Evolution Of Atmospheric Oxygen Research Articles

Insights from a dynamical system approach into the history of atmospheric oxygenation

Atmospheric oxygen levels are traditionally viewed to have been relatively stable throughout Earth’s history with several-step increases. Emerging evidence, however, suggests extremely dynamic atmospheric oxygen levels through large swaths of Earth’s history. Here, we provide a new perspective on atmospheric oxygen evolution using a dynamical analysis to explore the relative importance of previously proposed feedbacks on the global oxygen and carbon cycles. Our results from a stochastic analysis of oxygen mass balance in this framework suggest there are multiple steady states for atmospheric oxygen, but only three stable states. One stable state under anoxic conditions (<10−5 present atmospheric level (PAL)), one at low (~10−3to 10−2 PAL), and one near modern value atmospheric oxygen levels. Our findings also suggest two unstable states (tipping points) for atmospheric oxygen: one around 10−5 and another one around 10−1 PAL.

Marble trace element and C-O isotope geochemistry of the Paleoproterozoic Jingshan Group, North China: Insights into BIF formation during the Lomagundi-Jatuli Event

The North China Craton, one of the oldest continental blocks in the world, comprises Paleoproterozoic metamorphic sedimentary formations that record the effects of multiple major events in early Earth history, including the Great Oxidation Event (GOE) and its consequent Lomagundi-Jatuli carbon isotope event (LJE). The lack of focused research on the geological record of LJE-equivalent rocks of the Jiaodong Peninsula, Shandong Province, eastern China, has hindered our understanding of the Precambrian atmospheric oxygen evolution recorded by rocks of the North China Craton. This paper considers carbon and oxygen isotope and trace element geochemical data recovered from marble deposits of the Paleoproterozoic Jingshan Group (2.20–1.90 Ga) of the Jiaolai Basin. The investigated strata display a pronounced positive δ13CV-PDB excursion greater than 4.2 ‰. δ13CV-PDB and δ18OV-SMOW values range from −3.7 ‰ to 5.4 ‰ and 4.5 ‰ to 18.7 ‰, respectively. Positive co-variance of carbon and oxygen isotope values of the analyzed Jingshan Group samples is interpreted to reflect the effects of metamorphism and water–rock reactions. Marble deposits of the Jingshan Group experienced recrystallization, metamorphism, and silicification resulting in strong decarbonation. High amphibolite facies metamorphism may have reduced δ13CV-PDB by 3 ‰ to 5 ‰. Thus, estimated original sedimentary δ13CV-PDB values of as great as +8.4 ‰ suggest the existence of a strong positive anomaly that may reflect the global Lomagundi-Jatuli event. Rare earth and other trace element geochemical results of the present study suggest that banded iron formation (BIF) deposits associated with the Jingshan Group marble formed in a redox stratified ocean in the Jiaolai Basin dominated by ferruginous conditions. Moreover, our results suggest that positive carbon isotope anomaly associated with the LJE is largely constrained to sediment deposited in shallow and coastal marine environments.

Long-term evolution of terrestrial weathering and its link to Earth's oxygenation

Terrestrial weathering releases phosphorus and other essential nutrients that fuel life in Earth's surface environments and sustain oxygenic photosynthesis. Despite previous suggestions that major changes in terrestrial chemical weathering might have played a role in the global oxygen cycle in the geological past, little is known about the Earth's weathering history. To date, the cause-and-effect relationship between weathering and the long-term evolution of atmospheric oxygen, and whether chemical weathering became more efficient after the initial rise of atmospheric oxygen in the early Paleoproterozoic, remained largely elusive. Here we report a reconstruction of the intensity of terrestrial weathering for the last 2.7 billion years, based on coupled neodymium-hafnium isotope (ΔεHf(i)CLAY) and elemental analyses of the fine-grained clay-size fraction of shales. A pronounced shift towards higher ΔεHf(i)CLAY values and rubidium/aluminium (Rb/Al) ratios indicates that preferential dissolution of phosphate-bearing minerals and biotite intensified between ∼2.5 and 2.4 billion years ago (Ga), following the emergence of continental landmasses and coinciding with the initiation of the Great Oxidation Event. After a long time interval characterized by a constant degree of low-intensity chemical weathering, between ∼2.3 to 0.7 Ga, terrestrial weathering further accelerated after the Neoproterozoic glaciations at ∼0.6 Ga, as inferred from markedly decreased Rb/Al ratios, coincident with the second rise of atmospheric oxygen. These findings support a link between the long-term intensity of chemical weathering and atmospheric oxygen level since the late Archean. We further propose that the 100-million-year-long period of enhanced terrestrial weathering from ∼2.5 Ga played a major role, via crustal recycling of phosphorus and export to the surface ocean, in the early expansion of the aerobic biosphere that ultimately led to the Great Oxidation Event.

