湖南澧阳平原黑褐色土的稀土元素地球化学特征及其环境意义

Rare earth elements in and the environmental significance of the dark brown soil in the Liyang Plain,Hunan Province,China

通过对澧阳平原大量的野外考察、典型剖面玉成(YC)黑褐色土壤剖面的系统取样和室内粒度、稀土元素地球化学分析,以及OSL和AMS14C年代测定,探讨了黑褐色土的稀土地球化学特征,揭示了黑褐色土的形成年代、形成过程与形成环境.研究结果表明,黑褐色土层ΣREE变化范围分布在184.63～264.26μg.g-1,其平均值为207.93μg.g-1;低于剖面的ΣREE总平均值219.87μg.g-1,与洞庭盆地的红土ΣREE平均值(211.6μg.g-1)大体相当,但明显高于镇江下蜀黄土(207.7μg.g-1)和黄土高原黄土(159.2μg.g-1),表明YC剖面黑褐色土上经过了相对较强的风化成壤作用.YC剖面黑褐色土的稀土分布模式与该剖面黄土层、黄土高原黄土、镇江下蜀黄土和洞庭盆地风成红土等典型风成沉积物的分布模式具有一致性,而与河流沉积物、湖泊沉积物和基岩风化红土在稀土的分布模式上具有明显差别.而且,YC剖面沉积物粒度频率曲线呈明显的单峰分布,与典型河流相的双峰粒度频率分布存在显著差异;YC剖面位于澧阳平原中部台地上,比周边高5～10m,不可能是洪水作用沉积物;野外调查进一步表明,沉积物无水流作用痕迹;OSL和AMS14C年代测定表明,黑褐色土层形成的年代对应于23.8～10.6kaBP,此年代对应于末次冰期的晚期,其形成于相对冷湿的气候环境.因此,黑褐色土层及其上、下部的黄土层均为风成堆积物.

The geochemical characteristics of the rare earth elements （REE） in the dark brown soil from the YC profile in the Liyang plain are discussed based on field surveys, sediment sampling and laboratory experiments such as grainsize analysis, REE analysis, and OSL and AMS14C dating. The time, process and environment of the formation of the dark brown soil are also discussed in this paper. The ΣREE in the clark brown soil from the YC profile varies from 184.63 μg·g^-1 to 264.26μg·g^-1. The average value of EREE in the dark brown soil （ 207.93μg·g^-1 ） is lower than that of the entire profile YC （219.87μg·g^-1 ） , comparable tu that of the red sull in the Dongting Basin （211.6 μg·g^-1）, and significantly higher than those of the Xiashu loess in Zhenjiang （207.7 μg·g^-1 ） and the loess in the Loess Plateau （ 159.2 μg·g^-1 ）. The above results indicate that the dark brown soil from YC profile in the Liyang Plain has experienced relatively strong weathering pedogenesis. The REE distribution pattern of the dark brown soil from the YC profile is similar to that of the loess in the Loess Plateau, the Xiashu loess in Zhenjiang, and the eolian red soil from the Dongting Basin. However, the REE distribution pattern of the dark brown soil is significantly different from that of fluvial sediments, lake sediments, and the red soil from bedrock weathering. Moreover, the grain - size frequency curves of sediments along the YC profile have a unimodal distribution, which is different from the typical blmodal distribution of fluvial sediments. The YC profile is located on the tableland 5 to 10 m higher than the surrounding areas, and no trace of fluvial processes was found during the field survey. Thus the black brown soil is not diluvium. The results of the OSL and AMS14C dating indicate that the black brown soil formed between 23800 a BP and 10600 a BP during the last glaciation, in a relatively cold and wet climate. Therefore, the black brown soil and its upper and lower layers of loess are eolian deposits.