Slow-Mode MHD Wave Penetration into a Coronal Null Point due to the Mode Transmission     

Andrey N. Afanasyev  Arkadiy M. Uralov 《Solar physics》2016,291(11):3185-3193

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Rare Earth Element and yttrium compositions of Archean and Paleoproterozoic Fe formations revisited: New perspectives on the significance and mechanisms of deposition   总被引：12，自引：0，他引：12  

Noah Planavsky  Andrey Bekker  Balz Kamber  Andrew Knudsen 《Geochimica et cosmochimica acta》2010,74(22):6387-6405

The ocean and atmosphere were largely anoxic in the early Precambrian, resulting in an Fe cycle that was dramatically different than today’s. Extremely Fe-rich sedimentary deposits—i.e., Fe formations—are the most conspicuous manifestation of this distinct Fe cycle. Rare Earth Element (REE) systematics have long been used as a tool to understand the origin of Fe formations and the corresponding chemistry of the ancient ocean. However, many earlier REE studies of Fe formations have drawn ambiguous conclusions, partially due to analytical limitations and sampling from severely altered units. Here, we present new chemical analyses of Fe formation samples from 18 units, ranging in age from ca. 3.0 to 1.8 billion years old (Ga), which allow a reevaluation of the depositional mechanisms and significance of Precambrian Fe formations. There are several temporal trends in our REE and Y dataset that reflect shifts in marine redox conditions. In general, Archean Fe formations do not display significant shale-normalized negative Ce anomalies, and only Fe formations younger than 1.9 Ga display prominent positive Ce anomalies. Low Y/Ho ratios and high shale-normalized light to heavy REE (LREE/HREE) ratios are also present in ca. 1.9 Ga and younger Fe formations but are essentially absent in their Archean counterparts. These marked differences in Paleoproterozoic versus Archean REE + Y patterns can be explained in terms of varying REE cycling in the water column.Similar to modern redox-stratified basins, the REE + Y patterns in late Paleoproterozoic Fe formations record evidence of a shuttle of metal and Ce oxides across the redoxcline from oxic shallow seawater to deeper anoxic waters. Oxide dissolution—mainly of Mn oxides—in an anoxic water column lowers the dissolved Y/Ho ratio, raises the light to heavy REE ratio, and increases the concentration of Ce relative to the neighboring REE (La and Pr). Fe oxides precipitating at or near the chemocline will capture these REE anomalies and thus evidence for this oxide shuttle. In contrast, Archean Fe formations do not display REE + Y patterns indicative of an oxide shuttle, which implies an absence of a distinct Mn redoxcline prior to the rise of atmospheric oxygen in the early Paleoproterozoic. As further evidence for reducing conditions in shallow-water environments of the Archean ocean, REE data for carbonates deposited on shallow-water Archean carbonate platforms that stratigraphically underlie Fe formations also lack negative Ce anomalies. These results question classical models for deposition of Archean Fe formations that invoke oxidation by free oxygen at or above a redoxcline. In contrast, we add to growing evidence that metabolic Fe oxidation is a more likely oxidative mechanism for these Fe formations, implying that the Fe distribution in Archean oceans could have been controlled by microbial Fe uptake rather than the oxidative potential of shallow-marine environments.  相似文献   

Significance of sea water supercooling and intrawater ice generation for the formation of water mass and ice cover in the East Siberian and Chukchi seas     

G. A. Lebedev  V. I. Fedotov  N. V. Cherepanov 《Russian Meteorology and Hydrology》2014,39(2):93-99

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Time-dependent uptake of NO3 by sea salt     

Vladislav V. Zelenov  Elena V. Aparina  Andrey V. Ivanov 《Journal of Atmospheric Chemistry》2014,71(1):33-53

Using a coated-insert flow tube reactor coupled to a low-energy electron-impact mass spectrometer with molecular beam sampling, we studied uptake of NO₃ by sea salt at room temperature and [NO₃]?=?8?10¹¹???4?10¹³ molecule cm^?3. The radical uptake coefficient γ(t) is time dependent: its initial value (γ _ini) decreases exponentially with the characteristic time (τ) to its steady-state value (γ _ss) at given [NO₃]. The parameters γ _ini, γ _ss and τ depend on [NO₃], whereas γ _ss is water vapor independent at [H₂O]?=?8?10¹²???1.6?10¹⁵ molecule cm^?3 and RH ≤ 0.5 %. HCl and NO₂ are uptake products detected in the gas phase. We used these findings to estimate γ values under tropospheric conditions for urban coastal and remote marine environments: at high NO₃ (~90 ppt), the time dependence becomes important, and the γ value averaged over the aerosol lifetime is 4?10^?3; at low NO₃ (~1 ppt), the radical uptake is time independent and proceeds faster with γ _ini?=?8?10^?3  相似文献   

Response of evapotranspiration and water availability to the changing climate in Northern Eurasia     

Yaling Liu  Qianlai Zhuang  Zhihua Pan  Diego Miralles  Nadja Tchebakova  David Kicklighter  Jiquan Chen  Andrey Sirin  Yujie He  Guangsheng Zhou  Jerry Melillo 《Climatic change》2014,125(3-4):413-427

