625 research outputs found
Responses of the biogeochemical sulfur cycle to Early Permian tectonic and climatic events
No full textThe late Paleozoic was characterized by a series of continental collisions and ice ages. Despite the drastic environmental changes, sparse sulfur isotope data hinder our understanding of the late Paleozoic biogeochemical sulfur cycle, especially during the Early Permian. To overcome this potential bias, we present a high-resolution sulfur isotope record of carbonate-associated sulfate (CAS) and pyrite from the Carboniferous-Permian successions of the Svalbard archipelago. Throughout the Carboniferous, our results are largely consistent with the global trend, although the development of restricted environments resulted in a regionally observed delta S-34(CAS) peak of +20 parts per thousand during the Gzhelian. The Early Permian delta S-34(CAS) data in Svalbard bridge the gap in the existing record, showing a steady increase contemporaneous with the closure of the Ural Seaway and Gondwana glaciation, albeit superimposed by short-term oscillations. The enhanced incorporation of diagenetic sulfate into authigenic carbonates may have caused small-scale oscillations during the regional regression in the Artinskian, but the long-term increasing trend of delta S-34(CAS) and its relation to known geological events can be best explained by the enhanced pyrite burial flux driven by a major shift in the locus of organic carbon burial from the continent to the ocean, with a lesser contribution from the dissolution of epicontinental seaway evaporites. Since the onset of the Middle Carboniferous Bashkirian delta S-34(CAS) excursion also corresponds in timing to the major glaciation event and the closure of the Rheic Seaway, the sulfur isotope record in the course of the consolidation of Pangea is apparently punctuated by the episodes of increased pyrite burial and evaporite sulfate weathering, delineating the links between paleogeography, paleoclimate, and biogeochemical cycles. (C) 2022 The Author(s). Published by Elsevier B.V.N
Natural analogue studies for water-rock interactions in uranium deposits: Overseas case studies and hydrogeochemical characteristics of uranium deposits in the Okcheon belt of Korea
No full textIn order to evaluate the long-term safety of deep underground disposal, which is proposed as a disposal method for high-level radioactive wastes, it is necessary to understand the behavior of uranium, a representative radionuclide. Uranium is involved in various geochemical reactions, such as oxidation-reduction, complexation, precipitation-dissolution, and adsorption-desorption; thus, it is important to understand the behavioral characteristics related to these geochemical reactions. For this purpose, experiments or reactive transport model can be used. However, behavioral analyses through them could be limited in time and space or there could be uncertainties in model input parameters such as hydraulic conductivity and sorption coefficient. Therefore, it is necessary to monitor and analyze the long-term behaviors of radioactive elements in natural conditions during the past geological time. These studies are called as 'natural analogue' studies. In this review, we tried to understand the behaviors of uranium during the water-rock interactions based on the studies examining the geochemical characteristics of the seven overseas natural analogue research sites and the Okcheon belt with high uranium contents in Korea. Geochemical characteristics of groundwater in uranium-containing aquifers in the overseas natural analogue research sites indicated reduced environments represented by low Eh values, and showed that more reduced U(IV) species were predominantly present. Accordingly, it was found that the transport of uranium in the groundwater is limited due to precipitation of uranium minerals such as uraninite or coffinite. Groundwater from the uranium-containing coaly slate layer in the Okcheon belt in Korea also indicated a reduced environment, and showed that the transport of uranium is limited due to the precipitation of U(IV) into uraninite. This review, which identified the geochemical characteristics of groundwater in aquifers containing uranium under natural conditions, can derive implications for the behavior of uranium in the deep geological repository for high-level radioactive wastes.N
Predictive isotope model connects microbes in culture and nature
