Archaean Atmosphere Research Articles

Bioavailable nitrogen is thought to be a requirement for the origin and sustenance of life. Before the onset of biological nitrogen fixation, abiotic pathways to fix atmospheric N2 must have been prominent to provide bioavailable nitrogen to Earth’s earliest ecosystems. Lightning has been shown to produce fixed nitrogen as nitrite and nitrate in both modern atmospheres dominated by N2 and O2 and atmospheres dominated by N2 and CO2 analogous to the Archaean Earth. However, a better understanding of the isotopic fingerprints of lightning-generated fixed nitrogen is needed to assess the role of this process on early Earth. Here we present results from spark discharge experiments in N2−CO2 and N2−O2 gas mixtures. Our experiments suggest that lightning-driven nitrogen fixation may have been similarly efficient in the Archaean atmosphere, compared with modern times. Measurements of the isotopic ratio (δ15N) of the discharge-produced nitrite and nitrate in solution show very low values of −6‰ to −15‰ after equilibration with the gas phase with a calculated endmember composition of −17‰. These results are much lower than most δ15N values documented from the sedimentary rock record, which supports the development of biological nitrogen fixation earlier than 3.2 billion years ago. However, some Paleoarchean records (3.7 billion years ago) may be consistent with lightning-derived nitrogen input, highlighting the potential role of this process for the earliest ecosystems. Spark discharge experiments suggest lightning was not the main source of bioavailable nitrogen for the established Archaean biosphere, but could have been significant for Earth’s earliest ecosystems.

The Archaean atmosphere may have been well oxygenated, according to a reconsideration of sulfur cycling at that time. This challenges the view that sedimentary sulfur records oxygen-poor conditions during Earth’s first two billion years.

The exchange of volatile species-water, carbon dioxide, nitrogen and halogens-between the mantle and the surface of the Earth has been a key driver of environmental changes throughout Earth's history. Degassing of the mantle requires partial melting and is therefore linked to mantle convection, whose regime and vigour in the Earth's distant past remain poorly constrained1,2. Here we present direct geochemical constraints on the flux of volatiles from the mantle. Atmospheric xenon has a monoisotopic excess of 129Xe, produced by the decay of extinct 129I. This excess was mainly acquired during Earth's formation and early evolution3, but mantle degassing has also contributed 129Xe to the atmosphere through geological time. Atmospheric xenon trapped in samples from the Archaean eon shows a slight depletion of 129Xe relative to the modern composition4,5, which tends to disappear in more recent samples5,6. To reconcile this deficit in the Archaean atmosphere by mantle degassing would require the degassing rate of Earth at the end of the Archaean to be at least one order of magnitude higher than today. We demonstrate that such an intense activity could not have occurred within a plate tectonics regime. The most likely scenario is a relatively short (about 300million years) burst of mantle activity at the end of the Archaean (around 2.5 billion years ago). This lends credence to models advocating a magmatic origin for drastic environmental changes during the Neoarchaean era, such as the Great Oxidation Event.

The Great Oxidation Event following the end of the Archaean eon (~2.4 Ga) was a profound turning point in the history of Earth and life, but the relative importance of various contributing factors remains an intriguing puzzle. Controls on methane flux to the atmosphere were of particular consequence; too much methane would have inhibited a persistent rise of O2, but too little may have plunged Earth into severe and prolonged ice ages. Here, we document a shift in the weathering reactions controlling the ocean-bound flux of nickel—an essential micronutrient for the organisms that produced methane in Precambrian oceans—by applying Ni stable isotope analysis to Mesoarchaean and Palaeoproterozoic glacial sediments. Although Ni flux to the ocean dropped dramatically as Ni content of the continental crust decreased, the onset of sulfide weathering delivered a small, but vital, flux of Ni to the oceans, sustaining sufficient methane production to prevent a permanent icehouse, while allowing O2 to rise. Oxidative weathering supplied a crucial flux of nutrients to the late Archaean oceans that sustained methanogenesis and kept the Archaean atmosphere in a methane sweet-spot, according to analyses of nickel isotopes from glacial deposits.

