Iron formations: A global record of Neoarchaean to Palaeoproterozoic environmental history
TL;DR: A review of the defining features of iron formations and their distribution through the Neo-archaean and Palaeoproterozoic is presented in this article, along with an update of previous reviews by Bekker et al. (2010, 2014).
About: This article is published in Earth-Science Reviews.The article was published on 2017-09-01 and is currently open access. It has received 280 citations till now.
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TL;DR: The Archean eon data imply that substantial loss of hydrogen oxidized the Earth, and detailed understanding of the coevolving solid Earth, biosphere, and atmosphere remains elusive, however.
Abstract: The atmosphere of the Archean eon-one-third of Earth's history-is important for understanding the evolution of our planet and Earth-like exoplanets. New geological proxies combined with models constrain atmospheric composition. They imply surface O2 levels <10-6 times present, N2 levels that were similar to today or possibly a few times lower, and CO2 and CH4 levels ranging ~10 to 2500 and 102 to 104 times modern amounts, respectively. The greenhouse gas concentrations were sufficient to offset a fainter Sun. Climate moderation by the carbon cycle suggests average surface temperatures between 0° and 40°C, consistent with occasional glaciations. Isotopic mass fractionation of atmospheric xenon through the Archean until atmospheric oxygenation is best explained by drag of xenon ions by hydrogen escaping rapidly into space. These data imply that substantial loss of hydrogen oxidized the Earth. Despite these advances, detailed understanding of the coevolving solid Earth, biosphere, and atmosphere remains elusive, however.
233 citations
01 Dec 2010
TL;DR: In this article, the authors reported the results of a study of the early Archean BIFs from the Hamersley Basin, Australia and the early Isua Supracrustal Belt (ISB), Greenland.
Abstract: Banded Iron-Formations (BIFs) are voluminous chemical sediments that are rich in iron-oxide, carbonate and silica and whose occurrence is unique to the Precambrian. Their preservation in the geological record offers insights to the surface chemical and biological cycling of iron and carbon on early Earth. However, many details regarding the role of microbial activity in BIF deposition and diagenesis are unresolved. Laboratory studies have shown that reaction between carbon and iron through microbial iron respiration [2Fe2O3∙ nH2O + CH2O + 7H+ → 4Fe2+ + HCO3− + (2n + 4)H2O + chemical energy] can impart fractionation to the isotopic compositions of these elements. Here, we report iron (δ56Fe, vs. IRMM-014) and carbon isotopic (δ13C, vs. V-PDB) compositions of magnetite and of iron-rich and iron-poor carbonates in BIFs from the late Archean (~ 2.5 Ga) Hamersley Basin, Australia and the early Archean (~ 3.8 Ga) Isua Supracrustal Belt (ISB), Greenland. The range of δ56Fe values measured in the Hamersley Basin, including light values in magnetite and heavy values in iron-rich carbonates (up to + 1.2‰), are incompatible with their precipitation in equilibrium with seawater. Rather, the data together with previously reported light δ13C values in iron-rich carbonates record evidence for diagenetic reduction of ferric oxide precursors to magnetite and carbonate through microbial iron respiration (i.e., dissimilatory iron reduction, DIR). Iron and carbon isotope data of iron-rich metacarbonates from the ISB are similar to those of late Archean BIFs. The isotopic signatures of these metacarbonates are supportive of an early diagenetic origin despite metasomatic overprint, and preserve evidence of microbial iron respiration within the oldest recognized sedimentary rocks on Earth.
151 citations
TL;DR: Elevated rates of reverse weathering within silica-rich oceans led to enhanced carbon retention within the ocean–atmosphere system, promoting a stable, equable ice-free climate throughout Earth’s early to middle ages.
Abstract: For the first four billion years of Earth’s history, climate was marked by apparent stability and warmth despite the Sun having lower luminosity1. Proposed mechanisms for maintaining an elevated partial pressure of carbon dioxide in the atmosphere (
$${p}_{{{\rm{CO}}}_{{\rm{2}}}}$$
) centre on a reduction in the weatherability of Earth’s crust and therefore in the efficiency of carbon dioxide removal from the atmosphere2. However, the effectiveness of these mechanisms remains debated2,3. Here we use a global carbon cycle model to explore the evolution of processes that govern marine pH, a factor that regulates the partitioning of carbon between the ocean and the atmosphere. We find that elevated rates of ‘reverse weathering’—that is, the consumption of alkalinity and generation of acidity during marine authigenic clay formation4–7—enhanced the retention of carbon within the ocean–atmosphere system, leading to an elevated $${p}_{{{\rm{CO}}}_{{\rm{2}}}}$$
baseline. Although this process is dampened by sluggish kinetics today, we propose that more prolific rates of reverse weathering would have persisted under the pervasively silica-rich conditions8,9 that dominated Earth’s early oceans. This distinct ocean and coupled carbon–silicon cycle state would have successfully maintained the equable and ice-free environment that characterized most of the Precambrian period. Further, we propose that during this time, the establishment of a strong negative feedback between marine pH and authigenic clay formation would have also enhanced climate stability by mitigating large swings in $${p}_{{{\rm{CO}}}_{{\rm{2}}}}$$
—a critical component of Earth’s natural thermostat that would have been dominant for most of Earth’s history. We speculate that the late ecological rise of siliceous organisms8 and a resulting decline in silica-rich conditions dampened the reverse weathering buffer, destabilizing Earth’s climate system and lowering baseline $${p}_{{{\rm{CO}}}_{{\rm{2}}}}$$
. Elevated rates of reverse weathering within silica-rich oceans led to enhanced carbon retention within the ocean–atmosphere system, promoting a stable, equable ice-free climate throughout Earth’s early to middle ages.
