北疆黄土的磁化率各向异性揭示末次冰期以来古风向的变化

Paleowind direction variations revealed by anisotropy of magnetic susceptibility of loess deposits in north Xinjiang since the last glacial period

黄土磁化率各向异性(AMS)被认为是重建古风向变化重要的指标之一,在黄土高原地区得到广泛的应用。然而新疆地区的黄土磁化率各向异性研究相对薄弱。通过对新疆塔城盆地库尔托别剖面磁化率各向异性参数和磁化率分析古风向和风力强度的变化,结果表明:塔城地区末次冰期以来以东南风为主,剖面从下至上,可分为5个阶段:第1阶段(12~14 m):对应MIS3c时期,磁组构特征受水流作用的影响明显,表现为东南风。第2阶段(6~12 m):对应MIS3b时期,出现西南风,但主要还是以东南风为主。第3阶段(4~6 m):对应MIS3a早中期,以东南风为主,西南风逐渐消失,并且风力强度逐渐减弱。第4阶段(0.5~4 m):对应MIS3a晚期和MIS2早期,表现为东南风,风力强度波动较大。第5阶段(0~0.5 m):磁组构特征受成壤作用影响强烈。

As a proxy of paleowind direction, the anisotropy of magnetic susceptibility （AMS） has been widely applied to reconstruct the paleowind history of aeolian loess deposits in Chinese Loess Plateau （CLP）. However, fe- wer attentions have been paid to AMS in Xinjiang loess. In this study,we present AMS results from an aeolian loess section near Tacheng in North Xinjiang, China. Based on magnetic susceptibility and AMS, we discussed variations of paleowind intensity and direction. In general, SE paleowind dominatesin this area since Marine Isotope Stage （MIS） 3. According to magnetic fabric characteristics, we divide the section into five stages of paleowind variations. The stage 1 （12 -14 m in depth） ,corresponding to late MIS3e ,is characterized by reworked aeolian sediment. The stage 2 （6 -12 m in depth） ,corresponding to MIS3b ,is characterized by dominate SE wind with the appearance of SW wind. During MIS3b ,large-scale glaciers in Altay Mountains may cause stronger cold high pressure in the Jung- gar Basin. Undoubtedly, the pressure gap between Junggar Basin and Tacheng Basin will be enlarged, which will produce stronger SE wind in Tacheng Basin during MIS3b. Glacier widely developed during MIS3b in Tianshan can also push cold high pressure to expand to the north and close to Tacheng Basin, which is in favor to form stronger SW wind in Tacheng Basin. The stage 3 （4 -6 m in depth） , corresponding to early and middle MIS3a, is character- ized by decreasing intensity of wind and disappearing SW wind. Magnetic fabric characteristics is disturbed to some extent. During this stage, temperature increased and glaciers in Tianshan retreated because of gradually increasing high-latitude （65~N） solar insolation. However, due to particular location, Altay glacier will still keep its larger scale under Mongolia and Siberian high-pressure system. Retreating glacier in Tianshan may cause gradually the dis- appearing of SW wind in Tacheng Basin. The stage 4 （0.5 -4 m in depth） ,corresponding to late MIS3a and early MIS2, is characterized by completely disappeared SE wind and rapid fluctuating intensity of wind. The stage 5 （0 - 0.5 m in depth）, corresponding to topsoil,is characterized by that the magnetic fabric characteristics is totally dis- turbed by pedogenesis. Systematical AMS investigation of Tacheng loess will provide a new insight in understanding dust source and mechanisms of climate changes in Tacheng Basin.