中国黄土研究新进展 (三)时间标尺

PROGRESS IN LOESS RESEARCH (PART 3):TIME SCALE

作者对宝鸡黄土剖面的粒度组成以10cm 为间隔进行了系统的采样和分析,并以这条粒度曲线为基础,建立了一条适合黄土高原各代表性剖面的轨道调谐时间标尺。据此对37个古土壤的顶、底年龄作了估计。在建立这条时间标尺的过程中,各古地磁界线的K-Ar 年龄值基本保持不变。时间标尺建立后,对地轴倾斜度和岁差的特征周期进行了滤波,发现滤波曲线与理论计算值有很好的匹配。对比已有的两条深海轨道调谐时间标尺,宝鸡剖面时间标尺在时间分辨率及滤波曲线与理论计算值相匹配方面具有一些明显的优点。

In Baoji loess-soil section,a total of 1630 samples was taken at an interval of 10cm.Grain size was analyzed for each sample using a Sedigraph 500 E Analyzer in Xi'an Lab ofLoess and Quaternary Geology,Chinese Academy of Sciences.The ratio of the fraction largerthan 10μm to that finer than 2μm is used as an indicator of the variation in wind velocitywhich transported the loess grains from deserts to the Loess Plateau.Visual examination showsthat loess units have larger grain size than soils,suggesting stronger winds in glacial periodsthan during interglacial periods.This grain-size record is then used to develop an astronomi-cal time scale for Chinese loess deposits by the following procedures.1.An initial time scale was established under paleomagnetic controls based on the assump-tion that loess deposition rate during glacial periods is twice higher than that in interglacialperiods.2.The maxima/minima of the grain-size record in the initial time scale is then matched byadding new time control points with maxima/minima of the theoretical variation in the or-bital parameters of the Earth calculated by Berger (1978).3.Using spectral filtering technique,the orbital components were filtered from the grain-size record,and were then designed to be superimposed on the orbital target by stretching andcompressing the initial time scale.The procedure 3 was repeated for about 55 times.An orbitally-tuned time scale is thenobtained with the magnetic boundaries remaining basically unchanged with respect to the ra-diometric ages.The filtered components of the orbital periods from the orbital time scale areexcellently superimposed on the theoretical variations.Based on the astronomical time scale,ages for the thirty-seven major soils in the Baoji section were estimated.