灵台红粘土-黄土-古土壤序列频率磁化率的古气候意义

Paleoclimatic Implication of Frequency Dependent Magnetic Susceptibility of Red Clay-Loess-Paleosol Sequences in the Lingtai Profile

通过对甘肃灵台晚新生代红粘土 -黄土 -古土壤序列的磁化率测量和频率磁化率分析 ,发现红粘土及黄土 -古土壤的磁化率和频率磁化率存有良好的正相关关系 ,表明两套沉积在成壤过程中由生物作用或化学作用而形成的超顺磁性矿物含量 ,对磁化率和频率磁化率增大有重要的贡献。相比磁化率而言 ,受影响因素较少的频率磁化率变化 ,反映了不同气候条件下形成的超顺磁粒级铁磁性矿物的相对含量 ,能更真实、敏感地记录不同时间尺度的古气候波动。灵台红粘土 -黄土 -古土壤序列的频率磁化率在 2 .6MaBP前后不同的变率特征 ,揭示了大冰期来临前后黄土高原的冷暖、干湿反差产生了显著变化 ,而与此相关的冬。

On the basis of rock magnetic properties, red clay has the similar composition of magnetic minerals as that of loess paleosols sequences, and the magnetic susceptibility value is well correlated to the content of ultra fine magnetic minerals. Magnetic susceptibility of loess paleosols has been regarded as a proxy index of summer monsoon intensity. However, there are different opinions about the paleoclimatic implications of magnetic susceptibility of red clay. Here we report the magnetic susceptibility and frequency dependent magnetic susceptibility measured for the late Cenozoic red clay loess paleosol sequences of the Lingtai profile in Gansu province. A positive correlation is observed between the magnetic susceptibility and frequency dependent magnetic susceptibility, indicating that the content of ultra fine magnetic minerals formed by bio chemical processes has a great contribution to the increase of magnetic susceptibility and frequency dependent magnetic susceptibility. Compared with the magnetic susceptibility, the frequency dependent magnetic susceptibility, which indicates the content of ultra fine magnetic minerals formed under different climatic conditions, can reliably and sensitively record the paleoclimatic variations on different time scales, and also can be used as a proxy index of pedogenic degree and summer monsoon intensity. The different variability of frequency dependent magnetic susceptibility before and after 2.6MaBP indicates the great change between the cold and warm, dry and humid climatic conditions on the Loess Plateau. Summer and winter monsoon variations show different combinations before and after the occurrence of major North Hemisphere Glaciations.