Rise Of Atmospheric Oxygen Research Articles

Fractionation between inorganic and organic carbon during the Lomagundi (2.22–2.1 Ga) carbon isotope excursion

The Lomagundi (2.22–2.1 Ga) positive carbon isotope excursion in shallow-marine sedimentary carbonates has been associated with the rise in atmospheric oxygen, but subsequent studies have demonstrated that the carbon isotope excursion was preceded by the rise in atmospheric oxygen. The amount of oxygen released to the exosphere during the Lomagundi excursion is constrained by the average global fractionation between inorganic and organic carbon, which is poorly characterized. Because dissolved inorganic and organic carbon reservoirs were arguably larger in the Paleoproterozoic ocean, at a time of lower solar luminosity and lower ocean redox state, decoupling between these two variables might be expected. We determined carbon isotope values of carbonate and organic matter in carbonates and shales of the Silverton Formation, South Africa and in the correlative Sengoma Argillite Formation, near the border in Botswana. These units were deposited between 2.22 and 2.06 Ga along the margin of the Kaapvaal Craton in an open-marine deltaic setting and experienced lower greenschist facies metamorphism. The prodelta to offshore marine shales are overlain by a subtidal carbonate sequence. Carbonates exhibit elevated 13C values ranging from 8.3 to 11.2‰ vs. VPDB consistent with deposition during the Lomagundi positive excursion. The total organic carbon (TOC) contents range from 0.01 to 0.6% and δ 13C values range from − 24.8 to − 13.9‰. Thus, the isotopic fractionation between organic and carbonate carbon was on average 30.3 ± 2.8‰ ( n = 32) in the shallow-marine environment. The underlying Sengoma shales have highly variable TOC contents (0.14 to 21.94%) and δ 13C values (− 33.7 to − 20.8‰) with an average of − 27.0 ± 3.0‰ ( n = 50). Considering that the shales were also deposited during the Lomagundi excursion, and taking δ 13C values of the overlying carbonates as representative of the δ 13C value of dissolved inorganic carbon during shale deposition, a carbon isotope fractionation as large as ~ 37‰ appears to characterize the production of bulk organic matter in the deeper part of the Pretoria Basin at that time. This enhanced fractionation relative to that observed in shallow-water environments likely reflects heterotrophic (secondary) and chemotrophic productivity at and below a pronounced redoxcline, consistent with the euxinic conditions inferred from independent evidence for the deeper part of the Pretoria Basin. Greater variability in organic carbon vs. carbonate carbon isotopic values on the shallow-marine carbonate platform suggests that the carbon cycling was dominated by a large dissolved inorganic carbon reservoir during the Lomagundi excursion. Our study suggests that in contrast to the Late Neoproterozoic and Phanerozoic, when carbon isotope fractionation between carbonate and organic carbon in the open ocean was mostly controlled by primary producers, in the Paleoproterozoic redox-stratified ocean heterotrophic and chemotrophic productivity overprinted a signal of primary productivity below the redoxcline. This strong imprint of heterotrophic and chemotrophic productivity on organic carbon isotope records complicates the reconstruction of spatial patterns and secular trends in the δ 13C values of dissolved inorganic carbon in the Paleoproterozoic seawater.

Rise in seawater sulphate concentration associated with the Paleoproterozoic positive carbon isotope excursion: evidence from sulphate evaporites in the ∼2.2–2.1 Gyr shallow‐marine Lucknow Formation, South Africa

Abstract Past oceanic sulphate concentration is important for understanding how the oceans’ redox state responded to atmospheric oxygen levels. The absence of extensive marine sulphate evaporites before ∼1.2 Gyr probably reflects low seawater sulphate and/or higher carbonate concentrations. Sulphate evaporites formed locally during the 2.22–2.06 Gyr Lomagundi positive δ13C excursion. However, the ∼2.2–2.1 Gyr Lucknow Formation, South Africa, provides the first direct evidence for seawater sulphate precipitation on a carbonate platform with open ocean access and limited terrestrial input. These marginal marine deposits contain evidence for evaporite molds, pseudomorphs after selenite gypsum, and solid inclusions of Ca‐sulphate in quartz. Carbon and sulphur isotope data match the global record and indicate a marine source of the evaporitic brines. The apparent precipitation of gypsum before halite requires ≥2.5 mm L−1 sulphate concentration, higher than current estimates for the Paleoproterozoic. During the Lomagundi event, which postdates the 2.32 Gyr initial rise in atmospheric oxygen, seawater sulphate concentration rose from Archean values of ≤200 μm L−1, but dropped subsequently because of higher pyrite burial rates and a lower oceanic redox state.

