Rock magnetism in loess from the middle Tian Shan: Implications for paleoenvironmental interpretations of magnetic properties of loess deposits in Central Asia

Abstract

[1] Considerable attention has recently been paid to the magnetic properties of loess deposits in Central Asia. However, paleoenvironmental interpretations of magnetic properties of loess deposits in South Tajikistan, the Ili Basin, and the Tarim Basin remain controversial. Current meteorological observations demonstrate that the mean annual precipitation in the Junggar Basin exceeds that in the Tarim Basin and falls below that in the Ili Basin and South Tajikistan. This study of rock magnetism in loess from the Junggar Basin improves our understanding of the complicated relationships between magnetic properties, climate, and source materials for loess deposits of Central Asia. This study involved multiparameter investigations of rock magnetism in a 71 m loess section in the Dongwan area, on the northern slope of the middle Tian Shan (in the Junggar Basin). Variations in concentration of coarse eolian ferrimagnetic minerals appear to provide the main control over the magnetic properties of loess in this area. The contribution of ultrafine pedogenically produced magnetic grains to the magnetic susceptibility (c) is minimal. Furthermore, long-term increasing trends in the c and the saturation isothermal remanent magnetization (SIRM) were found in the Dongwan loess section. These trends may be linked with intensified aridification of Central Asia in the mid-Pleistocene. This study suggests that reduced precipitation of westerly moisture from Tajikistan into the Tarim Basin might play a critical role in the paleoenvironmental interpretations of magnetic properties in the loess deposits of Central Asia.

Long‐Term Variations in the Lithogenic Susceptibility of Chinese Eolian Deposits Since the Late Pliocene

“…Moreover, recent major element chemical analyses of the Baishui eolian sequence confirmed that the ratios of MgO/Al 2 O 3 , Na 2 O/Al 2 O 3 , and K 2 O/Al 2 O 3 increased significantly after 0.6 Ma, probably because of the increasing input of fresh silicates, which was closely related to active tectonics and intensive erosion under global cooling (Xiong et al, ). In addition, loess deposits from the northern slope of the West Kunlun Mountains and the northern slope of the middle Tian Shan, where paleosols are very weakly developed due to the low precipitation, and detrital magnetic minerals provide the main control on the magnetic properties of the eolian deposits; both exhibit a significant increase in magnetic susceptibility values at around 0.6–0.5 Ma (Zan et al, , ). Furthermore, the increase in rock magnetic concentration parameters is accompanied by a coarsening of the magnetic grain size and a “softening” of the magnetic mineralogy (i.e., an increased representation of low‐coercivity ferrimagnetic minerals), which may be associated with the increasing input of fresh detritus from the surrounding mountains due to intensive physical weathering and erosional processes.…”

Climatic fluctuations inferred for the Middle and Late Pleniglacial (MIS 2) based on high-resolution (∼ca. 20 y) preliminary environmental magnetic investigation of the loess section of the Madaras brickyard (Hungary)

“…However, often there is only a negligible contribution of ultrafine, pedogenicallyproduced magnetic grains to the magnetic susceptibility signal (see e.g. Zan et al 2012 and references therein). Measurement of magnetic susceptibility of loessic deposits has quickly become an effective tool of correlation between marine and terrestrial records both in international (Thompson and Oldfield 1986;Heller et al 1987;Kukla et al 1988) as well as in Hungarian practice (Horváth and Bradák 2004).…”

“…Zan et al 2012 and references therein). Measurement of magnetic susceptibility of loessic deposits has quickly become an effective tool of correlation between marine and terrestrial records both in international (Thompson and Oldfield 1986;Heller et al 1987;Kukla et al 1988) as well as in Hungarian practice (Horváth and Bradák 2004). Anisotropy of magnetic susceptibility within the magnetic minerals of loess sediments provides additional information relating to paleoenvironmental conditions and wind direction (Bradák 2009).…”

“…Environmental magnetism in loessic sediments is based on the enhancement of magnetic minerals, primarily iron oxides and hydroxides, through either pedogenesis (Babanin et al 1975;Maher 1984;Singer and Fine 1989;Heller and Evans 1995;Maher 1998;Evans and Heller 2001;Hambach et al 2008;Hambach 2010) or the accumulation of coarse eolian ferrimagnetic minerals (Zan et al 2012). Since pedogenesis is controlled by climate, tectonism, dust input as well as geomorphic position, variations in the intensity of laboratory-induced remanent magnetization and magnetic susceptibility, which result from this process, can be linked to orbitally-tuned records of climatic change from marine archives (Heller and Liu 1982, 1984.…”

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

“…All of these observations indicate that the medium‐coarse silt fraction provides the main control on the magnetic properties of the samples. This result is consistent with previous rock magnetic investigations of bulk samples, which suggest that the magnetic properties of Central Asian loess and paleosol deposits are dominated by larger PSD and MD ferrimagnetic grains [ Xia et al ., ; Song et al ., ; Zan et al ., ; Liu et al ., ]. However, we also note that the >75 μm fraction exhibits the minimum values of χ, χ ARM , and SIRM in all of the particle‐sized fractions and its contribution to bulk magnetic properties is negligible.…”