No full textIn PNAS, Wing and Halevy present a new model that quantitatively describes the magnitude of sulfur isotope fractionation produced by dissimilatory microbial sulfate reduction (MSR). MSR is a major player in the global biogeochemical cycles and is responsible for the respiration of up to 30% of organic matter in marine sediments. This metabolism produces large isotope effects, in which the product, sulfide, is depleted in the heavy isotopes ([superscript 33]S, [superscript 34]S, and [superscript 36]S) relative to the most abundant isotope [superscript 32]S (3), enriching modern seawater sulfate in [superscript 34]S by about 21‰ (parts per thousand) compared with mantle sulfur. Sedimentary sulfur minerals preserve a record of this effect and are used to track changes in the sulfur isotope composition of seawater and the biogeochemical sulfur, carbon, and oxygen cycles through geologic time (4). Such reconstructions require an understanding of factors that control the magnitude of sulfur isotope effects and dictate the fractionation of sulfur isotopes by sulfate reducers under a range of growth conditions
Multiple-sulfur isotope effects during photolysis of carbonyl sulfide
No full textLaboratory experiments were carried out to determine sulfur isotope effects during ultraviolet photolysis of carbonyl sulfide (OCS) to carbon monoxide (CO) and elemental sulfur (S[superscript 0]). The OCS gas at 3.7 to 501 mbar was irradiated with or without a N₂ bath gas using a 150 W Xe arc lamp. Sulfur isotope ratios for the product S[superscript 0] and residual OCS were analyzed by an isotope ratio mass-spectrometer with SF₆ as the analyte gas. The isotope fractionation after correction for the reservoir effects is −6.8‰ for the ratio [superscript 34]S/[superscript 32]S, where product S[superscript 0] is depleted in heavy isotopes. The magnitude of the overall isotope effect is not sensitive to the addition of N2 but increases to −9.5‰ when radiation of λ > 285 nm is used. The measured isotope effect reflects that of photolysis as well as the subsequent sulfur abstraction (from OCS) reaction. The magnitude of isotope effects for the abstraction reaction is estimated by transition state theory to be between −18.9 and −3.1‰ for [superscript 34]S which gives the photolysis isotope effect as −10.5 to +5.3‰. The observed triple isotope coefficients are ln(δ[superscript 34]S + 1)/ln(δ[superscript 34]S + 1) = 0.534 ± 0.005 and ln(δ[superscript 36]S + 1)/ln(δ[superscript 34]S + 1) = 1.980 ± 0.021. These values differ from canonical values for mass-dependent fractionation of 0.515 and 1.90, respectively. The result demonstrates that the OCS photolysis does not produce large isotope effects of more than about 10‰ for [superscript 34]S/[superscript 32]S, and can be the major source of background stratospheric sulfate aerosol (SSA) during volcanic quiescence.United States. National Aeronautics and Space Administration (Exobiology program, Grant No. NNX10AR85G
Atomistic insights into catalytic role of platinum-graphene nanostructures in decomposition of high-energy-density fuels
No full textTo advance the cooling performance critical for hypersonic vehicles, high-energy-density fuels have emerged as promising candidates, with platinum-graphene (Pt@FGS) nanocatalysts demonstrating significant potential for enhancing their regenerative cooling efficiency. However, the underlying catalytic mechanisms of these nanocatalysts, particularly their influence on reaction pathways and carbonization processes, remain insufficiently understood. This study employs a ReaxFF-based hybrid simulation approach to investigate the effects of Pt@FGS nanocatalysts on the decomposition of exo-tetrahydrodicyclopentadiene (exo-THDCPD) across a broad temperature range (900-2000 K). The Pt@FGS nanocatalysts were modeled as a partially oxidized graphene structure with six platinum atoms anchored at defect sites. ReaxFF molecular dynamics (MD) simulations were performed to capture real-time pyrolysis pathways and nanocatalyst-fuel interactions at the atomic scale. To extend the timescale and observe low-temperature pyrolysis relevant to experimental conditions, the collective variabledriven hyperdynamics (CVHD) method was employed. Nudged elastic band (NEB) calculations quantified key bond dissociation energy barriers, providing insight into catalytic dehydrogenation mechanisms. The MD results revealed that Pt@FGS nanocatalysts reduce the activation energy by approximately 33 % compared to neat fuel, significantly enhancing fuel conversion rates by up to a factor of four through catalytic dehydrogenation. Heat sink capacity improvements were observed at lower temperature ranges, attributed to nanocatalyst-promoted dehydrogenation, as confirmed by NEB analysis. The CVHD approach enabled pyrolysis simulations under experimentally relevant conditions, yielding activation energies and product distributions consistent with those obtained from high-temperature MD simulations. Interestingly, additional MD simulations demonstrated Pt@FGS nanocatalysts can delay carbonization onset effectively suppressing the formation of carbon deposits. By combining MD, CVHD, and NEB analyses, we elucidated the reaction mechanisms of exo-THDCPD decomposition over Pt@FGS nanocatalysts. The results demonstrate at the atomistic scale that Pt suppresses coke formation by interacting with intermediates and hindering aromatic ring closure, providing insights into the design of fueldispersible catalysts for regenerative fuel cooling.