Tracing the stepwise oxygenation of the Proterozoic ocean

Biogeochemical signatures preserved in ancient sedimentary rocks provide clues to the nature and timing of the oxygenation of the Earth's atmosphere. Geochemical data suggest that oxygenation proceeded in two broad steps near the beginning and end of the Proterozoic eon (2,500 to 542 million years ago). The oxidation state of the Proterozoic ocean between these two steps and the timing of deep-ocean oxygenation have important implications for the evolutionary course of life on Earth but remain poorly known. Here we present a new perspective on ocean oxygenation based on the authigenic accumulation of the redox-sensitive transition element molybdenum in sulphidic black shales. Accumulation of authigenic molybdenum from sea water is already seen in shales by 2,650 Myr ago; however, the small magnitudes of these enrichments reflect weak or transient sources of dissolved molybdenum before about 2,200 Myr ago, consistent with minimal oxidative weathering of the continents. Enrichments indicative of persistent and vigorous oxidative weathering appear in shales deposited at roughly 2,150 Myr ago, more than 200 million years after the initial rise in atmospheric oxygen. Subsequent expansion of sulphidic conditions after about 1,800 Myr ago (refs 8, 9) maintained a mid-Proterozoic molybdenum reservoir below 20 per cent of the modern inventory, which in turn may have acted as a nutrient feedback limiting the spatiotemporal distribution of euxinic (sulphidic) bottom waters and perhaps the evolutionary and ecological expansion of eukaryotic organisms. By 551 Myr ago, molybdenum contents reflect a greatly expanded oceanic reservoir due to oxygenation of the deep ocean and corresponding decrease in sulphidic conditions in the sediments and water column.

Carbon isotope excursions and the oxidant budget of the Ediacaran atmosphere and ocean

A possible global drop in marine carbon isotope values to as low as −12‰ Peedee belemnite (PDB), recorded in the Ediacaran Shuram Formation of Oman, has been attributed to the non-steady-state oxidation of oceanic dissolved organic carbon (DOC) resulting from the rise in atmospheric oxygen to near modern values at the end of the Precambrian. Geologic constraints indicate that the excursion lasted between 25 and 50 m.y., requiring a DOC pool thousands of times to 10,000 times the modern inventory to conform with carbon isotope mass balance calculations for a −12‰ excursion. At the consequent rates of DOC oxidation, oceanic sulfate and oxygen in the atmosphere and oceans are exhausted on a time scale of ~800 k.y. Oxidant depletion is incompatible with independent geochemical and biological indicators that show oceanic sulfate and oxygen levels were maintained or increased during the Shuram excursion. Furthermore, a DOC-driven excursion does not explain strong covariation between the carbon and oxygen isotope record. These indicators show that negative isotope excursions recorded in the Shuram and other Ediacaran sections are unlikely to represent a global ocean signal.

Trace element and isotopic characterization of Neoarchean and Paleoproterozoic iron formations in the Black Hills (South Dakota, USA): Assessment of chemical change during 2.9–1.9 Ga deposition bracketing the 2.4–2.2 Ga first rise of atmospheric oxygen

