The advent of oxygenic photosynthesis set the stage for the evolution of complex life on an oxygenated planet, but it is unknown when this transformative biochemistry emerged. The existing hydrocarbon biomarker record requires that oxygenic photosynthesis and eukaryotes emerged more than 300 million years before the Great Oxidation Event [∼2.4 billion years ago (Ga)]. We report that hopane and sterane concentrations measured in new ultraclean Archean drill cores from Australia are comparable to blank concentrations, yet their concentrations in the exteriors of conventionally collected cores of stratigraphic equivalence exceed blank concentrations by more than an order of magnitude due to surficial contamination. Consequently, previous hydrocarbon biomarker reports no longer provide valid evidence for the advent of oxygenic photosynthesis and eukaryotes by ∼2.7 Ga.
High methane concentrations are thought to have helped sustain warm surface temperatures on the early Earth (~3 billion years ago) when the sun was only 80% as luminous as today. However, radiative transfer calculations with updated spectral data show that methane is a stronger absorber of solar radiation than previously thought. In this paper we show that the increased solar absorption causes a redcution in the warming ability of methane in the Archaean atmosphere.
We test this idea with Se data from the 2.5 Ga Mount McRae Shale (Hamersley Basin, Australia), which records temporary enrichments in abundances and isotopes of other redox-sensitive elements indicating a “whiff of oxygen” in Earth’s atmosphere before the Great Oxidation Event. Se isotopic ratios expressed as δ82/78Se and abundances relative to crustal background show significant positive excursions of up to 1.1‰ and an enrichment 13 times above background, respectively, overlapping with excursions in Mo and N isotopes and abundances. Because Se has a relatively high redox potential and photosynthetic oxidation pathways are unknown, our data thus suggest that Se was mobilized by free O2 during this interval. The isotopic fractionation likely occurred during transport of Se oxyanions from the site of weathering to the outer shelf.
Here we propose that NH3 volatilization is largely responsible for δ15N values of up to +50% at high C/N ratios in the late Archean Tumbiana Formation. This sequence of sedimentary rocks represents a system of lakes that formed on subaerial flood basalts and were partly filled by basaltic volcanic ash. Aqueous alteration of volcanic glass followed by evaporative concentration of ions should have led to the development of high alkalinity with a pH of 9 or higher, as in modern analogues. In this sedimentologically unusual setting, nitrogen isotope ratios thus provide indirect evidence for the oldest alkaline lake system in the rock record. These very heavy lacustrine δ15N values contrast markedly with those of Archean marine sedimentary rocks, making a Precambrian “soda ocean” unlikely. Today, alkaline lakes are among the most productive ecosystems on Earth. Some nutrients, in particular molybdenum, are more soluble at high pH, and certain prebiotic reactions would likely have been favored under alkaline conditions in similar settings earlier in Earth’s history. Hence alkaline lakes in the Archean could have been significant for the origin and early evolution of life.
Here we present nitrogen isotope ratios with a mean of 0.0 ± 1.2‰ from marine and fluvial sedimentary rocks of prehnite-pumpellyite to greenschist metamorphic grade between 3.2 and 2.75 billion years ago. These data cannot readily be explained by abiotic processes and therefore suggest biological nitrogen fixation, most probably using molybdenum-based nitrogenase as opposed to other variants that impart significant negative fractionations. Our data place a minimum age constraint of 3.2 billion years on the origin of biological nitrogen fixation and suggest that molybdenum was bioavailable in the mid-Archaean ocean long before the Great Oxidation Event.
We propose several changes to the method, the most important of which is increasing the maximum possible drop size from to be consistent with new large datasets of raindrop observations. With these changes, our upper bound on modern surface density becomes , a valid limit. The upper bound on Archean atmospheric density is then revised to . In general, we find that raindrop imprint size distribution depends much more strongly on rainfall rate than atmospheric density, which translates into large errors. At best, the precision of raindrop palaeopycnometry will be a factor of a few to an order of magnitude.
