澧阳平原岩板垱剖面地球化学特征与风化强度研究

Geochemical Characteristics and Weathering Intensity of the Yanbandang Profile in Liyang Plain, the Middle Reach of the Changjiang River

通过对岩板垱剖面氧化物含量分析及年代测定,揭示其常量地球化学特征,探讨澧阳平原晚更新世晚期至全新世中期的化学风化过程和环境变化。结果表明,岩板垱剖面沉积物出现不同程度Na、Ca元素亏损及TFe和Mn元素富集,其化学风化作用程度明显高于黄土高原黄土,稍高于镇江下蜀黄土,处于中等的风化阶段。多个风化指标的垂向变化表明岩板垱剖面化学风化强度表现为风化较强、风化较弱、风化加强和风化较强4个阶段的变化。化学风化强度指示澧阳平原27~6.0 ka B.P.间,气候环境经历了相对暖湿-凉湿-气候回暖-温暖湿润的演变过程。

Based on detailed field survey, oxide contents analysis and optically stimulated luminescence （OSL） dating of the samples from the Yanbandang（YBD） profile in Liyang Plain, the geochemical characteristics of major elements, the weathering process and environmental change from late Pleistocene to mid-Holocene were discussed. The results showed that the content of oxide is mainly composed of SiO2, A1203 and TFe203 （Fe203＋ FeO） and their percentage are between 86% and 90.8%. Compared to the average upper continental crust com- position, Na and Ca have been leached out strongly, while total Fe and Mn relatively enriched in the sediments of the YBD profile. The intensity of chemical weathering in the YBD sequence was significantly higher than that of loess and paleosol in Loess Plateau, slightly higher than Xiashu loess in Zhenjiang, almost equal to the Yucheng profile in Liyang Plain, but obviously lower than the red soil from Xuancheng profile in Anhui Prov- ince, which indicated a moderate chemical weathering intensity under warm and moist conditions. Changes of the Chemical Index of Alteration （C/A）, the Eluvial Coefficient, the Coefficient of Weathering and Eluviations （BA）, the Index of Compositional Variation （ICV） and the analysis of A-CN-K and A-CNK-FM ternary dia- grams suggested palpable differences in weathering intensity of the strata. There are four stages of weathering process in the YBD profile： relatively stronger weathering, relatively weaker weathering, intensifying weather-ing, and relatively stronger weathering. The weathering intensity of the yellowish brown soil at the top and bot- tom of YBD profile were clearly higher than that of the black brown soil and the black soil. Since the chemical weathering intensity is sensitive to climate change, the geochemical parameters can be used to unravel the envi- ronmental change to some extent. The higher CIA value reveals that the climate is relatively warmer and wet- ter in the end of the MIS3a stage. It becomes cooler during the LGM, but gradually retums to be warm again in early-Holocene. The climate in the mid-Holocene was the wannest and most humid compared to the aforemen- tioned periods.