Nitrogen evolution within the Earth's atmosphere–mantle system assessed by recycling in subduction zones

Evolution of homochirality requires an initial enantiomeric excess (EE) between right and left-handed biomolecules. We show that magnetic circular dichroism (MCD) of sun’s ultraviolet C light by oxygen in Archaean earth’s anoxic atmosphere followed by chirally selective damage of biomolecules due to circular dichroism (CD) can generate EE of correct handedness. Our calculation of EE uses published data for CD of biomolecules and accepted magnitude for Archaean earth’s magnetic field. Independent of atmospheric oxygen concentration calculated EE has the same sign for all pyrimidine nucleosides which is opposite to that for amino-acids. Purine nucleosides have smaller EE values with opposite sign to pyrimidines but are less susceptible to UV damage. Homochirality is explained by origin of prebiotic life in one hemisphere of earth and its evolution to EE ~ ± 1 before reversal of terrestrial magnetic field. Chirality of biomolecules is decided by the direction of magnetic field where prebiotic life originated on Archaean earth.

Siderite cannot be used as CO2 sensor for Archaean atmospheres

In his comments on our paper (Depine et al. 2013) on the chemistry of uraninite grains from the auriferous conglomerates of the Mesoarchaean Witwatersrand Supergroup, Oberthur (2013) highlights a number of points, some of which had not been fully discussed by us due to the limitation on space in a short “letter”. We are therefore grateful for the opportunity to expand on some of the implications of our new data in this discussion here. The abundance of major and minor elements in uraninite from the Witwatersrand has been the subject of several previous studies. We could add to this knowledge, as acknowledged by Oberthur (2013), by presenting the first data on trace element concentrations in Witwatersrand uraninite as obtained by LA-ICPMS. The main message we tried to convey in our letter has been that the mineral chemistry of the uraninite grains, now also including a range of elements hitherto not or only poorly analysed for, provides unequivocal evidence of the detrital nature of these grains. We are satisfied, though not surprised, that Oberthur, an expert with livelong experience with the studied rocks, fully concurs with this principal conclusion. For many readers, this conclusion may not be worth receiving further attention after dozens of papers have been published over the past decades on the topic of the genesis of the world’s largest gold province and the U-ores therein, in which a strong case for a modified palaeoplacer model has been made. Since the last review (Frimmel et al. 2005), the discussion on the genesis of the Witwatersrand ores has been, however, by no means closed, and views opposing to those of Oberthur and many other workers have been presented more recently. The papers by Phillips and Powell (2011), who advocate a postdepositional syn-metamorphic introduction of the gold into the host conglomerates, or by Horscroft et al. (2011), who argue for a syngenetic, microbially mediated chemical precipitation of gold, uraninite and pyrite, may serve as examples. We therefore believe that it remains timely and opportune to continue this discussion on the Witwatersrand metallogeny with new data. Oberthur (2013) dismisses any model involving syngenetic or epigenetic-hydrothermal introduction of U into the host conglomerates because of the lack of oxidising conditions under an Archaean atmosphere. We could not agree more but want to point out that this has been a circular argument for a long time. After all, the evidence for a postulated reducing atmosphere in the Archaean has come primarily from the interpretation of the ore components, specifically rounded massive pyrite and uraninite, in the Witwatersrand goldfields (Krupp et al. 1994; Frimmel 2005). Only in the last decade, largely based on the discovery of mass-independent S isotope fractionation in Archaean rocks, independent evidence in favour of an O2-deficient Archaean atmosphere could be brought forward (Farquhar et al. 2007), thus supporting the original contention of the detrital Witwatersrand mineralogy. As effectively all points raised by Oberthur (2013) are discussed in greater detail in a full paper currently under review elsewhere (Frimmel et al. 2013), in which we present Editorial handling: B. Lehmann

Evolutionary mechanisms adopted by the photosynthetic apparatus to modifications in the Earth's atmosphere on a geological time-scale remain a focus of intense research. The photosynthetic machinery has had to cope with continuously changing environmental conditions and particularly with the complex ionizing radiation emitted by solar flares. The photosynthetic D1 protein, being the site of electron tunneling-mediated charge separation and solar energy transduction, is a hot spot for the generation of radiation-induced radical injuries. We explored the possibility to produce D1 variants tolerant to ionizing radiation in Chlamydomonas reinhardtii and clarified the effect of radiation-induced oxidative damage on the photosynthetic proteins evolution. In vitro directed evolution strategies targeted at the D1 protein were adopted to create libraries of chlamydomonas random mutants, subsequently selected by exposures to radical-generating proton or neutron sources. The common trend observed in the D1 aminoacidic substitutions was the replacement of less polar by more polar amino acids. The applied selection pressure forced replacement of residues more sensitive to oxidative damage with less sensitive ones, suggesting that ionizing radiation may have been one of the driving forces in the evolution of the eukaryotic photosynthetic apparatus. A set of the identified aminoacidic substitutions, close to the secondary plastoquinone binding niche and oxygen evolving complex, were introduced by site-directed mutagenesis in un-transformed strains, and their sensitivity to free radicals attack analyzed. Mutants displayed reduced electron transport efficiency in physiological conditions, and increased photosynthetic performance stability and oxygen evolution capacity in stressful high-light conditions. Finally, comparative in silico analyses of D1 aminoacidic sequences of organisms differently located in the evolution chain, revealed a higher ratio of residues more sensitive to oxidative damage in the eukaryotic/cyanobacterial proteins compared to their bacterial orthologs. These results led us to hypothesize an archaean atmosphere less challenging in terms of ionizing radiation than the present one.

