Constraining the surface environment of the early Earth is essential for understanding the origin and evolution of life. The release of cations from silicate weathering depends on climatic temperature and urn:x-wiley:ggge:media:ggge22102:ggge22102-math-0001, and such cations sequester urn:x-wiley:ggge:media:ggge22102:ggge22102-math-0002 into carbonate minerals in or on the seafloor, providing a stabilizing feedback on climate. Previous studies have suggested that this carbonate?silicate cycle can keep the early Earth’s surface temperature moderate by increasing urn:x-wiley:ggge:media:ggge22102:ggge22102-math-0003 to compensate for the faint young Sun. However, the Hadean Earth experienced a high meteorite impactor flux, which produced ejecta that is easily weathered by carbonic acid. In this study, we estimated the histories of surface temperature and ocean pH during the Hadean and early Archean using a new model that includes the weathering of impact ejecta, empirically justified seafloor weathering, and ocean carbonate chemistry. We find that relatively low urn:x-wiley:ggge:media:ggge22102:ggge22102-math-0004 and surface temperatures are probable during the Hadean, for example, at 4.3 Ga, urn:x-wiley:ggge:media:ggge22102:ggge22102-math-0005 (in bar) is urn:x-wiley:ggge:media:ggge22102:ggge22102-math-0006 urn:x-wiley:ggge:media:ggge22102:ggge22102-math-0007 and temperature is urn:x-wiley:ggge:media:ggge22102:ggge22102-math-0008 urn:x-wiley:ggge:media:ggge22102:ggge22102-math-0009 K. Such a low urn:x-wiley:ggge:media:ggge22102:ggge22102-math-0010 would result in a circumneutral to basic pH of seawater, for example, urn:x-wiley:ggge:media:ggge22102:ggge22102-math-0011 urn:x-wiley:ggge:media:ggge22102:ggge22102-math-0012 at 4.3 Ga. A probably cold and alkaline marine environment is associated with a high impact flux. Hence, if there was an interval of an enhanced impact flux, that is, Late Heavy Bombardment, similar conditions may have existed in the early Archean. Therefore, if the origin of life occurred in the Hadean, life likely emerged in a cold global environment and probably spread into an alkaline ocean.
We examined standard biogeochemical proxies, including organic carbon and nitrogen isotopes, iron speciation, metal abundances and carbonate-associated sulfate. Much of the primary information has been lost because the rocks of the Callanna Group have experienced extensive metamorphism up to amphibolite facies and are altered by modern weathering. However, relics of these proxies, combined with sedimentological features, preserve evidence of redox stratification within this basin. Furthermore, our observations, in particular weakly fractionated nitrogen isotopes and abundant gypsum pseudomorphs, are incompatible with the interpretation of high alkalinity. The high salt content and occurrences of tidal indicators are most parsimoniously explained by frequent incursions of seawater. Thus, the Callanna Group cannot speak straightforwardly to environmental conditions in non-marine habitats at this time. Lastly, the absence of a large carbon isotope anomaly indicates that these rocks do not correlate with the Bitter Springs Formation.
We find that in the Late Pennsylvanian units, an estuarine nutrient trap on the Midcontinent Shelf enabled vigorous selenium recycling, leading to very high concentrations in sediments and enrichment of heavy isotopes in the aqueous selenium reservoir. In contrast, we find that among the Late Devonian units, differences in local basinal hydrography led to a gradient in selenium abundance and isotopic fractionation, with the more restricted basins depleting their selenium reservoirs and causing enrichment of heavy isotopes in the residual aqueous reservoir. In both of these case studies, the additional context provided by complementary paleo-environmental proxies was critical for distinguishing between possible drivers of selenium isotopic variability. When extending such studies to other paleo-environmental settings, we suggest that the continued use of complementary datasets will enable the most robust use of the selenium paleo-redox proxy. Moreover, further development of techniques for high-precision and phase-specific selenium isotope measurements will greatly improve the ability to deduce subtle redox fluctuations with this proxy.
If the process of microbial N2 fixation records the δ15N value of atmospheric N2 in cycad foliage, the fossil record of cycads may provide an archive of atmospheric δ15N values. To explore this potential proxy, we conducted a survey of wild cycads growing in a range of modern environments to determine whether cycad foliage reliably records the isotopic composition of atmospheric N2. We find that neither biological nor environmental factors significantly influence the δ15N values of cycad foliage, suggesting that they provide a reasonably robust record of the δ15N of atmospheric N2. Application of this proxy to the record of carbonaceous cycad fossils may not only help to constrain changes in atmospheric nitrogen isotope ratios since the late Paleozoic, but also could shed light on the antiquity of the N2‐fixing symbiosis between cycads and cyanobacteria.
