渭南黄土剖面碳酸盐和铁变化的古气候记录

PALEOCLIMATE RECORDS OF VARIATION OF CARBONATE AND IRON OXIDES IN WEINAN LOESS SECTION

作者对渭南黄土剖面高密度连续采集的样品进行了碳酸盐和各种形态铁的系统分析。在研究黄土和古土壤中的碳酸盐和各种形态铁的变化及其影响因素的基础上，选择ＣａＣＯ_３丰度和Ｆｅ_２Ｏ_３／ＦｅＯ值（铁氧化度）分别作为降水量和温度变化的替代性指标，游离Ｆｅ_２ｏ_３／Ｆｅ_２ｏ_３值（铁游离度）能反映成壤作用的程度。它们在剖面上的分布表明距今约１４２ｋａ以来，该区的古气候变化经历了六个大时期：（１）１４２－１３５ｋａＢ．Ｐ．，湿润转温；（２）１３５－７３ｋａＢ．Ｐ．，温暖湿润；（３）７３－５２ｋａＢ，Ｐ．，寒冷干燥；（４）５２－２２ｋａＢ，Ｐ．，偏湿偏凉；（５）２２－１１ｋａＢ．Ｐ．，寒冷干燥；（６）１１ｋａＢ．Ｐ．以来，偏湿偏温。碳酸盐和铁记录的古气候演化序列完全可以与深海氧同位素阶段以及大陆冰期－间冰期系列相对比。在各个大的气候周期中，次一级的气候变化在曲线上也有良好体现。

The contents of CaCO_3, Fe_2O_3, FeO and free Fe_2O_3 in the sediments of the Weinan Loess Section have been analyzed in order to investigate the geochemical feature of different layers and to explore paleoclimate change during formation of the section The results show that the contents of CaCO_3 and the ratios of Fe_2O_3/FeO (oxidation index of iron) may be taken as substitute indexes for precipitation and temperature changes during strata formation, respectively. And the degree of pedogenesis may be reflected by the ratio variation of free Fe_2O_3/FeO (free index of iron). Based on the distribution of CaCO_3 contents and the variation of ratio Fe_2O_3/FeO and free Fe_2O_3/Fe_2O_3 in the section, the paleoclimate since 142ka B. P. may be divided into six stages in this region: in the first stage (about 142-135ka B. P.), the paleoclimate was characterized by a more humid and slight warm climate; in the second stage (abuot 135-73ka B. P.), by a humid and warm climate; in the third stage (abuot 73-52ka B. P.), by a dry and cold climate; in the fourth stage (about 52 22ka B. P.), by a cool and slight humid climate with a stable temperature and larger variation of precipitation than that in other stages; in the fifth stage (about 22-11ka B. P.), by a very cold and dry climate and in the sixth stage (about 11ka B. P.-present), the climate was relatively humid and warm. The precipitation was suddenly changed when the cold stage converted to warm one and it was gradually changed when the warm stage converted to a cold one. The temperature was always gradually changed at both conversions.