Exploring mechanical performance in Al-Pt binary alloys through molecular dynamics simulations
No full textThis study systematically explores the impact of environmental factors on the mechanical properties of an Al-Pt binary alloy using molecular dynamics simulations. By varying Pt content and examining key conditions such as temperature, strain rate, and vacancy defects, we delve into their combined effects on the alloy's fracture behavior and overall mechanical performance. Our simulations demonstrate that increasing strain rates enhance fracture strength, while higher temperatures and vacancy concentrations notably reduce it. In contrast, the elastic modulus remained relatively insensitive to these environmental changes. Furthermore, our study highlights the crucial role of point vacancies in accelerating fracture initiation, providing new insights into the failure mechanisms of Al-Pt alloys. These findings have significant implications for the design and optimization of highperformance alloy materials, particularly for applications requiring resilience under extreme operational conditions. The detailed analysis of fracture strength across various environmental scenarios offers a pathway to developing alloys with improved durability and mechanical integrity.
ReaxFF molecular dynamics simulations of high-energy-density fuel combustion catalyzed by Pt-graphene hybrids
No full textThe enhancement of combustion performance and ignition characteristics of high-energy-density fuels is crucial for the advancement of hypersonic propulsion systems. In particular, addressing long ignition delay times and incomplete fuel oxidation remains a key challenge. This study investigates the combustion reaction mechanisms of exo-tetrahydrodicyclopentadiene (exo-THDCPD) dispersed with Pt-graphene nanocatalysts using ReaxFF molecular dynamics simulations. Simulations were conducted at various temperatures to analyze the effects of Pt-graphene on fuel decomposition, ignition delay, and intermediate species formation. The results demonstrate that the presence of Pt-graphene significantly reduces ignition delay by accelerating radical formation and enhancing early-stage oxidation reactions. Additionally, the nanocatalyst promotes more complete combustion by facilitating CO oxidation to CO2 and suppressing intermediate hydrocarbon accumulation. Reaction pathway analysis further confirms that Pt-graphene shifts fuel breakdown mechanisms toward oxidation-driven pathways, resulting in improved fuel consumption and combustion efficiency. These findings provide valuable insight into the role of nanocatalysts in optimizing fuel performance for high-speed propulsion applications.
