This study focuses on a Barton County loess-soil section in central Kansas in an attempt to retrieve the geochemical signature of the late Quaternary environmental change in the arid-semiarid region and provide a data base of loess geochemical characteristics in the Great Plains. The data indicate that the elements (expressed as oxides) of the windborne fraction (<63 mm) that are easily solubilized and lost from the soil by chemical weathering and leaching are relatively sensitive to soil-forming processes, whereas the insoluble elements of that fraction are controlled primarily by the chemical composition of the parent material. The geochemical imprint of pedogenesis superimposed on that of the loess parent material can be distinguished by comparing the magnetic susceptibility signature with the Fe2O3/Al2O3 ratio. Based on the geochemical imprint and radiometric dates (14C and thermoluminescence), the Bignell Loess (8000-5000 yr) is poorly weathered and the Brady Soil (10 000-8000 yr) is strongly weathered. The Peoria Loess (20 000-10 000 yr) has neither a traceable leaching nor weathering marker, and the Gilman Canyon pedocomplex (35 000- 20 000 yr) is strongly leached. The reddish pedocomplex (70 000- 35 000 yr) is the most strongly weathered, probably due to stable land surface conditions in a mild climate, and the Barton sand (92 000-70 000 yr) seems to have been subjected to a moderate weathering. Within the exposed portion of the Loveland Loess (260 000-92 000 yr), weathering indices are inversely correlated with the carbonate content and the carbonate-rich soils were developed under warm dry environments before 92 000, around 193 000, and before 260 000 yr. The data also show that the geochemical background of Fe2O3, and Al2O3 is high in two portions of the section: from the Brady Soil to the Gilman Canyon pedocomplex and from LL1 to LL2 within the Loveland Loess. The geochemical background of Fe2O3 and AI2O3 in the loess-soil sequence might have been dictated by that of eolian sources.

We present the results of high-resolution multi-proxy climate studies of the S1 palaeosol, corresponding to oxygen isotope stage (OIS) 5, from the northwestern margin of the Chinese Loess Plateau area. Here, S1 is much thicker (ca.6-8m) than in the central Loess Plateau areas (ca. 2 m), where most previous studies have been conducted. Hence, much higher-resolution stratigraphic studies are possible, yielding more insight into the temporal variations of the East Asian monsoon during MIS 5. The frequency-dependent magnetic susceptibility, as well as the concentration of secondary carbonate, is used as an indicator of the summer monsoon intensity, and the median particle size as an indicator of the winter monsoon intensity. The results suggest that the northwestern margin of the Chinese Loess Plateau experienced the strongest summer monsoon intensity in sub-stage (OISS) 5e and the weakest in OISS 5a, among the three warmer periods during stage 5. The summer monsoon was weaker in OISS 5b than in OISS 5d. A dusty interval interrupted the second warmer period (5c) and a soil-forming event interrupted the "rst colder period (5d). The results also suggest that the directions of changes in the intensities of summer and winter monsoons may not always have been proportionately opposite. For example, the weakest summer monsoon occurred in OISS 5a during which the winter monsoon was not the strongest. We further conclude that the winter monsoon during the last interglacial was probably driven by global ice volume #uctuations, while the summer monsoon was primarily controlled by the northern hemisphere solar insolation and was probably modi"ed by a feedback mechanism. That is, the climatic buffering effect of low-latitudinal oceans may have distorted the response of the summer monsoon to insolation variations. Finally, our results do not show the degree of climatic instability comparable to that recorded in the GRIP ice core for the last interglacial (OISS 5e), even though the study area is situated in a region which has been sensitive to climatic changes. ( 1999 Elsevier Science Ltd. All rights reserved.

Several thick last glacial loess sequences in the western part of the Chinese Loess Plateau and an eolian-colluvial sequence in the central Mongolian Plateau are used to understand how the ummer and winter monsoons responded to global forcing factors during marine isotope stages MIS.4 and 3. The magnetic susceptibility data are compared with other climatic proxy data to improve the well-publicized reconstruction of the past monsoons. This study suggests that different sensitivities between the summer and winter monsoons in responding to the northern insolation resulted in transitional climates between marine isotope stages, which were characterized by cold and wet climates in the western part of Chinese Loess Plateau. After forest-steppe in MIS 5, coniferous forest emerged at the MIS 5/MIS 4 transition in cold and moist conditions. These imply that the lower latitude oceans were still capable of supplying water vapour to the continental interior, and that the Mongolian High became dominant when the northern insolation entered a stadia lstate MIS 4.. Coniferous forest dominated MIS 4, but thereafter deteriorated to a steppe towards the end of the MIS 4, probably because the lower latitude oceans failed to supply water vapour to the interiors. A coniferous forest emerged again at the MIS 3rMIS 2 transition after forest-steppe in MIS 3. Consequently, the southern boundary of the Gobi Desert retreated northwards during the coniferous tree-dominated periods between 73 and 60 ka and between 27 and 19 ka, and expanded to maxima between 60 and 50 ka and between 19 and 10 ka. The extent of the Gobi Desert was considerably smaller for several times (around 47 ka and 41 ka, and 27q ka) during MIS 3 than during the Holocene, and the northern boundary retreated 5 times (around 20 ka, 24 ka, 29 ka, 31 ka, and 34 ka. in just the later part of MIS 3. q1998 Elsevier Science B.V. All rights reserved.