Isotopic reconstruction of iron oxidation-reduction process based on an Archean Ocean analogue

The chemical composition and redox conditions of the Precambrian ocean are key factors for reconstructing the temporal evolution of atmospheric oxygen through time. In particular, the isotopic composition of iron are useful proxies for reconstructing paleo-ocean environments. Yet, respective processes and related signatures are poorly constrained, hindering the reconstruction of iron redox mechanisms in the Archean ocean. This study centers on Sihailongwan Lake, a stratified water body with a euxinic lower water column considered as an Archean ocean analogue. Results show that the anaerobic oxidation layer is so different from other similar lakes in which dissolved Fe oxidation is present in redoxcline layer. And the fractionation factor between ferrous Fe and iron hydroxide observed in nature water body of Sihailongwan Lake reaches to 2.6‰, which would benefit the production of the oxidations of BIF in sediment. By the spatial distribution of Fe isotope, the benthic water in autumn and the hypolimnetic anoxic water in spring has been identified as iron sulfide zone, where iron isotopic fractionation factor during iron sulfide formation is 1.16‰, accounting for partial scavenging of dissolved Fe(II) with an associated isotopic fractionation. However, pyrite in the sediment records the iron isotopic signal from the redoxcline but not in the iron sulfide or oxide zones of the water column. Above findings indicate that neither the iron isotope fractionation during partial transfer of ferrous iron to iron sulfide nor the partial oxidation of ferrous iron are recorded as pyrite in sedimentary rock. Importantly, the signal of Fe isotopic fractionation in water was archived in the suspended particulate matter and transferred into the sediment, rather than via ferrous iron directly deposited in the sediment. This study reveals that Fe isotopes from modern natural environments are useful proxies for reconstructing iron oxidation-reduction process during Earth's early history.

REE redistributions during granite weathering: Implications for Ce anomaly as a proxy for paleoredox states

Abstract Different responses of Ce to the redox state from those of the other light rare earth elements (LREEs) can be used to understand paleoredox states. To establish the possibility of using the Ce anomaly as a proxy for paleo-environments, we examined the mineralogical and chemical characteristics of bulk samples and REE-bearing minerals of a modern weathering profile developed on granite, by X-ray fluorescence analysis, laser-ablation inductively coupled plasma mass spectrometry, field emission electron microprobe analysis, field emission scanning electron microscopy, and X-ray diffractometry. Bulk samples showed no significant Ce-anomalies except for the topmost layer that had a positive Ce-anomaly reflecting significant loss of LREEs except for Ce. Allanite-(Ce), primary REE-bearing mineral, contributed to ~100% of La, Ce, Pr, and Nd in the parent rock, and gradually decreased in amount toward the topmost layer. Secondary cerianite-(Ce) [Ce(IV)O2] was observed in the weathering profile, especially at shallower depths. Secondary rhabdophane-(La), -(Ce), -(Nd), and -(Y) were also observed in the weathering profile but in less amounts in the topmost layer. The occurrences of rhabdophane-(La) and -(Nd) in contact with halloysite, a secondary clay mineral, suggest probable adsorption of REEs onto halloysite prior to their formation. Similar formation mechanisms are likely for rhabdophane-(Ce) that commonly occurred in grain boundaries and was usually formed in contact with halloysite. Rhabdophane-(Y) occurred in association with fluorapatite. The ratios of La, Pr, and Nd of rhabdophane-(La), -(Ce), and -(Nd) were similar to that of allanite-(Ce), suggesting that these LREEs are inherited from allanite-(Ce) and behave similarly before the formation of rhabdophane. Different negative Ce-anomaly values of rhabdophane [i.e., ~0.03–0.34 for rhabdophane-(La), -(Nd), and -(Y), and ~0.6 for rhabdophane-(Ce)] can result from a difference in intensity of the formation of cerianite-(Ce) prior to the precipitation of rhabdophane. We have classified LREE redistributions in both secondary minerals and bulk weathered samples during oxic weathering and suggested that Ce anomaly can provide useful information on anoxic weathering and thus atmospheric oxygen evolution in the Precambrian if Ce anomalies of both bulk samples and secondary REE-bearing minerals are determined.

