摘要
佳县红粘土剖面的磁性地层学研究证明,黄土高原北部风尘堆积的起始年代可以追溯到8 35Ma以前,通过进一步的磁化率测量和频率磁化率分析,发现佳县红粘土及上覆部分午城黄土 古土壤序列的磁化率和频率磁化率之间存在良好的正相关关系,表明成壤作用过程中由生物作用或风化作用形成的超顺磁颗粒和磁性矿物对磁化率和频率磁化率的增大具有重要贡献;同时,剖面中红粘土频率磁化率的百分含量明显高于上部午城黄土,指示了红粘土堆积较强的成壤作用,表明黄土高原普遍发育的红粘土堆积具有相似的磁学性质和古气候意义,并在空间尺度上可以对比。佳县风尘堆积的频率磁化率年代序列指示了3 6Ma以来,尤其是2 7Ma后气候波动加剧和北方干旱化增强。
Detailed magnetostratigraphic investigation of the Jiaxian red clay sequences indicated that widely spreading eolian succession in the northern Chinese Loess Plateau could be dated back to 835 Ma. On the basis of magnetic susceptibility measurement, we find that Jiaxian red clay sequences have similar composition of magnetic minerals to those of upper Wucheng loesspaleosol sequences, and the magnetic susceptibility value is well correlated to the content of ultrafine minerals. A positive correlation is observed between the magnetic susceptibility and the frequencydependent magnetic susceptibility, indicating that the content of the ultrafine magnetic minerals formed by biochemical processes has a significant contribution to the increase of magnetic susceptibility and frequencydependent magnetic susceptibility. Compared with the upper part of Wucheng loess sequence, red clay sequence shows a higher percentage of the frequencydependent magnetic susceptibility, which indicates more ultrafine magnetic minerals formed during relatively strong pedogenesis. We could conclude that the similar magnetism and paleoclimatic implications are recorded in the widely spreading red clay over the Chinese Loess Plateau and they can be used as a proxy index of pedogenic degree and the summer monsoon intensity. Changes of the frequencydependent magnetic susceptibility since 36 Ma, especially since 27 Ma, indicate the great changes between the cold and warm, the commencement dry and humid climatic conditions on the Loess Plateau and marked aridity in the inner northern China.
出处
《海洋地质与第四纪地质》
CAS
CSCD
北大核心
2003年第3期91-96,共6页
Marine Geology & Quaternary Geology
基金
国家自然科学基金资助项目(40121303
40202005
90102017)
中国科学院知识创新项目(KZCX2 108
KZCX1 10 01)
关键词
红粘土堆积
频率磁化率
古气候
黄土高原北部
red clay sequences
frequency-dependent magnetic susceptibility
paleoclimatic significance
northern Chinese Loess Plateau
