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Published January 2004 | public
Book Section - Chapter

Isotopic Constraints on Biogeochemical Cycling of Fe


Cycling of redox-sensitive elements such as Fe is affected by not only ambient Eh-pH conditions, but also by a significant biomass that may derive energy through changes in redox state (e.g., Nealson 1983; Lovely et al. 1987; Myers and Nealson 1988; Ghiorse 1989). The evidence now seems overwhelming that biological processing of redox-sensitive metals is likely to be the rule in surface- and near-surface environments, rather than the exception. The Fe redox cycle of the Earth fundamentally begins with tectonic processes, where "juvenile" crust (high-temperature metamorphic and igneous rocks) that contains Fe which is largely in the divalent state is continuously exposed on the surface. If the surface is oxidizing, which is likely for the Earth over at least the last two billion years (e.g., Holland 1984), exposure of large quantities of Fe(II) at the surface represents a tremendous redox disequilibrium. Oxidation of Fe(II) early in Earth's history may have occurred through increases in ambient O2 contents through photosynthesis (e.g., Cloud 1965, 1968), UV-photo oxidation (e.g., Braterman and Cairns-Smith 1987), or anaerobic photosynthetic Fe(II) oxidation (e.g., Hartman 1984; Widdel et al. 1993; Ehrenreich and Widdel 1994). Iron oxides produced by oxidation of Fe(II) represent an important sink for Fe released by terrestrial weathering processes, which will generally be quite reactive. In turn, dissimilatory microbial reduction of ferric oxides, coupled to oxidation of organic carbon and/or H2, is an important process by which Fe(III) is reduced in both modern and ancient sedimentary environments (Lovley 1991; Nealson and Saffarini 1994). Recent microbiological evidence (Vargas et al. 1998), together with a wealth of geochemical information, suggests that microbial Fe(III) reduction may have been one of the earliest forms of respiration on Earth. It therefore seems inescapable that biological redox cycling of Fe has occurred for at least several billion years of Earth's history.

Additional Information

© 2004 by the Mineralogical Society of America. Reviews by Francis Albarede, Ariel Anbar, and Susan Glasauer are appreciated. Sue Brantley is thanked for sharing several preprints and for additional comments on the paper. We also thank the Fe isotope group at UW Madison for their discussions and comments on drafts of the manuscript. Financial support for the research embodied here was provided by NASA, NSF, the Packard Foundation, and the University of Wisconsin. In particular, the NASA Astrobiology Institute supported a large portion of our work on Fe isotope fractionations in biologic systems. Collaborations with Carmen Aguiar, Nic Beukes, Paul Braterman, Lea Cox, Laura Croal, Andreas Kappler, Kase Klein, Hiroshi Ohmoto, Rebecca Poulson, Silke Severmann, Joseph Skulan, Henry Sun, Sue Welch, Rene Wiesli, and Kosei Yamaguchi have added greatly to our understanding of Fe isotope geochemistry in experimental and natural systems.

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