Archean Ocean Research Articles

Tracing the Early Emergence of Microbial Sulfur Metabolisms

Hydrogen sulfide (nH2S) and sulfur oxide (SO n ; n = 1, 2, 3) gases in early Earth’s globally anoxic atmosphere were subjected to gas-phase chemical transformations by UV light. A principal photolysis pathway at that time produced elemental sulfur aerosols with mass-independently fractionated (MIF) isotopic values carrying variable minor isotope (33S, 36S) compositions. These rained into the sulfate-deficient Archean (ca. 3.85–2.5 Ga) oceans to react with [Fe2+]aq and form sedimentary sulfides. The MIF-bearing sulfides were incorporated into Archean sediments, including banded iron formations (BIF). Such aerosols may also have fueled microbial sulfur metabolisms, and thus are traceable by the MIF sulfur isotopes. Yet, data show that before ∼3.5 Ga mass-dependent34S/32S values in Early Archean sediments tend to fall within a narrow (±0.1%) range even as they carry mass-independent values. By about 3.5 Ga, 34S/32S values show much greater changes (>1%) in range congruent with microbial metabolic processing. Here, we trace probable pathways of elemental sulfur aerosols into Archean sediments, and couple our study with analysis of the evolutionary relationships of enzymes involved in sulfur metabolism to explain the observed trends. Our model explains why elemental sulfur aerosols were apparently not utilized by the Eoarchean (pre-3.65 Ga) biosphere even though an immediate precursor to the required enzyme may have already been present. Highlights Evolution of microbial sulfur metabolisms is tracked by multiple sulfur isotopes Alkaline hydrothermal vents were an abode for early microbial life Sulfite detoxification prompted anaerobic respiration Reversal of respiratory electron transport chain (ETC) stimulated photothiotrophy Surplus e- acceptors permitted the emergence of elemental sulfur reduction

Mafic volcanic rocks of western Iron Ore Group, Singhbhum Craton, eastern India: Geochemical evidence for ocean–continent convergence

Precambrian mafic magmatism is an important global episode which played a significant role in the crustal evolution. In India, Singhbhum Craton being the oldest craton, witnessed significant occurrences of Precambrian geological activity, marked by several episodes of volcanism, plutonism, sedimentation spanning from Palaeoarchean to Mesoproterozoic age. Here we present petrological and geochemical characteristics of Precambrian mafic volcanic rocks (occurring in western Iron Ore Group (IOG), Singhbhum Craton, eastern India) to evaluate their petrogenetic aspects, tectonic setting, and magma generation. The mafic volcanic rocks are porphyritic in nature with the phenocrysts of plagioclase and groundmass composed of clinopyroxene, plagioclase, ilmenite, and volcanic glass. These rocks are mostly tholeiitic, sometimes with a transitional behaviour towards calc‐alkaline nature and display basalt‐basaltic andesite affinity. These mafic volcanic rocks also preserve geochemical signatures (high Nb/U, Nb/La, [Nb/Th]pm ratios) in support of Nb‐enriched basalts and are classified as Nb‐enriched basalts (NEB; Nb > 7 ppm) and high‐Nb basalts (HNB; Nb > 20 ppm) on the basis of Nb concentrations and mantle normalized Nb/La ratios (>0.5). The NEBs and HNBs are marked by lesser magnitude of negative Nb anomalies with high (Nb/Th)pm, (Nb/La)pm, and Nb/U ratios as compared to normal arc basalts. Several major element oxides, trace elements, and selected element ratios (like SiO2, CaO/Al2O3, Y, V/Cr, Zr/Nb, and ∑REE) show systematic variations with MgO which suggests role of magmatic fractionation. Chondrite‐normalized REE patterns for NEB and HNB rocks exhibit uniform LREE enrichment with distinct Eu negative anomalies while primitive mantle‐normalized incompatible trace element patterns reflect enrichment in LILE and LREE with prominent Nb‐Ta anomalies. Different HFSE ratios corroborate a subduction related setting for magma generation formed by ∼10%–20% melting in the domain of garnet lherzolite. Relative enrichment of LILE and LREE with depleted HFSE characteristics attest a garnet‐bearing mantle source and melt extraction with garnet in the residue. Geochemical signatures suggest that the genesis of NEB and HNB is attributable to slab‐melting and wedge hybridization processes during matured stages of subduction. Selected incompatible trace element ratios for the studied mafic volcanic rocks invoke an enriched (EM1‐EM2 type) mantle source and unequivocally suggest effects of continental crustal assimilation of the parent magma. Liquid immiscibility has played an important role as a differentiation mechanism leading to presence of high and low FeO types. The IOG mafic volcanic rocks preserve distinct geochemical signatures of matured stage of subduction, slab melting, crustal contamination and magma generation at an Archean ocean–continent convergent margin setting.

