Iron isotope fractionation during microbially stimulated Fe(II) oxidation and Fe(III) precipitation
Iron isotope fractionation during microbially stimulated Fe(II) oxidation and Fe(III) precipitation
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DOI：
10.1016/j.gca.2005.09.025
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发表时间：
2006-02-01
影响因子：
5
通讯作者：
Mandernack, KW
中科院分区：
文献类型：
--
作者：
Balci, N;Bullen, TD;Mandernack, KW
基于铁同位素比值的现代和古代岩石中保存的含铁矿物的起源解释取决于我们区分生物和非生物铁同位素分馏过程的能力。在这项研究中，我们比较了在低pH（< 3）的实验条件下，由亚铁氧化产生的共存的含水铁（Fe（II）（aq）、Fe（III）（aq））和羟基氧化铁沉淀（Fe（III）（ppt））的Fe-56/Fe-54比率。实验进行了使用两个纯的，培养物的氧化亚铁硫杆菌和无菌对照，以评估可能的生物套印的非生物分馏，和两个SO 42-和Cl-盐作为Fe（II）的来源，以确定可能的离子/形态的影响，可能与氧化/沉淀反应。此外，在pH范围从1.9至3.5进行了一系列的三价铁沉淀实验，以确定不同的沉淀速率是否导致铁羟基氧化物的同位素组成的差异。在硫酸盐和氯化物系统中微生物刺激的Fe（II）氧化期间，在时间序列中采样的残留Fe（II）a的Fe-56/Fe-54比率沿着明显的瑞利趋势演变，其特征在于分馏因子α（Fe（III）aq-Fe（II）aq）类似于1.0022。该分馏因子显著小于我们的无菌对照实验中测得的分馏因子（类似于1.0034）以及Fe（II）（aq）和Fe（III）（aq）之间同位素平衡的预测分馏因子（类似于1.0029），因此可以解释为反映生物同位素效应。然而，在我们的生物学实验中，在N2气氛下在每个时间点通过向最终溶液中加入NaOH而分离为固体的Fe（III）（水溶液）与Fe（II）（水溶液）之间的Fe-56/Fe-54比率的测量差异在大多数情况下平均接近2.9ppm（α（Fe（III）aq-Fe（II）aq）类似于1.0029），与Fe（II）（aq）和Fe（III）（aq）之间的同位素平衡一致。三价铁沉淀实验表明，Fe（III）（aq）的Fe-56/Fe-54比一般等于或大于Fe（III）（ppt），这些阶段之间的同位素分馏随着沉淀速率的增加和晶粒尺寸的减小而减小。综合考虑，这些数据证实，在我们的微生物实验中观察到的铁同位素变化主要是由非生物平衡和动力学因素控制的，这一结果有助于我们解释当今铁循环过程的能力，但使我们单独使用铁同位素来识别岩石记录中的生物过程的能力进一步复杂化。(c)2005年爱思唯尔公司All rights reserved.
Interpretation of the origins of iron-bearing minerals preserved in modern and ancient rocks based on measured iron isotope ratios depends on our ability to distinguish between biological and non-biological iron isotope fractionation processes. In this study, we compared Fe-56/Fe-54 ratios of coexisting aqueous iron (Fe(II)(aq), Fe(III)(aq)) and iron oxyhydroxide precipitates (Fe(III)(ppt)) resulting from the oxidation of ferrous iron under experimental conditions at low pH (< 3). Experiments were carried out using both pure,cultures of Acidothiobacillus ferrooxidans and sterile controls to assess possible biological overprinting of non-biological fractionation, and both SO42- and Cl- salts as Fe(II) sources to determine possible ionic/speciation effects that may be associated with oxidation/precipitation reactions. In addition, a series of ferric iron precipitation experiments were performed at pH ranging from 1.9 to 3.5 to determine if different precipitation rates cause differences in the isotopic composition of the iron oxyhydroxides. During microbially stimulated Fe(II) oxidation in both the sulfate and chloride systems, Fe-56/Fe-54 ratios of residual Fe(II)a, sampled in a time series evolved along an apparent Rayleigh trend characterized by a fractionation factor alpha(Fe(III)aq-Fe(II)aq) similar to 1.0022. This fractionation factor was significantly less than that measured in our sterile control experiments (similar to 1.0034) and that predicted for isotopic equilibrium between Fe(II)(aq) and Fe(III)(aq) (similar to 1.0029), and thus might be interpreted to reflect a biological isotope effect. However, in our biological experiments the measured difference in Fe-56/Fe-54 ratios between Fe(III)(aq), isolated as a solid by the addition of NaOH to the final solution at each time point under N-2-atmosphere, and Fe(II)(aq) was in most cases and on average close to 2.9 parts per thousand (alpha(Fe(III)aq-Fe(II)aq) similar to 1.0029), consistent with isotopic equilibrium between Fe(II)(aq) and Fe(III)(aq). The ferric iron precipitation experiments revealed that Fe-56/Fe-54 ratios of Fe(III)(aq) were generally equal to or greater than those of Fe(III)(ppt), and isotopic fractionation between these phases decreased with increasing precipitation rate and decreasing grain size. Considered together, the data confirm that the iron isotope variations observed in our microbial experiments are primarily controlled by non-biological equilibrium and kinetic factors, a result that aids our ability to interpret present-day iron cycling processes but further complicates our ability to use iron isotopes alone to identify biological processing in the rock record. (c) 2005 Elsevier Inc. All rights reserved.