n anoxic, sulfidic ocean that may have existed during the Proterozoic Eon (0.54–2.4 Ga) would have had limited trace metal abundances because of the low solubility of metal sulfides. The lack of copper, in particular, could have had a significant impact on marine denitrification. Copper is needed for the enzyme that controls the final step of denitrification, from N2O to N2. Today, only about 5–6% of denitrification results in release of N2O. If all denitrification stopped at N2O during the Proterozoic, the N2O flux could have been 15–20 times higher than today, producing N2O concentrations of several ppmv, but only if O2 levels were relatively high (>0.1 PAL).
We used numerical models to study whether such O2 concentrations might have been provided by atmospheric photochemistry. Results show that disproportionation of H2O2 near the surface might have yielded enough O2 to satisfy this constraint. Alternatively, poleward transport of O2 from the equatorial stratosphere into the polar night region, followed by downward transport in the polar vortex, may have brought O2 directly to the surface. Thus, our calculations indicate that this “early respiration” hypothesis might be physically reasonable.
We investigate the role which clouds could play in resolving the Faint Young Sun Paradox (FYSP). Lower solar luminosity in the past means that less energy was absorbed on Earth (a forcing of −50 W m−2 during the late Archean), but geological evidence points to the Earth having been at least as warm as it is today, with only very occasional glaciations. We perform radiative calculations on a single global mean atmospheric column. We select a nominal set of three layered, randomly overlapping clouds, which are both consistent with observed cloud climatologies and reproduced the observed global mean energy budget of Earth.
Despite the importance of the anaerobic oxidation of methane (AOM) to global biogeochemical cycles, the relationship between sulfate concentration and the rate of AOM has not been previously experimentally constrained. Here, we present measurements showing substantial methane oxidation at low sulfate concentrations, with no significant decrease in the rate of AOM until sulfate levels are well below 1 mm. At sulfate levels below 1 mm, there appears to be a strong decoupling of AOM and sulfate reduction, with a 13C‐label transferred from methane to carbon dioxide occurring at a rate almost an order of magnitude faster than the observed rate of sulfate reduction. These results allow for the possibility that high rates of AOM occurred in the Archean oceans and that high rates of AOM may be found in freshwater environments (lakes, rivers, etc.) and deep ocean sediments today.
Because perchlorate‐rich deposits in the Atacama desert are closest in abundance to perchlorate measured at NASA’s Phoenix Lander site, we made a preliminary study of the means to produce Atacama perchlorate to help shed light on the origin of Martian perchlorate. We investigated gas phase pathways using a 1‐D photochemical model. We found that perchlorate can be produced in sufficient quantities to explain the abundance of perchlorate in the Atacama from a proposed gas phase oxidation of chlorine volatiles to perchloric acid. The feasibility of gas phase production for the Atacama provides justification for future investigations of gas phase photochemistry as a possible source for Martian perchlorate.
Here we use a radiative–convective climate model to show that more N2 in the atmosphere would have increased the warming effect of existing greenhouse gases by broadening their absorption lines. With the atmospheric CO2 and CH4 levels estimated for 2.5 billion years ago, a doubling of the present atmospheric nitrogen (PAN) level would cause a warming of 4.4 ∘C. Our new budget of Earth’s geological nitrogen reservoirs indicates that there is a sufficient quantity of nitrogen in the crust (0.5 PAN) and mantle (>1.4 PAN) to have supported this, and that this nitrogen was previously in the atmosphere. In the mantle, N correlates with 40Ar, the daughter product of 40K, indicating that the source of mantle N is subducted crustal rocks in which NH4+ has been substituted for K+. We thus conclude that a higher nitrogen level probably helped warm the early Earth, and suggest that the effects of N2 should be considered in assessing the habitable zone for terrestrial planets.