Is the Faint Young Sun Problem for Earth Solved?

Stellar evolution models predict that the solar luminosity was lower in the past, typically 20-25% lower during the Archean (3.8-2.5 Ga). Despite the fainter Sun, there is strong evidence for the presence of liquid water on Earth’s surface at that time. This “faint young Sun problem” is a fundamental question in paleoclimatology, with important implications for the habitability of the early Earth, early Mars and exoplanets. Many solutions have been proposed based on the effects of greenhouse gases, atmospheric pressure, clouds, land distribution and Earth’s rotation rate. Here we review the faint young Sun problem for Earth, highlighting the latest geological and geochemical constraints on the early Earth’s atmosphere, and recent results from 3D global climate models and carbon cycle models. Based on these works, we argue that the faint young Sun problem for Earth has essentially been solved. Unfrozen Archean oceans were likely maintained by higher concentrations of CO2, consistent with the latest geological proxies, potentially helped by additional warming processes. This reinforces the expected key role of the carbon cycle for maintaining the habitability of terrestrial planets. Additional constraints on the Archean atmosphere and 3D fully coupled atmosphere-ocean models are required to validate this conclusion.

Authigenic precipitation of ferric oxyhydroxides and greenalite in Archean oceans

The initial mineral phases of banded iron formations (BIFs) were mostly altered during diagenesis and metamorphism. The initial iron mineral, whether it was hydrous ferric oxides (HFO) or hydrous ferrous silicates (HFS) is a matter of debate. Each of those being the primary iron mineral offers us considering completely different biogeochemical conditions of Archean oceans. Some Archean greenstone BIFs of Badampahar (~3.2 Ga), Singhbhum Craton and Nuvvuagittuq (~3.8 Ga), Northern Québec consist alternating grunerite-, magnetite- and quartz-rich authigenic sedimentary bands. The minerals appear to be co-crystallized during metamorphism and show evidence of being originated from different precursor minerals, e.g. magnetite from HFO and grunerite from HFS. The process of such episodic HFO and HFS precipitations apparently related to two different Fe-Si cycles in the Archean oceans is not understood yet. In-situ trace element geochemistry of the minerals proves their authigenic origin and shows some characteristic geochemical differences, e.g. grunerite having higher contents of hydrothermally sourced trace elements, whereas magnetite being rich in elements involved in marine biogenic cycles. The origin of mineralogically different (e.g. quartz-, grunerite- and magnetite-rich) bands of the studied BIFs is explained with a model of HFS rich precipitation during episodic hydothermalism, subsequent HFO rich precipitation by microbial oxidation, and silica rich precipitation during no hydrothermal activity or non-depositional period of iron minerals.

Generation of reactive oxygen species on pyrite surfaces: A likely oxidation mechanism for near-vent, hydrothermal fluid-dominated BIFs

The key to understand the origin of banded iron-formations (BIFs) formed in 3.8–2.5 Ga is to identify the oxidation mechanism of Fe2+ in anoxic oceans. In this study, we proposed a likely pyrite-induced Fe2+ oxidation mechanism for near-vent, hydrothermal fluid-dominated BIFs. A series of important environmental conditions including light intensity, wavelength and pH were considered and monitored to check their effects on the production of reactive oxygen species (ROS) in anaerobic pyrite suspensions. By quantifying the concentration of OH and H2O2, we observed a stable ROS production as induced by pyrite in both dark and light under a wide pH range. Interestingly, light significantly promoted the producing rate of ROS, which increased with the light intensity, but was poorly correlated with the light wavelength. In addition, the producing rate of ROS remarkably increased as the pH decreased from 6.0 to 3.0, suggesting the acidic conditions were more beneficial for ROS generation. The production of ROS could be attributed to the semiconducting photocatalysis of pyrite, and the sulfur-deficient defect on its surface. Based on the sulfur content of volatiles in modern submarine basalts and black smokers, we assumed that the concentration of H2S in Archean hydrothermal fluids ranged from 0.5 to 55 mmol/kg. Combined with the experimental results of H2O2 producing rates under 150 mW/cm2 irradiation at pH 3.0 and under dark at pH 6.0, the H2O2 flux of 6.9 × 105–1.1 × 109 mol/yr could be estimated in a thermal flow. This value covered the amount of H2O2 required for Fe2+ oxidation in Archean oceans to form the near-vent, hydrothermal fluid-dominated BIFs (1.7 × 106–1.3 × 107 mol/yr). The reaction kinetics revealed that the conversion of amorphous FeS into pyrite under hydrothermal conditions was very quick, while the consumption of H2O2 during the back-reaction with pyrite was quite limited. This view can explain why this type of BIFs typically occurs as intercalated layers in volcanic rocks and is associated with volcanogenic massive sulfide ore (VMS)-type deposits.

