陕西洛川黄土剖面的Rb/Sr值及其气候地层学意义

Rb/Sr VARIATIONS AND ITS CLIMATIC STRATIGRAPHICAL SIGNIFICANCE OF A LOESS-PALEOSOL PROFILE FROMLUOCHUAN,SHAANXI PROVINCE

本文系统地测试了陕西洛川黄土-古土壤剖面中的Rb，Sr含量和磁化率。通过对比该剖面2．50MaB．P．以来Rb／Sr曲线和磁化率曲线，发现两者的形状和变化的相位十分相似，然而两者的相对振幅却有明显的差别。Rb／Sr曲线记录了0．80MaB．P．左右一次重要的气候转型界面，揭示了黄士和古土壤内部次一级的气候波动旋回，反映了早更新世黄土高原较温暖潮湿的特点以及更新世以来气候由湿向干的变化趋势。研究表明，Rb／Sr值可以作为衡量东亚夏季风环流强度变化的较为敏感的替代性指标。

The loess-paleosol sequence in centyal China conshtutes an excellent long-termproxy record of East Asian monsoon climate variations during the Quaremary. Manyinvestigations have indicated that magnetic susceptibility and grain size variations ofthe sequence are good measures of summer and winter monsoon intensity respectively.However, these monsoon records need to be verified by other associated indices thatare independent and vary synchronology. Some trace element ratios have been provedto be of potential impoftance in reconstructing history of the regional. climate. Herewe present the vertical distribution of the Rb/Sr ratio in the Luochuan loess-paleosolsequence and discuss its climatic significance.The concentrations of Rb and Sr, and magnetic susceptibility have beenmeasured in a 140m thick loess-paleosol sequence from the Luochuan profile. Resultsshow that the Rb/ Sr ratio vary from 0.39 in loess to 1.20 in paleosols.The averagevalue of Rb/ Sr ratios is lower than that in paleosols with 0.55 and 0.73 respectively.Similar to magnetic susceptibility signal, the Rb/Sr ratio can be used to distinguishloess and paleosol layers.The difference of Rb/Sr between loess and paleosols is mainly due topostdepositional processes of loess deposits, which may cause the marked fractionationof Rb and Sr in the profile. The significant process was weathering and pedogenesis.Studies show that Rb is stable element and appears to retain in the profile during thestage of weathering and pedogenesis, while Sr is mobile and its concentrationdecreases with the enhancement of weathering and pedogenesis intensities. Therefore,variation of the Rb/Sr ratio essentially reflects strength of leaching of loess depositsduring weathering and pedogenesis. It should be pointed out that extent of leachingduring pedogenesis is related to amount of precipitation which represents the intensityof summer monsoon. During the domination of summer monsoon, the precipitationon the Loess Plateau is heavier, leaching and eluviation are stronger and thedepletion of Sr is greater, resulting in higher Rb/Sr ratios in paleosols. This meansthe Rb/Sr ratio can be served as an indicator for the intensity of summer monsoon inthe Loess Plateau.It can be seen that the distribution curves of Rb/Sr ratios and magneticsusceptibility along the whole profile show much similar pattern in both curve shapeand phase transition, further proving the reliability of the Rb/Sr ratio as an indicatorfor summer monsoon. On the other hand, perhaps owing to various degrees inresponding to climate changes, the two parameters have striking differences inrelative amplitude, as shown in the following:First The Rb/Sr signal confirms that 0.80Ma B. P. was an impotent boundary(within the layer S8) at which monsoon climate have changed sharply, i.e. from high-frequency, low- amplitude fluctuations to low-frequency, high-amplitude variations.This corresponds well with the variation of global ice volume recorded in deep-seaoxygen isotope of ODP sites (e. g. 659, 677, 607). This significant boundary ofclimate changes was also found in other proxy indices in loess Plateau,such asmagnetic susceptibility in Xifeng and grain-size in Baoji.Second, the Rb/Sr ratio can provide more detailed information than magneticsusceptibility. For example, magnetic susceptibility curve shows only one sub-peak inUpper Silt Layer (L9). While there are two obvious peaks shown in the Rb/Sr ratio inL9, which correspond to two sub-variations in isotope stage 23 indicated from ODPsite 607 and 659.Furthermore, the fluctuation of Rb/Sr ratios in the whole section shows that theclimate in the Loess Plateau during Early Pleistocene was warm and moist andbecame cold and dry laaer. This is in agreement with the global climate variations,suggesting that the Rb/Sr parameter is much better than magnetic susceptibility intracing paleoclimare change recorded in the Loess Plateau with global climaticsignificance.