Volcanic emissions and the early Earth atmosphere

Despite uncertainties in our understanding of early Earth volcanism and atmospheric composition, thermodynamic modelling is able to offer estimates of the global production of reactive trace species (NO, OH, SO₃, Cl, Br and I) from early Earth volcanism, and thereby to shed light on processes which may have been different in Earth’s early atmosphere. Model results show that thermal decomposition of magmatic H₂O, CO₂ and SO₂ in high-T mixtures of magmatic and atmospheric gases (at T > 1400 °C) generate high levels of reactive trace gas species. Production of these reactive trace species is insensitive to atmospheric CO₂ in mixtures where the atmospheric gas is the minor component and will hence continue during periods of low atmospheric CO₂. Fluxes of NO, OH, Cl, Br and I from early Earth volcanism are predicted to exceed those from modern Earth volcanism as the higher temperature of early Earth emissions compensates for lower levels of O₂ in the atmosphere, compared to the modern Earth. Under certain conditions, the volcanic NO flux from early Earth volcanism is found to be comparable to other sources of reactive N such as lightning NO and photochemical HCN. This is one possible source of fixed nitrogen which may alleviate any postulated Archean nitrogen crisis. Our thermodynamic model reveals that production of SO₃ (a potential precursor for near-source volcanic sulphate and hence ‘primary’ volcanic aerosol) is likely to be significantly lower from early Earth volcanism. Uncertainty in the pathway to near-source sulphate in modern volcanism (i.e., the reaction of SO₃ with water or direct emission) introduces a large uncertainty into the production rate of near-source volcanic sulphate on the early Earth.     

Degassing history and carbon cycle of the Earth: From an impact-induced steam atmosphere to the present atmosphere

The recent theoretical studies on the formation and evolution of the atmosphere and oceans of the Earth are reviewed. Impact degassing during accretion of the Earth would probably generate an impact-induced steam atmosphere on the proto-Earth. At the end of accretion, the steam atmosphere became unstable and condensed to form the proto-ocean with almost the present mass of ocean. The steam atmosphere would have thus evolved to the proto-CO₂ atmosphere during the earliest history of the Earth because CO in the proto-atmosphere may be photochemically converted to CO₂. However, CO₂ in the proto-atmosphere has decreased with time through the global carbon cycle which may have stabilized the terrestrial environment against an increase in the solar luminosity. The continental growth during Hadean and Archean would therefore have a significant influence on the carbon cycle and the surface temperature. It is also suggested that the continental growth is a necessary condition for the terrestrial environment to evolve to the present state. Both the impact degassing and the subsequent continuous degassing are suggested to have played a major role in the formation and evolution of the atmosphere and ocean. In particular, most of N₂ may have been produced by the impact degassing during accretion, and the contribution of the subsequent continuous degassing is at most 10% for N₂. As a consequence, after the CO₂ level decreased to less than 1 bar, the atmosphere may have been at about 1 bar and composed mainly of N₂ for most of the subsequent history of the Earth.     

The electrical conductivity of olivine under highly reducing conditions

The electrical conductivity of San Carlos olivine has been measured at 1100 °C under reducing conditions at controlled oxygen fugacity, inside and outside the olivine stability field, in order to study the kinetics of olivine destabilization. Electrical conductivity increases along the direction [010] and decreases along [001]. as oxygen fugacity decreases. To understand these dependences, electrical conductivity transitory regimes were studied. In response to decreases in oxygen fugacity, two transient regimes with different time scales have been observed. A fast (≈1–2 min) increase of electrical conductivity is first observed, followed by a slower decrease (1–10 h, depending on the crystal orientation). After a few hours of annealing, precipitation of metallic iron and nickel and formation of amorphous silica can be observed at the crystal surface. The fast conductivity increase in the first transient regime is ascribed to an increase in the population of electrons at the olivine surface. Two effects: (1) equilibration of surface defects with the bulk of the crystal, and (2) iron loss from the olivine due to metal precipitation, could explain the subsequent decrease of electrical conductivity. Anisotropic diffusion of surface defects to the bulk of the crystal, by a process faster than atomic diffusion is the most likely. Received: 3 September 1997 / Revised, accepted: 16 April 1998     

