RECONNAISSANCE SURVEYS OF CONTEMPORARY PERMAFROST ENVIRONMENTS IN CENTRAL ICELAND USING GEOELECTRICAL METHODS: IMPLICATIONS FOR PERMAFROST DEGRADATION AND SEDIMENT FLUXES     

CHRISTOF KNEISEL  Þ. SÆMUNDSSON  A.A. BEYLICH 《Geografiska Annaler: Series A, Physical Geography》2007,89(1):41-50

Four different sites in the highlands of central Iceland have been investigated for permafrost occurrence using two‐dimensional resistivity imaging. The results of the surveys indicate the presence of shallow permafrost of low to medium resistivity. The distribution pattern is spatially heterogeneous which is consistent with permafrost at the fringe of seasonal frost. These sites are likely to react rapidly to changes of the environmental boundary conditions, therefore future research should include monitoring for detecting the early impact of climate change on permafrost degradation. The extent to which periglacial morphodynamics and sediment fluxes are influenced by permafrost and/or seasonal frost and potential permafrost degradation is hard to determine. Hence, long‐term monitoring approaches for both permafrost and sediment dynamics are essential.  相似文献   

Moment analysis for compounds undergoing sequential chain reactions with first-order decay     

R. Parashar  Rao S. Govindaraju 《Stochastic Environmental Research and Risk Assessment (SERRA)》2006,20(1-2):95-105

Temporal and spatial moment analysis of one-dimensional equations governing fate and transport of parent compounds along with their transformation products is useful for parameter estimation of model parameters, and for understanding the average attributes of contaminant behavior. The objective of this paper is to present analytical expressions for the lower order moments of members in a sequential chain reaction, where members undergo first-order decay to produce the next member in the chain. Specifically, moments up to second order are discussed for the first two members. For the case of purely advective transport (Peclet number tending to infinity), temporal moment expressions are provided for more members of the chain. The sensitivity of temporal moments is examined with respect to Peclet number and transformation rates. Spatial moments are derived by two methods—one using Fourier transforms and another using moment generating differential equations. The behavior of lower order moments for the first few members of a chain can be elucidated from their mathematical expressions. However, expressions for higher order moments tend to be very complicated especially for members further down the chain.  相似文献   

用贝叶斯分类方法和本体实现空间信息语义互操作   总被引：1，自引：0，他引：1  

谭喜成  边馥苓 《武汉大学学报(信息科学版)》2006,31(8):724-727

构建了异构空间信息系统语义互操作的框架，通过基于本体的语义异构空间信息的概念级匹配机制，并利用贝叶斯分类方法完成空间信息实例级的重分类，实现了语义互操作。  相似文献   

The Effect of Temperature and Humidity on the Reaction of Ozone with Combustion Soot: Implications for Reactivity near the Tropopause     

A. R. Chughtai  J. M. Kim  D. M. Smith 《Journal of Atmospheric Chemistry》2003,45(3):231-243

The effect of temperature (296–238 K) on the reaction of combustion soot (n-hexane) with ozone at low concentration (6–8 ppm) has been measured. Long optical path FTIR spectroscopy has revealed the rate law for ozone loss beyond initial stages, second order in O₃, to be the same over this range of conditions. The reaction rate is 3.5 times lower at 238 K than at 296 K, and reveals an activation energy of 12.9 ± 0.5 kJ mol^–1. The effect of humidity on the reaction has been estimated using its recently determined rate law dependence (p^0.2). These data, differing from O₃ reaction kinetics obtained from other types of carbonaceous particles used as surrogates for atmospheric soot, have implications for the role of combustion soot in atmospheric chemistry. Any involvement of aircraft soot in ozone depletion near the tropopause, for example, should be estimated using these temperature and humidity dependences.  相似文献   

The 1.05-μm feature in the spectrum of the Type Ia supernova 1994D: He in SNe Ia?     

P. A.Mazzali  L. B.Lucy 《Monthly notices of the Royal Astronomical Society》1998,295(2):428-436

A P Cygni profile with absorption at 1.05 μm was observed in three pre-maximum J -band spectra of the Type Ia supernova (SN) 1994D. The feature was not present in two post-maximum spectra. The line was attributed to He I 10830 ... or Mg II 10926 ..., based on a local thermodynamic equilibrium (LTE) treatment. The detection of He in the ejecta of a SN Ia would be useful for determining the pre-SN evolution and the explosion mechanism of SNe Ia.
In this paper, synthetic spectra are presented for both the He and Mg models. The population of the He levels has been computed in non-local thermodynamic equilibrium (NLTE), including non-thermal excitation and ionization effects resulting from the deposition of γ-rays from the decay of ⁵⁶Ni and ⁵⁶Co.
The J -band feature in the pre-maximum spectra can be reproduced either assuming the presence of a narrow shell, between 10000 and 12500 km s⁻¹, containing about 0.01 M⊙ of He, or increasing the abundance of Mg by about a factor of 5 with respect to the W7 value, implying a Mg mass of about 0.08 M⊙ above 10000 km s⁻¹. Both models are in good agreement with the optical spectrum. In particular, a strong He I 10830-... line does not imply a strong 5876-... line, because the departure coefficients of the 2p and 2s levels of He I differ by about an order of magnitude.
Unfortunately, neither model is able to reproduce the sudden disappearance of the J -band feature in the post-maximum spectra. Possible explanations are discussed.  相似文献   