Cycling phosphorus on the Archean Earth: Part II. Phosphorus limitation on primary production in Archean ecosystems

Several lines of evidence point to low rates of net primary production (NPP) in Archean oceans. However, whether Archean NPP was limited by electron donors or nutrients, particularly phosphorus (P), and how these factors might have changed over a billion years of recorded Archean history, remains contentious. One major challenge is to understand quantitatively the biogeochemical cycling of P on the early Earth. In Part I of this series (Hao et al., 2020), we estimated the weathering flux of P to the oceans as a function of temporally increasing continental emergence and elevation through Archean time. In Part II, we conduct thermodynamic and kinetic simulations to understand key processes of P cycling within the Archean ocean, including seafloor weathering, recycling of organic P, the solubility and precipitation of secondary phosphate minerals, and the burial diagenesis of P precipitates. Our calculations suggest low solubilities of apatite minerals in Archean seawater, primarily due to nearly neutral pH and high levels of Ca. This low solubility, in turn, implies a negligible contribution of apatite dissolution to P bioavailability in Archean seawater.We also simulate the solubility limits of common secondary P-bearing minerals, showing that vivianite would have been the least soluble P mineral in ferruginous Archean seawater (0.1–0.3 µM), even at moderate supersaturation states (Ω = 100 or 1000). If vivianite precipitation was kinetically favorable by microbial activities and mineral adsorption, the sinking flux of P as vivianite in Archean seawater could have reached the modern sinking flux, implying that vivianite precipitation was a potentially major sink for P in Archean oceans. During burial diagenesis, however, vivianite in porewater would have become less stable than Ca-phosphates of lower solubility. At elevated temperatures (>100 °C) associated with burial diagenesis and low-grade metamorphism, vivianite is predicted to react irreversibly with calcite to form apatite.Optimistic assumptions about the recycling efficiency of P on the Archean Earth lead us to estimate that by the end of the eon the total flux of P (continental weathering + recycling) could have supported NPP at levels up to 7% of the modern. The total flux of P would have been much lower on the early and middle Archean Earth, whereas fluxes of electron donors could have been higher, suggesting very low productivity and P-limitation of marine ecosystems during much of the eon. Comparing our estimates of NPP as limited by P supply with the estimate by Ward et al. (2019), in which NPP was limited by electron donors and metabolic efficiency, there could have been a transition between P-limited productivity in the early to middle Archean to electron donor-limitation closer to the eon’s end (assuming no oxygenic photosynthesis). Once oxygenic photosynthesis reached ecological significance, probably near the end of the Archean, our estimated flux of P would allow rapid oxidation of atmosphere.