The membranes of the first protocells on the early Earth were likely self-assembled from fatty acids. A major challenge in understanding how protocells could have arisen and withstood changes in their environment is that fatty acid membranes are unstable in solutions containing high concentrations of salt (such as would have been prevalent in early oceans) or divalent cations (which would have been required for RNA catalysis). To test whether the inclusion of amino acids addresses this problem, we coupled direct techniques of cryoelectron microscopy and fluorescence microscopy with techniques of NMR spectroscopy, centrifuge filtration assays, and turbidity measurements. We find that a set of unmodified, prebiotic amino acids binds to prebiotic fatty acid membranes and that a subset stabilizes membranes in the presence of salt and Mg2+. Furthermore, we find that final concentrations of the amino acids need not be high to cause these effects; membrane stabilization persists after dilution as would have occurred during the rehydration of dried or partially dried pools. In addition to providing a means to stabilize protocell membranes, our results address the challenge of explaining how proteins could have become colocalized with membranes. Amino acids are the building blocks of proteins, and our results are consistent with a positive feedback loop in which amino acids bound to self-assembled fatty acid membranes, resulting in membrane stabilization and leading to more binding in turn. High local concentrations of molecular building blocks at the surface of fatty acid membranes may have aided the eventual formation of proteins.
The partial pressure of atmospheric hydrogen (pH2) on the early Earth is important because it has been proposed that high pH2 warmed the planet or allowed prebiotic chemistry in the early atmosphere. However, such hypotheses lack observational constraints on pH2 . Here, we use the existence of detrital magnetites in (? 3.0 Ga) Archean riverbeds to constrain pH2 . Under the condition of high pH2 , magnetite should disappear via reductive dissolution. We investigated the timescale for a magnetite particle in a river to dissolve, which depends on pH2 and pCO2 . Using published estimates of Archean pCO2 and assuming the presence of Fe(III)-reducing microbes, the survival timescale is ? 1 kyr when pH2 is ?10-2 bar , and decreases as pH2 increases. Considering that the residence time of a particle in a short river (< 1000 km) is ? 1 kyr , the existence of detrital magnetite particles in Archean riverbeds likely indicates that pH2 was below ?10-2 bar . Such a level would preclude H2 as a greenhouse gas or a strongly reducing Archean atmosphere. It is also consistent with limits imposed on H2 by consumption by methanogens because conversion to CH4 is thermodynamically favored.
Cyanide plays a critical role in origin of life hypotheses that have received strong experimental support from cyanide-driven synthesis of amino acids, nucleotides, and lipid precursors. However, relatively high cyanide concentrations are needed. Such cyanide could have been supplied by reaction networks in which hydrogen cyanide in early Earths atmosphere reacted with iron to form ferrocyanide salts, followed by thermal decomposition of ferrocyanide salts to cyanide. Using an aqueous model supported by new experimental data, we show that sodium ferrocyanide salts precipitate in closed-basin, alkaline lakes over a wide range of plausible early Earth conditions. Such lakes were likely common on the early Earth because of chemical weathering of mafic or ultramafic rocks and evaporative concentration. Subsequent thermal decomposition of sedimentary sodium ferrocyanide yields sodium cyanide (NaCN), which dissolves in water to form NaCN-rich solutions. Thus, geochemical considerations newly identify a particular geological setting and NaCN feedstock nucleophile for prebiotic chemistry.
Orbital phase-dependent variations in thermal emission and reflected stellar energy spectra can provide meaningful constraints on the climate states of terrestrial extrasolar planets orbiting M dwarf stars. Spatial distributions of water vapor, clouds, and surface ice are controlled by climate. In turn, water, in each of its thermodynamic phases, imposes significant modulations to thermal and reflected planetary spectra. Here we explore these characteristic spectral signals, based on 3D climate simulations of Earth-sized aquaplanets orbiting M dwarf stars near the habitable zone. By using 3D models, we can self-consistently predict surface temperatures and the location of water vapor, clouds, and surface ice in the climate system. Habitable zone planets in M dwarf systems are expected to be in synchronous rotation with their host star and thus present distinct differences in emitted and reflected energy fluxes depending on the observed hemisphere. Here we illustrate that icy, temperate, and incipient runaway greenhouse climate states exhibit phase-dependent spectral signals that enable their characterization.
