Pleniglacial millennium-scale climate variations in northern China based on records from the Salawusu River Valley

Situated in the Salawusu River Valley, southeast of China's Mu Us Desert, the MGS2 (Milanggouwan section) portion of the Milanggouwan stratigraphic section records 5.5 sedimentary cycles consisting of alternations between dune sand deposits and fluvial or lacustrine facies. We analyzed the grain-size and CaCO3 distributions in MGS2, and found that Mz (mean particle diameter) and a (standard deviation) displayed clear variations in peaks and valleys within different sedimentary facies. The CaCO3 content averaged 0.4% in the dune sand deposits, 1.43% in the fluvial facies, and 8.82% in the lacustrine facies. Both the grain-size distribution and CaCO3 contents, which equal the indicators for the alternation among the sedimentary fades, suggest the occurrence of 5.5 cycles. These results suggest that the observed cycles mainly resulted from fluctuations between a cold and dry winter monsoon climate and a warm and humid summer monsoon climate, and that the MGS2 portion experienced at least 5.5 fluctuations between these two extremes. This high-frequency climatic fluctuation indicates a strong influence of millennium-scale variations in the strength of the East Asian winter and summer monsoons in our study area during the Pleniglacial.