Mechanisms of Fe isotope fractionation during biological Fe(III) reduction

同位素地球化学一般提供了一种在地球化学过程中追踪原子交换和转移的手段，拟议的工作将集中在同位素地球化学发展最快的"新"领域之一，即Fe同位素。岩石记录中显著的铁同位素变化可以追溯到太古代，这表明在矿物-微生物界面处断裂铁同位素的过程可能记录了生物铁循环回到早期地球，但是这些特征仍然很难解释，直到在机理水平上理解生物过程中的铁同位素分馏。是密切相关的微生物和铁Fe矿物之间的界面，反映了氧化还原的变化和条件，允许分离的移动的，同位素不同的成分，如水溶液Fe（II）。实验工作将调查Fe同位素分馏水溶液中的Fe（II）和Fe（III）和Fe吸附到矿物表面。 此外，基于新的发现，Fe（III）在铁Fe矿物中的作用是开放的氧化还原循环和同位素交换将进行研究。将使用来自吉氏菌和希瓦氏菌组的细菌，部分用于对比底物分离实验中有机配体的产生。其他变量，将探讨包括基板矿物学（水铁矿，赤铁矿，针铁矿，lepidocrocite，和铁的铁粘土），以及腐殖酸的存在或不存在。除了研究同位素交换和分馏机制之间的高活性池的Fe（II）aq，Fe（III）aq，FeSORB，和Fe（III）在基板在沉积过程中，我们还将进行实验，产生铁碳酸盐和磁铁矿，因为这些矿物是无处不在的岩石记录。重要的是，将集中精力探索碳酸盐中的Ca-Mg-Fe组合物的同位素效应，给出了强有力的证据，碳酸盐化学计量在Fe同位素分馏中起着重要作用。除生物还原实验外，还将使用各种途径进行非生物还原实验，以提供与生物还原过程中产生的影响进行比较的基础。在许多实验中，除了“正常”同位素组成外，还将使用富集的-57 Fe示踪剂来量化同位素交换的动力学;这些信息对于理解哪些途径可能在平衡条件下进行，哪些可能不进行很重要。最后，将研究两个正在生产生物菱铁矿和磁铁矿的现场，以提供与受控实验室条件下获得的结果进行比较的基础。

EAR-0525417Isotope geochemistry in general provides a means for tracing atom exchange and transfer during biogeochemical processes, and the proposed work will focus on one of the most rapidly developing "new" fields of isotope geochemistry, Fe isotopes. That significant Fe isotope variations in the rock record extend back to the Archean suggests that the processes which fractionate Fe isotopes at the mineral-microbe interface may record biological Fe cycling back to the early Earth, but these signatures will remain difficult to interpret until Fe isotope fractionations during biological processing are understood at a mechanistic level.Iron isotope fractionation and exchange during dissimilatory iron reduction (DIR) is intimately related to the interface between microbes and ferric Fe minerals, reflecting both redox changes and conditions that allow separation of mobile, isotopically distinct components such as aqueous Fe(II). Experimental work will investigate Fe isotope fractionation among aqueous Fe(II) and Fe(III) and Fe that is sorbed to the mineral surface. Moreover, based on new findings, the role of Fe(III) in the ferric Fe minerals that is open to redox cycling and isotopic exchange will be investigated. Bacteria from the Geobacter and Shewanella groups will be used, in part to contrast production of organic ligands in substrate-isolation experiments. Other variables that will be explored include substrate mineralogy (ferrihydrite, hematite, goethite, lepidocrocite, and ferric Fe clays), and the presence or absence of humic acids. In addition to studying the mechanisms of isotopic exchange and fractionation among the highly reactive pools of Fe(II)aq, Fe(III)aq, Fesorb, and Fe(III) in the substrate during DIR, we will also conduct experiments that produce Fe carbonates and magnetite because these minerals are ubiquitous in the rock record. Importantly, a concentrated effort will be made to explore the isotopic effects of Ca-Mg-Fe compositions in carbonates, given the strong evidence that carbonate stoichiometry plays an important role in Fe isotope fractionations. In addition to the DIR experiments, abiologic reduction experiments will be done using a variety of pathways to provide a basis for comparison with the effects produced during biological reduction. In many of the experiments, enriched-57Fe tracers will be used, in addition to "normal" isotope compositions, to quantify the kinetics of isotope exchange; this information is important for understanding which pathways are likely to proceed under equilibrium conditions and which may not. Finally, two field sites will be studied where biogenic siderite and magnetite are being produced to provide a basis for comparison to the results obtained under controlled laboratory conditions.