古地理过程研究的方法与实例——以洛川黄土剖面为例

METHODOLOGY FOR PALEOGEOGRAPHICAL STUDIES AND SPECIFIC CASE STUDIES──A CASE STUDY OF OESS PROFILE IN LUOCHUAN

本文选取陕西省洛川县黑木沟黄土剖面中CaCO_3含量、粉粘比(kd值)、FeO/Fe_2O_3以及SiO_2/Al_2O_3等指标,运用相关分析、聚类分析、投影变换等数学方法,研究这一地区中、晚更新世古地理演变过程。结果表明,洛川地区在中、晚更新世经历了5种类型的自然环境和9次地理旋迴,地理旋迴的周期和变化幅度不断增大,自然环境朝干冷方向演变。

Contemporary physical geography has been attempting to form a five dimension united dynamic system by combining matter, energy, information and temporal and spatial changes. In this system, the changes of various geographical elements over temporal series constitute geogra-phical processes. The research on geographical processes is one of the most important taks in geography. The thickness of loess, accumulated on the red clay of late Pliocene, is 130 meters in Hei-mugou of Luochuan. Heilu soils lie on Malan loess of late Pleistocene. Lishi loess of middle Pleistocene is composed of 14 layers of loess and 14 layers of paleosols. Our study is about the loess of middle-late Pleistocene (1.05 millions of years ago to 10 thousands of years ago). Fourvariables, i.e. CaCOs content, powder sand-clay ratio (Kd), FeO-Fe2Os ratio and SiO2-Al2O3 ratio, are chosen to study the environmental changes in this area. All of these variables have dis-tinct paleogeographical significances, CaCOs content indicates leaching extent of chemical ele-ments of soil layers. FeO-FejOs ratio is an indicator of the oxidation-reduction process in the environment. SiO2-Al2Os ratio reflects complex precipitation-temperature conditions in the en-vironment. Ed reflects environmental characteristics in the loess sources areas and loess accumu-lation areas. Generally speaking, the greater the values of these variables are, the more dry-cool the environment will be. In order to examine the environmental evolution process in the area, a linear correlation analysis was carried out among four variables. The results of correlation analysis provided a sound basis for cluster analysis. All of 26 samples were then divided into 5 groups by using cluster analysis. These groups are different from each other in CaCOs content, Kd, FeO-FdOj ratio and SiOa-AUOs ratio and represent 5 different types of paleoenvironmental conditions. And then, 5 types of samples were projected on a projective axis by using projective transfor-mation. Thus, the geographical explanation of the 5 types can be done on the projective axis. The results of cluster analysis and projective transformation show that type 2 represents the dry-cool environmental conditions, type 3 represents the wet-warm environmental conditions, and the other three types represent transitional environmental conditions from the dry-cool en-vironment to the wet-warm environment. Combining the results from cluster analysis with the results from projective transformation, we also found the Luochuan region had 9 geogra-phical cycles during middle-late Pleistocene. Finally, the results from the analysis of deve-loping trend of CaCOs content, Kd, FeO-FesOs ratio and SiOj-AUOs ratio show that the values of 4 variables increase gradually, which means that the natural environment changes to the dry-cool direction in this area during middle-late Pleistocene. At the same time, the periods of geographical cycles gradually become longer and the changing extents of geographical cycles become greater due to the dry-cool trend of natural environment. Although helpful progress has been obtained by applying correlation analysis, cluster ana-lysis, projective transformation and trend analysis to paleogeographical process study, there are still some problems to the resulved, for example, the response of changing extent of various geographical elements to environment conditions and the subjectivity in choosing variables in cluster analysis.