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In the eastern Loess Plateau region of northern China, the Quaternary loess-soil sequences of the last 2.6 Ma are underlain by the Hipparion Red-Earth Formation. Magneto-stratigraphic studies suggest that it is a near continuous terrestrial record for the period from similar to7 to similar to2.6 Ma BP, and therefore, has great potential for the study of Asian paleoclimates. The origin of this formation is controversial and needs further study. In this work, the Xifeng Red-Earth section, a type section for the eastern Loess Plateau region, was studied using optical microscopic analysis, grain-size measurements, and both major- and trace-element geochemical properties to address questions of its origin and environmental implications. The results show that this formation consists of three parts. The Lower part (>6.2 Ma BP) is a water-reworked deposit related to alluvial and slope processes; the Middle part (from similar to6.2 to similar to3.6 Ma BP) was derived from in situ eolian dust deposition, but was significantly affected by groundwater oscillations; and the Upper part (from similar to3.6 to similar to2.6 Ma BP) is an eolian formation, similar to the Quaternary loess-soil sequence. The grain-size of the Upper part is significantly coarser than for the middle part, but finer than for the overlying loess.

Because the eolian dust deposits in the middle reaches of the Yellow River were mainly transported from the deserts in northern China by the northwesterly winter monsoon wind and the westerlies, we interpret the eolian origin of the Middle and Upper Red-Earth formation, beginning approximately at 6.2 Ma BP, as an indication of the strengthening of the aridification of the Asian continent. Desert lands in Central Asia must have been formed at or by that time to provide a significant source of dust. However, the generally finer grain-size of the Red-Earth suggests a weaker transporting wind, and/or a more remote source. The latter implies a smaller extent of deserts than in the Quaternary. The high degree of similarity between the geochemical properties of the Red-Earth and loess samples seems to suggest a rather similar source areas and comparable dust-transporting trajectories. The removal of the dust deposits from groundwater influence and the increase in grain-size at similar to3.6 Ma BP are attributable to both tectonic and climatic causes. The initiation of these phenomena corresponds to an intensive uplift of the Tibetan Plateau and also to a simultaneous uplift of the sedimentary basins in the surrounding areas. It is also approximately synchronous with an increase in eolian mass accumulation rate in the North Pacific, both suggesting an increased continental aridity in the Asian dry lands, and an increasing intensity of transporting winds. (C) 2001 Elsevier Science B.V. All rights reserved.