Millimeter-scale variations of stable isotope abundances in carbonates from banded iron-formations in the Hamersley Group of Western Australia

Dissimilatory Fe(III) and Mn(IV) Reduction

“…For example, the extensive deposition of magnetite in banded iron formations probably resulted from the activity of dissimilatory Fe(III)-reducing microorganisms (23,185,205,328). There is a strong correlation between the accumulation of magnetite in banded iron formations and the accumulation of isotopically light carbonates (23,254).…”

Iron formations: A global record of Neoarchaean to Palaeoproterozoic environmental history

“…12B). Significantly, coupling the reduction of some Fe(III) minerals to the oxidation of organic matter not only explains the low content of organic carbon in IFs (<0.5 wt.%; Gole and Klein, 1981), but it also explains highly negative δ 13 C values of the early diagenetic Fe(II)-rich carbonates Walker, 1984;Baur et al, 1985;Heimann et al, 2010;Craddock and Dauphas, 2011), the small-scale heterogeneity in δ 56 Fe values (e.g., Steinhofel et al, 2010;W. Li et al, 2013a), the presence of Fe(II)-bearing minerals in IF (e.g., magnetite, siderite, greenalite -recall section 2.3), and the general lack of microfossils preserved in the Fe-rich layers lacking silicification.…”

“…6), Nd isotope data suggest a mixture of continental and hydrothermal REE+Y sources. Further, the Marra Mamba IF is dominated by magnetite, ankerite, and fibrous silicate (Baur et al, 1985), which suggests excess organic carbon was present during diagenesis. Support for this interpretation is found in negative δ 13 C values that increase with increasing magnetite abundance (Baur et al, 1985).…”

Micro-scale tracing of Fe and Si isotope signatures in banded iron formation using femtosecond laser ablation

“…The Fe(II) released by reduction reacts either with residual ferric (hydr)oxides to produce magnetite or with bicarbonate to form siderite. Several studies of C isotope compositions in Precambrian BIFs indicate that extensive magnetite and iron carbonate deposition resulted from organic carbon oxidation coupled to Fe(III) reduction (Perry et al, 1973;Walker, 1984;Baur et al, 1985;Kaufman et al, 1990). In this process, layering is of diagenetic origin that is controlled by the relative amount of organic matter to ferric (hydr)oxide in a given layer.…”

“…The investigation of light stable isotope systems in BIFs, namely O, C and S isotope ratios, has a long tradition (e.g. Clayton, 1972, 1976;Perry et al, 1973Perry et al, , 1978Goodwin et al, 1976;Baur et al, 1985;Beukes et al, 1990;Kaufman et al, 1990;Mojzsis et al, 1996;Fedo et al, 2006). In recent years, advances in analytical techniques have provided the opportunity to study the stable isotope fractionation of the two major elements in BIFs, Fe and Si.…”