Behaviors of trace elements in Neoarchean and Paleoproterozoic paleosols: Implications for atmospheric oxygen evolution and continental oxidative weathering

The behaviors of redox-sensitive and/or bio-essential trace elements in Neoarchean and Paleoproterozoic paleosols (ancient weathering profiles) were investigated to better understand atmospheric oxygen evolution. The loss or retention of individual trace elements in the paleosols can show how continental oxidative weathering, and thus atmospheric oxygen evolution, took place against age mainly due to their various redox potentials. The V, Cr, Ni, Cu, Zn and Mo concentrations of two Paleoproterozoic paleosols were measured by inductively coupled plasma optical emission spectrometry and mass spectrometry, and those, as well as Co, W and U concentrations, of nine Neoarchean and Paleoproterozoic paleosols were obtained from the literature. The trace element behaviors were constrained by their degrees of loss or retention in the paleosols. We applied two methods to the estimation: (i) retention fraction of element M (a mass ratio of element M of paleosol to parent rock using immobile elements such as Ti) and (ii) element-element (in particular, Si-element) correlations at different profile depths of a paleosol.There are two distinct groups in trace element behavior in the Neoarchean and Paleoproterozoic paleosols: Co, Ni, Zn and W were lost from weathering profiles until the early Paleoproterozoic but retained in the middle and late Paleoproterozoic, while V, Cr, Cu, Mo and U were retained in the profiles until the early Paleoproterozoic or slightly later but lost from the profiles in the middle and late Paleoproterozoic. More precisely, the timings of such loss and retention were different between trace elements during the Paleoproterozoic. The characteristics of these changes from retention to loss or from loss to retention indicate that the changes occurred and lasted throughout the Paleoproterozoic. The trace element behaviors, accordingly, suggest that continental weathering became oxidative progressively with age during almost the whole Paleoproterozoic, and thus, atmospheric oxygen increased similarly with age in the same time period but not suddenly in a relatively short term. The loss and retention of the trace elements may infer how they were released from the continent to the ocean against age in the Neoarchean and Paleoproterozoic.

Seeking a geochemical identifier for authigenic carbonate.

Authigenic carbonate was recently invoked as a third major global carbon sink in addition to primary marine carbonate and organic carbon. Distinguishing the two carbonate sinks is fundamental to our understanding of Earth's carbon cycle and its role in regulating the evolution of atmospheric oxygen. Here, using microscale geochemical measurements of carbonates in Early Triassic strata, we show that the growth of authigenic carbonate follows a different trajectory from primary marine carbonate in a cross-plot of uranium concentration and carbon isotope composition. Thus, a combination of the two geochemical variables is able to distinguish between the two carbonate sinks. The temporal distribution of authigenic carbonates in the Early Triassic strata suggests that the increase in the extent of carbonate authigenesis acted as a negative feedback to the elevated atmospheric CO2 concentration.