What fascinates people about astrobiology is that it seeks answers to long-standing unsolved questions: How quickly did life evolve on Earth and why did life persist here? Is there life elsewhere in the Solar System or beyond? Astrobiology: A Very Short Introduction explores some of the big unanswered questions about the universe, considers the origins of life on Earth and its evolution, and brings together the ideas of microbiologists, astronomers, planetary scientists, and geologists. It introduces the origins of astrobiology and demonstrates its impact on current astronomical research and…
The Archean atmosphere was likely a weakly reduced mixture composed predominantly of N2 and CO2, with smaller concentrations of H2, CO, and CH4. Both CO2 and N2 may have been present in abundances exceeding today’s values, by a factor of 2 or more for N2 and by factors of 100 or more for CO2. Published upper limits on CO2 from paleosols and banded iron formations are probably invalid; hence, CO2 could have been the dominant greenhouse gas that compensated for the fainter young Sun. The Archean greenhouse effect was likely supplemented by CH4, which could have risen to levels of 1000 ppmv or more once methanogens had evolved. Warming by CH4 was limited to approximately 10–12°, however, by formation of organic haze. The key to analyzing Archean atmospheric composition is to understand the hydrogen budget of the atmosphere in which outgassing of H2 and other reduced gases from volcanoes was balanced by loss of hydrogen to space and burial of organic carbon in sediments. The mixing ratio of O2 in such a weakly reduced atmosphere would have been extremely low, roughly 10− 13 at the surface, increasing to ~ 10− 3 in the upper stratosphere. A rise in O2 just after the end of the Archean may have eliminated the methane greenhouse and triggered the Paleoproterozoic glaciations.
Here we argue that the second condition was the oxidation of the surface and crust to the point where O2 became more stable than competing reduced gases such as CH4. The cause of Earth’s surface oxidation would be the same cause as it is for other planets with oxidized surfaces: hydrogen escape to space. The duration of the interregnum would have been determined by the rate of hydrogen escape and by the size of the reduced reservoir that needed to be oxidized before O2 became favored. We suggest that continental growth has been influenced by hydrogen escape, and we speculate that, if there must be an external bias to biological evolution, hydrogen escape can be that bias.
Detection of antineutrinos from U and Th series decay within the Earth (geoneutrinos) constrains the absolute abundance of these elements. Marine detectors will measure the ratio over the mantle beneath the site and provide spatial averaging. The measured mantle Th/U may well be significantly below its bulk earth value of ~4; Pb isotope measurements on mantle-derived rocks yield low Th/U values, effectively averaged over geological time. The physics of the modern biological process is complicated, but the net effect is that much of the U in the mantle comes from subducted marine sediments and subducted upper oceanic crust. That is, U subducts preferentially relative to Th. Oxygen ultimately from photosynthesis oxidizes U(IV) to U(VI), which is soluble during weathering and sediment transport.
The Great Oxidation Event (GOE) was an increase in atmospheric oxygen levels from less than 1 ppm to 0.2–2% by volume 2.4–2.2 billion years ago. In atmospheric chemistry, hydrogen-bearing reduced gases, such as methane and hydrogen, are adversaries of O2. When the concentration of hydrogen-bearing reduced gases goes up, O2 declines, and vice versa. Thus, the pre-GOE atmospheric redox chemistry should have been dominated by methane and hydrogen. Before the GOE, oxygen was driven to trace levels by reactions with volcanic and metamorphic reductants, including dissolved cations (e.g., Fe2 +) in surface waters and reducing gases (H2, CH4, CO, SO2, and H2S). Rapid escape of hydrogen to space from such an atmosphere would have slowly oxidized the Earth. A ‘tipping point’ was reached when the flux of O2 associated with the burial of organic carbon exceeded O2 losses. Oxidative weathering then became significant. Models suggest that methane level fell before the GOE and such loss of greenhouse gases plausibly caused global cooling. Multiple glaciations during 2.45–2.22 Ga hint that the climate and atmospheric composition oscillated until permanent oxygenation was established. Subsequent levels of O2 were sufficient to protect the Earth’s surface from harmful ultraviolet with a stratospheric ozone layer.
Here, we present a study of the four sulfur isotopes obtained using secondary ion MS that seeks to reconcile a number of features seen in the Neoarchean sulfur isotope record. We suggest that Neoarchean ocean basins had two coexisting, significantly sized sulfur pools and that the pathways forming pyrite precursors played an important role in establishing how the isotopic characteristics of each of these pools was transferred to the sedimentary rock record. One of these pools is suggested to be a soluble (sulfate) pool, and the other pool (atmospherically derived elemental sulfur) is suggested to be largely insoluble and unreactive until it reacts with hydrogen sulfide. We suggest that the relative contributions of these pools to the formation of pyrite depend on both the accumulation of the insoluble pool and the rate of sulfide production in the pyrite-forming environments. We also suggest that the existence of a significant nonsulfate pool of reactive sulfur has masked isotopic evidence for the widespread activity of sulfate reducers in the rock record.
