Records On Paleoenvironment In The Downstream Of Salawusu River Valley Since150Ka BP

The world has experienced a series of rapid changes of centuries to millennium, as well as the last interglacial-glacial cycle of large time scale since The Late Pleistocene. The Salawusu River Valley is located in the transitional zone between the Mu Us Desert and the Loess Plateau, and in the northern marginal area of the East Asian Monsoon. So it is very sensitive to climate changes and becomes the ideal area to study the Quaternary stratigraphy and paleoenvironment. In the article, we selected Jiufangtai section as research object in the downstream of Salawusu River Valley, and established the believable time scale by the method of AMS-14C, OSL and ESR. What’s more, the history of climate variations since150ka BP was reconstructed with grain size, magnetic susceptibility, geochemistry, pollen and some other regular proxies. The results can keep pace with the former records in the valley, regional area and the world.At first, we get the stratigraphic division in term of sedimentary environments and chronology. Holocene Series is made up of Dishaogouwan Formation and Dagouwan Formation. The former is mainly clayey silt, since2ka BP, while the latter is silt, formed during2-11ka BP. The upper part of Late Pleistocene, named Chengchuan Formation, is mainly composed of aeolian sand but lacustrine phase in the middle. The time of Chengchuan Formation is11～72ka BP. The bottom part of Late Pleistocene, named Salawusu Formation, is the typical Late Pleistocene strata sequence in North China. It is mostly composed of clay of lacustrine phase, which can be divided into5segments. The time of Chengchuan Formation is72～153ka BP. In the bottom of Jiufangtai section is Lishi Formation, which is uniform composition of silty sand, and is enriched in carbonate nodule. The time of Chengchuan Formation is153-207ka BP.With the relatively comprehensive analysis of proxies’ records, we restored the different classification scheme from different methods, and chose some representative indexes to comparison and comprehensive research. The whole evolution of environment and some significant events are consistent with the former study result. Especially, the paleoclimate divisions of Jiufangtai section are similar with the stage divisions of deep-sea oxygen isotope (MIS1-5). This illustrated the Salawusu River Valley almost corresponded global change, under the influence of alternation of winter and summer monsoons. During MIS5stage (72～152ka BP), the climate was warm and humid which is suitable for plants. The winter monsoon strengthened, the desert zone moved south, and climate was too cold and dry to plants during MIS4(54～72ka BP). And then, the summer monsoon strengthen and rainfall increase, which make lots of grassland vegetation recovery in MIS3stage (38～54ka BP). During MIS2stage (11～28ka BP), it’s under the interaction of the winter monsoon and the paleoclimate made wide fluctuation. But the environment of the stage is considered to be cold and dry and the vegetation was conifer forest-grassland. Lastly, records revealed there were wet and warm subdivisions in relatively arid MIS1stage (from11ka BP) and the vegetation landscape is mainly grassland.On the other hand, there are some little divergence reflections in different indexes, which is own to the difference of the implications and analytical conditions of the proxies. In the article, the results indicated oxide ratios of primary elements and coarse grain size are better to reflect the paleoenvironment change than the other proxies.