地球系统动力学与取热减灾减排

在综合分析大量地质、地球物理资料的基础上,建立了以热能为驱动力的开放地球系统发生多级物质循环的地球系统动力学新模式,据此可以合理地解释洋陆系统及其相关的盆山体系的动力学机制,阐明地球系统资源灾害环境发展的内在关系。在地球系统动力学创新思想指导下,总结了活动地壳热构造系统的特征,初步研究了华北和西南热灾害链的结构和强震发展趋势,系统地分析了取热减灾减排的重要性、必要性和可行性,在此基础上阐明了系统开发干热岩的思路和方法及其可持续发展的途径。从而为即将到来的新地学革命和能源革命提供了一个新思路。     

Thermodynamical constraints on the crystallization of a deep magma-ocean on Earth

It has been argued that the crystallization of the magma ocean (MO) after the Moon-forming impact led to the formation of a basal magma ocean (BMO). We search which primordial conditions of pressure, temperature and chemical composition could be compatible with such scenario, based on thermodynamical constraints. The major requirement is an early formation of a viscous layer (VL) of mantle material (i.e. bridgmanite (Bg)) at mid lower-mantle depth, which could insulate thermally and chemically the BMO from the rest of the mantle. To produce such VL, Bg grains should be: (i) neutrally buoyant at mid lower-mantle depths, (ii) sufficiently abundant to produce an efficient insulating layer, and (iii) aggregated to the boundary layer from above and below. The first and the second require a large amount of MO crystallization, up to more than 45%, even in the most favorable case of all Fe partitioning into the melt. The latter is very questionable because the Bg grains have a very small settling velocity. We also investigate different scenarios of MO crystallization to provide constraints on the resulting core temperature. Starting from a fully molten Earth, a temperature as high as ～4725 K could be found at the core–mantle boundary (CMB), if the Bg grains settle early atop the CMB. Such a basal layer of Bg can efficiently decouple from each other the cooling rates of the core and the mantle above the VL. If the settling velocity of Bg grains is too low and/or the MO is too turbulent, such basal VL may not form. In this case, the CMB temperature after MO solidification should stabilize at ～4350 K. At this temperature, enough Bg grains are crystallized to make the mushy mantle viscous at any mantle depth.     

CO2 windows from mantle to atmosphere: Models on ultrahigh-temperature metamorphism and speculations on the link with melting of snowball Earth

We attempt here to correlate the melting phase of major snowball Earth events in the planet with the processes associated with extreme crustal metamorphism and formation of ultrahigh-temperature (UHT) granulite facies rocks. While the dry mineral assemblages that characterize UHT granulites can result from different mechanisms, the direct evidence for the involvement of CO₂-rich fluids in generating diagnostic UHT assemblages has been recorded from the common occurrence of pure CO₂ fluid inclusions in several terranes. Here we evaluate the tectonic settings under which UHT rocks are generated using modern analogues and show that divergent tectonics—both post-collisional extension and rifting—play a crucial role. In an attempt to speculate the link among CO₂ liberation from the carbonated tectosphere, UHT metamorphism and major earth processes, we address some of the important issues such as: (a) how the subcontinental mantle i.e., the tectosphere, had become carbonated; (b) how and when the tectosphere degassed; and (c) what is the difference between Proterozoic orogens and those of the present day. The fate of the Earth as a habitable planet was possibly dictated by a reversal of the fundamental process of formation of oceans through the selective removal of CO₂ into mantle in the Hadean times, carbonation of the Archean mantle wedge, and subsequent decarbonation of the carbonated mantle through divergent metamorphism and water infiltration since the Late Proterozoic.The abundant CO₂ liberated by subsolidus decarbonation along consuming plate boundaries was probably one of the factors that contributed to the greenhouse effect thereby triggering the deglaciation of snowball Earth. Based on an evaluation of the distribution of carbonated subcontinental mantle in global reconstructions of the Proterozoic supercontinent assembly, and their link with crustal domains that have undergone CO₂-aided dry metamorphism at extreme conditions, we speculate that the UHT rocks might represent windows for the transfer of CO₂ from the mantle into the mid crust and ultimately to the atmosphere.     

Instability of the thermal bar formed in a gravity current

The results of experiments concerning fresh water at the temperature more than 4°C released along a free surface into water with a temperature less than 4°C are presented. As a result of Relay-Taylor instability caused by the nonmonotonic dependence of water density on the temperature, the denser water initially sank to the inclined bottom. At the same time, a vertical front separating water masses less and greater than 4°C formed. In limnology such an effect is called the “thermal bar.” Under certain conditions along with the Relay-Taylor instability, the Kelvin-Helmholtz instability tending to destroy the vertical front was also observed. Corresponding illustrations are presented.     