We present measurements of three different organic-rich shales of varying ages prepared with eight different sample preparation protocols and identify a method with which high selenium yields are obtained for all three samples while the concentration of germanium is greatly reduced. We further investigate the quantitative importance of isobaric interferences and present new post-analytical data correction protocols. If selenium concentrations in standards and samples are matched to within 5%, the ratios of five isotopes of selenium (74Se, 76Se, 77Se, 78Se and 82Se) can be measured with precisions better than 0.2‰ for δ76/78Se, δ77/78Se and δ82/78Se and 0.5‰ for δ74/78Se, allowing analytical accuracy to be monitored with three-isotope diagrams and thus enabling the detection of any mass-independent isotopic fractionation.
Here, we report the occurrence of phosphite in early Archean marine carbonates at levels indicating that this was an abundant dissolved species in the ocean before 3.5 Ga. Additionally, we show that schreibersite readily reacts with an aqueous solution of glycerol to generate phosphite and the membrane biomolecule glycerol–phosphate under mild thermal conditions, with this synthesis using a mineral source of P. Phosphite derived from schreibersite was, hence, a plausible reagent in the prebiotic synthesis of phosphorylated biomolecules and was also present on the early Earth in quantities large enough to have affected the redox state of P in the ocean. Phosphorylated biomolecules like RNA may, thus, have first formed from the reaction of reduced P species with the prebiotic organic milieu on the early Earth.
We present a new dynamic thresholding method for computationally separating amygdules from their basaltic matrix in X-ray images that is based on a technique used in seismology. The technique is sensitive to the gradient of the gray-scale value, rather than an absolute threshold value often applied to an entire set of X-ray images. Additionally, we present statistical methods for extrapolating the volumetric measurement mean and standard deviation of amygdules in the measured samples to the entire population in the flow. To do so, we create additional amygdule sample sets from the original sample set in the process of ‘bootstrap’ resampling, and use the Central Limit Theorem to calculate the mean and standard deviation of the amygdule population from these sample sets. This suite of methods allows the extension of bubble-size distribution studies typically done on modern flows to the ancient rock record and potentially has many other uses in geosciences where quantitative discrimination between materials with a range of densities is required.
We present multiple sulfur isotopes for 2.71 Ga pyritic black shales derived from the Kidd Creek area, Ontario, Canada. These samples display high positive Δ33S values up to 3.8‰ and the typical late Archean slope in Δ36S/Δ33S of −0.9. In contrast, the time period before (3.2–2.73 Ga) is characterized by greatly attenuated MIF-S magnitudes and a slope in Δ36S/Δ33S of −1.5. We attribute the increase in Δ33S magnitude as well as the contemporaneous change in the slope of Δ36S/Δ33S to changes in the relative reaction rate of different MIF-S source reactions and changes in atmospheric sulfur exit channels. Both of these are dependent on atmospheric CH4:CO2 and O2 mixing ratios. We propose a distinct change in atmospheric composition at 2.7 Ga resulting from increased fluxes of oxygen and methane as the best explanation for the observed Neoarchean MIF-S record. Our data and modeling results suggest that oxygenic photosynthesis was a major contributor to primary productivity 2.7 billion years ago.
Why do some gases cause greenhouse warming, whereas others do not? H2O is a greenhouse gas because it has a permanent electric dipole moment (a charge separation within the molecule) that allows it to interact strongly with electromagnetic radiation. CO2 also has an electric dipole moment, but it has to bend or stretch asymmetrically to create it, because, unlike H2O, it is a linear molecule. N2 and O2 are not normally considered to be greenhouse gases, because these symmetric, diatomic molecules have no electric dipole moment and cannot bend or stretch to create one. But as Wordsworth and Pierrehumbert show on page 64 of this issue (1), N2 and molecular hydrogen (H2) can be greenhouse gases under the right conditions; H2 may have been important for Earth’s Archean climate (before 2.5 billion years ago).
