渭河流域典型黄土剖面的环境磁学特征及其古气候意义

对黄土高原南部耀县全新世黄土地层的形成及保存环境进行了研究.结果表明:地层中磁性矿物主要为软磁性矿物,硬磁性矿物对地层的磁学性质贡献小;随着成壤强度的增加,硬磁性矿物的相对含量在减少.本区全新世地层的形成和保存环境为氧化环境,适宜用磁学手段反演全新世古气候.对磁化率和频率磁化率、无磁滞剩磁磁化率的对比分析,可以看出:成壤强度较低时,细粒磁性矿物相对于磁性矿物总量的增加速率相对较快;成壤强度较高时,细粒磁性矿物的相对含量会达到或趋近饱和,相对于磁性矿物总量的增加速率相对较慢.研究表明,在黄土地层中,χfd%和χARM/χ等指示细粒磁性矿物的磁学参数可以显示更细微的气候变化;在古土壤中,χlf的气候指示意义相对更好.

Typical Loess/Paleosol Sequences in Weihe Drainage Areas: Environmental Magnetism Features and Paleoclimate Interpretation

The Loess Plateau, covered with aeolian sediments called loess, lies in the north of East Asian monsoon areas. Previous studies indicated that the loess deposits are an excellent archive of palaeo-environmental information, which may provide some of the most complete continental records of climate change. Environment magnetism is one of common substitute parameters of paleoclimate, including χlf, χfd%, χARM, χARM/χ, SIRM, SIRM/χ, Hcr, and Hc and so on. But the recent studies have showed that the χlf not always relate positively to humidity, and the reductive-oxidative conditions maybe infect it. In the south of Loess Plateau, some loess profiles also have this phenomenon. In this paper, a study is presented, which choices the Yaoxian profile as the research object. The profile is located in the south of the Loess Plateau. The environment magnetism parameters showed the soft magnetism mineral controlling the sample's property, and the pedogenesis being negative relation to the content of hard magnetism mineral. This relation is more approximate to the typical loess profiles in Loess Plateau. Thus, the Yaoxian profile is considered forming and saving in an oxidative condition. Comparing the differences of χlf, χfd% and χARM/χ curves' changes in loess and paleosol stratum discovers that the increasing rate of fine magnetic grain mineral relative to the total content of magnetized mineral is much pronounced in weak pedogenesis loess stratum. Thus, the χfd% and χARM/χ are much useful to recover the paleoclimate in arid period, but the χlf , rather than χfd% and χARM/χ, is much useful to recover the paleoclimate in humid period.