Stable isotope geochemistry of early Protezozoic carbonate concretions in the Animikie group of the Lake Superior region: evidence for anaerobic bacterial processes

Iron Formations: Their Origins and Implications for Ancient Seawater Chemistry

“…Studies of carbonates from the Brockman IF (Baur et al, 1985) show that isotopically light carbon and oxygen isotope values correlate with concentrations of iron. The negative carbonate carbon isotope values have been interpreted as evidence for direct carbonate (siderite) precipitation from an iron-rich water column, stratified with respect to carbon isotope composition of total dissolved inorganic carbon and influenced by a deep-water, hydrothermal flux associated with submarine magmatic activity (e.g., Winter and Knauth, 1992). Although a stratification of several per mil in the carbon isotope composition of dissolved inorganic carbon is present in the modern ocean (e.g., Kroopnick, 1985), a much smaller gradient is preserved in the early Precambrian rock record and is expected under the high pCO 2 conditions required to compensate for a lower solar luminosity during early Earth history (Hotinski et al, 2004).…”

“…Most carbon isotope studies have focused on the carbonate fraction, due in part to the generally low organic carbon content in IFs. The most extensive studies have been undertaken on the low metamorphic grade deposits of the late Neoarchean to early Paleoproterozoic Transvaal Supergroup in South Africa (e.g., and the $2.5 Ga Brockman IF in Western Australia (e.g., Baur et al, 1985;Becker and Clayton, 1972;Kaufman et al, 1990), as well as the $1.88 Ga Biwabik and Gunflint IFs in the United States and Canada Winter and Knauth, 1992). Numerous siderite-rich IFs also have been analyzed for carbonate carbon isotopes (e.g., Ohmoto et al, 2004).…”

1.1 Tectonic Evolution and Major Global Earth-Surface Palaeoenvironmental Events in the Palaeoproterozoic

“…The first known appearance, and then worldwide development, of microbially meditated, 13 C-depleted, diagenetic carbonate concretions outside a "BIF-environment" are documented in published literature at around 2.0 Ga (Winter and Knauth 1992;Melezhik et al 1998Melezhik et al , 1999bFallick et al 2008). In c. post-2.0 Ga sedimentary successions, the concretions are varied and abundant ( Fig.…”