It is commonly assumed that the building blocks of the terrestrial planets were derived from a cosmochemical reservoir that is best represented by chondrites, the so-called chondritic Earth model. This view is possibly a good approximation for refractory elements (although it has been recently questioned; e.g., Caro et al. 2008), but for volatile elements, other cosmochemical reservoirs might have contributed to Earth, such as the solar nebula gas and/or cometary matter (Owen et al. 1992; Dauphas 2003; Pepin 2006). Hence, in order to get insights into the origin of the carbon in Earth, it is…
We find that terrestrial oxidation of pyrite by microbes using oxygen has contributed a substantial fraction of the total sulphur weathering flux since at least 2.5 Gyr ago, with probable evidence of such activity 2.7–2.8 Gyr ago. The late Archaean onset of terrestrial sulphur cycling is supported by marine molybdenum abundance data and coincides with a shift to more sulphidic ocean conditions5. We infer that significant microbial land colonization began by 2.7–2.8 Gyr ago. Our identification of pyrite oxidation at this time provides further support for the appearance6 of molecular oxygen several hundred million years before the Great Oxidation Event.
Oxygenic photosynthesis appears to have evolved well before O2 levels increased in the atmosphere, at around 2.4 Ga. This has led to numerous suggestions as to what may have kept O2 suppressed and then eventually allowed it to rise. These suggestions include changes in the recycling of carbon and sulfur relative to water (or hydrogen), a switch from dominantly submarine to dominantly subaerial volcanism, gradual oxidation of the continents and a concomitant decrease in reduced metamorphic gases, a decline in deposition of banded iron-formations, a decline in nickel availability, and various proposals to increase the efficiency of photosynthesis. Several of these different mechanisms could have contributed to the rise of O2, although not all of them are equally effective. To be considered successful, any proposed mechanism must make predictions that are consistent with the carbon isotope record in marine carbonates, which shows relatively little change with time, apart from transient (but occasionally spectacular) excursions. The reasons for this constancy are explored here, but are not fully resolved. In the process of making these comparisons, a self-consistent redox balance framework is developed which will hopefully prove useful to others who may work on this problem and to astronomers who may one day try to decipher spectral signatures of oxygen on Earth-like exoplanets.
We find evidence for oxygen production in microbial mats and localized oxygenation of surface waters. Carbon and sulphur isotopes indicate that this oxygen production occurred under a reduced atmosphere that was periodically rich in methane, consistent with the prediction of a hydrocarbon haze. We use a photochemical model to corroborate our geochemical data. Our simulations predict transitions between two stable atmospheric states, one with organic haze and the other haze-free. The transitions are presumably governed by variations in the amount of biological methane production during the Archaean eon. We find that the isotopic signatures we observe are evident in other data sets from this period and conclude that methane was an important component of the atmosphere throughout the Archaean.
Here we show that raindrop imprints in tuffs of the Ventersdorp Supergroup, South Africa, constrain surface air density 2.7 billion years ago to less than twice modern levels. We interpret the raindrop fossils using experiments in which water droplets of known size fall at terminal velocity into fresh and weathered volcanic ash, thus defining a relationship between imprint size and raindrop impact momentum. Fragmentation following raindrop flattening limits raindrop size to a maximum value independent of air density, whereas raindrop terminal velocity varies as the inverse of the square root of air density. If the Archaean raindrops reached the modern maximum measured size, air density must have been less than 2.3 kg m(-3), compared to today’s 1.2 kg m(-3), but because such drops rarely occur, air density was more probably below 1.3 kg m(-3). The upper estimate for air density renders the pressure broadening explanation possible, but it is improbable under the likely lower estimates. Our results also disallow the extreme CO(2) levels required for hot Archaean climates.
Here we show that when this transport-determined limitation is incorporated into the COPSE biogeochemical model6, the stabilization time is substantially longer, >107 years. When we include a simple ice-albedo feedback, the model produces greenhouse–icehouse oscillations on this timescale that are compatible with observations. Our simulations also indicate positive carbon isotope excursions and an increased flux of oxygen to the atmosphere during interglacials, both of which are consistent with the geological record7,8. We conclude that the long gaps between snowball glaciations can be explained by limitations on silicate weathering rates.
The Sun was fainter when the Earth was young, but the climate was generally at least as warm as today; this is known as the ‘faint young Sun paradox’. Rosing et al.1 claim that the paradox can be resolved by making the early Earth’s clouds and surface less reflective. We show that, even with the strongest plausible assumptions, reducing cloud and surface albedos falls short by a factor of two of resolving the paradox. A temperate Archean climate cannot be reconciled with the low level of CO2 suggested by Rosing et al.1; a stronger greenhouse effect is needed.