The nitrogen cycle provides essential nutrients to the biosphere, but its antiquity in modern form is unclear. In a drill core though homogeneous organic-rich shale in the 2.5-billion-year-old Mount McRae Shale, Australia, nitrogen isotope values vary from +1.0 to +7.5 per mil (‰) and back to +2.5‰ over ∼30 meters. These changes evidently record a transient departure from a largely anaerobic to an aerobic nitrogen cycle complete with nitrification and denitrification. Complementary molybdenum abundance and sulfur isotopic values suggest that nitrification occurred in response to a small increase in surface-ocean oxygenation. These data imply that nitrifying and denitrifying microbes had already evolved by the late Archean and were present before oxygen first began to accumulate in the atmosphere.
Here, we revisit this conclusion. Correction of an error in the CH4 absorption coefficients, combined with the predicted early onset of climatically cooling organic haze, suggest that the amount of greenhouse warming by CH4 was more limited and that pCO2 must therefore have been 0.03 bar, at or above the upper bound of the value obtained from paleosols. Enough warming from CH4 remained in the Archean, however, to explain why Earth’s climate cooled and became glacial when atmospheric O2 levels rose in the Paleoproterozoic. Our new model also shows that greenhouse warming by higher hydrocarbon gases, especially ethane (C2H6), may have helped to keep the Late Archean Earth warm.
The atmosphere has apparently been oxygenated since the ‘Great Oxidation Event’ ca 2.4 Ga ago, but when the photosynthetic oxygen production began is debatable. However, geological and geochemical evidence from older sedimentary rocks indicates that oxygenic photosynthesis evolved well before this oxygenation event. Fluid-inclusion oils in ca 2.45 Ga sandstones contain hydrocarbon biomarkers evidently sourced from similarly ancient kerogen, preserved without subsequent contamination, and derived from organisms producing and requiring molecular oxygen. Mo and Re abundances and sulphur isotope systematics of slightly older (2.5 Ga) kerogenous shales record a transient pulse of atmospheric oxygen. As early as ca 2.7 Ga, stromatolites and biomarkers from evaporative lake sediments deficient in exogenous reducing power strongly imply that oxygen-producing cyanobacteria had already evolved. Even at ca 3.2 Ga, thick and widespread kerogenous shales are consistent with aerobic photoautrophic marine plankton, and U-Pb data from ca 3.8 Ga metasediments suggest that this metabolism could have arisen by the start of the geological record. Hence, the hypothesis that oxygenic photosynthesis evolved well before the atmosphere became permanently oxygenated seems well supported.
Pre-photosynthetic niches were meagre with a productivity of much less than 10−4 of modern photosynthesis. Serpentinization, arc volcanism and ridge-axis volcanism reliably provided H2. Methanogens and acetogens reacted CO2 with H2 to obtain energy and make organic matter. These skills pre-adapted a bacterium for anoxygenic photosynthesis, probably starting with H2 in lieu of an oxygen ‘acceptor’. Use of ferrous iron and sulphide followed as abundant oxygen acceptors, allowing productivity to approach modern levels. The ‘photobacterium’ proliferated rooting much of the bacterial tree. Land photosynthetic microbes faced a dearth of oxygen acceptors and nutrients. A consortium of photosynthetic and soil bacteria aided weathering and access to ferrous iron. Biologically enhanced weathering led to the formation of shales and, ultimately, to granitic rocks. Already oxidized iron-poor sedimentary rocks and low-iron granites provided scant oxygen acceptors, as did freshwater in their drainages. Cyanobacteria evolved dioxygen production that relieved them of these vicissitudes. They did not immediately dominate the planet. Eventually, anoxygenic and oxygenic photosynthesis oxidized much of the Earth’s crust and supplied sulphate to the ocean. Anoxygenic photosynthesis remained important until there was enough O2 in downwelling seawater to quantitatively oxidize massive sulphides at mid-ocean ridge axes.
