Loess magnetic properties in the Ili Basin and their correlation with the Chinese Loess Plateau

Abstract

Over the past two decades, magnetoclimatological studies of loess-paleosol sequences in the Chinese Loess Plateau (CLP) have made outstanding achievements, which greatly promote the understanding of East Asian paleomonsoon evolution, inland aridification of Asia, and past global climate changes. Loess magnetic properties of the CLP have been well studied. In contrast, loess magnetic properties from outside the CLP in China have not been fully understood. We have little knowledge about the magnetic properties of loess in the Ili Basin, an intermontane depression of the Tianshan (or Tien Shan) Mountains. Here, we present the results of rock magnetic measurements of the Ili loess including mass magnetic susceptibility (χ) and anhysteretic remanent magnetization (ARM), high/low temperature dependence of susceptibility (TDS) and hysteresis, as well as X-ray diffraction (XRD) for mineral analysis. Based on the comparison with loess-paleosol sequences in the CLP (hereafter referred to as the Chinese loess), we discuss the possible magnetic susceptibility enhancement mechanism of the Ili loess. The results show that 1) the total magnetic mineral concentration of the Ili loess is far lower than that of the Chinese loess, though they have similar magnetic mineral compositions. The ferrimagnetic minerals in the Ili loess are magnetite and maghemite, and the antiferromagnetic mineral is hematite; XRD analysis also identifies the presence of ilmenite. The ratio of maghemite is lower in the Ili loess than in the Chinese loess, but the ratios of magnetite and hematite are higher in the Ili loess than in the Chinese loess.2) The granularity of magnetic minerals in the Ili loess, dominated by pseudo-single domain (PSD) and multi-domain (MD) grains, is generally much coarser than that of the Chinese loess. Ultrafine pedogenically-produced magnetic grains have a very limited contribution to the susceptibility enhancement. Rather, PSD and MD particles of magnetite and maghemite are the main contributors to the enhancement of susceptibility in the Ili loess. 3) The susceptibility enhancement mechanism for the Ili loess is complicated and superimposes both a wind velocity/vigor model (Alaskan or Siberian model) and the in situ ultrafine grain pedogenic model; the former might play an important role in the Ili loess. 4) Magnetic susceptibility enhancements of the Ili loess are related not only to the eolian input of the source area, but also to the local climate, landform, and geological background. Therefore, great care should be taken when reconstructing paleoclimate using magnetic susceptibility data from the Ili loess. loess, magnetic property, magnetic susceptibility, the Ili Basin, the Chinese Loess Plateau Citation: Song Y G, Shi Z T, Fang X M, et al. Loess magnetic properties in the Ili Basin and their correlation with the Chinese Loess Plateau.

Susceptibility variations of multiple origins of loess from the Ily Basin (NW China)

“…However, the sedimentation rates from all the paleosols studied are quite different, with thicknesses ranging from ca. 0 to more than 2 m. S1 paleosols from some sections by the Gongnaishi river valley were thought to exhibit a decrease in  [21][22][23][24][25][26][27]. However, the sharp and uneven boundaries between the  highs and lows cannot be correlated to the strata or the pedogenetic intensity.…”

“…However, pedogenesis influences not only the magnetic concentration but also the magnetic mineralogy and contributes at least partly to susceptibility variations in loess from . A negative relationship between susceptibility and the degree of pedogenesis was found in the Ily Basin, northwestern China [21][22][23][24][25][26][27]. In that location the susceptibility decreased dramatically in paleosols except S0, but was rela- Chen Q, et al Chin Sci Bull May (2012) Vol.57 No.15 1845 tively high and constant in other horizons.…”

“…A negative relationship between susceptibility and the degree of pedogenesis was found in the Ily Basin, northwestern China [21][22][23][24][25][26][27]. In that location the susceptibility decreased dramatically in paleosols except S0, but was rela-tively high and constant in other horizons.…”

“…In that location the susceptibility decreased dramatically in paleosols except S0, but was rela- Chen Q, et al Chin Sci Bull May (2012) Vol.57 No.15 1845 tively high and constant in other horizons. YE Wei, SHI ZhengTao, SONG YouGui, XIA DunSheng et al gave different explanations for the susceptibility variations but they did not adequately explain this depletion of susceptibility [21][22][23][24][25][26][27]. In this work, during September 2007 to May 2010, we collected loess samples in the Ily Basin during 3 field seasons.…”

The magnetic properties of Serbian loess and its environmental significance

“…In this area, the climate is extremely arid and pedogenesis is weak. Thus, physical weathering is dominant, such as in loess paleosols in most parts of Xinjiang, where magnetic susceptibility and grain size show a positive correlation [39,40]. This area should be a good place to apply the "wind theory" hypothesis.…”