2 items with the tag “sediment

  • Project 4C: Iron Isotope Geochemistry in Biogenic Magnetite-Bearing Lake Sediments
    NAI 2011 University of Wisconsin Annual Report

    The production of magnetite as a byproduct of dissimilatory microbial iron oxide reduction (DIR) has been hypothesized to be an important pathway in the early diagenesis of chemically-precipitated sediments on early Earth, leading ultimately to the preservation of large quantities of magnetite in banded iron formations (BIFs). A significant fraction of the magnetite (and other Fe-bearing minerals such as siderite and pyrite) in BIFs is isotopically light, likely due to Fe isotope fractionation between biogenic Fe(II) and residual Fe(III) oxides. Only one modern environmental setting has reported possible in situ magnetite formation resulting from DIR: the Bay of Vidy in Lake Geneva, Switzerland. Previous work has characterized a widespread magnetic susceptibility anomaly in the Bay of Vidy sediments stemming from an influx of amorphous Fe(III) oxide from a nearby sewage treatment plant, and determined the presence of fine-grained magnetite apparently produced via DIR. In this study, we examined the Fe isotope composition of distinct pools of solid-phase Fe contained in sediments from the Bay of Vidy. Significant Fe(III) reduction has taken place, resulting in the reduction of nearly all reactive (non-silicate) Fe. Very little Fe isotope variation was observed within sediment Fe pools, including magnetite. The lack of sediment heterogeneity, along with the highly reduced nature of the sediments, suggests that DIR has carried through to completion in this deposit. The absence of spatial Fe redox gradients accompanying complete Fe(III) reduction has prevented the segregation of Fe isotopes during microbial reduction. This case study provides a basis for interpreting instances in the rock record where DIR was active but no Fe isotope fractionation was preserved.