ACS Earth and Space Chemistry最新文献

“Ovchinnikovite”, a unique iron oxysulfide from the burned dumps of the Chelyabinsk coal basin (Ural, Russia), is a product of high-temperature (1000–1200 °C) alteration of xenoliths of siderite and ankerite rocks by the sulfide-rich gases emanating from the interior of the dumps. According to chemical and crystal-structure analysis, the compound is orthorhombic, with the chemical formula Ca₄Fe²⁺₃Fe³⁺₂O₆S₄ (Z = 8). Among five Fe positions with octahedral coordination, two Fe³⁺ sites are bonded to four O and two S atoms each, while three Fe sites occupied by Fe²⁺ are bonded to two O and four S atoms each. The structure is based upon a three-dimensional framework of Fe-centered octahedra, with two types of layers alternating along the a axis. One layer type is built from corner-sharing Fe³⁺O₄S₂ octahedra (connected through O atoms). In the second layer, Fe²⁺O₂S₄ octahedra share faces and corners. S atoms connect the adjacent layers, and Ca atoms with 9-fold coordination are in between the layers. Density functional theory calculations indicate that Fe₄₀Ca₃₂O₄₈S₃₂ (i.e., Fe_2.5Ca₂O₃S₂) is a magnetic semimetal material. Other physical characteristics (spin magnetic moment; band structure; optical and dielectric properties) are presented. Crystal chemical relationships with synthetic Ca–Fe oxysulfides are discussed. Conditions of formation of “ovchinnikovite” are compared to those of rare oxysulfides in natural combustion metamorphic rocks as well as in metallurgical processes. The study of “ovchinnikovite” shows that man-made geochemical environments such as those occurring in burned coal dumps serve as another source of novel crystal chemistries almost unprecedented among synthetic systems.

Growing evidence shows that water networks in agricultural watersheds, including rivers, paddy fields (PFs), and ditches (DCs), are hotspots of aquatic greenhouse gas emissions globally. However, the knowledge of the role of natural processes and anthropogenic activities, including agricultural practices, in promoting CO₂ production and emissions still remains unclear. In this study, sampling and analysis of different surface waters during the agricultural drainage period were conducted to clarify the production mechanism and emission of CO₂. The results showed that all of the surface waters in the Nongjiang River (NJR) watershed acted as sources with respect to atmospheric CO₂, while a few in the Honghe Wetland and the estuary acted as CO₂ sinks. Longitudinal variations of CO₂ in mainstreams of the NJR indicated that fertilizer application, manure, and sewage discharges stimulated the CO₂ production, while those in the Yalu River (YLR) were mainly affected by the natural wetlands. The accumulation of carbon and nitrogen in waters of PF and DC have enhanced CO₂ emissions during drainage periods. The high ratio of ΔCO₂/ΔO₂ revealed the important role of the extensive respiration in CO₂ production in the NJR. Furthermore, the correlation between dissolved oxygen and CO₂ demonstrated that respiration and photosynthesis dominated CO₂ production and consumption in all types of water bodies. This study implied that agricultural water networks might be vague sources for aquatic systems, and effort is still urgently needed to quantitatively assess the CO₂ emissions in the context of wetland–farmland shifts regionally and worldwide.

Explaining the formation pathways of amides on ice-grain mantels is crucial to understanding the prebiotic chemistry in an interstellar medium. In this computational study, we explore different radical-neutral formation pathways for some of the observed amides (formamide, acetamide, urea, and N-methylformamide) via intermediate carbamoyl (NH₂CO) radical precursors and their isomers. We assess the relative energy of four NH₂CO isomers in the gas phase and evaluate their binding energy on small water clusters to discern the affinity that the isomers present to an ice model. We consider three possible reaction pathways for the formation of the carbamoyl radicals, namely, the OH + HCN, CN + H₂O, and NH₂ + CO reaction channels. We computed the binding energy distribution for the HCN and CH₃CN precursors on an ice model consisting of a set of clusters of 22 water molecules each to serve as a starting point for the reactivity study on the ice surface. The computations revealed that the lowest barrier to the formation of an NH₂CO isomer corresponds to the NH₂ + CO reaction (12.6 kJ mol^–1). The OH + HCN reaction pathway results in the exothermic formation of the N-radical form of carbamoyl HN(C═O)H with a reaction barrier of 26.7 kJ mol^–1. We found that the CN + H₂O reaction displays a high energy barrier of 70.6 kJ mol^–1. Finally, we also probed the direct formation of the acetamide radical precursor via the OH + CH₃CN reaction and found that the most probable outcome on interstellar ices is the H-abstraction reaction to yield CH₂CN and H₂O. Based on these results, we believe that including alternative reaction pathways, leading to the formation of amides via the N-radical form of carbamoyl, would provide an improvement in the prediction of the amide abundances in astrochemical models, especially regarding the chemistry of star-forming regions.

