Haitao Wei

Changing intensity of human activity over the last 2,000 years recorded by the magnetic characteristics of sediments from Xingyun Lake, Yunnan, China

AbstractWe report mineral magnetic measurements (magnetic susceptibility, anhysteretic and isothermal remanence, magnetic hysteresis and thermomagnetic properties) from a 2.46-m-long sediment core taken in Xingyun Lake, Yunnan, China. Results demonstrate that magnetic minerals in the lake sediment are dominated by stable single-domain (SSD) ferrimagnetic minerals derived mainly from in-washed catchment soil. Magnetic susceptibility increases sharply during the period AD ~500–1050, and the grain size of magnetic minerals decreases. These changes are accompanied by increases in the concentrations of Fe, Ti, Zr and Si, and by decreased tree pollen percentages. δ18O measures in speleothems from South China demonstrate that climate was relatively uniform during this interval. Documentary evidence, however, indicates a large increase in the human population of Yunnan Province at that time. We conclude that the late Holocene sediment record from Xingyun Lake reflects a dramatic increase in soil erosion that was caused by a growing human population, which in turn was responsible for increased deforestation and agricultural activity. Our results from Xingyun Lake are consistent with sediment magnetic records from Erhai Lake, located near our study site. We therefore conclude that the interval AD ~500–1050 corresponded to a regional-scale increase in the intensity of human activity on the Yunnan-Guizhou Plateau.

Early Pleistocene climate in western arid central Asia inferred from loess-palaeosol sequences

Arid central Asia (ACA) is one of the most arid regions in the mid-latitudes and one of the main potential dust sources for the northern hemisphere. The lack of in situ early Pleistocene loess/dust records from ACA hinders our comprehensive understanding of the spatio-temporal record of aeolian loess accumulation and long term climatic changes in Asia as a whole. Here, we report the results of sedimentological, chronological and climatic studies of early Pleistocene loess-palaeosol sequences (LPS) from the northeastern Iranian Golestan Province (NIGP) in the western part of ACA. Our results reveal that: 1) Accumulation of loess on the NIGP commenced at ~2.4–1.8 Ma, making it the oldest loess known so far in western ACA; 2) the climate during the early Pleistocene in the NIGP was semi-arid, but wetter, warmer, and less windy than during the late Pleistocene and present interglacial; 3) orbital-scale palaeoclimatic changes in ACA during the early Pleistoceneare in-phase with those of monsoonal Asia, a relationship which was probably related to the growth and decay of northern hemisphere ice sheets.

Holocene shorelines and lake evolution in Juyanze Basin, southern Mongolian Plateau, revealed by luminescence dating

Holocene environment change in the Gobi Desert of Mongolia and northwestern China is of global significance as it is one of the key dust source areas of the world. The Juyanze Basin, located in the central Gobi Desert of southern Mongolian Plateau, is one of the three terminal basins of the Heihe River that flows north from the Qilian Mountain. A series of geomorphic shorelines at different elevations around the basin indicate that large lake levels fluctuated during the past. In this study, we measured the exact elevation of shorelines in the Juyanze Basin using Digital Global Positioning System and found seven shorelines at ~37, ~36, ~34, ~30, ~28, ~24, and ~22 m above modern basin floor (a.m.b.f.). Optically stimulated luminescence dating was employed to date the deposition of beach sand and gravel sequences at these shorelines. Results indicate a paleolake developed in the basin at ~5 ka when lake levels reached ~26 m a.m.b.f.. The lake level then gradually increased to ~29 m a.m.b.f. at ~3.3 ka and reached its highest Holocene level of~37 m a.m.b.f. at ~2.7 ka. The lake environment during 1.1–0.8 ka was characterized by frequent lake-level fluctuations at ~30 m a.m.b.f. The lake disappeared from both East and West Juyanze Basin after ~0.6 ka. This sequence is out-of-phase with other regional Holocene lake records which indicate Holocene high stands occurred during the early to middle Holocene. We suggest that out-of-phase lake high stands at ~5–1 ka in the Juyanze Basin are related to geomorphological shifts of the Heihe River channels across its fan/delta, feeding the three sub-basins of the Ejina at different times. Using paleolake shorelines in this region to reconstruct the climate changes needs to be very careful.

Magnetic property of loess strata recorded by Kansu profile in Tianshan Mountains

Kansu (KS) profile is located in the east of Yili basin, western Xinjiang, where typical loess sediments are distributed. The magnetic parameters (such as IRM, SIRM SOFT, and M) and grain size in the KS profile were analyzed in the study. The results showed that the magnetic property of KS loess is dominated by ferrimagnetic minerals, such as magnetite and maghemite. Antiferromagnetic and superparamagnetic minerals also exist in the profile, but had less impact on magnetic susceptibility. Compared with the typical loess sediments of the central Loess Plateau in China, the strata of Kansu profile contained more magnetic minerals and hard magnetic minerals. The analysis of grain size for magnetic minerals indicated that the properties of loess and paleosol were respectively dominated by PSD/MD and coarse SSD magnetite. The research found that the contents of magnetic minerals in loess and paleosol sequences in Kansu profile were similar, but the proportion of fine grained magnetite and soft magnetic minerals were varying, which implies a positive relationship between the value of magnetic susceptibility and intensity of pedogenesis.