Gobi dynamics in the Northern Mongolian Plateau during the past 20,000+yr:preliminary results

This study focuses on the Khyaraany sand/loess/soil section (50.2 degreesN 106.7 degreesE) in the Northern Mongolian Plateau with the aim of decipheri ng the paleoenvironmental records and inferring the last glacial Gobi dynam ics. The main findings are as follows: (1) silt percentage (with a negligib le clay percentage) is positively correlated with the organic matter conten t. (2) The silt (%) and frequency-dependent magnetic susceptibility (%) are negatively correlated. (3) The magnetic susceptibility is positively relat ed to sand percentage and negatively to silt percentage. Based on C-14 date s and the extrapolated ages, the following observations can be made. (1) Un like 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 pal eosols (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 be tter vegetation conditions generally dominated the later part of the last g lacial (15,090-8300 yr BP), during which three paleosols (incipient histoso ls) were formed (15,090, 13,030, 8300 yr BP). (3) Windy and/or poor-vegetat ion conditions generally dominated the early part of the last glacial (simi lar to 24,000-16,000 yr BP). (4) This section documents approximately thous and-year-long redox cycles. Two tentative conclusions can be drawn from thi s and previous (Feng et al., 1998) studies. (1) The northern boundary of th e Gobi has shrunk as many as nine times during the past 40,000 yr: around 3 4:400, 30,700, 28,900, 24,500, 15,090, 13,030, 8300, 4070 yr BP and the rec ent 1000 + yr. (2) Based on the negative correlation of the susceptibility with silt percentage and organic matter content, it is proposed that the re ducing 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. (C) 2001 Elsevier Science Ltd and INQUA. All rights reserved.