Banded iron formations (BIFs) were deposited in the Black Hills, South Dakota, prior to and after the first main oxygenation of the atmosphere at ∼2.4–2.2 Ga. The oldest, Neoarchean horizon (Nemo Iron Formation) is an oxide-facies, detritus-free BIF deposited at ∼2.9–2.6 Ga, in association with mafic phyllite. These BIFs show rare earth and yttrium elemental (REY) signatures typical of ambient seawater, including Pr/Yb < 1 and positive La, Eu, and Y anomalies in Post-Archean-Australian-Shale (PAAS)-normalized REE diagrams, but they lack clear Ce anomalies. Deposition of an Early Paleoproterozoic horizon (Benchmark Iron Formation) is constrained to the interval between 2.56 and 2.48 Ga. These BIFs also show the REE distribution patterns typical of ambient seawater. Benchmark BIFs also exhibit slightly negative Ce anomalies, but BIFs included within the younger, 2.10–2.02 Ga Estes metaconglomerate exhibit mostly positive Ce anomalies, which are probably the result of oxidative supergene remobilization of Ce during transport. Mixed, silicate–carbonate–sulfide–oxide–facies BIFs with slightly elevated detrital inputs were deposited in Middle Paleoproterozoic time, between 2.02 and 1.97 Ga (Homestake Iron Formation and equivalents) and between 1.89 and 1.87 Ga (Rochford Iron Formation). These BIFs are characterized by variably fractionated, PAAS-normalized REY patterns, which include: Pr/Yb ratios of 0.2–4.1; absence of La anomalies; and presence of positive Eu, positive and negative Ce, and mostly negative Y anomalies. Sm–Nd isotopes from individual microbands of the Neoarchean BIF define Eo- to Mesoarchean, Nd T DM ages of 3.6–2.9 Ga, which are generally compatible with crustal residence ages inferred from elsewhere in the Wyoming craton (i.e., in the 2.72–2.67 Ga Atlantic City BIF, Wind River Range). Model ages of microbands from the Early and Middle Paleoproterozoic BIFs are highly variable, ranging from 3.8 to 2.15 Ga. The youngest components in the Neoarchean and Middle Paleoproterozoic BIFs, inferred from the model ages, correspond to associated, mafic metatuffs and metabasalts, which yield ∼2.9 and ∼2.15 Ga model ages, respectively. Further, the oldest (i.e., ≤3.9–3.6 Ga) components indicate that Eo- to Mesoarchean crust was exposed during the entire BIF depositional time frame, between ∼2.9 and 1.88 Ga. Pb isotopes distinguish the Neoarchean from Early and Middle Paleoproterozoic BIFs. Whereas all BIFs reflect the generally high- μ ( 238U/ 204Pb) values typical of the Wyoming craton, thorogenic-versus-uranogenic Pb-isotope data arrays constrain sources of Pb in the various iron formations, particularly those deposited in Early and Middle Paleoproterozoic time. Early Paleoproterozoic BIFs exhibit low 208Pb/ 204Pb relative to 206Pb/ 204Pb ratios, implying a highly U/Th-fractionated source. This source is compatible with the peraluminous, ∼2.60 Ga Bear Mountain granite exposed in the W-central Black Hills, which itself was derived from even older crust. In contrast, the Middle Paleoproterozoic BIFs define a Pb-isotopic array that is compatible with a moderately fractionated source resembling the metaluminous, ∼2.56 Ga Little Elk granite exposed in the NE Black Hills. The Neoarchean BIFs are generally less radiogenic than their younger counterparts, and plot near the intersection of the two Paleoproterozoic arrays on the thorogenic diagram. Although the ∼2.9–1.9 Ga Black Hills BIFs bracket the worldwide, 2.4–2.0 Ga BIF depositional gap, which curiously coincides with the coupled, first atmospheric–biospheric oxygenation and subsequent, positive δ 13C excursion, we are unable to delineate major changes in BIF geochemical compositions that might reflect concomitant changes in atmospheric oxygenation. However, slightly negative Ce anomalies recorded in the 2.56–2.48 Ga, oxide–facies BIFs, and an overall trend of decreasing, positive Eu anomalies between ∼2.9 and 1.9 Ga among Black Hills BIFs generally (in agreement with the trend observed worldwide), might argue for the onset of bottom-water oxidation at the Archean-Proterozoic boundary and a decrease of high-temperature hydrothermal activity over the worldwide, BIF depositional gap. In the Black Hills, the 2.4–2.0 Ga depositional gap reflects either a ∼400 Myr absence of seawater between incipient, intracratonic rifting of supercraton Superia (or supercontinent Kenorland) at ∼2.5–2.4 Ga and its final breakup at ∼2.1 Ga, a ∼400 Myr missing rock record across a mapped unconformity, or both.

Increased subaerial volcanism and the rise of atmospheric oxygen 2.5 billion years ago

The hypothesis that the establishment of a permanently oxygenated atmosphere at the Archaean-Proterozoic transition (approximately 2.5 billion years ago) occurred when oxygen-producing cyanobacteria evolved is contradicted by biomarker evidence for their presence in rocks 200 million years older. To sustain vanishingly low oxygen levels despite near-modern rates of oxygen production from approximately 2.7-2.5 billion years ago thus requires that oxygen sinks must have been much larger than they are now. Here we propose that the rise of atmospheric oxygen occurred because the predominant sink for oxygen in the Archaean era-enhanced submarine volcanism-was abruptly and permanently diminished during the Archaean-Proterozoic transition. Observations are consistent with the corollary that subaerial volcanism only became widespread after a major tectonic episode of continental stabilization at the beginning of the Proterozoic. Submarine volcanoes are more reducing than subaerial volcanoes, so a shift from predominantly submarine to a mix of subaerial and submarine volcanism more similar to that observed today would have reduced the overall sink for oxygen and led to the rise of atmospheric oxygen.

