Iron isotope fractionation in oxic soils by mineral weathering and podzolization
Iron isotope fractionation in oxic soils by mineral weathering and podzolization
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DOI：
10.1016/j.gca.2007.07.023
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发表时间：
2007-12
影响因子：
5
通讯作者：
J. Wiederhold;N. Teutsch;S. Kraemer;A. Halliday;R. Kretzschmar
中科院分区：
文献类型：
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作者：
J. Wiederhold;N. Teutsch;S. Kraemer;A. Halliday;R. Kretzschmar
采用MC-ICPMS测量了3种土壤（2种灰壤和1种Cambisol）的稳定铁同位素比值，探讨了氧条件下成土铁转化和转运过程中的铁同位素分异。灰化作用是指在有机配体的作用下，氧化铁在底土中溶解、迁移和富集的成土过程。Cambisol是一种由化学风化形成的土壤，没有明显的铁转运。采用三步顺序提取程序从土壤样品中分离出可操作定义的铁矿池（即，贫晶氧化铁、结晶氧化铁、硅酸盐结合铁）。比较了土壤总消化物与分离铁矿池的铁同位素比值。质量平衡计算表明，顺序提取的结果与土壤总消化的结果非常一致。在灰长岩剖面中发现了铁同位素特征的系统变化。在沉积Bh层位的全土壤消化物中，发现δ57Fe轻铁同位素富集约0.6‰，这可以解释为轻铁同位素优先易位。分离出的铁矿池在灰长岩剖面中显示出δ57Fe值范围广，超过3‰。风化残余物硅酸盐结合铁中重铁同位素富集，表明轻铁同位素在风化过程中优先转化。灰化化过程中的铁同位素分馏可能与配体控制的铁转运过程有关。通过对比坡积岩剖面和坡积岩剖面的铁同位素数据，发现一定有铁从坡积岩剖面中浸出。然而，母体材料的初始铁含量和铁同位素组成的不确定性阻碍了剖面内铁通量的彻底质量平衡计算。与灰长岩剖面相比，Cambisol剖面的δ57Fe值在不同土壤深度表现出均匀性，在0.5M HCl萃取的弱结晶氧化铁池中，轻铁同位素的富集量较小，约为0.4‰。这项工作表明，在有机配体的影响下，在氧环境下的成土过程中可能发生显著的铁同位素分馏。我们的发现为铁同位素的分馏机制提供了新的见解，并将有助于开发稳定的铁同位素作为自然界生物地球化学铁循环的示踪剂。
Stable iron isotope ratios in three soils (two Podzols and one Cambisol) were measured by MC-ICPMS to investigate iron isotope fractionation during pedogenic iron transformation and translocation processes under oxic conditions. Podzolization is a soil forming process in which iron oxides are dissolved and iron is translocated and enriched in the subsoil under the influence of organic ligands. The Cambisol was studied for comparison, representing a soil formed by chemical weathering without significant translocation of iron. A three-step sequential extraction procedure was used to separate operationally-defined iron mineral pools (i.e., poorly-crystalline iron oxides, crystalline iron oxides, silicate-bound iron) from the soil samples. Iron isotope ratios of total soil digests were compared with those of the separated iron mineral pools. Mass balance calculations demonstrated excellent agreement between results of sequential extractions and total soil digestions. Systematic variations in the iron isotope signature were found in the Podzol profiles. An enrichment of light iron isotopes of about 0.6‰ in δ57Fe was found in total soil digests of the illuvial Bh horizons which can be explained by preferential translocation of light iron isotopes. The separated iron mineral pools revealed a wide range of δ57Fe values spanning more than 3‰ in the Podzol profiles. Strong enrichments of heavy iron isotopes in silicate-bound iron constituting the residue of weathering processes, indicated the preferential transformation of light iron isotopes during weathering. Iron isotope fractionation during podzolization is probably linked to the ligand-controlled iron translocation processes. Comparison of iron isotope data from eluvial and illuvial horizons of the Podzol profiles revealed that some iron must have been leached out of the profile. However, uncertainties in the initial iron content and iron isotopic composition of the parent materials prevented thorough mass balance calculations of iron fluxes within the profiles. In contrast to the Podzol profiles, the Cambisol profile displayed uniform δ57Fe values across soil depth and showed only a small enrichment of light iron isotopes of about 0.4‰ in the poorly-crystalline iron oxide pool extracted by 0.5M HCl. This work demonstrates that significant iron isotope fractionations can occur during pedogenesis in oxic environments under the influence of organic ligands. Our findings provide new insights into fractionation mechanisms of iron isotopes and will help in the development of stable iron isotopes as tracers for biogeochemical iron cycling in nature.