Chemical archeoceanography

Elemental fluxes to the ocean are expected to increase with the surface area of continental exposure to weathering and atmospheric PCO2. The record of phosphorus in sediments, which has no notable source within the ocean, and the radiogenic strontium isotopes in Archean carbonates indicate that, prior to the Great Oxidation Event (GOE), subaerial expanses represented only about 20% of the modern continental surface area, i.e. 7% of the surface of the Earth. Because these simple first-order observations, in contrast to the low oxygen content of the pre-GOE atmosphere, have so far received only little attention in the appraisal of the marine chemistry of the early Earth, a reassessment of the chemistry of the pre-GOE ocean is warranted. Here we discuss some of the geochemical cycles of the Archean world, including protons, alkalinity, electrons, and other electrolytes, and attempt to build a first conceptual framework for Chemical Archeoceanography. The smaller subaerial exposures characterizing the Archean and the low abundance of Archean carbonates and mudstones imply that the flux of alkalinity to the ocean was much weaker than today and therefore that the capacity of the runoff to neutralize the high-temperature hydrothermal fluids was less important. Such a reduced flux in turn entails lower seawater pH and higher chlorinity. The lack of atmospheric O2 allowed iron to be in its soluble Fe(II) form and to reduce HCO3− to CH4 while liberating large amounts of protons. The low pH of the pre-GOE ocean and the reduced alkalinity input, reinforced by scant P supply, account for reduced carbonate precipitation. Ocean chemistry evolved under the control of two changing factors: (i) the balance between chemical weathering and hydrothermal fluxes and (ii) the oxygen pressure in the ocean and the atmosphere. Seawater iron concentration was controlled by high [Fe2+]/[H+]2 ratios. With the temperature of hydrothermal fluids at mid-ocean ridges and other active volcanic edifices being determined by the expansion properties of seawater, the chemistry of hydrothermal fluids is controlled by the chlorinity of the ocean. Of all the major element cycles in seawater, those of Na, Mg, and P seem to be unbalanced when runoff falls below the modern value. The very low P content of banded iron formations is inconsistent with a major role of biological activity in the oxidation of Fe2+ dissolved in the Archean ocean. Prior to the GOE, banded iron formations were the major sedimentary sink of seawater cations, a role now played by carbonates. The controls of seawater alkalinity, which today are exerted by the Ca2+–CaCO3 couple, was exerted by dissolved Fe2+-magnetite or Fe2+-ferrihydrite. Plain language summaryWe here evaluate the constraints on seawater chemistry before free oxygen was introduced into the atmosphere some 2.4 billion years ago. We suggest a simple conceptual framework which we validated on modern seawater and which is based on two so-far largely overlooked observations, namely that the phosphorus concentration and strontium isotopic records in ancient sediments require that emerged continental land masses were covering only ~7% of Earth's surface at the time of the Great Oxidation Event instead of 30% today. Atmospheric and seawater chemistry critically depend on this proportion as well as on the chlorinity of the ocean. The oxygen-free atmosphere allowed iron to remain in solution and to reduce atmospheric carbon dioxide to methane. The pH of the ancient oceans must have been lower because less river water was available to neutralize the acidic submarine high-temperature hydrothermal fluids emitted at mid-ocean ridges. Limited continental surface starved the sedimentary column for clays. The low pH and scant phosphorus drainage led to rare carbonate deposition. We conclude by proposing a new scheme for Archean marine chemistry and geochemical cycles that can explain the chemical and geological observations from the Archean and early Proterozoic rock record.

Perspectives on Atmospheric Evolution from Noble Gas and Nitrogen Isotopes on Earth, Mars & Venus

The composition of an atmosphere has integrated the geological history of the entire planetary body. However, the long-term evolutions of the atmospheres of the terrestrial planets are not well documented. For Earth, there were until recently only few direct records of atmosphere's composition in the distant past, and insights came mainly from geochemical or physical proxies and/or from atmospheric models pushed back in time. Here we review innovative approaches on new terrestrial samples that led to the determination of the elemental and isotopic compositions of key geochemical tracers, namely noble gases and nitrogen. Such approaches allowed one to investigate the atmosphere's evolution through geological period of time, and to set stringent constraints on the past atmospheric pressure and on the salinity of the Archean oceans. For Mars, we review the current state of knowledge obtained from analyses of Martian meteorites, and from the direct measurements of the composition of the present-day atmosphere by rovers and spacecrafts. Based on these measurements, we explore divergent models of the Martian and Terrestrial atmospheric evolutions. For Venus, only little is known, evidencing the critical need for dedicated missions.

Photoferrotrophy, deposition of banded iron formations, and methane production in Archean oceans.

Banded iron formation (BIF) deposition was the likely result of oxidation of ferrous iron in seawater by either oxygenic photosynthesis or iron-dependent anoxygenic photosynthesis-photoferrotrophy. BIF deposition, however, remains enigmatic because the photosynthetic biomass produced during iron oxidation is conspicuously absent from BIFs. We have addressed this enigma through experiments with photosynthetic bacteria and modeling of biogeochemical cycling in the Archean oceans. Our experiments reveal that, in the presence of silica, photoferrotroph cell surfaces repel iron (oxyhydr)oxides. In silica-rich Precambrian seawater, this repulsion would separate biomass from ferric iron and would lead to large-scale deposition of BIFs lean in organic matter. Excess biomass not deposited with BIF would have deposited in coastal sediments, formed organic-rich shales, and fueled microbial methanogenesis. As a result, the deposition of BIFs by photoferrotrophs would have contributed fluxes of methane to the atmosphere and thus helped to stabilize Earth's climate under a dim early Sun.

