黄土高原靖边剖面岩石磁学性质及其古环境意义  

Mineral magnetism of the Jingbian loess-paleosol sequence and its paleoclimatic significance

作　　者：董良 沈中山 邓成龙[1,2] DONG Liang;SHEN ZhongShan;DENG ChengLong(State Key Laboratory of Lithospheric Evolution,Institute of Geology and Geophysics,Chinese Academy of Sciences,Beijing 100029,China;College of Earth and Planetary Sciences,University of Chinese Academy of Sciences,Beijing 100049,China)

机构地区：[1]中国科学院地质与地球物理研究所岩石圈演化国家重点实验室,北京100029 [2]中国科学院大学地球与行星科学学院,北京100049

出　　处：《地球物理学报》2024年第8期3060-3074,共15页Chinese Journal of Geophysics

基　　金：国家自然科学基金项目(41888101)资助。

摘　　要：中国黄土在东亚第四纪气候环境变化研究领域占有重要地位,靖边黄土剖面位于黄土高原北部边缘,对东亚季风变化敏感.本文对靖边黄土剖面进行了详细的岩石磁学和环境磁学研究.剖面沉积物中的主要磁性矿物为磁铁矿、磁赤铁矿以及赤铁矿,黄土中的碎屑磁铁矿普遍经历了低温氧化作用的初始阶段,形成磁赤铁矿外壳包裹磁铁矿内核的结构,导致矫顽力大幅增加.磁化率随温度变化曲线(X-T)指示成壤作用生成的单畴颗粒的亚铁磁性矿物(磁铁矿和磁赤铁矿)的含量在L15(约1.25Ma)后整体减少,表明东亚夏季风强度发生阶段性减弱.黄土层中的磁化率与非磁滞磁化率比值(X/X_(ARM))与来自物源区粗粒磁性矿物含量相关,冬季风增强则X/X_(ARM)值越高,靖边剖面黄土层X/X_(ARM)在L14以后整体增加,L6以来逐渐增加,表明东亚冬季风发生过两次增强.矫正后磁化率与非磁滞磁化率((X-X_(0))/X_(ARM))指示了来自物源区磁性矿物含量,靖边剖面自2.6Ma以来,(X-X_(0))/X_(ARM)逐渐增加,源区的磁性矿物含量增多表明冬季风逐渐增强.非磁滞剩磁与饱和等温剩磁中值退磁场比值(MDF_(ARM)/MDF_(SIRM))与样品相关,样品磁性矿物粒度越粗,低温氧化作用越弱,矫顽力越高,MDF_(ARM)/MDF_(SIRM)越高,靖边剖面MDF_(ARM)/MDF_(SIRM)分别在L14和L6后整体升高,表明磁性矿物粒度整体变粗,代表了沙漠两次向南扩张.综上磁学参数一致表明,靖边黄土剖面有效地记录了东亚季风演化过程,对研究亚洲内陆干旱环境演变有重要意义.Chinese loess plays an important role in the study of Quaternary climate and environmental changes in East Asia.The Jingbian loess section is located on the northern margin of the Loess Plateau and is sensitive to the East Asian monsoon.The main magnetic minerals in Jingbian loess are magnetite,maghemite,and hematite.The low-temperature oxidation of detrital magnetite in loess has formed a structure in which the maghemite rim encapsulates the magnetite core,resulting in a significant increase in coercivity.Magnetic susceptibility versus temperature curves(χ-T)show that content of fine-grained single domain pedogenic ferrimagnetic minerals(magnetite and maghemite)decreased after at least L15(approximately 1.25 Ma),indicating a periodic weakening of the East Asian summer monsoon intensity.The ratio of magnetic susceptibility to anhysteretic susceptibility(χ/χ_(ARM))in the loess layers is related to the content of coarse-grained magnetic minerals from the source area,which increases with the increasing East Asian winter monsoon(EAWM)strength.The χ/χ_(ARM) of the loess layers in the Jingbian section increases overall after L14,and gradually increases since L6,indicating two episodes of stepwise enhancements of EAWM.The corrected ratio of magnetic susceptibility to anhysteretic susceptibility((χ-χ_(0))/χ_(ARM))directly represents the magnetic mineral content from the source area.Since 2.6 Ma,the(χ-χ_(0))/χ_(ARM) gradually increased in the Jingbian section,and the increase of magnetic mineral content from the source area reveals a long-term strengthening of EAWM.The ratio of median destructive field of anhysteretic remanent magnetization to saturation isothermal remanent magnetization(MDF_(ARM)/MDFSIRM)is related to the microcoercivity of the sample.The coarser magnetic minerals have high coercivity because of a weaker magnitude low-temperature oxidation,and they always have high MDF_(ARM)/MDFSIRM.The MDF_(ARM)/MDFSIRM of the Jingbian section increased after L14 and L6,respectively,indicating that the partic

关 键 词：黄土高原 靖边剖面 岩石磁学 环境磁学 东亚季风 沙漠扩张 

分 类 号：P318[天文地球—固体地球物理学]