陕西旬邑新近系化学地层与环境变迁

陕西旬邑下墙剖面化学组成的系统测试资料、年均温度与年均降水量的定量计算揭示出 :该区红土堆积时的气候总体上比第四纪黄土堆积时的要暖湿 ,在暖湿背景下 ,早期相对较凉干 ,中期最暖湿 ,晚期较暖湿。进入第四纪后 ,气候突然变冷变干。如更进一步细分 ,新近系红土段自下向上存在着由相对凉干—相对暖湿的四个百万年级气候旋回 ,其中第二、三旋回之间 ,即第 17与第 16层组之间 ,有气候较明显降温变干现象 ,它有可能主要是北极冰盖形成的影响所致。进入第四纪的突然明显变冷变干可能主要受控于当时青藏高原剧烈隆升作用。剖面上化学组成的变化在地层划分上有重要意义 ,从旬邑剖面的地化资料出发 ,并参考岩性、磁化率等将第四系底划在第 13层组顶 ,上新统底划在第 17层组与第 18层组之间

THE NEOGENE CHEMICAL STRATIGRAPHY AND ENVIRONMENTAL CHANGES OF XUNYI, SHAANXI PROVINCE

For understanding the environmental characteristics and changes of Neogene red earth deposit period in the Loess Plateau, the authors of this paper investigated the nearly continuous Late Cenozoic earthy profile at Xiaqiang Village, Xunyi County, Shaanxi Province, and collected fifty samples in the whole Neogene red earth beds and the lowest Quaternary loess paleosol in that profile. The geochemical analysis results provide very useful data for analyzing the paleoclimate and conditions from the Late Miocene to the very early Pleistocene. These results might also be one of the useful supplementary factors of subdividing strata. The following conclusions are drawn from Figs.2 and 3: 1. All of the curves in Figs.2 and 3 could be divided into four parts from bottom to top. The first part is composed of the 18th—20th bed group. The main features of this part are that the content of stable compositions such as SiO 2, Al 2O 3, Fe 2O 3 and Rb are lower than those of the comparison lines, while the content of soluble and easy soluting compositions such as FeO, CaO, Na 2O and Sr are higher than those of the comparison lines. The content ratio between stable compositions and the soluble and easy soluting such as Rb/Sr, Fe 2O 3/FeO, Al 2O 3/CaO and Al 2O 3/Na 2O in Fig.3 is a little lower than that of the comparison lines. The second part formed by 15th—17th bed group shows that the contents of stable compositions and those of soluble and easy soluting ones are respectively higher and lower than those of the comparison lines, and the general content of every composition fluctuates obviously. The third part composed of the 13th—14th bed group is a relatively stable one. The forth part formed by L 43 , S 43 and L 44 , the lowest part of loess and paleosol deposit sequence of Quaternary, has a sharp change which appeared right at the beginning of this part. The content of the stable compositions in this part is lower and that of soluble and easy soluting ones is higher than those of the previous three parts, the content ratio of these two groups of compositions is obviously lower. All these features are quite different from those of Neogene. Based on these data mentioned above, these three parts belong to the period of Neogene, in which the climate of the 1st and 3rd parts was cool and dry, while that of the 2nd might be the best climate stage of the Late Miocene, warmer and humid. There were minor climate changes against the warmer and humid climatic background in Neogene. When the geologic history entered the Quaternary, that is, the stage of loess accumulation in the loess Plateau, the climate became obviously colder and drier than ever. 2. With more careful observation of the curves of Neogene, one can find four cycle changes, the stable compositions changing from low to high, while the soluble and easy soluting ones from high to low, which may suggest that the climate when Neogene red earth were deposited was generally warmer and moister, but there were still many minor climatic changes, probably caused by the intermittent uplift of Qinghai Tibet Plateau. The change feature of geochemistry seems coincident with those of petrology and susceptibility. 3. From the geochemical data, we could imagine that the accumulation of Neogene red earth means that the monsoon had been initially formed, but the winter monsoon at that time was much weaker than summer monsoon. Between the 2nd and 3rd cycles of climate change, or between the 17th and 16th bed groups, at about 5.2Ma B P, existed an obvious cool and dry climate change, which was related to the formation of Antarctic ice sheet. And then, the sharp cold and dry climatic change at 2.6Ma B P, the beginning of 13th bed group, was affected by the intensive uplift of Qinghai Tibet Plateau and the surface of that area had been raised quite high. The winter monsoon was then intensified.