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

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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.