Jinbo Zan

A rock magnetic study of loess from the West Kunlun Mountains

West Kunlun Mountains have continuous loess deposits. However, very few papers have explored the rock magnetic characteristics of loess deposited there. Thus, although Kunlun Mountains loess is one of the useful paleoclimate archives, its importance has not been emphasized. Here we present detailed rock magnetic and bulk grain-size studies on a 207 m loess core on the northern slope of the West Kunlun Mountains. We find that magnetic susceptibility of loess from the northern slope of the West Kunlun Mountains is not controlled by changes in the concentration of ultrafine pedogenic magnetite and maghemite, but instead controlled by changes in the concentration of aeolian multidomain magnetite and maghemite. This is unsurprising given the low rainfall of the Kunlun region and the lack of postdepositional weathering and soil formation in these aeolian sequences. In addition, we find that magnetic susceptibility values increase dramatically at 111 m corresponding to an age of similar to 0.5 Ma. The magnetic susceptibility increase is accompanied by coarsening of both bulk and magnetic grain size and softening of magnetic mineralogy. We tentatively tie these changes in rock magnetic and grain-size parameter records to an expansion of desert environment.

Bulk particle size distribution and magnetic properties of particle‐sized fractions from loess and paleosol samples in Central Asia

Recent studies demonstrate that particle size separation based on gravitational settling and detailed rock magnetic measurements of the resulting fractionated samples constitutes an effective approach to evaluating the relative contributions of pedogenic and detrital components in the loess and paleosol sequences on the Chinese Loess Plateau. So far, however, similar work has not been undertaken on the loess deposits in Central Asia. In this paper, 17 loess and paleosol samples from three representative loess sections in Central Asia were separated into four grain size fractions, and then systematic rock magnetic measurements were made on the fractions. Our results demonstrate that the content of the 75 μm) exhibits the minimum values of χ, χARM, and SIRM, demonstrating that the concentrations of ferrimagnetic grains are not positively correlated with the bulk particle size in the Central Asian loess deposits.

ROCK-MAGNETIC CHARACTERISTICS OF THE TIANSHAN LOESS AND THE MECHANISM FOR ENHANCING MAGNETIC SUSCEPTIBILITY

The large area of eolian loess distributed over the northern slope of the middle Tianshan between the altitude of 2 400~2 700m has provided a good opportunity to study the loss rock magnetic properties of Central Asia and the evolutionary process of the extreme arid areas of NW China.Our research profile is located on the sixth terrace of Qingshui river on the northern slope of Tianshan.We collected 580 of bulk samples for analysis of the rock magnetic susceptibility at a 10~20cm interval and selected several samples for analyses of other magnetic parameters.Results show that the magnetic properties of the Tianshan Loess are mainly controlled by low-coercivity ferrimagnetic minerals(such as magnetite and maghemite),significantly different from the loess sequence of the Loess Plateau,and the contribution of ultrafine pedogenically-produced magnetic grains to loess magnetic susceptibility is very limited,while the increased coarse ferrimagnetic minerals from the source region play a decisive role in increasing magnetic susceptibility in the Tianshan region.The gradual increasing trend in magnetic susceptibility and the coarsening upwards trend of grain size since 0.8Ma may be the response to the aridification process in the inland Asia since the mid-Pleistocene.

MEASUREMENT AND PALEOCLIMATIC SIGNIFICANCE OF SOIL BULK DENSITY FOR LOESS IN EXTREME ARID REGION: MEASUREMENT AND PALEOCLIMATIC SIGNIFICANCE OF SOIL BULK DENSITY FOR LOESS IN EXTREME ARID REGION

Soil bulk density of the loess-paleosol can not only reflect variations in strength of East Asian winter and summer monsoon,but also has important significance to study paleoclimate and reconstruct drying trend of Asian heartland.Now,most of researches on the soil bulk density concentrate their attention on Chinese Loess Plateau(CLP),while in the arid regions of Northwest China there are relatively fewer researches on the methods and paleoclimatic significance of the soil bulk density.Based on previous studies,an ideal method for measurement of the soil bulk density in arid region of Asian heartland is put forward in this paper,and we also find the soil bulk density value in negative correlation with variations in the grain sizes and magnetic susceptibility(MS),which is very different from that in Chinese Loess Plateau.Further analysis suggested that variations in the soil bulk density of extreme arid region loess was mainly controlled by the drought extent of source region,rather than monsoon intensity.