The tree of terrestrial life probably roots in non‐photosynthetic microbes. Chemoautotrophs were the first primary producers, and the globally dominant niches in terms of primary productivity were determined by availability of carbon dioxide and hydrogen for methanogenesis and sulfite reduction. Methanogen niches were most abundant where CO2‐rich ocean water flowed through serpentinite. Black smoker vents from basalt supplied comparable amount of H2. Hydrogen from arc volcanoes supported a significant methanogenic niche at the Earth’s surface. SO2 from arc volcanoes reacted with organic matter and hydrogen, providing a significant surface niche. Methane ascended to the upper atmosphere where photolysis produced C‐rich haze and CO, and H escaped into space. The CO and C‐rich haze supported secondary surface niches. None of these ecologies were bountiful; less than 1% of the CO2 vented by ridge axes, arcs, and metamorphism became organic matter before it was buried in carbonate. In contrast, a photosynthetic biosphere leaves copious amounts of organic carbon, locally concentrated in sediments. Black shales are a classic geologic biosignature for photosynthesis that can survive subduction and high‐grade metamorphism.
To investigate the weathering of sedimentary organic matter and its role in regulating atmospheric oxygen, a theoretical modeling study is presented that addresses the fundamental controls on atmospheric oxygen uptake: erosion rate, organic matter content, and reaction rate. We compare model results with the previous part of this study that analyzed a drill core of black shale from the New Albany formation (Upper Devonian, Clay City, KY) for total and organic carbon, pyrite sulfur, porosity, permeability and specific surface area. As was observed in the field study, the model predicts that the loss of organic matter by oxidative weathering takes place across a reaction “front” where organic carbon content decreases sharply toward the land surface along with pyrite loss.
We consider time‐dependent fluxes that include organic carbon burial and associated oxygen production, reducing gases from metamorphic and volcanic sources, oxidative weathering, and the escape of hydrogen to space. We find that the oxic transition occurs in a geologically short time when the O2‐consuming flux of reducing gases falls below the flux of organic carbon burial that produces O2. A short timescale for the oxic transition is enhanced by a positive feedback due to decreasing destruction of O2 as stratospheric ozone forms, which is captured in our atmospheric chemistry parameterization. We show that one numerically self‐consistent solution for the rise of O2 involves a decline in flux of reducing gases driven by irreversible secular oxidation of the crust caused by time‐integrated hydrogen escape to space in the preoxic atmosphere, and that this is compatible with constraints from the geological record. In this model, the timing of the oxic transition is strongly affected by buffers of reduced materials, particularly iron, in the continental crust. An alternative version of the model, where greater fluxes of reduced hydrothermal cations from the Archean seafloor consume O2, produces a similar history of O2 and CH4. When climate and biosphere feedbacks are included in our model of the oxic transition, we find that multiple ‘Snowball Earth’ events are simulated under certain circumstances, as methane collapses and rises repeatedly before reaching a new steady‐state.
A coupled photochemical‐ecosystem model has been developed to simulate the early Archean biosphere. The model incorporates kinetic and nutrient limitations on biological productivity, along with constraints imposed by metabolic thermodynamics. We have used this model to predict the biogenic CH4 flux and net primary productivity (NPP) of the marine biosphere prior to the advent of oxygenic photosynthesis. Organisms considered include chemotrophic and organotrophic methanogens, H2‐, H2S‐, and Fe‐using anoxygenic phototrophs, S‐reducing bacteria, CO‐using acetogens, and fermentative bacteria.
Here, we make use of 110 publicly available complete genome sequences to understand how the core components of nitrogenase, including NifH, NifD, NifK, NifE, and NifN proteins, have evolved. These genes are universal in nitrogen fixing organisms-typically found within highly conserved operons-and, overall, have remarkably congruent phylogenetic histories. Additional clues to the early origins of this system are available from two distinct clades of nitrogenase paralogs: a group composed of genes essential to photosynthetic pigment biosynthesis and a group of uncharacterized genes present in methanogens and in some photosynthetic bacteria. We explore the complex genetic history of the nitrogenase family, which is replete with gene duplication, recruitment, fusion, and horizontal gene transfer and discuss these events in light of the hypothesized presence of nitrogenase in the last common ancestor of modern organisms, as well as the additional possibility that nitrogen fixation might have evolved later, perhaps in methanogenic archaea, and was subsequently transferred into the bacterial domain.