Organic sulfur was integral to the Archean sulfur cycle

The chemistry of the Early Earth is widely inferred from the elemental and isotopic compositions of sulfidic sedimentary rocks, which are presumed to have formed globally through the reduction of seawater sulfate or locally from hydrothermally supplied sulfide. Here we argue that, in the anoxic Archean oceans, pyrite could form in the absence of ambient sulfate from organic sulfur contained within living cells. Sulfides could be produced through mineralization of reduced sulfur compounds or reduction of organic-sourced sulfite. Reactive transport modeling suggests that, for sulfate concentrations up to tens of micromolar, organic sulfur would have supported 20 to 100% of sedimentary pyrite precipitation and up to 75% of microbial sulfur reduction. The results offer an alternative explanation for the low range of δ34S in Archean sulfides, and raise a possibility that sulfate scarcity delayed the evolution of dissimilatory sulfate reduction until the initial ocean oxygenation around 2.7 Ga.

Temperature fluctuations in the Archean ocean as trigger for varve-like deposition of iron and silica minerals in banded iron formations

Abstract Banded iron formations (BIFs) are Fe- and Si-rich chemical sediments that were deposited in the oceans throughout the Archean Eon (4.0–2.5 billion years ago; Ga). Despite extensive research pertaining to their composition, mineralogy and depositional settings, the mechanism(s) leading to their characteristic layering is still a matter of debate. Recently, it has been proposed that temperature fluctuations could have acted as the unifying trigger for the precipitation and deposition of Fe(III)-mineral-rich layers mediated by phototrophic Fe(II)-oxidizing bacteria during warm periods and the deposition of silica (SiO2)-rich layers by abiotic precipitation of dissolved silica during cold periods. To verify the feasibility of such a mechanism, we conducted laboratory experiments with the marine phototrophic Fe(II)-oxidizing bacterium Rhodovulum iodosum under simulated Archean ocean conditions. The temperature was cycled between 26 °C (warm period) and 5 °C (cold period) to mimic temperature fluctuations in the past. Our results showed that R. iodosum readily oxidized Fe(II) during warm periods resulting in the formation of an orange-brown Fe(III) mineral layer, whereas it was inactive during cold periods. Conversely, silica largely stayed in solution during warm periods but was precipitated abiotically as an amorphous, gel-like layer during cold periods, enhanced by Si addition that led to Si oversaturation. Most importantly, during Fe(III) mineral precipitation, most silica stayed in solution leading to an independent precipitation of the Fe- and Si-rich layers. This is due to inhibition of silica sorption onto the biogenic Fe-minerals caused by sorption of microbially derived organic matter that contains negatively-charged carboxyl/phosphodiester groups. Analyses of precipitation rates and theoretical sedimentological considerations suggest that this process could explain the banding in BIFs on the microband level and produce sediments of similar thicknesses as found in nature.

Archean seawater composition and depositional environment – Geochemical and isotopic signatures from the stromatolitic carbonates of Dharwar Craton, India

The well-preserved stromatolitic carbonates from Neoarchean Sandur, Chitradurga and Shimoga greenstone belts of Dharwar Craton were studied to constrain the depositional conditions, geochemical, isotopic characteristics and the Archean seawater temperatures. These rocks display limited diversity in their morphological features such as columnar (occasionally branched), pseudo-columnar, domal, crinkly laminated, and oval/elliptical shapes. The stromatolites are associated with Fe-Mn formations along with arenites, argillites, limestones and carbonaceous shales. These stromatolites are cherty dolomites having variable SiO2, MgO and CaO contents. The stromatolitic carbonates of the three greenstone belts possess low total Rare earth elements plus Ytterium (∑REY) along with positive La, Eu, Gd, Y anomalies and Y/Ho ratios reflecting their deposition in marine conditions with variable hydrothermal and terrigenous inputs. These stromatolites display predominantly negative δ13C values ranging from −1.61‰ to 0.01‰ VPDB of Sandur, −2.53‰ to 1.35‰ VPDB of Chitradurga and positive δ13C of Shimoga varying from −0.30‰ to 1.29‰ VPDB combined with the negative δ18O values (−20.95‰ to −7.72‰ VPDB). Our studies indicate fluctuating oxic to anoxic; acidic to alkaline environment responsible for the deposition of these stromatolites with locally varying Archean ocean temperatures from 25 to 75 °C. These conditions favoured the deposition of cherty dolomitic stromatolites in a shallow marine environment ranging from supra to subtidal zones with the minor influence of hydrothermal activity which is evident for the existence of Archean smaller plate tectonics.

Metabolic potential of microbial communities from ferruginous sediments.

