南京老虎山黄土剖面的磁化率及Rb和Rb/Sr对古气候的指示意义

PALEOCLIMATIC IMPLICATION OF DISTRIBUTION OF Rb, Rb/Sr AND MAGNETIC SUSCEPTIBILITY IN LOESS AND PALEOSOLS FROM LAOHUSHAN PROFILE,NANJING

宁镇地区的下蜀黄土记录了古气候变化的丰富信息,作为中国北方风尘堆积的南部边缘相,下蜀黄土在全球环境变化研究中占有重要的地位。以10cm间距系统分析了典型的下蜀黄土剖面——老虎山剖面的Rb、Sr含量和频率磁化率,结果表明,磁化率对冬季风场强变化有较好的指示意义,但在指示下蜀黄土的风化成壤强度方面不很理想;Rb含量可以作为指示东亚冬季风强度的替代性指标;Rb/Sr比可以作为下蜀黄土化学风化强度的替代性指标,Rb/Sr比由剖面下部至上部升高是长江中下游地区12万年以来成壤作用增强的结果。

Xiashu loess has become an ideal natural record for cold and warm climate change, especially at the middle and lower reaches of the Yangtze River. As a marginal facies of eolian sediments, the Xiashu loess holds the regional promise of providing a record of variations in the East Asian Monsoon. Compared over a spatial and time span, the Xiashu loess record can give a picture of East Asian Monsoon variations on different time scales, its circulation process and mechanism. Paleoclimatic records from the Xiashu loess could reveal intensities of both summer and winter monsoon climate changes.In this paper, Rb and Sr concentrations as well as magnetic susceptibility have been analyzed of the 3 loess layers and 3 paleosol layers of the Laohushan profile, Nanjing. Rb contents are relatively constant, with the mean value lower than that of Luochuan loess in the Loess Plateau. This supports the fact that loess of Xiashu formation and that of Loess Plateau are both eolian dust accumulations from similar source regions. A similar curve between rubidium concentration and magnetic susceptibility suggests that they should be effective proxy for winter monsoon intensity.  Rb/Sr ratios of paleosol layers are higher than those of its underlying loess layers,indicating that Rb/Sr ratios may be regarded as proxy indicator of the intensity of postdepositional chemical weathering and pedogenesis. The general increase trend of Rb/Sr ratios from the bottom upward indicates the increasing intensity of summer monsoon in the lower reach area of the Yangtze River since 120 ka.