Coping with cyclic oxygen availability: evolutionary aspects

Both the gradual rise in atmospheric oxygen over the Proterozoic Eon as well as episodic fluctuations in oxygen over several million-year time spans during the Phanerozoic Era, have arguably exerted strong selective forces on cellular and organismic respiratory specialization and evolution. The rise in atmospheric oxygen, some 2 billion years after the origin of life, dramatically altered cell biology and set the stage for the appearance of multicelluar life forms in the Vendian (Ediacaran) Period of the Neoproterozoic Era. Over much of the Paleozoic, the level of oxygen in the atmosphere was near the present atmospheric level (21%). In the Late Paleozoic, however, there were extended times during which the level of atmospheric oxygen was either markedly lower or markedly higher than 21%. That these Paleozoic shifts in atmospheric oxygen affected the biota is suggested by the correlations between: (1) Reduced oxygen and the occurrences of extinctions, a lowered biodiversity and shifts in phyletic succession, and (2) During hyperoxia, the corresponding occurrence of phenomena such as arthropod gigantism, the origin of insect flight, and the evolution of vertebrate terrestriality. Basic similarities in features of adaptation to hyopoxia, manifest in living organisms at levels ranging from genetic and cellular to physiological and behavioral, suggest the common and early origin of a suite of adaptive mechanisms responsive to fluctuations in ambient oxygen. Comparative integrative approaches addressing the molecular bases of phenotypic adjustments to cyclic oxygen fluctuation provide broad insight into the incremental steps leading to the early evolution of homeostatic respiratory mechanisms and to the specialization of organismic respiratory function.

Anaerobic methanotrophy and the rise of atmospheric oxygen

In modern marine sediments, the anoxic decomposition of organic matter generates a significant flux of methane that is oxidized microbially with sulphate under the seafloor and never reaches the atmosphere. In contrast, prior to ca 2.4Gyr ago, the ocean had little sulphate to support anaerobic oxidation of methane (AOM) and the ocean should have been an important methane source. As atmospheric O2 and seawater sulphate levels rose on the early Earth, AOM would have increasingly throttled the release of methane. We use a biogeochemical model to simulate the response of early atmospheric O2 and CH4 to changes in marine AOM as sulphate levels increased. Semi-empirical relationships are used to parameterize global AOM rates and the evolution of sulphate levels. Despite broad uncertainties in these relationships, atmospheric O2 concentrations generally rise more rapidly and to higher levels (of order approx. 10(-3) bar versus approx. 10(-4) bar) as a result of including AOM in the model. Methane levels collapse prior to any significant rise in O2, but counter-intuitively, methane re-rises after O2 rises to higher levels when AOM is included. As O2 concentrations increase, shielding of the troposphere by stratospheric ozone slows the effective reaction rate between oxygen and methane. This effect dominates over the decrease in the methane source associated with AOM. Thus, even with the inclusion of AOM, the simulated Late Palaeoproterozoic atmosphere has a climatologically significant level of methane of approximately 50ppmv.

Oxidative forcing of global climate change: A biogeochemical record across the oldest Paleoproterozoic ice age in North America

Carbon isotope compositions of organic matter in fine-grained siliciclastic units deposited above and below glacial diamictite at the base of the ca. 2.45–2.22 Ga Huronian Supergroup in Ontario, Canada were studied to constrain relationships between profound fluctuations in the exogenic carbon cycle and dramatic climate changes at the beginning of the Proterozoic Eon. In both drill core and outcrop sections the organic matter preserved in proximal lithofacies, dominated by coarse-grained sand and silt, are enriched in 13C relative to distal lithofacies, dominated by argillites. In the drill core, sand-dominated lithofacies of the McKim Formation beneath the glacial diamictite of the Ramsay Lake Formation have a narrow range of δ 13C values (− 28.4 to − 26.0‰ V-PDB), but organic matter in argillite-dominated lithofacies of the outcrop section ∼ 40 km to the southeast is somewhat more 13C-depleted with values ranging from − 34.5 to − 26.4‰. Similarly, sand-dominated lithofacies of the Pecors Formation above the glacial diamictite in the drill core section with δ 13C values of ca. − 28‰ are notably 13C-enriched relative to argillite-dominated lithofacies, which record values as low as − 40.5‰. The sand-dominated lithofacies of the Pecors Formation in the outcrop sections have δ 13C compositions ranging from − 34.4 to − 27.9‰. The isotopic differences appear to be unrelated to organic carbon abundances, so we suggest that these are controlled by environmental differences in proximal and distal settings. The strong 13C-depletion in the organic-lean McKim and Pecors argillites, especially in the drill core section of the Pecors Formation, is consistent with significant biological methane production and oxidative recycling by methanotrophs both before and after the ice age in shallow-water environments stratified with respect to oxygen. The rise of atmospheric oxygen and subsequent enhanced biogeochemical methane cycling in shallow-water settings likely contributed to unstable climate conditions during the Paleoproterozoic glacial epoch.