Ferruginous (Fe-rich, SO4 -poor) conditions are generally restricted to freshwater sediments on Earth today, but were likely widespread during the Archean and Proterozoic Eons. Lake Towuti, Indonesia, is a large ferruginous lake that likely hosts geochemical processes analogous to those that operated in the ferruginous Archean ocean. The metabolic potential of microbial communities and related biogeochemical cycling under such conditions remain largely unknown. We combined geochemical measurements (pore water chemistry, sulfate reduction rates) with metagenomics to link metabolic potential with geochemical processes in the upper 50 cm of sediment. Microbial diversity and quantities of genes for dissimilatory sulfate reduction (dsrAB) and methanogenesis (mcrA) decrease with increasing depth, as do rates of potential sulfate reduction. The presence of taxa affiliated with known iron- and sulfate-reducers implies potential use of ferric iron and sulfate as electron acceptors. Pore-water concentrations of acetate imply active production through fermentation. Fermentation likely provides substrates for respiration with iron and sulfate as electron donors and for methanogens that were detected throughout the core. The presence of ANME-1 16S and mcrA genes suggests potential for anaerobic methane oxidation. Overall our data suggest that microbial community metabolism in anoxic ferruginous sediments support coupled Fe, S and C biogeochemical cycling.

Gravitational Potential Energy per Unit Area as a Constraint on Archean Sea Level

AbstractTo constrain the density structure of Archean lithosphere, I assume that gravitational potential energy (GPE) per unit area at Archean spreading centers equals that for continents whose surfaces lay near sea level. The present‐day balance of GPE limits average density deficits in Archean mantle lithosphere due to depletion of iron during its formation to ∼30–45 kg/m3. For an Archean volume of seawater equal to or greater than that today and heat loss approximately three times that today (e.g., at ∼3.5 Ga), plausible Archean structures, constrained by GPE balance, favor submerged spreading centers and continents. Emergent Archean continents would be allowed by a combination of the following (a) heat loss only twice that today, (b) less than approximately half of the present‐day volume of thick crust, either continental or oceanic plateaus, and (c) deep Archean spreading centers ( 2 km below continents), which would require a thickness of Archean oceanic crust 25 km. If the volume of Archean seawater were 50% greater than that today, emergent spreading centers could have existed only as isolated unusual features, and emergent continents would require both limited extent of thick crust and spreading centers deeper that ∼2 km. The observed widespread development of continental margins and carbonate platforms at ∼2.7–2.5 Ga is consistent with heat loss roughly twice that today, a volume of seawater comparable with that today, and either (a) a volume of continental crust comparable to that at present and oceanic crust 30 km thick or (b) thicker oceanic crust, possibly 40–50 km, but with a volume of continental crust less than approximately half that today. Calculated temperatures at the Archean Moho are ∼700–900 °C. These calculations do not rule out a hot early Archean ocean (∼50–80 °C) and do not favor an early Archean emergence of life on land above sea level.

Superflare Ultraviolet Impact on Kepler-96 System: A Glimpse of Habitability When the Ozone Layer First Formed on Earth.

Kepler-96 is an active solar-type star harboring a Super-Earth planet in close orbit. Its age of 2.3 gigayears is the same as the Sun when there was a considerable increase of oxygen in Earth's atmosphere due to micro-organisms living in the ocean. We present the analysis of superflares seen on the transit light curves of Kepler-96b. The model used here simulates the planetary transit in a flaring star. By fitting the observational data with this model, it is possible to infer the physical properties of the flares, such as their duration and the energy released. We found three flares within the energy range of superflares, where the biggest superflare observed was found to have an energy of 1.81 × 1029 J (1.81 × 1035 ergs). The goal is to analyze the biological impact of these superflares on a hypothetical Earth in the habitable zone of Kepler-96, assuming this planet has protection through different scenarios: an Archean and present-day atmospheres. Also, we compute the attenuation of the flare ultraviolet (UV) radiation through an Archean ocean. The conclusion is that considering the increase in the UV flux by the strongest superflare emission, Escherichia coli and Deinococcus radiodurans could survive on the surface of the planet only if there was an ozone layer present on the planet atmosphere. However, they could escape from the hazardous UV effects at a depth of 28 and 12 m below the ocean surface, respectively. For smaller superflares contribution, D. radiodurans could survive in the surface even in an Archean atmosphere with no ozone.

Homochirality through Photon-Induced Denaturing of RNA/DNA at the Origin of Life.