Two major geological problems regarding the origin of oxygenic photosynthesis are (i) identifying a source of oxygen pre-dating the biological oxygen production and capable of driving the evolution of oxygen tolerance, and (ii) determining when oxygenic photosynthesis evolved. One solution to the first problem is the accumulation of photochemically produced H(2)O(2) at the surface of the glaciers and its subsequent incorporation into ice. Melting at the glacier base would release H(2)O(2), which interacts with seawater to produce O(2) in an environment shielded from the lethal levels of ultraviolet radiation needed to produce H(2)O(2). Answers to the second problem are controversial and range from 3.8 to 2.2 Gyr ago. A sceptical view, based on the metals that have the redox potentials close to oxygen, argues for the late end of the range. The preponderance of geological evidence suggests little or no oxygen in the Late Archaean atmosphere (less than 1 ppm). The main piece of evidence for an earlier evolution of oxygenic photosynthesis comes from lipid biomarkers. Recent work, however, has shown that 2-methylhopanes, once thought to be unique biomarkers for cyanobacteria, are also produced anaerobically in significant quantities by at least two strains of anoxygenic phototrophs. Sterane biomarkers provide the strongest evidence for a date 2.7 Gyr ago or above, and could also be explained by the common evolutionary pattern of replacing anaerobic enzymes with oxygen-dependent ones. Although no anaerobic sterol synthesis pathway has been identified in the modern biosphere, enzymes that perform the necessary chemistry do exist. This analysis suggests that oxygenic photosynthesis could have evolved close in geological time to the Makganyene Snowball Earth Event and argues for a causal link between the two.

1. The Earth System. Introduction. The Nature of the Early Geological Record. Archaean Lithological Associations. The Oldest Rocks. How Much do we Really Know About the Early Earth?. 2. The Origin and Differentiation of the Earth. The Origin and Early History of the Universe. Star Formation. The Condensation of the Solar System. Earth Differentiation - the First Earth System. 3. The Evolution of the Earth's Mantle. Understanding the Mantle. The Earth's Earliest Mantle. Mantle Models. 4. The Origin of the Continental Crust. Modern Crust Formation - Models and Mechanisms. First Order Constraints on the Origin of the Continental Crust. The Secular Evolution of the Earth's Continental Crust. Crustal Growth During the Archaean. Crust-Mantle Interactions: Reservoirs and Fluxes. 5. The Origin of the Earth's Atmosphere and Oceans. The Volatile Budget of the Modern Earth. The Origin of the Earth's Atmosphere and Oceans. The Nature of the Archaean Atmosphere. The Nature of the Archaean Oceans. 6. The Origin of Life. Setting the Scene for Life. Geochemical Signals of Biological Activity. The Geological Record of Life's Origins. The Microbial Record of Life's Origins. In the Beginning... 7. Postscript. References. Index

Integrated Pb- and S-isotope investigation of sulphide minerals from the early Archaean of southwest Greenland

Archaean atmospheric evolution: evidence from the Witwatersrand gold fields, South Africa

Ohmoto et al. argue that carbon dioxide was abundant in the late Archaean and early Proterozoic atmosphere and that methane was probably scarce, based on a reanalysis of the occurrence of siderite, FeCO3, in ancient rocks. Here I consider several factors that may undermine their conclusions.

Ohmoto et al. reply - The idea of a methane-rich Archaean atmosphere has become popular since Rye et al. assumed in their calculation1 that siderite was absent in pre-2.2-Gyr palaeosols. We have concluded that the absence of siderite in some Archaean palaeosols does not constrain the atmospheric pCO2, but the presence of much siderite in sedimentary rocks does2. Sleep's recognition3 that siderite occurs in Archaean palaeosols substantiates our arguments2: although siderite should be absent in well aerated soils of all geological ages, it may form in waterlogged soils where pO2 became less than about 10−60 atm owing to the abundant anaerobic production of H2. In fact, we have reported this in a 2.6-Gyr soil profile at Schagen, South Africa4: abundant ferric-rich minerals formed while the soil was exposed to air, but ferrous-rich carbonate formed while it was apparently submerged under an anoxic pond.