Rate law of Fe(II) oxidation under low O2 conditions

Despite intensive studies on Fe(II) oxidation kinetics, the oxidation rate law has not been established under low O2 conditions. The importance of Fe(II) oxidation under low O2 conditions has been recently recognized; for instance, the Fe(II)/Fe(III) compositions of paleosols, ancient soils formed by weathering, can produce a quantitative pattern of the atmospheric oxygen increase during the Paleoproterozoic. The effects of partial pressure of atmospheric oxygen (Po2) on the Fe(II) oxidation rate were investigated to establish the Fe(II) oxidation rate – Po2 relationships under low O2 conditions. All oxidation experiments were carried out in a glove box by introducing Ar gas at ∼10−5–∼10−4atm of Po2, pH 7.57–8.09 and 22°C. Luminol chemiluminescence was adopted to measure low Fe(II) concentrations (down to ∼2nM). Combining previous data under higher Po2 conditions (10−3–0.2atm) with the present data, the rate law for Fe(II) oxidation over a wide range of Po2 (10−5–0.2atm) was found to be written as:d[Fe(II)]dt=-k[Fe(II)][O2]x[OH-]2where the exponent of [O2], x, and the rate constant, k, change from x=0.98 (±0.04) and logk=15.46 (±0.06) at ∼6×10−3–0.2atm of Po2 to x=0.58 (±0.02) and logk=13.41 (±0.03) at 10−5–∼6×10−3atm of Po2. The most plausible mechanism that explains the change in x under low O2 conditions is that, instead of O2, oxygen-derived oxidants, H2O2 and to some extent, O2-, dominate the oxidation reactions at <∼10−3atm of Po2. The rate law found in the present study requires us to reconsider distributions of Fe redox species at low Po2 in natural environments, especially in paleoweathering profiles, and may provide a deeper understanding of the evolution of atmospheric oxygen in the Precambrian.

Weathering model for the quantification of atmospheric oxygen evolution during the Paleoproterozoic

A weathering model has been developed to quantify atmospheric oxygen evolution during the Paleoproterozoic. The weathering model calculates the concentrations of Fe2+ dissolved from Fe2+-bearing primary minerals and oxidized Fe3+ out of the dissolved Fe2+ at a given partial pressure of atmospheric oxygen (Po2) during weathering and establishes the relationships between Po2 and ϕ, where ϕ is the ratio of oxidized and then precipitated Fe3+ out of the Fe2+ dissolved from primary minerals to the dissolved Fe2+ in a whole weathering profile. The weathering model considers controlling factors of the redistribution of Fe during weathering, that is, the dissolution rate of Fe2+-bearing primary minerals, the oxidation rate of Fe2+, and the groundwater flow rate. The validity of the model was confirmed by applying the model to the experimental data of olivine dissolution carried out under low O2 conditions. The sensitivity analysis of the model has revealed that the formation time of weathering, the mineral dissolution rate and the diffusion of O2 into a weathering profile have no or slight influence on ϕ, resulting in ∼0, 0 and 0.3 changes in log(Po2) caused by four orders of magnitude change of the formation time, more than 10 orders change of the mineral dissolution rate, and assumed change of the O2 diffusion, respectively. On the other hand, the temperature, the pH and the groundwater flow rate have moderate to large effects on ϕ: 0.6, 1.4 and 1.5 changes in log(Po2) for changes of 5°C in temperature, 0.5 in pH, and one order of magnitude in groundwater flow rate, respectively. Using possible surface temperature, pH and groundwater flow rate estimated from the literature, we calculated the ϕ-Po2 relationships which were then applied to the ϕ values of paleosols (fossil weathering profiles) formed between 2.5 and 1.8Ga. Taking account of the constraints given by the records of mass independent fractionation in sulfur isotopes and other geological proxies (i.e., <∼10−6atm prior to 2.45Ga, >∼10−6atm at 2.32Ga and >∼10−3atm at 2.0Ga), our model implies that the Po2 levels were <10−6atm but >10−9atm at ∼2.46Ga, 10−4.5–10−2atm at ∼2.25–2.0Ga and >10−2atm at ∼1.85Ga. The present weathering model can constrain Po2 levels during the possible Paleoproterozoic hot periods but not those during the glacial periods when temperature was <0°C. It has been found that the Cooper Lake, Pronto/NAN, Gaborone and Drakenstein paleosols with ages of ∼2.5–2.1Ga did not form under extreme conditions such as those in glacial and hot periods but formed under moderate conditions.