An agent-based simulation model of a primitive agricultural society

The paper describes a model of an agricultural society in which agents live in a single settlement and use the surrounding area to produce essential and non-essential goods. Agents make, and attempt to fulfil, consumption and production plans but markets do not always clear and goods can change hands at different prices between different pairs of agents. The model generates a wide range of agricultural landscapes, including those of a classical von Thünen economy. Demographically, it produces outcomes varying from logistic growth to periodic collapse caused by cyclical famines.     

Al-Mg systematics of chondrules in a primitive CO chondrite

We have conducted systematic investigations of formation age, chemical compositions, and mineralogical characteristics of ferromagnesian chondrules in Yamato-81020 (CO3.05), one of the most primitive carbonaceous chondrites, to get better understanding of the origin of chemical groups of chondrites. The ²⁶Al-²⁶Mg isotopic system were measured in fourteen FeO-poor (Type I), six FeO-rich (Type II) and two aluminum-rich (Al-rich) chondrules using a secondary ion mass spectrometer. Excesses of ²⁶Mg in plagioclase (1.0-13.5‰) are resolved with sufficient precision (mostly 0.4-6.6‰ at 2σ level) in all the chondrules studied except one. Chemical zoning of Mg and Na in plagioclase were investigated in detail in order to evaluate the applicability of ²⁶Al-²⁶Mg chronometer. We conclude that the Al-Mg isotope system of the chondrules in Y-81020 have not been disturbed by parent-body metamorphism and can be used as chronometer assuming homogeneous distribution of ²⁶Al. Assuming an initial ²⁶Al/²⁷Al ratio of 5 × 10⁻⁵ in the early solar system, ²⁶Al-²⁶Mg ages were found to be 1.7-2.5 Ma after CAI formation for Type I, 2.0-3.0 Ma for Type II and 1.9 and 2.6 Ma for Al-rich chondrules.The formation ages of ferromagnesian chondrules in Y-81020 are in good agreement with those of L and LL (type 3.0-3.1) chondrites in the literature, which indicates that common chondrules in the CO chondrite were formed contemporaneously with those in L and LL chondrites. The concurrent formation of chondrules of CO and L/LL chondrites suggests that the chemical differences between CO and L/LL chondrites might be caused by spatial separation of chondrule formation environments in the protoplanetary disk.     

Effects of instability of the Earth and celestial coordinate systems on Earth orientation parameters

Unknown secular and long-term changes in the Earth orientation parameters attributed to instability (possible rotation) of both the Earth and celestial coordinate systems (ECS and CCS) are studied. Rotation of the CCS due to changes in the coordinates of extragalactic sources resulting from gravitational lensing can lead to errors of the order of several microarcseconds in the orientation parameters. The rotation of the ECS due to the crust pressing on the mantle diminishes the tidal retardation of the Earth's rotation and produces long-term variations in the duration of the day (with a period of about 1500 years) and in the motion of the pole relative to the Earth's surface.     

Influence of photoelectrons on the structure and dynamics of the upper atmosphere of a hot Jupiter

A self-consistent, aeronomic model of the upper atmosphere of a “hot Jupiter” including reactions involving suprathermal photoelectrons is presented. This model is used to compute the height profiles of the gas density, velocity, and temperature in the atmosphere of the exoplanet HD 209458b. It is shown that including suprathermal electrons when computing the heating and cooling functions reduces the mass loss rate of the atmosphere by a factor of five.     

Nitrogen isotopes in ophiolitic metagabbros: A re-evaluation of modern nitrogen fluxes in subduction zones and implication for the early Earth atmosphere