The ethynyl radical, C₂H, is found in a variety of different environments ranging from interstellar space and planetary atmospheres to playing an important role in the combustion of various alkynes under fuel-rich conditions. Hydrogen-atom abstraction reactions are common for the ethynyl radical in these contrasting environments. In this study, the C₂H + HX → C₂H₂ + X, where HX = HNCO, trans-HONO, cis-HONO, C₂H₄, and CH₃OH, reactions have been investigated at rigorously high levels of theory, including CCSD(T)-F12a/cc-pVTZ-F12. For the stationary points thus located, much higher levels of theory have been used, with basis sets as large as aug-cc-pV5Z and methods up to CCSDT(Q), and core correlation was also included. These molecules were chosen because they can be found in either interstellar or combustion environments. Various additive energy corrections have been included to converge the relative enthalpies of the stationary points to subchemical accuracy (≤0.5 kcal mol^–1). Barriers predicted here (2.19 kcal mol^–1 for the HNCO reaction and 0.47 kcal mol^–1 for C₂H₄) are significantly lower than previous predictions. Reliable kinetics were acquired over a wide range of temperatures (50–5000 K), which may be useful for future experimental studies of these reactions.

The “Boring Billion” (BB, ∼1800–800 Ma) is characterized by the perceived stasis of carbon isotopes within the geological record of that time. However, geochemical data obtained from global Paleo-Mesoproterozoic strata indicate heterogeneity and complexity of oxygen contents in the oceans, which hinder paleoenvironmental reconstructions from this period. Furthermore, there has been a dearth of studies focused on the Paleoproterozoic strata in the western and southern parts of the North China Craton (NCC). In this paper, we report elemental abundances and Fe speciation data from the Huangqikou Formation in the Ordos Basin, western NCC, and the Dagushi, Bingmagou, Baicaoping, and Puyu Formations in the Xiong’er Basin, southern NCC. Our latest findings, integrated with prior research, indicate that sedimentary environments in these parts of the NCC in the late Paleoproterozoic were uniformly anoxic and ferruginous. During a marine transgression, we see limited evidence of oxic surface waters entering the sedimentary water and resulting in intermittent oxygenation events in the Ordos Basin. However, microbial respiration of oxygen and/or limited oxygen replenishment under sluggish circulation in the basin might have caused the consistently anoxic conditions in the Xiong’er Basin during the late Paleoproterozoic.

Thermokarst ponds (thaw lakes) are ubiquitous in northern landscapes. They are hotspots for the biogeochemical processing of elements, such as carbon (C), nitrogen (N), sulfur (S), iron (Fe) and manganese (Mn). In turn, those elementary cycles may control the mobility of selenium (Se), an essential micronutrient. To unravel these coupled biogeochemical cycles and identify processes controlling Se mobility, we studied four thermokarst ponds in a subarctic peatland valley influenced by permafrost thaw. The data set comprises of water column and sediment porewater concentration profiles collected during both summer and winter. Physicochemical parameters and dissolved concentrations of major elements, nutrients, and Se were measured and used to model fluxes at the sediment–water interface and to calculate Se speciation. The results suggest that the proximity of the pond from the permafrost structures influenced their biogeochemical dynamics. In the ponds close to permafrost, Se concentrations are 2-fold higher in winter compared to summer, accompanied by an increase in sediment fluxes from 13 to 149 pmol cm^–2 yr^–1 between summer and winter. The combination of comparatively older dissolved organic matter and of oxygenated conditions explain the seasonal variation in Se concentrations. In the ponds further from the permafrost, Se concentrations are higher, remain unchanged in the water column across seasons, and are linearly correlated with both DOC (R² = 0.64, p < 0.01, n = 50) and Fe (R² = 0.60) concentrations. Thermodynamic calculations show that Se(IV) dominates Se speciation in the porewater at all sites, while the water column reaches saturation with respect to elemental zerovalent Se, suggesting that precipitation of elemental Se could mediate dissolved Se concentrations. Collectively, our results point to the strong control that redox conditions exert on Se mobility, via DOC and Fe, and to the linkages between landscape features, pond physicochemistry, and Se dynamics.

Organic molecules are ubiquitous in primitive solar system bodies such as comets and asteroids. These primordial organic compounds may have formed in the interstellar medium and in protoplanetary disks (PPDs) before being accreted and further transformed in the parent bodies of meteorites, icy moons, and dwarf planets. The present study describes the composition of primordial organics analogs produced in a laboratory simulator of the PPD (the Nebulotron experiment at the CRPG laboratory) with nitrogen contents varying from N/C < 0.01 to N/C = 0.63. We present the first Fourier transform ion cyclotron resonance mass spectrometry analysis of these analogs. Several thousands of molecules with masses between m/z 100 and 500 are characterized. The mass spectra show a Gaussian shape with maxima around m/z 250. Highly condensed polyaromatic hydrocarbons (PAH) are the most common compounds identified in the samples with lower nitrogen contents. As the amount of nitrogen increases, a dramatic increase of the chemical diversity is observed. Nitrogen-bearing compounds are also dominated by polyaromatic hydrocarbons (PANH) made of 5- and 6-membered rings containing up to four nitrogen atoms, including triazine and pyrazole rings. Such N-rich aromatic species are expected to decompose easily in the presence of water at higher temperatures. Pure carbon molecules are also observed for samples with relatively small fractions of nitrogen. MS peaks compatible with the presence of amino acids and nucleobases, or their isomers, are detected. When comparing these Nebulotron samples with the insoluble fraction of the Paris meteorite organic matter, we observe that the samples with intermediate N/C ratios bracketing that of the Paris insoluble organic matter (IOM) display relative proportions of the CH, CHO, CHN, and CHNO chemical families also bracketing those of the Paris IOM. Our results support that Nebulotron samples are relevant laboratory analogs of primitive chondritic organic matter.