Since a racemic mixture of chiral nucleotides frustrates the enzymeless extension of RNA and DNA, the origin of homochirality must be intimately connected with the origin of life. Homochirality theories have elected to presume abiotic mechanisms for prebiotic enantiomer enrichment and post amplification, but none, so far, has been generally accepted. Here I present a novel hypothesis for the procurement of homochirality from an asymmetry in right- over left-circularly polarized photon-induced denaturing of RNA and DNA at the Archean ocean surface as temperatures descended below that of RNA and DNA melting. This asymmetry is attributed to the small excess of right-handed circularly polarized submarine light during the afternoon, when surface water temperatures were highest and thus most conducive to photon-induced denaturing, and to a negative circular dichroism band extending from 230 to 270 nm for small oligos of RNA and DNA. Because D-nucleic acids have greater affinity for L-tryptophan due to stereochemistry, and because D-RNA/DNA+L-tryptophan complexes have an increased negative circular dichroism band between 230 and 270 nm, the homochirality of tryptophan can also be explained by this hypothesis. A numerical model is presented, demonstrating the efficacy of such a mechanism in procuring homochirality of RNA or DNA from an original racemic solution in as little as 270 Archean years.

Depositional setting of the Late Archean Fe oxide- and sulfide-bearing chert and graphitic argillite in the Shaw Dome, Abitibi greenstone belt, Canada

Interbedded chert-rich exhalite and graphitic argillite are the only sedimentary rocks deposited in deep-water settings during long-lived hiatuses in mafic to ultramafic volcanism within the Hart area of the Shaw Dome in the Late Archean Abitibi greenstone belt in Canada. The Fe oxide- and sulfide-bearing, but predominantly cherty, exhalite lithological unit in the Hart area can be traced laterally to iron formation elsewhere in the Shaw Dome. Whole-rock as well as Fe and S isotope geochemistry suggest that the exhalite unit was formed as a result of direct precipitation from seawater, distally from hydrothermal centres. Fractionation of Fe isotopes through the precipitation of iron oxyhydroxides in a neutrally buoyant hydrothermal plume removed the heavier isotopes of Fe, resulting in the negative δ56Fe values observed in the exhalite in the Hart area. Archean seawater is generally considered to be anoxic, but moderate Mn enrichments (up to 1.87 wt% MnO) in exhalite along with negative Fe isotope values resulting from partial Fe(II) oxidation suggest the presence of oxygen in the upper part of the water column along the pathway of hydrothermal plumes from their source to the depositional site in the Abitibi greenstone belt. In contrast, the graphitic argillite contains abundant pyrite nodules and bands that exhibit systematic negative Fe isotope values, but does not show Mn enrichment. This unit likely formed in a zone of upwelling of nutrient-rich waters from deeper parts of the basin resulting in high organic productivity. Both exhalite and graphitic argillite have negative Δ33S values, suggesting that sulfur was derived from seawater sulfate, which is consistent with an anoxic atmosphere with sulfate aerosols produced by photochemical reactions. Combined, our data indicates disequilibrium between anoxic atmosphere and partially oxygenated upper part of the water column during periods of volcanic quiescence in the ∼2.7 Ga Abitibi greenstone belt supporting the existence of oxidized oases within the Archean ocean.

Archean phosphorus liberation induced by iron redox geochemistry

The element phosphorus (P) is central to ecosystem growth and is proposed to be a limiting nutrient for life. The Archean ocean may have been strongly phosphorus-limited due to the selective binding of phosphate to iron oxyhydroxide. Here we report a new route to solubilizing phosphorus in the ancient oceans: reduction of phosphate to phosphite by iron(II) at low (<200 °C) diagenetic temperatures. Reduction of phosphate to phosphite was likely widespread in the Archean, as the reaction occurs rapidly and is demonstrated from thermochemical modeling, experimental analogs, and detection of phosphite in early Archean rocks. We further demonstrate that the higher solubility of phosphite compared to phosphate results in the liberation of phosphorus from ferruginous sediments. This phosphite is relatively stable after its formation, allowing its accumulation in the early oceans. As such, phosphorus, not as phosphate but as phosphite, could have been a major nutrient in early pre-oxygenated oceans.

Kinetics of Decomposition of Thiocyanate in Natural Aquatic Systems.

Rates of thiocyanate degradation were measured in waters and sediments of marine and limnic systems under various redox conditions, oxic, anoxic (nonsulfidic, nonferruginous, nonmanganous), ferruginous, sulfidic, and manganous, for up to 200-day period at micromolar concentrations of thiocyanate. The decomposition rates in natural aquatic systems were found to be controlled by microbial processes under both oxic and anoxic conditions. The Michaelis-Menten model was applied for description of the decomposition kinetics. The decomposition rate in the sediments was found to be higher than in the water samples. Under oxic conditions, thiocyanate degradation was faster than under anaerobic conditions. In the presence of hydrogen sulfide, the decomposition rate increased compared to anoxic nonsulfidic conditions, whereas in the presence of iron(II) or manganese(II), the rate decreased. Depending on environmental conditions, half-lives of thiocyanate in sediments and water columns were in the ranges of hours to few dozens of days, and from days to years, respectively. Application of kinetic parameters presented in this research allows estimation of rates of thiocyanate cycling and its concentrations in the Archean ocean.