135 citations
01 Dec 2011
TL;DR: In this article, a combination of Fe and Mo isotope systematics of Ca-Mg carbonates and shales from the 2.68 to 2.50 Ga Campbellrand-Malmani carbonate platform of the Kaapvaal Craton in South Africa was used to constrain free O2 levels in the photic zone of a Late Archean marine basin by the combined use of Fe-Mo isotope systems.
Abstract: Most geochemical proxies and models of atmospheric evolution suggest that the amount of free O2 in Earth’s atmosphere stayed below 10−5 present atmospheric level (PAL) until the Great Oxidation Event (GOE) that occurred between ∼2.2 and 2.4 Ga, at which time free O2 in the atmosphere increased to approximately 10−1 to 10−2 PAL. Although photosynthetically produced “O2 oases” have been proposed for the photic zone of the oceans prior to the GOE, it has been difficult to constrain absolute O2 concentrations and fluxes in such paleoenvironments. Here we constrain free O2 levels in the photic zone of a Late Archean marine basin by the combined use of Fe and Mo isotope systematics of Ca–Mg carbonates and shales from the 2.68 to 2.50 Ga Campbellrand–Malmani carbonate platform of the Kaapvaal Craton in South Africa. Correlated Fe and Mo isotope compositions require a key role for Fe oxide precipitation via oxidation of aqueous Fe(II) by photosynthetically-derived O2, followed by sorption of aqueous Mo to the newly formed Fe oxides. A dispersion/reaction model illustrates the effects of Fe oxide production and Mo sorption to Fe oxides, and suggests that a few to a few tens of μM free O2 was available in the photic zone of the Late Archean marine basin, consistent with some previous estimates. The coupling of Fe and Mo isotope systematics provides a unique view into the processes that occurred in the ancient shallow ocean after production of free O2 began, but prior to oxygenation of the deep ocean, or significant accumulation of free O2 in the atmosphere. These results require oxygenic photosynthesis to have evolved by at least 2.7 Ga and suggest that the Neoarchean ocean may have had a different oxygenation history than that of the atmosphere. The data also suggest that the extensive iron formation deposition that occurred during this time was unlikely to have been produced by anoxygenic photosynthetic Fe(II) oxidation. Finally, these data indicate that the ocean had significant amounts of O2 at least 150 Myr prior to previously proposed “whiffs” of O2 at the Archean to Proterozoic transition.
121 citations
References
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TL;DR: Pore water profiles of total CO 2, pH, PO 3−4, NO − 3 plus NO − 2, SO 2− 4, S 2−, Fe 2+ and Mn 2+ have been obtained in cores from pelagic sediments of the eastern equatorial Atlantic under waters of moderate to high productivity as mentioned in this paper.
3,045 citations
2,776 citations
TL;DR: In this article, it was shown that organic matter appears to be the major control on pyrite formation in normal (non-euxinic) terrigenous marine sediments where dissolved sulfate and iron minerals are abundant.
2,234 citations
"Iron formations: A global record of..." refers background in this paper
...Given that the accumulation of sulfide and subsequent pyrite formation requires iron limitation (Berner, 1984; Poulton and Canfield, 2011), Bekker et al. (2010) suggested that sulfide-facies IF should no longer be considered a variety of IF, sensu stricto....
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...Given that the accumulation of sulfide and subsequent pyrite formation requires iron limitation (Berner, 1984; Poulton and Canfield, 2011), Bekker et al....
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TL;DR: Negative carbon isotope anomalies in carbonate rocks bracketing Neoproterozoic glacial deposits in Namibia, combined with estimates of thermal subsidence history, suggest that biological productivity in the surface ocean collapsed for millions of years.
Abstract: Negative carbon isotope anomalies in carbonate rocks bracketing Neoproterozoic glacial deposits in Namibia, combined with estimates of thermal subsidence history, suggest that biological productivity in the surface ocean collapsed for millions of years. This collapse can be explained by a global glaciation (that is, a snowball Earth), which ended abruptly when subaerial volcanic outgassing raised atmospheric carbon dioxide to about 350 times the modern level. The rapid termination would have resulted in a warming of the snowball Earth to extreme greenhouse conditions. The transfer of atmospheric carbon dioxide to the ocean would result in the rapid precipitation of calcium carbonate in warm surface waters, producing the cap carbonate rocks observed globally.
2,233 citations
"Iron formations: A global record of..." refers background in this paper
...This low-oxygen redox state in the deep oceans formed a prelude to Snowball glaciations, the rise of atmospheric and oceanic oxygen, and the emergence of complex life; the only return of massive IFs was in association with the Sturtian glaciation (e.g., Klein and Beukes, 1993; Hoffman et al., 1998)....
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TL;DR: The average chemical composition of the upper continental crust (UC) as a function of age is estimated from chemical analyses, geologic maps, stratigraphic sections and isotopic ages as discussed by the authors.
1,916 citations
"Iron formations: A global record of..." refers background in this paper
...…invoked a continental source of iron, where Fe (II) would have been mobile during weathering in the absence of atmospheric O2 (e.g., James, 1954; Lepp and Goldich, 1964), as well as the expectation that early continents would have been more mafic than today, and hence richer in iron (Condie, 1993)....
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..., James, 1954; Lepp and Goldich, 1964), as well as the expectation that early continents would have been more mafic than today, and hence richer in iron (Condie, 1993)....
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