北京邻区枣沟、斋堂和赤峰黄土-古土壤剖面粒度和磁化率变化在幅度和相位上都有明显的差异.在两个冰期-间冰期旋回,磁化率记录具有冰量变化模式,而粒度记录具有轨道周期特点,粒度和磁化率之间的相位关系在从冰期向间冰期转换和冰期向间冰期过渡的过程中,既有滞后也有超前的表现,很难单纯以成土作用来解释.由于黄土粒度主要反映粉尘源区环境和古风场强度的变化,而磁化率主要记录的是沉积区的成壤作用,反映夏季风的强度变化,因此剖面粒度和磁化率变化的不协调说明古风场和粉尘源区变化与粉尘沉积区气候变化之间不统一.这可能意味着控制夏季风强度变化和控制冬季风强度变化的机制存在差异.
The studies for the loess deposits of China have been obviously progressed in recent two decades and many paleoclimatic signals were detected by the analyses of a lot of climatic proxies. An important understanding by now is that the depositional features of the loess are mainly controlled by the paleomonsoon and the variations in summer and winter monsoon strength can be deciphered from the fluctuations of the grain size and magnetic susceptibility of the loess. However, the paleoclimatic proxies of the loess are not yet well understood and more loess sequences need to be studied in detail. Here we report the analysis result of three sections in the vicinity of Beijing region, focusing on the comparison of the variations in the grain size and in susceptibility along the sequence. The results show that the variations in the grain size and susceptibility are not simultaneous both during the deglaciation and glaciation. During glaciations (from paleosol to loess), the grain size exhibits lagged variations than that of the susceptibility. However, during deglaciations, the variations of the grain size either lead or lag that of the susceptibility. Meanwhile, the curve of the susceptibility from S2 to S0 (in Zhaogou section) is featured by 100ka cycles (ice volume variation pattern) but the grain size variation has the cycles of the orbital parameters in general. This may imply that the winter monsoon is relevant to the orbital cycles whereas the summer monsoon is cooperated with the ice volume changes. It’s not yet properly explained why the susceptibility and the grain size exhibited different patterns of variations along the loess sequences. The susceptibility of the loess reflected the variations in the strength of the summer monsoon which are controlled by the low latitudes machanisms, whereas the grain size recorded the fluctuations in the winter monsoon winds from high latitudes. The different patterns of variations in the susceptibility and the grain size may imply that the mechanisms that controlled the climate changes of the low latitudes and the high latitudes are different.
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