Mo isotopes in the Lower Cambrian formation of southern China and its implications on paleo-ocean environment

Over the past decade, as one of nontraditional stable isotopes, Mo isotope has developed rapidly and now become an important geochemical proxy to trace paleo-oceanic and atmospheric evolution through geological history. In this paper, Early Cambrian formations in southern China are investigated. The results indicate that δ97/95Mo values of Early Cambrian seawater may have been larger than 1.4%, values that are close to those of the modern ocean. It was also found that the variations in Mo isotope composition in samples from two sections (Huangjiawan and Gezhongwu in Guizhou) were closely related to changes in redox conditions during sedimentary processes. Combining our results with existing data, a preliminary model for the evolution of seawater Mo isotope composition through geological history was provided. It indicated that Mo isotopic variations were generally consistent with the evolution of atmospheric oxygen.

Formation of rare earth phosphate minerals in 2.45-Ga paleosol

In order to understand whether Ce in rare earth (RE) phosphate minerals can be used as a proxy for the estimation of atmospheric oxygen evolution in the Precambrian, we examined the formation mechanisms of RE phosphate minerals in 2.45-Ga paleosol (ancient soil formed by weathering), Pronto, Canada. The RE phosphate minerals were observed and analyzed by scanning electron microscopy with energy-dispersive X-ray spectroscopy and electron back-scattered diffraction analysis, and by transmission electron microscopy. The textures and chemistries of RE phosphate minerals have revealed that monazite-[light RE elements (REE)] and xenotime-(Y) at the rims of apatite and pores are secondary products. Monazite-(light REE) and xenotime-(Y) were found to have been formed by either of two mechanisms: the direct formation during metasomatism occurring after weathering or the formation of rhabdophane containing light REE and Y during weathering and its subsequent decomposition into monazite-(light REE) and xenotime-(Y) during metasomatism. If the latter was the actual reaction pathway, the Ce contents in the secondary monazite suggest a very low atmospheric oxygen level at ∼ 2.45 Ga.

Early evolution of atmospheric oxygen from multiple-sulfur and carbon isotope records of the 2.9 Ga Mozaan Group of the Pongola Supergroup, Southern Africa

Sulfur isotope mass-independent fractionation (S-MIF) is a unique geologic record of Archean atmospheric chemistry that provides important constraints on the evolution of the early Earth’s atmosphere and its impact on early life. In this contribution, we report multiple-sulfur (33S/32S, 34S/32S, and 36S/32S) isotope ratios of sulfide minerals and carbon (13C/12C) isotope ratios of organic carbon for shale in the ~2.96 to ~2.84 Ga Mozaan Group of the Pongola Supergroup, Southern Africa. The δ13C of organic carbon shows two populations: one with δ13C of ~ −26 ‰ and another with δ13C of −32 ‰. The Δ33S values from nine samples ranges from −0.49 to +0.36 ‰, which is considerably smaller than what was measured for the sulfide and sulfate minerals from other Archean intervals but outside the range of Δ33S values measured for post-2.0 Ga sulfide and sulfate minerals. Moreover, some samples from the Mozaan Group yield Δ36S/Δ33S ratios that are different from Phanerozoic sulfides, suggesting sulfide sulfur from the Mozaan Group carries mass-independent isotope fractionation originated from atmospheric photochemistry. The relatively small Δ33S values for the Mozaan Group may suggest that the atmosphere became slightly oxidized at ~2.9 Ga with oxygen level above 10−5 but below 10−2 times present atmospheric level. This intermediate oxygen level would allow production of S-MIF in atmospheric chemistry but prohibit preservation of large S-MIF signatures in surface deposits. Our hypothesis implies the evolution of oxygenic photosynthesis as early as ~2.9 Ga. Such an ephemeral oxidation event could have triggered the Mozaan-Witwatersrand glaciation by destabilizing an existing methane-rich Archean atmosphere.