Nuclear burning rates and Population II stellar models     

E. Brocato  V. Castellani  & F. L. Villante 《Monthly notices of the Royal Astronomical Society》1998,298(2):557-561

We discuss theoretical predictions concerning the evolution of globular cluster Population II stars with respect to current estimates of standard errors in the determination of nuclear burning rates. Numerical evaluations are given for the dependence of the turn-off and horizontal branch luminosities on the rate of the relevant nuclear reactions. We conclude that evolutionary predictions appear rather solid in this respect, with a maximum 3 σ error of about 1 Gyr in the evaluation of cluster ages derived from the calibration of the difference in luminosity between the horizontal branch and the turn-off. However, current evaluation of the original He content, as given on the basis of the R -parameter, will need to wait for a much better determination of the ¹²C(α, γ)¹⁶O reaction before reaching a satisfactory accuracy.  相似文献   

On the Exchange of NO3 Radicals with Aqueous Solutions: Solubility and Sticking Coefficient     

Karin Thomas  Andreas Volz-Thomas  Djuro Mihelcic  Herman G. J. Smit  Dieter Kley 《Journal of Atmospheric Chemistry》1998,29(1):17-43

The exchange of NO₃ radicals with the aqueous-phase was investigated at room temperature (293 K) in a series of wetted denuders. From these experiments, the uptake coefficient of NO₃ was determined on 0.1 M NaCl solutions and was found to be (NO₃) 2 × 10^-3 in good agreement with recent studies. The Henry coefficient of NO₃ was estimated to be K_H(NO₃) = 1.8 M · atm^-1, with a (2) uncertainty of ±3 M · atm^-1. From the upper limit for the Henry coefficient (K_H = 5 M · atm^-1) and available thermodynamic data, the redox potential of dissolved NO₃/NO ₃ ^– is estimated to be in the range of 2.3 to 2.5 V. This range is at the lower boundary of earlier estimates. The results are discussed in the light of a recent publication. Based on our data and a model of the transport and chemistry in the liquid film, an upper limit is derived for the product of the Henry coefficient K_H and the rate coefficient k ₁₀ of the potential reaction NO₃ + H₂O HNO₃ + OH. For K_H = 0.6 M · atm^-1, we find k ₁₀ < 0.05 s^-1 · atm^-1, i.e., about 100 times smaller than what was suggested by Rudich and co-workers. Because of its small solubility, heterogeneous removal of NO₃ is only important under conditions where the dissolved NO₃ is removed quickly from equilibrium, for example by reactions with Cl^– or HSO ₃ ^– ions in the liquid-phase. Otherwise, heterogenous removal should mainly proceed via N₂O₅.  相似文献   

Primary Product Distribution from the Reaction of Hydroxyl Radicals with Toluene at ppb NOX Mixing Ratios     

D. F. Smith  C. D. McIver  T. E. Kleindienst 《Journal of Atmospheric Chemistry》1998,30(2):209-228

A study has been conducted to examine the OH-initiated degradation products of toluene in the presence of sub part-per-million levels of NO_X. The experiments were conducted in a dynamic reactor to minimize the conversion of the aromatic compounds while allowing a sufficient mass of products to be collected for analysis. The major primary products detected in the toluene system (with molar yields) include glyoxal (0.238), methylglyoxal (0.167), o-cresol (0.120), benzaldehyde (0.06), 4-oxo-2-pentenal (0.03), and p-cresol (0.03). Six other reaction products, most being ring cleavage products, were measured at yields below 3%. Corrections for secondary OH reactions with the products were made where necessary. The formation of the cresol isomers was found to be invariant to the NO₂ concentrations which indicates that under atmospheric conditions the initial hydroxycyclohexadienyl radical reaction with NO₂ is a minor process and that most of the reaction occurs with O₂. The product yields found in this study are expected to be representative of those that occur in the urban atmosphere at ambient NO₂ concentrations.  相似文献   

Carbonatite melt–CO₂ fluid inclusions in mantle xenoliths from Tenerife, Canary Islands: a story of trapping, immiscibility and fluid–rock interaction in the upper mantle     

Maria Luce Frezzotti  Tom Andersen  Else-Ragnhild Neumann  Siri Lene Simonsen 《Lithos》2002,64(3-4):77-96

