中国东北典型黄土-古土壤序列铁同位素组成及其环境意义

The formation and development process of loess-paleosol sequence is complex, and its pedogenesis is different from traditional understanding of soil formation. The migration and transformation of iron in the loess-paleosol has not been well addressed. The analysis of iron isotope composition can be used to track the migration direction of iron, reveal the mechanism of iron migration, and accurately reflect the information of iron migration and transformation in the process of soil formation. This provides a novel method for further study on soil development process. The research on iron isotope fractionation characteristics of loess-paleosol sequence is helpful to further study the development process and sedimentary environment of loess-paleosol. Iron isotope compositions (δ56Fe) in both soils and different iron forms by sequential chemical extractions from loess and paleosol with different development stages in northeastern China will be evaluated. Following the comparison of the δ56Fe content difference of iron coatings and iron nodules with their surroundings, the characteristics of iron isotope fractionation of loess-paleosol sequence and its pedogenesis will be explored. This project will then focus on the changes of δ56Fe within the soil profile. Combined with results from our previous studies on basic properties of loess-paleosol sequence, such as morphology, grain size distribution, and mineral composition, the environmental significances of iron isotope for reconstruction of precipitation during loess sedimentary period and determination of pedogenic weathering degrees after loess deposition, are expected to be revealed. The result of this project can enhance methods to better elucidate the forming processes of loess-paleosols, and help to reconstruct the paleoenvironment of loess sedimentary period more accurately.

黄土-古土壤序列形成发育过程复杂，其成土作用过程有别于传统土壤发生过程，对其铁的迁移和转化了解不清。铁同位素组成分析明确铁元素迁移的方向，揭示铁迁移的机理，更准确详尽地反映成土过程中铁的迁移和转化信息，为深入研究土壤发育过程提供新的研究手段。开展黄土-古土壤序列铁同位素分馏特征研究有助于深化对黄土-古土壤的发育过程及其沉积环境研究。为此，本项目拟通过分析东北典型黄土-古土壤序列中处于不同发育阶段黄土和古土壤及其中不同形态铁的同位素组成（δ56Fe）；比较铁质胶膜或结核与其基质中δ56Fe含量，分析黄土-古土壤序列中铁同位素分馏特征，探讨黄土-古土壤序列的发育过程及其变化规律。在此基础上，对比分析δ56Fe曲线与剖面形态特征、粒度、矿物等指标，探讨铁同位素组成变化对降水量和风化成土强度的指示意义。研究结果有助于更好地理解黄土-古土壤序列的形成过程，为准确恢复黄土沉积时期的古环境奠定科学基础。

铁同位素组成分析能够明确铁元素迁移的方向，探究铁迁移的机理，为研究典型黄土-古土壤序列复杂成土过程提供新的研究手段。本研究以中国东北典型黄土-古土壤序列及不同形态铁锰新生体为研究材料，明确了典型黄土-古土壤序列中不同形态铁的组成特征，系统分析了地层、微域、铁锰新生体尺度铁同位素组成特征，揭示了东北典型黄土-古土壤序列的铁同位素分馏机理，探讨了铁同位素组成的成土强度及环境指示意义。研究结果表明：.1.典型黄土-古土壤序列全铁以硅酸盐结合态铁为主，非硅酸盐结合态铁以晶质铁为主，成土过程铁形态发生较大变化，古土壤的不同形态铁的含量高于黄土。δ56Fe含量与全铁呈显著负相关关系，地层尺度黄土δ56Fe含量高于古土壤，但无明显分馏；硅酸盐结合态铁中重铁富集，游离铁中轻铁富集，铁形态变化过程同位素分馏明显。微域尺度Fe淋失、Fe富集的δ56Fe含量有明显变化但分馏不明显。铁锰新生体尺度，形成铁锰胶膜阶段，铁以毫米级迁移为主，迁移量有限，δ56Fe含量存在变化但分馏不明显；形成铁锰结核阶段，铁迁移量较大，轻铁优先迁移并随着铁锰结核形成而积累，δ56Fe含量变化明显、分馏明显，δ56Fe含量随粒径变大呈降低趋势。.2.典型黄土-古土壤序列铁迁移至形成铁锰结核阶段铁同位素出现明显分馏，δ56Fe含量主要取决于干湿交替土壤水分条件下铁的还原溶解迁移量，随铁积累量（迁移量）增加呈降低趋势。研究结果表明δ56Fe含量变化示踪了黄土-古土壤序列铁的迁移过程，记录了表生过程氧逸度的变化，与降水量存在非线性关系，有望成为一项古环境解译的有效指标。建议后续相关研究考虑采用地层、微域、铁锰新生体尺度综合采样法来揭示黄土-古土壤序列铁同位素分馏过程，有助于更好地揭示黄土-古土壤序列的形成过程。