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.

ABSTRACTBedded carbonate rocks from the 3.45 Ga Warrawoona Group, Pilbara Craton, contain structures that have been regarded either as the oldest known stromatolites or as abiotic hydrothermal deposits. We present new field and petrological observations and high‐precision REE + Y data from the carbonates in order to test the origin of the deposits. Trace element geochemistry from a number of laminated stromatolitic dolomite samples of the c. 3.40 Ga Strelley Pool Chert conclusively shows that they precipitated from anoxic seawater, probably in a very shallow environment consistent with previous sedimentological observations. Edge‐wise conglomerates in troughs between stromatolites and widespread cross‐stratification provide additional evidence of stromatolite construction, at least partly, from layers of particulate sediment, rather than solely from rigid crusts. Accumulation of particulate sediment on steep stromatolite sides in a high‐energy environment suggests organic binding of the surface. Relative and absolute REE + Y contents are exactly comparable with Late Archaean microbial carbonates of widely agreed biological origin. Ankerite from a unit of bedded ankerite–chert couplets from near the top of the stratigraphically older (3.49 Ga) Dresser Formation, which immediately underlies wrinkly stromatolites with small, broad, low‐amplitude domes, also precipitated from anoxic seawater. The REE + Y data of carbonates from the Strelley Pool Chert and Dresser Formation contrast strongly with those from siderite layers in a jasper–siderite–Fe‐chlorite banded iron‐formation from the base of the Panorama Formation (3.45 Ga), which is clearly hydrothermal in origin. The geochemical results, together with sedimentological data, strongly support: (1) deposition of Dresser Formation and Strelley Pool Chert carbonates from Archaean seawater, in part as particulate carbonate sediment; (2) biogenicity of the stromatolitic carbonates; (3) a reducing Archaean atmosphere; (4) ongoing extensive terrestrial erosion prior to ∼3.45 Ga.

Abstract The Hadean Earth (before c. 4 Ga) was abiotic, possibly sterring a bumpy course between brief periods of hot inferno after meteorite impacts, and long episodes of Norse icehell. The earliest Archaean life would probably not have been planet-altering, but restricted to particular habitats. One of the first may have been hot regions around hydrothermal systems where redox contrasts between ocean water and magmatic fluids could be exploited. Molecular evidence suggests that with the evolution of anoxygenic photosynthesis, life became able to occupy wider regions, although focused in the vicinity of hydrothermal systems. Oxygenic photosynthesis by cyanobacteria allowed life fully to occupy the planet, not only forming coastal microbial mats but also possibly inhabiting the broad oceans with abundant photosynthetic bacterial picoplankton, underlain by deeper archaeal picoplankton. In the Belingwe belt, Zimbabwe, textural and isotopic evidence suggests that a complex microbial ecology existed in the late Archaean (2.7 Ga), which was essentially modern in its biochemical abilities and which sequestered into the biosphere the same fraction of primitive carbon emitted from mantle as today. To do this, by the late Archaean the biological productivity must have been significant; not necessarily as large as today, but capable of managing the global carbon budget. When this began is unknown, possibly earlier than 3.5 Ga ago. The controls on the oxidation state of the late Archaean atmosphere—ocean system are not self-evident. Although inorganic controls dominate the long-term balance, short-term biological management of the air may have been crucial. Methane may have played a major role in the pre-metazoan biosphere. The modern atmosphere is a biological construct: oxygen and its reverse, carbon dioxide, are managed by rubisco; nitrogen, its oxides and hydrides mainly by nitrifying and denitrifying bacteria, with a small input from lightning in an oxygen-rich atmosphere; and water (itself the most important greenhouse gas) by its complex interdependence with other greenhouse gases and albedo, including clouds. Earth’s air is highly improbable. In controlling surface temperature a subtle interplay between organic and inorganic controls has operated, perhaps to the extent that it is invalid to ask which was the dominant factor. But there is a reasonable uniformitarian argument that life has constructed the air in the past as now, and that, within the broad constraints of the physical setting, this biologically shaped atmosphere has been the dominant control on the planet’s surface temperature. In turn, the surface temperature has been one of the various controlling factors on the tectonic evolution of the planet. Thus to a significant extent life has helped shape the physical evolution of the planet.

Cindy Ebinger reports on an RAS discussion meeting on 10 and 11 February 2000 that fostered lively debate on the early Earth, from workers in a range of specialisms.

Late-orogenic basins in the Archaean Superior Province, Canada: characteristics and inferences