泥河湾层古环境演变的分析模式

ANALYTICAL MODELS FOR THE PALAEOENVIRONMENTAL EVOLUTION IN THE NIHEWAN BEDS

本文以泥河湾古湖小渡口剖面为例,选取指示古环境的特征标志:CaCO_3、C/P_2O_5、Fe^(2+)/Fe^(3+)、pH值、Eb值,进行几种统计数学模型分析,以便筛选变量,确定定量性指标。并结合其他因素,探讨其古环境演变特征。通过分析:泥河湾层在冷的气候条件下,往往以还原作用为主,介质pH值为偏碱性。第三系、第四系的界线可能在第13层,此界线与古地磁的松山反向极性世—高斯正向极性世界线(M/G界线)相一致,年代约2.48Ma,并与其它项界线大致吻合。地球化学带Ⅰ包括冷带1;带Ⅱ包括冷带2、3;带Ⅲ包括冷带4;带Ⅳ包括冷带5。

In this paper, the evolutional characteristics of palaeoclimate and oxidation-reduction conditions as well as acidity-alkalinity environments are discussed by means of the step-regression, cluster, optimal partitioning and correlative analyses of CaCO_3, C/P_2O_5, Fe^(2+)/Fe^(3+), pH values and Eh values, taking the Xiaodukou section in the Nihewan basin as an example. CaCO_3, C/P_2O_5 and pH values were calculated respectively by using the optimal partitioning method. Thus five cold zones and six warm zones and five reduction and six oxidation zones have been distinguished; and then the inductive method was used to produce four numerical groups: 8.10, 8.3—8.4, 8.6—8.7 and 8.9—8.97. The above-mentioned results were obtained respectively based on the CaCO_3 content, C/P_2O_5 and pH values. From Fig. 3 (Table 1) it can be seen (Table 2) that the Nihewan Beds formed mainly under reduction and slightly alkaline environments of the cold zone, with pH values of 8.3—8.4. The Fig. 3 shows that bed 35 is approximately near the boundary of the Brunhes and Matuyama polarity epochs, dated at 0.73 Ma; bed 26 is near the Jaramillo event (base), dated at 0.97 Ma. Bed 18 was dated at 2.00 Ma B. P. (?), coinciding with the magnetostratigraphic age boundary. Bed 13 may be the Pleistocene-Pliocene boundary bed, dated at 2.48 Ma B. P. (?) age. Thus geochemical zones Ⅰ, Ⅱ, Ⅲ and Ⅳ include cold zones 1, 2 and 3, 4 and 5, respectively.