Decoupling photochemical Fe(II) oxidation from shallow-water BIF deposition

Oxidized Fe minerals in Archean–Paleoproterozoic banded iron formations (BIFs) are commonly taken to indicate the presence of biogenic O 2 or photosynthetic Fe(II)-oxidizing bacteria in the oceans' photic zone. However, at least one viable abiogenic oxidation mechanism has been proposed. Prior to the rise of atmospheric oxygen and the development of a protective ozone layer, the Earth's surface was subjected to high levels of ultraviolet radiation. Bulk ocean waters that were anoxic at this time could have supported high concentrations of dissolved Fe(II). Under such conditions, dissolved ferrous iron species, such as Fe 2+ and Fe(OH) +, would have absorbed radiation in the 200–400 nm range, leading to the formation of dissolved ferric iron [Fe(III)], which in turn, would have hydrolyzed to form ferric hydroxide [Fe(OH) 3] at circumneutral pH [Cairns-Smith, A.G., 1978, Precambrian solution photochemistry, inverse segregation, and banded iron formations. Nature 76, 807–808; Braterman, P.S., Cairns-Smith, A.G., and Sloper, R.W., 1983, Photo-oxidation of hydrated Fe 2-Significance for banded iron formations. Nature 303, 163–164]. This process has been invoked to account for BIF deposition without need for biology [François, L.M., 1986, Extensive deposition of banded iron formations was possible without photosynthesis. Nature 320, 352–354]. Here, we evaluate the potential importance of photochemical oxidation using a combination of experiments and thermodynamic models. The experiments simulate the chemistry of ambient Precambrian seawater mixing with Fe(II)-rich hydrothermal fluids with, and without, UV irradiation. We find that if Fe(II) was effused from relatively shallow seamount-type vent systems directly into an anoxic photic zone, the photochemical contribution to solid-phase precipitation would have been negligible. Instead, most of the Fe(II) would have precipitated rapidly as an amorphous precursor phase to the ferrous silicate mineral greenalite ((Fe) 3Si 2O 5(OH) 4), and/or the ferrous carbonate, siderite (FeCO 3), depending on different simulated atmospheric pCO 2 levels. Conversely, in experiments where Fe(II) was exposed either to phototrophic Fe(II)-oxidizing bacteria or to O 2, ferric hydroxide formed rapidly, and the precipitation of ferrous iron phases was not observed. If, as suggested on mass balance grounds, BIF deposition requires that Fe be sourced from shallow seamount-type systems, then we are driven to conclude that oxide-facies BIF are the product of a rapid, non-photochemical oxidative process, the most likely candidates being direct or indirect biological oxidation, and that a significant fraction of BIF could have initially been deposited as ferrous minerals.

Multiple sulfur isotopes from Paleoproterozoic Huronian interglacial sediments and the rise of atmospheric oxygen

Mass-dependently fractionated (MDF) sulfur isotopes in sedimentary sulfides and sulfates can provide information on the past activity of microbial metabolisms and serve as a proxy for the concentration of seawater sulfate. In conjunction with these proxies, mass-independently fractionated (MIF) sulfur isotopes can be used to track the rise of atmospheric oxygen. The anoxic–oxic transition of the surface environment in the Paleoproterozoic has been constrained by MIF sulfur in 2.47 Ga sulfides from Western Australian banded iron-formations, and the absence of MIF in diagenetic sulfides from 2.32 Ga black shales in the Transvaal Supergroup (South Africa). We report new multicollector ion microprobe data for individual sulfides from water-lain sedimentary units in the 2.45–2.22 Ga Huronian Supergroup (Ontario, Canada). The authigenecity of sulfides in these units is interpreted on the basis of host lithology, mode of distribution, sulfide chemistry and MDF sulfur isotopes. Sulfides interpreted to have an authigenic-sedimentary origin from the McKim Fm. have a range of 0.7‰ in Δ 33S values and those in the Pecors Fm. preserve small magnitude MIF Δ 33S values up to + 0.88‰, which we propose represents the last remnants of an anoxic atmosphere. High δ 34S values (up to + 31.2‰) and near-zero Δ 33S values in authigenic sulfides from the stratigraphically higher Espanola and Gordon Lake Fms. suggest local variations of seawater sulfate concentrations and/or δ 34S sulfate, consistent with increased atmospheric O 2 levels. MIF sulfur is absent from the marine record after the second Huronian glaciation, which is analogous to the record of the Transvaal Supergroup where MIF is absent from the upper part of the interglacial sequence. Our results suggest that the possible simultaneous demise of MIF sulfur isotopes at these two localities may serve as a useful global geochemical marker to correlate Paleoproterozoic supergroups. We propose a new biogeochemical model where enhanced weathering rates during Paleoproterozoic post-glacial thawing served as a critical stimulus for interglacial blooms of oxygenic photosynthesis, the demise of methane, and ultimately to the irreversible rise in atmospheric oxygen.