SIMS microanalysis of the Strelley Pool Formation cherts and the implications for the secular-temporal oxygen-isotope trend of cherts

The significance of oxygen isotope ratios in Archean chert has long been debated. Cherts from the c. 3.4 Ga Strelley Pool Formation (SPF) (Pilbara Craton, Western Australia) host some of the oldest stromatolite and microfossil evidence for life, but the genesis and timing of silica cements has been unclear. Field relations, petrography and a combination of laser fluorination and in-situ SIMS measurements of δ18O in quartz show that bedded cherts of the SPF were originally precipitated as carbonates and were later widely replaced by quartz. Three localities were studied and analyzed for δ18O(Qz) in chert: 1) Camel Creek: foliated, metamorphosed, bedded cherts and meter-scale black chert veins; 2) Unconformity Ridge and ABDP8 drill core: stromatolitic and bedded chert overlying basal detrital quartz sandstone; and 3) the Trendall locality: “bedded” stromatolitic chert replacing original dolomite, low temperature hydrothermal quartz, and mm- to decimeter-scale chert-quartz veins. Laser fluorination (mm-scale) values of δ18O(Qz) range from: 14.2 to 18.2‰ VSMOW at Camel Creek; 9.3 to 18.9‰ at Unconformity Ridge; and 13.7 to 25.7‰ at Trendall. Values of δ18O(Qz) in cm to decimeter-scale hydrothermal chert veins cutting bedded carbonates at Trendall range from ca. 15 to 16‰, whereas “bedded cherts” are 17 to 26‰. These laser data include the highest δ18O values reported for cherts in Paleoarchean sediments and are up to 4‰ higher than the upper limit of ∼22‰ reported in other studies, in apparent contrast to the long-standing secular-temporal trend which shows such high δ18O only in younger chert. However, analysis by laser fluorination at the 1-mm scale cannot resolve microtextures seen petrographically. In contrast, in-situ SIMS analyses can resolve petrographic microtextures and show δ18O(Qz) at 10-μm scale have an even greater range of 7–31‰ in “bedded” cherts at the Trendall locality, up to 9‰ above the secular-temporal trend.Textures observed optically at the Trendall locality were classified as: microquartz, mesoquartz, chalcedony, megaquartz veins, and cavity megaquartz. SEM-CL imaging shows two generations of meso- and megaquartz; bright CL with well-developed growth zoning, and dark CL with massive or mottled texture. Microquartz is the earliest textural generation of quartz and has a maximum δ18O(Qz) of ∼22‰ by SIMS. Dark-CL mesoquartz has similar δ18O to microquartz and is interpreted to also be early. Bright-zoned-CL mesoquartz, which formed post-Archean, has even higher δ18O, up to 29‰. Vein megaquartz crosscuts most quartz generations and has a restricted range of δ18O, mostly from 16 to 19‰. Chalcedony pseudomorphs rhombic cavities and fractures, lines the edges of veins, and has similar δ18O to veins (16–19‰). Late cavity megaquartz is bright and zoned by CL, grows into late open cavities, and has the highest δ18O(Qz) values reported from the Pilbara, up to 31.3‰. Thus, the highest-δ18O quartz cements at the Trendall locality are the youngest and may be related to weathering. Early silicification and the formation of microquartz, chalcedony and low δ18O mesoquartz occurred during low temperature hydrothermal activity in the Archean.None of the SPF quartz examined is interpreted to have formed as a direct precipitate from Paleoarchean seawater. Thus, values of δ18O(Qz) do not record either water chemistry or temperature of Archean oceans. In-situ SIMS analysis shows that high-δ18O(Qz) values above 22‰ are only found in late-forming cavity megaquartz and bright-zoned-CL mesoquartz at the Trendall locality.The SPF results from our sample suite demonstrate the ability to resolve complex history using detailed petrography and SIMS analysis. Similar studies may show equal complexity of δ18O(Qz) data for other localities that are interpreted to show secular-temporal trends for chert. The apparent increase of δ18O(Qz) through time may reflect differences in diagenesis, and/or an inherent and previously unrecognized sampling bias that compares fundamentally different populations of quartz, such as Archean hydrothermal chert from volcanic greenstone belts, with unrelated Phanerozoic biogenic quartz.