Three types of fluid inclusions have been identified in olivine porphyroclasts in the spinel harzburgite and lherzolite xenoliths from Tenerife: pure CO₂ (Type A); carbonate-rich CO₂–SO₂ mixtures (Type B); and polyphase inclusions dominated by silicate glass±fluid±sp±silicate±sulfide±carbonate (Type C). Type A inclusions commonly exhibit a “coating” (a few microns thick) consisting of an aggregate of a platy, hydrous Mg–Fe–Si phase, most likely talc, together with very small amounts of halite, dolomite and other phases. Larger crystals (e.g. (Na,K)Cl, dolomite, spinel, sulfide and phlogopite) may be found on either side of the “coating”, towards the wall of the host mineral or towards the inclusion center. These different fluids were formed through the immiscible separations and fluid–wall-rock reactions from a common, volatile-rich, siliceous, alkaline carbonatite melt infiltrating the upper mantle beneath the Tenerife. First, the original siliceous carbonatite melt is separated from a mixed CO₂–H₂O–NaCl fluid and a silicate/silicocarbonatite melt (preserved in Type A inclusions). The reaction of the carbonaceous silicate melt with the wall-rock minerals gave rise to large poikilitic orthopyroxene and clinopyroxene grains, and smaller neoblasts. During the metasomatic processes, the consumption of the silicate part of the melt produced carbonate-enriched Type B CO₂–SO₂ fluids which were trapped in exsolved orthopyroxene porphyroclasts. At the later stages, the interstitial silicate/silicocarbonatite fluids were trapped as Type C inclusions. At a temperature above 650 °C, the mixed CO₂–H₂O–NaCl fluid inside the Type A inclusions were separated into CO₂-rich fluid and H₂O–NaCl brine. At T<650 °C, the residual silicate melt reacted with the host olivine, forming a reaction rim or “coating” along the inclusion walls consisting of talc (or possibly serpentine) together with minute crystals of NaCl, KCl, carbonates and sulfides, leaving a residual CO₂ fluid. The homogenization temperatures of +2 to +25 °C obtained from the Type A CO₂ inclusions reflect the densities of the residual CO₂ after its reactions with the olivine host, and are unrelated to the initial fluid density or the external pressure at the time of trapping. The latter are restricted by the estimated crystallization temperatures of 1000–1200 °C, and the spinel lherzolite phase assemblage of the xenolith, which is 0.7–1.7 GPa.  相似文献   

Precambrian granulites of the Aldan shield, eastern Siberia, USSR   总被引：17，自引：0，他引：17  

L. L. PERCHUK  L. Ya. ARANOVICH  K. K. PODLESSKII  I. V. LAVRANT'EVA  V. Yu GERASIMOV  V. V. FED'KIN  V. I. KITSUL  L. P. KARSAKOV  N. V. BERDNIKOV 《Journal of Metamorphic Geology》1985,3(3):265-310

Abstract Precambrian granulites of the Aldan shield in southern Yakutia, USSR, form a massif of 200,000 km² bounded by younger fold-belts to the south, west and east. The massif consists of several blocks that reflect a primary heterogeneity of composition and differences in structural and thermodynamic evolution of different parts of the area. According to structural and petrological data the massif can be divided into two megablocks: eastern Aldan and western Aldan. They are separated by a narrow meridional fold-belt. Structural evolution of this central zone was determined by the geodynamics of the mega-blocks and was completed in the late Archaean. Towards the south, this central zone is ‘transformed’into the relatively small Sutam block adjoining the Stanovoy fold-belt that bounds the Aldan shield on the south. The Sutam block is separated from the other structural units of the Aldan shield by a system of north trending grabens filled by post-Archaean sediments. The Aldan shield is composed of Archaean high-grade granulites, while the Stanovoy fold-belt, to the south, consists of highly foliated Proterozoic rocks metamorphosed under relatively lower-grade conditions. However, relics of the granulites are mapped within the fold-belt. They contain high-grade assemblages (e.g. Opx + Sil + Qz, Sap + Qz, Opx + Gr + Sil, etc.). One of the relics, the Tokskii block, which is only slightly touched by diaphthoresis, is located in the southeastern part of the Stanovoy fold-belt. Metamorphic conditions of the Tokskii block are compared with those of the Sutam block and a similar evolution of the units is revealed. Mineral assemblages and mineral compositions do not vary within each unit, but they change in a north-south direction. The Opx + Sil + Qz assemblage has been found only in Sutam and Tok, but not in eastern Aldan and western Aldan. The Sap + Qz assemblage has been found in the Tokskii block but has not yet been found in the Sutam block. The pyrope content in garnets, from metapelites of both blocks, is significantly higher than that from the Aldan (eastern and western blocks) rocks to the north. The most important assemblages from different units of the Aldan shield have been studied using the electron microprobe in order to unravel the metamorphic evolution of the granulites and thus to deduce the thermodynamic regime of this evolution. A geodynamic model for the Aldan shield is discussed in terms of Archaean island arc development.  相似文献