Black smokers and density currents: A uniformitarian model for the genesis of banded iron-formations

Banded iron-formations (BIFs) were comparatively abundant and widespread marine sedimentary rocks in the Archean and Lower Proterozoic eras, but thereafter they appear to be restricted to the Neoproterozoic and Paleozoic eras, although there are indications of similar rocks forming at present. BIFs are important as the major source of iron ore for industry and have also been used to support hypotheses regarding the evolution of life, oceans, and the atmosphere in the Archean and Proterozoic. They apparently formed in deep water and consisted of a semi-regular alternation of quartz (chert) and iron-rich minerals with little or no terrigenous sediment and low (< 1 wt.%) alumina content, in contrast to Proterozoic to Phanerozoic oolitic ironstones (Clinton–Minette style) that formed in shelf environments, were comparatively small and rare and contain abundant iron aluminosilicates and hydroxides but little or no chert. Although ooliths may be locally abundant in granular iron-formations (GIFs) they differ from the Clinton–Minette style oolitic ironstones that formed in shelf seas during periods of slow sedimentation. A common mode of origin for marine deep-water iron-formations is proposed, in which hot fluids, consisting of marine and connate water leaching iron, silica and other elements from mafic and ultramafic rocks associated with mantle plumes or mid-oceanic ridges and active spreading centres are released into the ocean at underwater hot springs (black smokers). On contact with cold marine water, the least soluble elements are precipitated in the form of colloidal hydrous silicates (clay minerals) and hydroxides close to the hydrothermal vent. The hydrothermal fluids are high in silica and low in alumina causing the precipitation of alumina-poor iron silicates (nontronite) that dissociate into iron hydroxide and amorphous silica during diagenesis. The amorphous silica is typically entrapped by iron oxide laminae to form bands of chert. Breaches of the iron oxide laminae permitted the escape of the gelatinous amorphous silica during compaction and dewatering leaving a chert-free residue as the protore of non-hydrothermal sedimentary high-grade iron ore. The rapid deposition and abundant included water formed unstable mounds and chimneys around the vents. Slumping of the mounds caused by compaction, dewatering, gravity sliding, and seismic events produced turbidity currents forming proximal fans of GIF, but colloidal particles remained suspended in longer-lived density flows to deposit the ultra fine-grained BIF over vast areas of the ocean floor. Episodes of density current deposition were separated by intervals of slow pelagic sedimentation and silicification of the sea floor. The density currents commonly caused minor erosion of the sea floor with rip-out clasts incorporated in the new layers. The iron-rich hydrothermal fluids triggered the precipitation of dissolved ferrous iron accumulated by anoxic weathering to produce the huge deposits of BIF in the Archean and Paleoproterozoic until the rise in atmospheric oxygen stopped the accumulation of ferrous iron in the oceans leaving only the hydrothermal source for later deposits.

Modern proteomes contain putative imprints of ancient shifts in trace metal geochemistry

Because of the rise in atmospheric oxygen 2.3 billion years ago (Gya) and the subsequent changes in oceanic redox state over the last 2.3-1 Gya, trace metal bioavailability in marine environments has changed dramatically. Although theorized to have influenced the biological usage of metals leaving discernable genomic signals, a thorough and quantitative test of this hypothesis has been lacking. Using structural bioinformatics and whole-genome sequences, the Fe-, Zn-, Mn-, and Co-binding metallomes of 23 Archaea, 233 Bacteria, and 57 Eukarya were constructed. These metallomes reveal that the overall abundances of these metal-binding structures scale to proteome size as power laws with a unique set of slopes for each Superkingdom of Life. The differences in the power describing the abundances of Fe-, Mn-, Zn-, and Co-binding proteins in the proteomes of Prokaryotes and Eukaryotes are similar to the theorized changes in the abundances of these metals after the oxygenation of oceanic deep waters. This phenomenon suggests that Prokarya and Eukarya evolved in anoxic and oxic environments, respectively, a hypothesis further supported by structures and functions of Fe-binding proteins in each Superkingdom. Also observed is a proliferation in the diversity of Zn-binding protein structures involved in protein-DNA and protein-protein interactions within Eukarya, an event unlikely to occur in either an anoxic or euxinic environment where Zn concentrations would be vanishingly low. We hypothesize that these conserved trends are proteomic imprints of changes in trace metal bioavailability in the ancient ocean that highlight a major evolutionary shift in biological trace metal usage.