Morphological record of oxygenic photosynthesis in conical stromatolites
No full textConical stromatolites are thought to be robust indicators of the presence of photosynthetic and phototactic microbes in aquatic environments as early as 3.5 billion years ago. However, phototaxis alone cannot explain the ubiquity of disrupted, curled, and contorted laminae in the crests of many Mesoproterozoic, Paleoproterozoic, and some Archean conical stromatolites. Here, we demonstrate that cyanobacterial production of oxygen in the tips of modern conical aggregates creates contorted laminae and submillimeter-to-millimeter-scale enmeshed bubbles. Similarly sized fossil bubbles and contorted laminae may be present only in the crestal zones of some conical stromatolites 2.7 billion years old or younger. This implies not only that cyanobacteria built Proterozoic conical stromatolites but also that fossil bubbles may constrain the timing of the evolution of oxygenic photosynthesis
Biophysical basis for the geometry of conical stromatolites
No full textStromatolites may be Earth's oldest macroscopic fossils; however, it remains controversial what, if any, biological processes are recorded in their morphology. Although the biological interpretation of many stromatolite morphologies is confounded by the influence of sedimentation, conical stromatolites form in the absence of sedimentation and are, therefore, considered to be the most robust records of biophysical processes. A qualitative similarity between conical stromatolites and some modern microbial mats suggests a photosynthetic origin for ancient stromatolites. To better understand and interpret ancient fossils, we seek a quantitative relationship between the geometry of conical stromatolites and the biophysical processes that control their growth. We note that all modern conical stromatolites and many that formed in the last 2.8 billion years display a characteristic centimeter-scale spacing between neighboring structures. To understand this prominent - but hitherto uninterpreted -organization, we consider the role of diffusion in mediating competition between stromatolites. Having confirmed this model through laboratory experiments and field observation, we find that organization of a field of stromatolites is set by a diffusive time scale over which individual structures compete for nutrients, thus linking form to physiology. The centimeter-scale spacing between modern and ancient stromatolites corresponds to a rhythmically fluctuating metabolism with a period of approximately 20 hr. The correspondence between the observed spacing and the day length provides quantitative support for the photosynthetic origin of conical stromatolites throughout geologic time. Keywords: geobiology; photosynthesis; cyanobacteria; microbialiteUnited States. National Aeronautics and Space Administration (Grant NNA08CN84A)National Science Foundation (U.S.) (Grant EAR–0420592
Physiology of multiple sulfur isotope fractionation during microbial sulfate reduction
No full textThesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences, 2012.Cataloged from PDF version of thesis.Includes bibliographical references.Microbial sulfate reduction (MSR) utilizes sulfate as an electron acceptor and produces sulfide that is depleted in heavy isotopes of sulfur relative to starting sulfate. The fractionation of S-isotopes is commonly used to trace the biogeochemical cycling of sulfur in nature, but a mechanistic understanding of factors that control the range of isotope fractionation is still lacking. This thesis investigates links between the physiology of sulfate reducing bacteria in pure cultures and multiple sulfur isotope (³², ³³, ³⁴34S, and ³⁶S) fractionation during MSR in batch and continuous culture experiments. Experiments address the influence of nutrient and electron donor conditions, including organic carbon, nitrogen, and iron, in cultures of a newly isolated marine sulfate reducing bacterium (DMSS-1). An actively growing culture of DMSS-1 produced sulfide depleted in ³⁴S by 6 to 66%o, depending on the availability and chemistry of organic electron donors. The magnitude of isotope effect correlated well with the cell specific sulfate reduction rate (csSRR), and the largest isotope effects occurred when cultures grew slowly on glucose, a recalcitrant organic substrate. These findings bridge the long-standing discrepancy between the upper limit for S-isotope effect in laboratory cultures and the corresponding observations in nature and indicate that the large (>46 %o) fractionation of S-isotopes does not unambiguously record the oxidative sulfurrecycling. When the availability of iron was limited, the increase in S-isotope fractionation was accompanied by a decrease in the cytochrome c content as well as csSRR. In contrast, growth in nitrogenlimited cultures increased both csSRR and S-isotope fractionation. The influence of individual enzymes and electron carriers involved in sulfate respiration on the fractionation of S-isotopes was also investigated in cultures of mutant strains of Desulfovibrio vulgaris Hildenborough. The mutant lacking Type I tetraheme cytochrome c₃ fractionated ³⁴S/³²S ratio 50% greater relative to the wild type. The increasing S-isotope fractionation accompanied the evolution of H2 in the headspace and the decreasing csSRR. These results further demonstrate that the flow of electrons to terminal reductases imparts the primary control on the magnitude of the fractionation of S-isotopes, suggested by culture experiments using DMSS-1.by Min Sub Sim.Ph.D
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