THE EARLY HISTORY OF ATMOSPHERIC OXYGEN: Homage to Robert M. Garrels

▪ Abstract This paper reviews the Precambrian history of atmospheric oxygen, beginning with a brief discussion of the possible nature and magnitude of life before the evolution of oxygenic photosynthesis. This is followed by a summary of the various lines of evidence constraining oxygen levels through time, resulting in a suggested history of atmospheric oxygen concentrations. Also reviewed are the various processes regulating oxygen concentrations, and several models of Precambrian oxygen evolution are presented. A sparse geologic record, combined with uncertainties as to its interpretation, yields only a fragmentary and imprecise reading of atmospheric oxygen evolution. Nevertheless, oxygen levels have increased through time, but not monotonically, with major and fascinating swings to both lower and higher levels.

Atmospheric oxygen evolution anticipated from behavior of light rare earths

Atmospheric oxygen evolution anticipated from behavior of light rare earths

Emerging views on the evolution of atmospheric oxygen during the Precambrian

The oxygenation of the atmosphere produced some irreversible changes in the Earth's history, including evolution of higher biological forms. Several aspects of this important process, such as its timing and causes, have remained subjects of debate. The present review is an attempt to provide an update on issues related to the evolution of atmospheric oxygen during the Precambrian. It is generally believed that the amount of atmospheric oxygen increased during the Paleoproterozoic despite the fact that photosynthesis originated much earlier in the Earth's history. The pattern of Fe retention in paleosols and the record of mass-independent fractionation in sulfur isotopes confirm that the transition to more oxidizing conditions took place during the Paleoproterozoic. Various mechanisms, ranging from an increase in the sources of oxygen to a decrease in its sinks, have been envisaged as processes causing the oxygen rise during the Paleoproterozoic. Conventionally, it is believed that the burial of photosynthetic carbon has allowed the establishment of oxygen. However, the transition in mantle oxidation states and the escape of hydrogen during photolysis of methane of biogenic origin have also been suggested as having played an important role in the establishment of free molecular oxygen in the atmosphere-hydrosphere system. The coincidence of timing of the oxygen rise and the positive excursions in carbon isotope compositions of carbonate rocks during the Paleoproterozoic suggests an important role for the carbon cycle in atmospheric oxygen evolution. Better quantitative modeling of atmospheric oxygen levels may be essential to allow full comprehension of the timing, causes and consequences of the oxygen rise in the Earth's history. Quantitative modeling is essential as it can lead to an unraveling of the co-evolutionary nature of the surface environment and the biosphere of the Earth.

A lower limit for atmospheric carbon dioxide levels 3.2 billion years ago.

The quantification of greenhouse gases present in the Archaean atmosphere is critical for understanding the evolution of atmospheric oxygen, surface temperatures and the conditions for life on early Earth. For instance, it has been argued that small changes in the balance between two potential greenhouse gases, carbon dioxide and methane, may have dictated the feedback cycle involving organic haze production and global cooling. Climate models have focused on carbon dioxide as the greenhouse gas responsible for maintaining above-freezing surface temperatures during a time of low solar luminosity. However, the analysis of 2.75-billion-year (Gyr)-old palaeosols--soil samples preserved in the geologic record--have recently provided an upper constraint on atmospheric carbon dioxide levels well below that required in most climate models to prevent the Earth's surface from freezing. This finding prompted many to look towards methane as an additional greenhouse gas to satisfy climate models. Here we use model equilibrium reactions for weathering rinds on 3.2-Gyr-old river gravels to show that the presence of iron-rich carbonate relative to common clay minerals requires a minimum partial pressure of carbon dioxide several times higher than present-day values. Unless actual carbon dioxide levels were considerably greater than this, climate models predict that additional greenhouse gases would still need to have a role in maintaining above-freezing surface temperatures.

