. The enhanced aeolian sediment mobilization during late Holocene is provoked by drier and cooler climatic conditions reducing the vegetation density. In addition, human impact on the vegetation by overgrazing of livestock and forest clearing enforced soil erosion processes and thus the supply of fine material.The modern soil and vegetation cover on top of this sediment unit are related to the modern climatic conditions. In the semi-arid region surrounding the lakes Nam Co and Siling Co alpine steppe vegetation and phaeozem soils occur. In the southern part which is more humid, alpine meadow vegetation with cambisols appears. All the paleosols are of cambisol type and therefore represent a more humid period from the upper boundary of mid-Holocene until the beginning of late Holocene, because of higher availability of effective moisture for the growth of dense vegetation cover and appropriate paleosol formation. For this period of paleosol development a general trend from north to south is obvious: from 5.5 to 3.8 ka BP in the northeastern TP to between 4.1 and 2 ka BP in the southern TP. This coincides with the observed retreat of monsoonal precipitation on the TP during that time. In contrast to the arid and semi-arid environment of the TP the amount of precipitation is not the primary limiting factor for vegetation density and soil development, but temperature and solar radiation have a great influence on local evapotranspiration and effective moisture.