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Micro-scale variations of iron isotopes in fossilized microorganisms

Published online by Cambridge University Press:  08 July 2008

Magnus Ivarsson*
Affiliation:
Department of Geology and Geochemistry, Stockholm University, Sweden
Seppo Gehör
Affiliation:
Department of Geology, University of Oulu, Finland
Nils G. Holm
Affiliation:
Department of Geology and Geochemistry, Stockholm University, Sweden

Abstract

Laser-ablation inductively coupled plasma mass spectroscopy analyses have been performed on carbonaceous filamentous structures that have been interpreted as fossilized microorganisms containing ~10–50 wt% Fe and on non-carbonaceous filamentous structures that have been interpreted to have been formed abiotically containing ~80 wt% Fe. The obtained laser-ablation profiles of the two structural types show a distinct difference in the iron isotopic variations. The centers of the carbonaceous filaments are enriched in 57Fe and 58Fe and depleted in 56Fe. The surficial parts of the filaments display an opposite behavior of the iron isotopes and are thus enriched in 56Fe and depleted in 57Fe and 58Fe. 54Fe usually follows 57Fe and 58Fe but in some cases it follows 56Fe instead. The outer, surficial parts enriched in 56Fe have been interpreted as iron oxides precipitated on the surfaces of the microorganisms as they mediate oxidation of the iron to achieve metabolic energy. The laser-ablation profiles of the abiotically formed non-carbonaceous filamentous structures do not show the same characteristics as the carbonaceous filaments but only irregular elevations of 56Fe. The characteristic profile patterns of the isotope variations in the microfossils suggest that microbially formed iron oxides may be enriched in 56Fe. If that is the case the isotopic profiles could be used to distinguish abiotically formed iron oxides from biologically formed oxides.

Type
Research Article
Copyright
Copyright © 2008 Cambridge University Press

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