Current evidence for oxygenated environments in the Mesoarchean is limited to the shallowest marine and fluvio-lacustrine settings. It is not until the Neoarchean that signs of oxygenated surface waters above outer shelf and basinal depositional environments become evident. In order to further explore the Mesoarchean redox landscape for signs of basinward surface water oxygenation, we present nitrogen and carbon isotope ratios from the turbiditic Mosquito Creek Formation of the Nullagine Group (?2.9?Ga). The ?15N and ?13Corg values are invariant around ?1.8 and ?32 respectively throughout a 70?m section of drill-core, suggesting an ecosystem dominated by nitrogen fixers (anaerobic nitrogen cycling) and CO2 fixation by the Calvin Cycle. When compared with other Archean isotopic data, these results (i) provide further evidence that the Mosquito Creek Formation was deposited in a marine basin, and (ii) contain ?15N values that highlight the prevalence of nitrogen fixation by Mo-nitrogenase and the dearth of aerobic nitrogen metabolisms in the Mesoarchean.
CO2?driven changes to climate have occurred during many epochs of Earth’s history when the solar insolation, atmospheric CO2 concentration, and surface temperature of the planet were all significantly different than today. Each of these aspects affects the implied radiative forcings, climate feedbacks, and resultant changes in global mean surface temperature. Here we use a three?dimensional climate system model to study the effects of increasing CO2 on Earth’s climate, across many orders of magnitude of variation, and under solar inputs relevant for paleo, present, and future Earth scenarios. We find that the change in global mean surface temperature from doubling CO2 (i.e., the equilibrium climate sensitivity) may vary between 2.6 and 21.6 K over the course of Earth’s history. In agreement with previous studies, we find that the adjusted radiative forcing from doubling CO2 increases at high concentrations up to about 1.5 bars partial pressure, generally resulting in larger changes in the surface temperature. We also find that the cloud albedo feedback causes an abrupt transition in climate for warming atmospheres that depends both on the mean surface temperature and the total solar insolation. Climate sensitivity to atmospheric CO2 has probably varied considerably across Earth’s history.
NASA Ames Research Scientist Niki Parenteau and NASA Goddard Civil Servant Giada Arney were recently featured in a Many Worlds article. Parenteau and Arney explain the…
VPL Scientist Dr. Shawn Domagal-Goldman (NASA GSFC) was recently featured on the Ask an Astrobiologist program! Dr. Domagal-Goldman is a…
VPL researchers Josh Krissansen-Totton and David Catling have published an article in the journal Nature titled “Constraining climate sensitivity and…
Giada Arney will start in her new position as a NASA Civil Servant Space Scientist at the Goddard Space Flight Center.…
Eddie Schwieterman began his NASA Postdoctoral Program Appointment at the University of California, Riverside with Dr. Tim Lyons of the NAI-Alternative…
Here, we calculate absolute Archaean barometric pressure using the size distribution of gas bubbles in basaltic lava flows that solidified at sea level ∼2.7 Gyr in the Pilbara Craton, Australia. Our data indicate a surprisingly low surface atmospheric pressure of Patm = 0.23 ± 0.23 (2σ) bar, and combined with previous studies suggests ∼0.5 bar as an upper limit to late Archaean Patm. The result implies that the thin atmosphere was rich in auxiliary greenhouse gases and that Patm fluctuated over geologic time to a previously unrecognized extent.
Ashes of ancient meteors recovered from a 2.7-billion-year-old lake bed imply that the upper atmosphere was rich in oxygen at a time when all other evidence implies that the atmosphere was oxygen-free
Here we present four new coupled carbon and quadruple sulphur isotope records from distal, time equivalent (2.7–2.5 Ga), sedimentary successions from South Africa and Western Australia. These extended records reveal similar chemostratigraphic trends, supporting a dynamic terminal-Neoarchaean atmosphere, oscillating between a hazy state at elevated methane concentrations, and a haze-free anoxic background state. We suggest these atmospheric aberrations were related to heightened biogenic methane fluxes fuelled by enhanced nutrient delivery from climatically or weathering induced feedbacks. These data question the canonical view of a simple, unidirectional planetary oxygenation and signify that the overture to the GOE was governed by complex feedbacks within the Earth–biosphere system.
Here, we modeled the amount of H2 O2 that could be produced in an Eoarchean atmosphere using updated solar fluxes and plausible CO2 , O2 , and CH4 mixing ratios. Irrespective of the atmospheric simulations, the upper limit of H2 O2 rainout was calculated to be <10(6) molecules cm(-2) s(-1) . Using conservative Fe(III) sedimentation rates predicted for submarine hydrothermal settings in the Eoarchean, we demonstrate that the flux of H2 O2 was insufficient by several orders of magnitude to account for IF deposition (requiring ~10(11) H2 O2 molecules cm(-2) s(-1) ). This finding further constrains the plausible Fe(II) oxidation mechanisms in Eoarchean seawater, leaving, in our opinion, anoxygenic phototrophic Fe(II)-oxidizing micro-organisms the most likely mechanism responsible for Earth's oldest IF.
