This study focuses on the Khyaraany sand/loess/soil section (50.23N 106.73E) in the Northern Mongolian Plateau with the aim of deciphering the paleoenvironmental records and inferring the last glacial Gobi dynamics. The main "ndings are as follows: (1) silt percentage (with a negligible clay percentage) is positively correlated with the organic matter content. (2) The silt (%) and frequency-dependent magnetic susceptibility (%) are negatively correlated. (3) The magnetic susceptibility is positively related to sand percentage and negatively to silt percentage. Based on C dates and the extrapolated ages, the following observations can be made. (1) Unlike the Marine Isotope Stage (MIS) 3 paleosols (24,500, 28,900, 30,700, 34,400 yr BP) that formed under oxidizing-dominated conditions, two MIS 2 paleosols (15,090, 13,030 yr BP) and two Holocene paleosols (8300, 4070 yr BP) were formed under dominantly reducing conditions. (2) Less windy and/or better vegetation conditions generally dominated the later part of the last glacial (15,090-8300 yr BP), during which three paleosols (incipient histosols) were formed (15,090, 13,030, 8300 yr BP). (3) Windy and/or poor-vegetation conditions generally dominated the early part of the last glacial (&24,000-16,000 yr BP). (4) This section documents approximately thousand-year-long redox cycles. Two tentative conclusions can be drawn from this and previous (Feng et al., 1998) studies. (1) The northern boundary of the Gobi has shrunk as many as nine times during the past 40,000 yr: around 34,400, 30,700, 28,900, 24,500, 15,090, 13,030, 8300, 4070 yrBP and the recent 2000+yr. (2) Based on the negative correlation of the susceptibility with silt percentage and organic matter content, it is proposed that the reducing conditions of incipient histosol formations have contributed to the alteration of magnetic minerals from strong forms of oxidized iron to weak forms. Therefore, the magnetic susceptibility is basically an indicator of redox cycles.

An and Porter (Geology 25 (1997) 603) reported six high dust-influx events of millennial timescales recovered from the last interglacial paleosol S1 and correlated them to six cool events of millennial timescale in the North Atlantic. However, the complexity of soil-forming processes may have made the chronological correlation with the North Atlantic records inadequate. To examine the complexity of the S1 formation, the S1 paleosol was traced laterally and identified based on the preserved characteristics observed in the field and analyzed in the laboratory. Our data show that fromthe northwest to the southeast, the S1 paleosol gradually converges fromthree distinctive soil profiles into a single welded profile because the net rate of loess accumulation was attenuated to the southeast and pedogenic development intensified southeastward during the last interglacial. Three soil-forming events within the S1 paleosol (S1S1, S1S2 and S1S3) separated by two loess units (S1L1 and S1L2) in the northwestern part of the Loess Plateau are stratigraphically coeval with a single soil profile in the southeastern margin of the Loess Plateau. In the southeast, the S1 paleosol developed into underlying older loess L2 (e.g., at the Lantian section). The three paleosols (S1S1, S1S2 and S1S3) are partially welded in the central part of the Chinese Loess Plateau (e.g., at the Tianshui section), where the lower portion of S1 paleosol developed in the underlying older loess unit L2. In the northwestern margin of the Chinese Loess Plateau (e.g., at the Lanzhou section), the preservation of the repeating soil–loess sequence (S1S1, S1L1, S1S2, S1L2 and S1S3) continuously documented the climatic events of the last interglacial. Our data also show that the magnetic signatures and particle-size information are more or less acceptable climatic proxies only for the northwestern sections, where the degree of pedogenesis was lower and the rate of eolian influx was greater during the last interglacial. It appears that in all cases investigated, the median grain size and the coarse fraction (>63 mm) content define the upper and lower boundaries of the S1 paleosol reasonably well and can be used to estimate the timetransgressive nature of the S1 paleosol relative to its parent material. Soil welding, bioturbation and material translocation within the S1 soil profiles make it impossible to preserve the detailed and high-resolution information of climate changes in those S1 profiles in the southeastern part (including the popularly called central part) of the Chinese Loess Plateau.

This study compares two pairs of adjacent lacustrine and eolian sections at sites in the southern and northern Mongolian Plateaus in order to test spatial climate variability during the Holocene. Based on the lithology, proxy data, and 14C dated and the interpolated ages, the following observations can be made. In the northern Mongolian Plateau, a best developed Holocene paleosol dated at 8672 14Cyr BP at the Shaamar section and the carbonate-rich laminated layer in the Gun Nuur lake core mark the interval of warmer and dryer climate during the early Holocene. Younger paleosols at the Shaamar section and corresponding organic-rich layers in the Gun Nuur core were formed under distinctly cooler and more humid conditions. The Baahar Nuur lake core in the southern Mongolian Plateau and the Dingxi-type section in the northern part of the Western Chinese Loess Plateau appear to indicate that a prolonged interval of maximum humidity prevailed in this region during the early and mid-Holocene (9000-4000 14Cyr BP). By contrast, in the northern Mongolian Plateau the most humid conditions seem to have occurred from 4500 to 2500 (possibly to 1650) 14Cyr BP. This discrepancy implies that the concept of the Holocene climatic optimum has limitations and may have to be reconsidered if it is intended to have a large-scale connotation.

This study deals with desertification and its impacts on environment in the source region of the Yellow River. Our objectives were to reveal and quantify desertifcation changes from 1990-2000 by remote sensing and GIS, and discuss its impacts on carbon cycle. The results indicate that the study area is one of the most desertified regions in northeast Qinghai-Tibetan Plateau. The study area is now a carbon source. The desertification would have contributed to CO2 emission from grasslands of Qinghai-Tibetan Plateau. Otherwise, there are great potential to resequestration carbon through controlling grazing, restoring ecosystem, and reducing desertification.