Iron oxidation state of a 2.45-Byr-old paleosol developed on mafic volcanics

The 2.45-Byr-old weathering profile developed on early Proterozoic mafic volcanics located near Cooper Lake, Ontario, Canada, was examined geochemically and mineralogically for a better understanding of the atmospheric oxygen evolution. Ferrous to ferric ion, Fe(II) and Fe(III), respectively, ratios of the bulk rock samples were analyzed by Mössbauer spectrometry. The total Fe (Fe(T)) and Fe(II) concentrations decrease from 12.0 and 11.2 wt.% to 1.85 and 0.89 wt.%, respectively, from the bottom to the top of the weathering profile. The Fe(T) and Fe(II) concentrations normalized to Ti and Zr, as well as the Fe(II)/Fe(III) ratio of raw data, linearly decrease with depth toward the top, while the Fe(III) concentration remains nearly constant throughout the profile. The linear decrease of Fe(II), accompanied by the nearly constant distribution of Fe(III), is difficult to be explained by the scenario of oxidizing weathering and subsequent reducing hydrothermal alteration. The behaviors of Fe(II) and Fe(III) can be simply explained by anoxic weathering. The anoxic weathering suggests that the 2.45-Ga atmosphere was anoxic. The slight increase of Fe/(Fe+Mg) in the octahedral sites of chlorite toward the top and no Ce anomaly in the REE patterns are also consistent with anoxic weathering.

Evidence for low or no oxygen in the late Archean atmosphere from the ∼2.76 Ga Mt. Roe #2 paleosol, Western Australia: Part 3

This paper is a contribution to the ongoing controversy regarding the evolution of atmospheric oxygen. The history of the oxidation state of paleosols can help to resolve the controversy, because the distribution of redox-sensitive elements in soils has been strongly influenced by the oxygen content of the ambient atmosphere, particularly before the advent of the higher land plants.The distribution of the major and minor elements in a section through the 2.76 Ga Mt. Roe #2 paleosol seems to have been unaffected by the presence of oxygen, and indicates that this soil developed under an atmosphere that contained very little or no oxygen.

The evolution of the ribonucleotide reductases: much ado about oxygen.

Ribonucleotide reduction is the only known biological means for de novo production of deoxyribonucleotides, the building blocks of DNA. These are produced from ribonucleotides, the building blocks of RNA, and the direction of this reaction has been taken to support the idea that, in evolution, RNA preceded DNA as genetic material. However, an understanding of the evolutionary relationships among the three modern-day classes of ribonucleotide reductase and how the first reductase arose early in evolution is still far off. We propose that the diversification of this class of enzymes is inherently tied to microbial colonization of aerobic and anaerobic niches. The work is of broader interest, as it also sheds light on the process of adaptation to oxygenic environments consequent to the evolution of atmospheric oxygen.

Detrital and Hydrothermal Origins for Quartz, Pyrite, and Uraninite in Au-U-rich Conglomerates of the Witwatersrand Gold Field, South Africa

Quartz conglomerate beds in the Witwatersrand gold field, South Africa, ranging from 3.0 Ga to 2.7 Ga in age, host many ore deposits containing gold, uraninite and pyrite. These minerals have been considered by most previous workers as detrital in origin (e.g. Viljoen et al., 1970). This 'model' for the origin of these minerals has been used as important evidence for a reduced atmosphere prior to 2.2 Ga because uraninite and pyrite are unstable under oxic conditions (e.g. Holland, 1984). However, some researchers (e.g. Barnicoat et al., 1996) have proposed hydrothermal theories, in which the pyrite and uraninite, as well as gold, are hydrothermal origin. Furthermore, from an examination of the Fe geochemistry of preand post-2.2 Ga palaeosols, Ohmoto (1996) suggested that the atmosphere was already oxic 3.0 Ga ago. Because Ohmoto's suggestion contradicts the currently accepted theory for the evolution of atmospheric oxygen, this study was initiated to examine the 'evidence' for detrital origin of uraninite, gold, and pyrite in these deposits.