Our data show a small positive excursion in δ82/78Se prior to the extinction, consistent with local euxinia. However, this is followed by a significant negative excursion with a minimum of −1.8‰ (relative to NIST SRM 3149), immediately preceding the principal extinction horizon. A net fractionation of this magnitude likely resulted from partial reduction of Se oxyanions dissolved in the water column. Due to their low abundance, Se oxyanions are rapidly scavenged in anoxic basins or regions of high biological productivity with little net isotopic fractionation. We therefore interpret the uniquely negative fractionations in this section as an indicator for relatively oxygenated conditions in this marine basin at the time when biological productivity declined.
Here we use Se isotopic and abundance measurements of marine and non-marine mudrocks to reconstruct the evolution of the biogeochemical Se cycle from ∼3.2 Gyr onwards. The six stable isotopes of Se are predominantly fractionated during redox reactions under suboxic conditions, which makes Se a potentially valuable new tool for identifying intermediate stages from an anoxic to a fully oxygenated world. δ82/78Se shows small fractionations of mostly less than 2‰ throughout Earth’s history and all are mass-dependent within error. In the Archean, especially after 2.7 Gyr, we find an isotopic enrichment in marine (+0.37 ± 0.27‰) relative to non-marine samples (−0.28 ± 0.67‰), paired with increasing Se abundances.
Our results show that the ∼3350 Ma Euro Basalt preserves a shallow magnetic inclination that appears to have formed as a result of early seafloor hydrothermal alteration, suggesting that the evaporitic carbonate platform of the conformably underlying Strelley Pool Formation was deposited in a near-equatorial location. This is consistent with (although does not require) late Paleoarchean climatic zoning, low orbital obliquity, and a geocentric axial dipole (GAD) field geometry similar to that of the Phanerozoic. The Euro Basalt paleopole overlaps with previously published Paleoarchean poles from the East Pilbara craton and with time-equivalent poles reported from the Barberton Greenstone Belt of the Kaapvaal craton, supporting the existence of a Paleoarchean Vaalbara continental aggregation.
The tectonic mechanisms of heat escape have evolved over time as the Earth’s interior cooled. The Earth condensed from rock vapor over liquid magma immediately following the Moon-forming impact, ~ 4.5 billion years ago. The liquid magma convected vigorously and cooled rapidly until solids formed in the deep mantle. Multiple layers of clouds made the atmosphere opaque, so heat escaped slowly. Tidal dissipation maintained a thin solid layer in the deep mantle over a few million years until the Moon moved far enough away that dissipation no longer balance the heat lost to space. Over a few more million years, the Earth cooled to mostly solid mush capped solid rock.
We comprehensively compile and review N content in geologic materials to calculate a new N budget for Earth. Using analyses of rocks and minerals in conjunction with N–Ar geochemistry demonstrates that the Bulk Silicate Earth (BSE) contains ~ 7 ± 4 times present atmospheric N (4 × 1018 kg N, or PAN), with 27 ± 16 × 1018 kg N. Comparison to chondritic composition, after subtracting N sequestered into the core, yields a consistent result, with BSE N between 17 ± 13 × 1018 kg to 31 ± 24 × 1018 kg N. Embedded in the chondritic comparison we calculate a N mass in Earth’s core (180 ± 110 to 30 ± 180 × 1018 kg) as well as present discussion of the Moon as a proxy for the early mantle.
Organic and inorganic carbon isotope records reflect the burial of organic carbon over geological timescales. Permanent burial of organic carbon in the crust or mantle oxidizes the surface environment (atmosphere, ocean and biosphere) by removing reduced carbon. It has been claimed that both organic and inorganic carbon isotope ratios have remained approximately constant throughout Earth’s history, thereby implying that the flux of organic carbon burial relative to the total carbon input has remained fixed and cannot be invoked to explain the rise of atmospheric oxygen (Schidlowski, 1988; Catling and others, 2001; Holland, 2002; Holland, 2009; Kump and others, 2009; Rothman, 2015). However, the opposite conclusion has been drawn from the same carbon isotope record (Des Marais and others, 1992; Bjerrum and Canfield, 2004). To test these opposing claims, we compiled an updated carbon isotope database and applied both parametric and non-parametric statistical models to the data to quantify trends and mean-level changes in fractional organic carbon burial with associated uncertainties and confidence levels.