Magnetic Study Of Loess From The Tianshan Mountains

A great deal of work has been made on the Chinese plateau and great progress has been made. The magnetic properties in loess records can be attributed to both original aeolian input, which is mainly coarse grained magnetite, and post-depositional pedogenesis, which produces fine grained maghemite. Post-depositional pedogenesis results in susceptibility enhancement in paleosols. The grain size distribution of pedogenetic ferrimagnetic grains is consistent, which is independent of pedogenetic intensity. There is a grain size threshold between ferrimagneitc grains of aeolian origin and those of pedogenetic origin. On the Chinese loess plateau, susceptibility is positively correlated to peodgenetic degree and can be used as a proxy for summer monsoon intensity and paleoprecipitation.However, the magnetic properties of the loess deposits in other areas can differ greatly from those on the Chinese loess plateau. There is reverse relationship between susceptibility and pedogenesis. The magnetic mechanism, magnetic mineralogy and ferrimagnetic grain size distribution of loess from the Tianshan mountain areas, northwest China has not been intensively investigated. The environmental implications of the loess magnetic properties, which is more complicated compared to that on the Chinese loess plateau, is to be further studied. Systematic studies were made on four sections in the Tianshan mountain areas. Various magnetic methods and grain size was used to study the origins of susceptibility variation, mineralogy and ferrimagetic grain size distribution of the loesss sections.The susceptibility of Tianchi loess section is constantly low, the change in which is attributed to wind strength or local source. Pedogenesis was observed in the lower part of Lujiaowan loess section, where χ and χfd% are low while Bcr and paramagnetic content are high, and ferrimagnetic grain size decreases. This might suggest overwet conditions during peodgenesis.Neleke loess section is characterized by its multiple origins of susceptibility varation. The loess horizons have multi-domain magnetite grains of aeolian origin, the modern soils are characterized by production of pedogenetic ultrafine-grained ferrimagnetics, and the other paleosols are characterized by the formation of nonferrimagnetics. The correlation between the low concentration of ferrimagnets, high paramagnetic content, high magnetic coercivity remanence, fine ferrimagnetic grain size and intensified pedogenesis suggest two competing processes of pedogenetic enhancement and pedogenetic depletion in the lower paleosols. Pedogenetic depletion dominates and is responsible for the low susceptibility.The magnetic properties of Zeketai section are similar to those of Neleke section, χ decreases in the lower part of ZKT section, which probably reflects local environment change but not climate change in a broad area. χfd% is more applicable to reflect climate change. Correlation between grain size and pedogenesis of the sections is observed but correlation between susceptibility and pedogenesis is absent, suggesting waterlogging rather than wind strength is responsible for susceptibility variation. The negative relationship between χand χfd of Zeketai brickyard secion, which is interpreted to be alternately controlled by waterlogging and wind strength, is suggestive of rapid climate change. Generally, grain size of bulk samples cannot be correlated to susceptibility in loess horizon, but magnetic grain size responses to magnetization. On one hand, it reveals that the relationship between ferrimagnetic grain size and magnetization can better reflect the effect of wind strength on magnetic accumulation in loess, on the other hand, it is demonstrated that wind strength is not directly linked to dust grain size or magnetic grain size. Coarse dust grain size leads to coarse magnetic grain size. However, intensified winds do not necessarily result in increased coarse grain size of aeolian deposits. This is supportive of the speculation that the atmospheric system in the Ily basin is complicated compared to other areas in northwest China, with the effect of the local airflows superimposed on the dominance of westerlies. A grain size parameter alone may not adequately reflect the change in the atmospheric system. Local source might contribute significantly to susceptibility variation.Spatial variation of loess magnetic properties demonstrates zonality.χ are dominated by precipitation and temperature, which vary with altitude.It was evidenced that the ferrimagnetic grain size distribution of loess from the Tianshan mountain areas is the same as that of the Chinese loess plateau. The grain size distribution of pedogenetic ferromagnetic grains is consistent, which is independent of pedogenetic intensity. There is a grain size threshold for ferrimagneitc grains of aeolian origin and those of pedogenetic origin. Waterlogging possibly results in higher concentration of SSD maghemite, and lower concentration of SP maghemite. It is suggested that the environmental implications of ferrimagnetic grain size of loess in Tianshan mountain area are applicable in other areas.