Metal limitation of cyanobacterial N2fixation and implications for the Precambrian nitrogen cycle

ABSTRACTNitrogen fixation is a critical part of the global nitrogen cycle, replacing biologically available reduced nitrogen lost by denitrification. The redox‐sensitive trace metals Fe and Mo are key components of the primary nitrogenase enzyme used by cyanobacteria (and other prokaryotes) to fix atmospheric N2into bioessential compounds. Progressive oxygenation of the Earth's atmosphere has forced changes in the redox state of the oceans through geologic time, from anoxic Fe‐enriched waters in the Archean to partially sulfidic deep waters by the mid‐Proterozoic. This development of ocean redox chemistry during the Precambrian led to fluctuations in Fe and Mo availability that could have significantly impacted the ability of prokaryotes to fix nitrogen. It has been suggested that metal limitation of nitrogen fixation and nitrate assimilation, along with increased rates of denitrification, could have resulted in globally reduced rates of primary production and nitrogen‐starved oceans through much of the Proterozoic. To test the first part of this hypothesis, we grew N2‐fixing cyanobacteria in cultures with metal concentrations reflecting an anoxic Archean ocean (high Fe, low Mo), a sulfidic Proterozoic ocean (low Fe, moderate Mo), and an oxic Phanerozoic ocean (low Fe, high Mo). We measured low rates of cellular N2fixation under [Fe] and [Mo] estimated for the Archean ocean. With decreased [Fe] and higher [Mo] representing sulfidic Proterozoic conditions, N2fixation, growth, and biomass C:N were similar to those observed with metal concentrations of the fully oxygenated oceans that likely developed in the Phanerozoic. Our results raise the possibility that an initial rise in atmospheric oxygen could actually have enhanced nitrogen fixation rates to near modern marine levels, providing that phosphate was available and rising O2levels did not markedly inhibit nitrogenase activity.

Gradual and continuous rise of atmospheric oxygen between 26 - 20 Ga

Gradual and continuous rise of atmospheric oxygen between 26 - 20 Ga

Multi-stage Ediacaran ocean oxidation and its impact on evolutionary radiation

High-resolution paired carbon (δ^(13)C_(carb) and δ^(13)C_(org)) and sulfur (δ^(34)S_(SO4) and δ^(34)S_(pyrite)) isotope records of the Ediacaran strata of the Huqf Supergroup, Sultanate of Oman record a three-stage oxidation of the Ediacaran ocean. Following the Marinoan glaciation, stage I oxidation lasts from ∼635 to 610 million years ago (Myr) and records an increase in marine sulfate concentrations above ∼200 μM, likely due to a rise in atmospheric oxygen. Stage II oxidation, from ∼575 to 550 Myr, coincides with the Shuram excursion, ɑ > 13‰ negative excursion in δ^(13)C_(carb)Burns and Matter, 1993 created by the oxidation of a large reservoir of deep ocean dissolved organic carbon (DOC). The onset of stage II oxidation coincides with the evolution of acanthomorph acritarchs in Australia and China Grey, 2005, and the appearance of the first Ediacaran metazoa, Charnia-type segmented fronds Narbonne, 2005, while the first motile macroscopic metazoa (e.g., Kimberella) appear at ∼555 Myr Narbonne, 2005 as the Shuram excursion draws to a close. Stage III oxidation, from ∼550 to 547 Myr, is marked by presence of sulfur disproportionating metabolisms and the onset of co-variation in δ^(13)C_(carb) and δ^(13)C_(org), which is absent in older strata. Coincident with the onset of stage III oxidation, we see the evolution of macroscopic multicellular algae Xiao, 2004, an increase in the diversity of the acanthomorph acritarchs Burns and Matter, 1993, and the appearance of the weakly calcifying metazoa Cloudina and Namacalathus Narbonne, 2005. Following stage III oxidation, there is a period of quiescence characterized by little perturbation in the carbon cycle and an absence of significant evolutionary events. At ∼542 Myr, anoxia at the Ediacaran–Cambrian boundary causes the extinction of many Ediacaran organisms, setting the stage for the Cambrian radiation.