Nitrogen contents and isotope compositions together with major and trace element concentrations were determined in a sequence of metagabbros from the western Alps (Europe) in order to constrain the evolution and behavior of N during hydrothermal alteration on the seafloor and progressive dehydration during subduction in a cold slab environment (8 °C/km). The rocks investigated include: (i) low-strain metagabbros that equilibrated under greenschist to amphibolite facies (Chenaillet Massif), blueschist facies (Queyras region) and eclogite facies (Monviso massif) conditions and (ii) highly-strained mylonites and associated eclogitic veins from the Monviso Massif. In all samples, nitrogen (2.6-55 ppm) occurs as bound ammonium () substituting for K or Na-Ca in minerals. Cu concentrations show a large variation, from 73.2 to 6.4 ppm, and are used as an index of hydrothermal alteration on the seafloor because of Cu fluid-mobility at relatively high temperature (>300 °C). In low-strain metagabbros, δ¹⁵N values of +0.8‰ to +8.1‰ are negatively correlated with Cu concentrations. Eclogitic mylonites and veins display Cu concentrations lower than 11 ppm and show a δ¹⁵N-Cu relationship that does not match the δ¹⁵N-Cu correlation found in low-strain rocks. This δ¹⁵N-Cu correlation preserved in low-strain rocks is best interpreted by leaching of Cu-N compounds, possibly of the form Cu(NH₃)₂²⁺, during hydrothermal alteration. Recognition that the different types of low-strain metagabbros show the same δ¹⁵N-Cu correlation indicates that fluid release during subduction zone metamorphism did not modify the original N and Cu contents of the parent hydrothermally-altered metagabbros. In contrast, the low Cu content present in eclogitic veins and mylonites implies that ductile deformation and veining were accompanied either by a loss of copper or that externally-derived nitrogen was added to the system.We estimate the global annual flux of N subducted by metagabbros as 4.2 (±2.0) × 10¹¹ g/yr. This value is about half that of sedimentary rocks, which suggests that gabbros carry a significant portion of the subducted nitrogen. The net budget between subducted N and that outgassed at volcanic arcs indicates that ∼80% of the subducted N is not recycled to the surface. On a global scale, the total amount of N buried to the mantle via subduction zones is estimated to be three times higher than that released from the mantle via mid-ocean ridges, arc and intraplate volcanoes and back-arc basins. This implies that N contained in Earth surface reservoirs, mainly in the atmosphere, is progressively transferred and sequestered into the mantle, with a net flux of ∼9.6 × 10¹¹ g/yr. Assuming a constant flux of subducted N over the Earth’s history indicates that an amount equivalent to the present atmospheric N may have been sequestered into the silicate Earth over a period of 4 billion years.     

采石场大气污染物源强分析研究

采石场环境影响评价的关键在于生产过程中产生的大气污染TSP、PM10的影响,而TSP、PM10的影响又取决于其产生的源强的大小及扩散模式.以襄樊市襄阳城南岘山两处采石场为例,采用模拟实验、实测法对采石场大气污染物扩散模式、扩散参数及源强的确定进行分析研究.结果表明:在通常气象(平均风速)条件下,各类稳定度的TSP小时平均浓度在下风向距离800 m以上均较低.基本上与实测值的TSP值相吻合.在D类稳定度情况下,距离污染源800~900 mTSP最大小时平均浓度为0.13~0.11 mg/hm3.     

Dispersion modeling of air pollutants in the atmosphere: a review

Modeling of dispersion of air pollutants in the atmosphere is one of the most important and challenging scientific problems. There are several natural and anthropogenic events where passive or chemically active compounds are emitted into the atmosphere. The effect of these chemical species can have serious impacts on our environment and human health. Modeling the dispersion of air pollutants can predict this effect. Therefore, development of various model strategies is a key element for the governmental and scientific communities. We provide here a brief review on the mathematical modeling of the dispersion of air pollutants in the atmosphere. We discuss the advantages and drawbacks of several model tools and strategies, namely Gaussian, Lagrangian, Eulerian and CFD models. We especially focus on several recent advances in this multidisciplinary research field, like parallel computing using graphical processing units, or adaptive mesh refinement.     

缓倾角顺层滑坡失稳机制及加固措施研究

本文通过某缓倾角顺层岩质滑坡实例,分析了该滑坡的变形破坏特征,并通过建立FLAC3D数值模型研究滑坡产生的原因、失稳机制和松弛区影响范围,结果表明:1.滑坡破坏模式属于张拉-剪切复合破坏,基于张拉-剪切复合破坏准则对岩质边坡进行稳定性分析更符合实际。2.FLAC3D数值计算能确定边坡开挖产生的松弛区影响范围,结合极限平衡法可用于分析该类型边坡的稳定性。3.此类工程采用预应力锚索对滑坡进行加固处理能够较好地控制滑坡位移,施工阶段对滑坡的扰动也较小,安全性较高,同时能够降低工程造价,较经济。