巴基斯坦Bahawalpur黄土岩石磁学特征及磁化率变化机制研究

风成黄土是陆地上分布最广泛的沉积物之一，记载了各种古气候演化信息.目前巴基斯坦的黄土研究甚少，磁化率与气候对应的变化机制研究尚未开展.本文对位于巴基斯坦印度河平原Bahawalpur地区新发现的黄土-古土壤剖面进行系统的岩石磁学研究，结合粒度和漫反射光谱(DRS)数据，讨论巴基斯坦黄土的磁化率变化机制.实验结果显示：Bahawalpur (BH)剖面黄土层主要的载磁矿物为磁铁矿，同时含有少量磁赤铁矿和针铁矿，磁性颗粒以原生的MD和PSD颗粒为主.相对于黄土层，古土壤层则是以针铁矿为主，含有顺磁性矿物和少量磁铁矿.BH剖面磁化率与成土作用关系和中国黄土高原典型剖面相反，磁化率的变化可能存在一个阈值12.8×10^-8m³·kg^-1，在阈值之上，强磁性矿物(磁铁矿、磁赤铁矿)占主导；阈值之下，以弱磁性矿物(主要是针铁矿)为主，这种磁性矿物的转变可能导致磁化率降低.本文可为今后利用磁化率解读该地区地层蕴含的古气候信息提供新线索.

Rock magnetic properties and magnetic susceptibility change mechanism of loess in Bahawalpur,Pakistan

Aeolian loess is one of the most widespread deposits on the land and records many kinds of information of paleoclimate changes.Currently,the studies of loess in Pakistan are rare,and the studies on the mechanism of magnetic susceptibility variation are still unavailable.In this study,the the rock magnetic parameters of the newly discovered loess-paleosol section(29°19′56″N,71°42′65″E)in the Bahawalpur area of the Indus Plain of Pakistan were systematiclly measured.Combined with the grain size characteristics and the diffuse reflectance spectrum(DRS),the mechanism of magnetic susceptibility variation was discussed.The results show that the main magnetic minerals in loess are magnetite,coupled with goethite and maghemite.And the magnetic domain particles in the magnetic minerals are Pseudo-single domain(PSD)and multi-domain(MD).Compared with the loess,the main magnetic minerals in paleosol are goethite,coupled with magnetite and paramagnetic minerals.This changing mechanism of the susceptibility is different from that of Chinese Loess Plateau(CLP).There is a possible threshold of magnetic susceptibility 12.8×10^-8 m³·kg^-1 in the study area,above which the main magnetic minerals are strongly magnetic minerals(magnetite and maghemite);below this value,the main magnetic minerals are weakly magnetic minerals(such as goethite).This transition of magnetic minerals may lead to a decrease in magnetic susceptibility.This study provides a new clue for further paleoclimatic research in Pakistan based on magnetic susceptibility in the future.