塔里木盆地南缘8.5 ka以来的黄土岩石磁学特征及其磁化率变化机制

黄土岩石磁学参数是古气候研究中的重要指标，其中磁化率应用最为广泛，并在黄土高原地区取得重大进展，其受控于成壤作用的变化机制也被普遍接受.然而在黄土高原外缘的新疆地区，磁化率的变化机制仍不明确，导致磁化率的古气候意义在该区存在较大争议.本文选取塔里木盆地南缘具有精确年代控制的典型黄土剖面（羊场剖面）开展岩石磁学和高分辨率磁化率研究，利用交叉小波分析方法并结合剖面粒度、矿物及元素特征对该地区磁化率变化机制进行初步探讨.结果显示，羊场剖面的岩石磁学性质主要由粗颗粒软磁性矿物所控制，同时也表现出一定的顺磁性特征.根据载磁矿物和磁化率变化特征可将剖面进一步划分为两个阶段：阶段Ⅰ（8.5~2.5 ka），载磁矿物以亚铁磁性的磁铁矿为主，磁化率值整体较高；阶段Ⅱ（2.5~0.2 ka），亚铁磁性矿物依然占据主导地位，但硬磁性矿物和以黄铁矿为代表的顺磁性矿物相对增多，磁化率值显著降低.相关性研究和交叉小波分析表明：阶段Ⅰ磁化率与粗颗粒组分的变化具有一致性，符合"风速论"模式；阶段Ⅱ磁化率不仅与粗颗粒组分具有明显的正相关关系，而且与指示成壤作用强度的频率磁化率百分含量呈现出显著的负相关关系，暗示了阶段Ⅱ的磁化率变化可能受到"风速论"和"还原性成壤"模式的共同影响.本文拓宽了对新疆地区黄土岩石磁学特征及其磁化率变化机制的深入理解，也为利用磁化率恢复新疆及中亚地区全新世以来的古气候变化历史提供了新的线索.

Rock magnetic characteristics and magnetic susceptibility change mechanism of the loess since 8.5 ka in the southern margin of Tarim Basin

Magnetic susceptibility, as a convenient and reproducible rock magnetic index, has been widely used in paleoclimatic reconstruction and has achieved great success in China Loess Plateau. The mechanism that pedogenesis intensity contributes to the magnetic susceptibility is generally accepted in China Loess Plateau. However, the magnetic susceptibility change mechanism in Xinjiang is still unclear, so that the paleoclimatic significance of magnetic susceptibility remains controversial. In this study, a typical loess section (Yangchang section) with accurate chronology from the southern margin of Tarim Basin was selected for rock magnetism and high resolution magnetic susceptibility study. Combined with the grain size characteristics, mineral components and elemental concentrations, the magnetic susceptibility change mechanism was discussed. The results show that the main magnetic-bearing mineral is coarse-grained magnetite with a small amount of hard magnetic minerals and paramagnetic minerals in the Yangchang loess sediments. According to the variation characteristics of magnetic-bearing minerals and magnetic susceptibility, the section can be further divided into two stages:Phase Ⅰ (8.5~2.5 ka), the content of soft magnetic minerals are relatively high, and the magnetic susceptibility stays in relatively high value. Phase Ⅱ (2.5~0.2 ka), hard magnetic minerals and paramagnetic minerals, represented by pyrite, increase, while the magnetic susceptibility decreases significantly. Correlation analysis and cross wavelet analysis results show that the change of magnetic susceptibility is consistent with that of coarse particles in phase Ⅰ, suggesting a so-called "wind velocity theory" model control of magnetic susceptibility in this stage. While in phase Ⅱ, besides the positive correlation between magnetic susceptibility and the contents of coarse grain, a strong negative correlation between the magnetic susceptibility and pedogenic proxy was also observed, indicating a combined effects of "wind velocity theory" and "reductive pedogenesis" processes on magnetic susceptibility change during phase Ⅱ. This study provides valuable information on rock magnetic characteristics and magnetic susceptibility change in this area, and gives a new clue on further paleoclimatic reconstruction in Xinjiang and Central Asia based on magnetic susceptibility.