Patterns of local and regional grain size distribution and their application to Holocene climate reconstruction in semi-arid Inner Mongolia, China

The semi-arid temperate steppe in northern central China is one of the main areas influenced by frequent dust and sand storms, and is at the same time a primary source of dust from deteriorated grasslands; thus, the sediment grain size distribution of inland lakes in this region can be a particularly useful indicator of palaeoenvrionmental change. The local pattern of grain size suggests that aeolian activity is the most important agent for sedimentation in the lake center in this region, as strong northwesterly winds prevail for most of the year and the surface runoff is very weak. Meanwhile, the regional pattern of topsoil grain size and its close association with mean annual precipitation (MAP) allows the establishment of a statistical model for palaeo-moisture reconstruction from sediment grain size. In this study, we reconstructed a humidity time series based on the sediment grain size sequence from Anguli Nuur Lake on the southern Inner Mongolian Plateau in China and found that it coincides very closely with the C/N ratio (carbon to nitrogen ratio) and other humidity indices revealed in previous studies of this temperate steppe region and from the δ18O values of stalagmite calcite in southern, monsoon-dominated China. This close relationship suggests that climate change in the semi-arid areas of Asia is strongly influenced by the Pacific summer monsoon and that it is reasonable to use sediment grain size as an indicator of humidity variability in the semi-arid steppe region. The reconstructed humidity increased during the early Holocene, and generally humid conditions lasted from about 10,400 until 7000 yr BP. The period from around 7000 to 5200 yr BP was a transition phase from humid to semi-arid conditions, and the monsoon intensity of that time may have been at the threshold for a semi-arid vegetation ecosystem. Finally, since approximately 5200 yr BP to present, the climate has become more arid, with corresponding vegetation deterioration and strong aeolian activity.