Cancer's sweet tooth

Even in the presence of an adequate oxygen supply, many tumors metabolize the majority of the glucose they take up through glycolysis. It has been a long-held belief that this glycolytic phenotype is due to cancer-specific defects in mitochondrial oxidative phosphorylation. In this issue of Cancer Cell, Fantin et al. now report that most tumor cells have a substantial reserve capacity to produce ATP by oxidative phosphorylation when glycolysis is suppressed. These new data add to mounting evidence that the high rate of glycolysis exhibited by most tumors is required to support cell growth rather than to compensate for defect(s) in mitochondrial function.

Carbon isotope record for the onset of the Lomagundi carbon isotope excursion in the Great Lakes area, North America

Carbonate units of the Chocolay Group in the Lake Superior area, USA and the Gordon Lake Formation of the correlative upper Huronian Supergroup, Ontario, Canada were deposited after the last Paleoproterozoic glacial event and an episode of intense chemical weathering. The Huronian Supergroup contains at the base ∼2.45 Ga volcanics and is intruded by the 2.22 Ga Nipissing diabase dykes and sills while the Chocolay Group is bracketed in age between ∼2.29 and 2.20 Ga. The Lomagundi (∼2.22–2.1 Ga) carbon isotope excursion started after the Paleoproterozoic glacial epoch and before a plume breakout event at 2.22 Ga. Therefore, the Chocolay and Upper Huronian carbonates were deposited either before or during the onset of the Lomagundi event. Notably, thin carbonates of the basal Gordon Lake Formation and thick carbonate succession of the Kona Dolomite in the northeastern exposures of the Chocolay Group record δ 13C values as high as +9.5‰ versus V-PDB. Similar to other successions deposited during the Lomagundi event, both units contain pseudomorphs and molds after sulfates. This observation suggests that seawater sulfate contents rose dramatically in association with the onset of the Lomagundi event and the rise of atmospheric oxygen. Carbonates in the western and southern exposures of the Chocolay Group (Randville and Bad River dolomites, and Saunders Formation) previously assumed to be equivalent to the Gordon Lake Formation and Kona Dolomite have carbon isotope values close to 0‰. Based on basin analysis, we infer that these carbonate units were deposited during a negative carbon isotope excursion after the Lomagundi event started and are slightly younger than the Kona Dolomite and Gordon Lake Formation. This interpretation implies that the carbonate platform in the Great Lakes area transgressed to the west over shallow-marine and fluvial deposits. The negative carbon isotope excursion in the Lake Superior area might correspond to similar δ 13C values of the Mooidraai Dolomite in the Griqualand West Basin, South Africa supporting correlation between Paleoproterozoic successions of North America and South Africa and the notion of three global glaciations in the Paleoproterozoic Era. Carbonates of the Mille Lacs Group (Trout Lake, Glen Township, and Denham formations) in Minnesota have δ 13C values ranging from −1.2 to +2.5‰. Combined with geochronologic constraints, these data suggest that these units were deposited after the Lomagundi excursion and are related to the rifting event that led to development of the so far unrecognized ∼2.0 Ga passive margin in the Lake Superior area.

The oxygenation of the atmosphere and oceans.

The last 3.85 Gyr of Earth history have been divided into five stages. During stage 1 (3.85-2.45 Gyr ago (Ga)) the atmosphere was largely or entirely anoxic, as were the oceans, with the possible exception of oxygen oases in the shallow oceans. During stage 2 (2.45-1.85 Ga) atmospheric oxygen levels rose to values estimated to have been between 0.02 and 0.04 atm. The shallow oceans became mildly oxygenated, while the deep oceans continued anoxic. Stage 3 (1.85-0.85 Ga) was apparently rather 'boring'. Atmospheric oxygen levels did not change significantly. Most of the surface oceans were mildly oxygenated, as were the deep oceans. Stage 4 (0.85-0.54 Ga) saw a rise in atmospheric oxygen to values not much less than 0.2 atm. The shallow oceans followed suit, but the deep oceans were anoxic, at least during the intense Neoproterozoic ice ages. Atmospheric oxygen levels during stage 5 (0.54 Ga-present) probably rose to a maximum value of ca 0.3 atm during the Carboniferous before returning to its present value. The shallow oceans were oxygenated, while the oxygenation of the deep oceans fluctuated considerably, perhaps on rather geologically short time-scales.

Response to Comment on "Iron Isotope Constraints on the Archean and Paleoproterozoic Ocean Redox State"

We reported a secular trend in iron isotope values of Precambrian sedimentary pyrite and related it to the changing redox state of Precambrian oceans. We restate that the iron cycle before 1.8 billion years ago was different from that now and reflected the rise of atmospheric oxygen and the subsequent moderate atmospheric oxygen level in the Paleoproterozoic.