We introduce a probabilistic framework for vulnerability analysis and use it to quantify current and future vulnerability of the US water supply system. We also determine the contributions of hydro-climatic and socio-economic drivers to the changes in projected vulnerability. For all scenarios and global climate models examined, the US Southwest including California and the southern Great Plains was consistently found to be the most vulnerable. For most of the US, the largest contributions to changes in vulnerability come from changes in supply. However, for some areas of the West changes in vulnerability are caused mainly by changes in demand. These changes in supply and demand result mainly from changes in evapotranspiration rather than from changes in precipitation. Importantly, changes in vulnerability from projected changes in the standard deviations of precipitation and evapotranspiration are of about the same magnitude or larger than those from changes in the corresponding means over most of the US, except in large areas of the Great Plains, in central California and southern and central Texas.  相似文献   

Chromium solubility in MgSiO3 ilmenite at high pressure     

Luca Bindi  Ekaterina A. Sirotkina  Andrey V. Bobrov  Tetsuo Irifune 《Physics and Chemistry of Minerals》2014,41(7):519-526

The crystal structure and chemical composition of crystals of (Mg_1?x Cr _x )(Si_1?x Cr _x )O₃ ilmenite (with x = 0.015, 0.023 and 0.038) synthesized in the model system Mg₃Cr₂Si₃O₁₂–Mg₄Si₄O₁₂ at 18–19 GPa and 1,600 °C have been investigated. Chromium was found as substitute for both Mg at the octahedral X site and Si at the octahedral Y site, according to the reaction Mg²⁺ + Si⁴⁺ = 2Cr³⁺. Such substitutions cause a shortening of the <X–O> and a lengthening of the <Y–O> distances with respect to the values typically observed for pure MgSiO₃ ilmenite and eskolaite Cr₂O₃. Although no high Cr contents are considered in the pyrolite model, Cr-bearing ilmenite may be the host for chromium in the Earth’s transition zone. The successful synthesis of ilmenite with high Cr contents and its structural characterization are of key importance because the study of its thermodynamic constants combined with the data on phase relations in the lower-mantle systems can help in the understanding of the seismic velocity and density profiles of the transition zone and the constraining composition and mineralogy of pyrolite in this area of the Earth.  相似文献   

X-ray single-crystal and Raman study of knorringite,Mg3(Cr1.58Mg0.21Si0.21)Si3O12, synthesized at 16 GPa and 1,600 °C     

Elena A. Bykova  Andrey V. Bobrov  Ekaterina A. Sirotkina  Luca Bindi  Sergey V. Ovsyannikov  Leonid S. Dubrovinsky  Yuriy A. Litvin 《Physics and Chemistry of Minerals》2014,41(4):267-272

The crystal structure of a knorringite-type compound, Mg₃(Cr_1.58Mg_0.21Si_0.21)Si₃O₁₂, synthesized in a multi-anvil press at P = 16 GPa and T = 1,600 °C, was refined from single-crystal X-ray diffraction data up to R = 2.36 % for 314 independent reflections. Garnet was found to be cubic and have space group Ia $\overline{3}$ d, with the unit cell parameters a = 11.5718 (1) Å, V = 1,549.54 (2) Å³. The knorringite crystal studied contains 21 mol% of majorite end-member. The structural characterization of knorringitic garnet is important because the study of its thermodynamic constants provides new constraints on thermobarometry of peridotitic garnet assemblages of the lowermost upper mantle. The Raman spectra of synthetic knorringite have been obtained for the first time.  相似文献   

The Study Internal Structure of the Annual Layers in Lake Sediments Using Synchrotron Radiation with X-ray Focusing Optics     

Fedor DARIN  Ivan KALUGIN  Andrey DARIN  Yakov RAKSHUN 《《地质学报》英文版》2014,88(Z1):5-6

正1 Introduction Annually laminated(varve)sedimentary deposits are considered as one of the most important archives,since they offer precise temporal information(years)in combination with high time resolutions.Bottom sediments of the lakes contain detailed geochemical information on  相似文献   

Monocotyledon-like Leaves from the Middle Jurassic of East Siberia (Russia)     

Andrey FROLOV  Ilya ENUSHCHENKO 《《地质学报》英文版》2022,96(6):1884-1896

Leaf fragments of Herbifolia antiqua A. Frolov et Enushchenko gen. et sp. nov. from the Middle Jurassic (Aalenian) of the Irkutsk Coal Basin (Eastern Siberia, Russia) are reported. The following features are characteristic of H. antiqua gen. et sp. nov.: the presence of leaf sheaths, linear smooth-edged leaves with parallel venation, anastomoses between the veins, anomocytic stomata, rhomb-shaped ordinary epidermal cells. Such a combination of characters is widespread in modern monocotyledonous plants and is absent in fossil and modern cryptogam and gymnosperms. Due to the lack of evidence of an enclosed ovule in H. antiqua gen. et sp. nov.we attribute it to a typological angiosperm, based on its unique leaf structure characteristic of monocotyledons. The leaf epidermal structure of Herbifolia gen. nov. is most similar to those of modern Asparagales and Liliales.  相似文献   

Gold and silver in PGE–Cu–Ni and PGE ores of the Noril’sk deposits,Russia     

Sergey F. Sluzhenikin  Andrey V. Mokhov 《Mineralium Deposita》2015,50(4):465-492

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