The origin of chondritic macromolecular organic matter: A carbon and nitrogen isotope study     

C. M. O'D. ALEXANDER  S. S. RUSSELL  J. W. ARDEN  R. D. ASH  M. M. GRADY  C. T. PILLINGER 《Meteoritics & planetary science》1998,33(4):603-622

Abstract— The N and C abundances and isotopic compositions of acid-insoluble carbonaceous material in thirteen primitive chondrites (five unequilibrated ordinary chondrites, three CM chondrites, three enstatite chondrites, a CI chondrite and a CR chondrite) have been measured by stepped combustion. While the range of C isotopic compositions observed is only ～δ¹³C = 30%, the N isotopes range from δ¹⁵N ' -40 to 260%. After correction for metamorphism, presolar nanodiamonds appear to have made up a fairly constant 3–4 wt% of the insoluble C in all the chondrites studied. The apparently similar initial presolar nanodiamond to organic C ratios, and the correlations of elemental and isotopic compositions with metamorphic indicators in the ordinary and enstatite chondrites, suggest that the chondrites all accreted similar organic material. This original material probably most closely resembles that now found in Renazzo and Semarkona. These two meteorites have almost M-shaped N isotope release profiles that can be explained most simply by the superposition of two components, one with a composition between δ¹⁵N = -20 and -40% and a narrow combustion interval, the other having a broader release profile and a composition of δ¹⁵N ～ 260%. Although isotopically more subdued, the CI and the three CM chondrites all appear to show vestiges of this M-shaped profile. How and where the components in the acid-insoluble organics formed remains poorly constrained. The small variation in nanodiamond to organic C ratio between the chondrite groups limits the local synthesis of organic matter in the various chondrite formation regions to at most 30%. The most ¹⁵N-rich material probably formed in the interstellar medium, and the fraction of organic N in Renazzo in this material ranges from 40 to 70%. The isotopically light component may have formed in the solar system, but the limited range in nanodiamond to total organic C ratios in the chondrite groups is consistent with most of the organic material being presolar.  相似文献   

Spectral surface albedo over Morocco and its impact on radiative forcing of Saharan dust     

By EIKE BIERWIRTH  MANFRED WENDISCH  RÉ EHRLICH  BIRGIT HEESE  MATTHIAS TESCHE  DIETRICH ALTHAUSEN  ALEXANDER SCHLADITZ  DETLEF MÜLLER  SEBASTIAN OTTO  THOMAS TRAUTMANN  TILMAN DINTER  WOLFGANG von HOYNINGEN-HUENE  RALPH KAHN ‡ 《Tellus. Series B, Chemical and physical meteorology》2009,61(1):252-269

In May–June 2006, airborne and ground-based solar (0.3–2.2 μm) and thermal infrared (4–42 μm) radiation measurements have been performed in Morocco within the Saharan Mineral Dust Experiment (SAMUM). Upwelling and downwelling solar irradiances have been measured using the Spectral Modular Airborne Radiation Measurement System (SMART)-Albedometer. With these data, the areal spectral surface albedo for typical surface types in southeastern Morocco was derived from airborne measurements for the first time. The results are compared to the surface albedo retrieved from collocated satellite measurements, and partly considerable deviations are observed. Using measured surface and atmospheric properties, the spectral and broad-band dust radiative forcing at top-of-atmosphere (TOA) and at the surface has been estimated. The impact of the surface albedo on the solar radiative forcing of Saharan dust is quantified. In the SAMUM case of 19 May 2006, TOA solar radiative forcing varies by 12 W m⁻² per 0.1 surface-albedo change. For the thermal infrared component, values of up to +22 W m⁻² were derived. The net (solar plus thermal infrared) TOA radiative forcing varies between −19 and +24 W m⁻² for a broad-band solar surface albedo of 0.0 and 0.32, respectively. Over the bright surface of southeastern Morocco, the Saharan dust always has a net warming effect.  相似文献   

Gravity waves in the mesosphere generated by tropospheric convention     

JAMES R. HOLTON  M. JOAN ALEXANDER 《地球，A辑:动力气象学与海洋学》1999,51(1):45-58

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Exploration of quantitative kinetic models for the evaporation of silicate melts in vacuum and in hydrogen     

C. M. O'D. ALEXANDER 《Meteoritics & planetary science》2001,36(2):255-283

Abstract— Two basic approaches (pure component reference (PCR) and equilibrium reference (EQR)) to modeling silicate melt evaporation are explored. The PCR model calculates the maximum possible evaporation rates of the pure oxides from their equilibrium vapor pressures and rescales these rates according to the activities of the oxides in the silicate melts and the melt densities. The EQR model calculates the maximum possible evaporation rates based on the equilibrium vapor pressures of the melts. Differences between the calculated and experimentally determined evaporation rates are accounted for with evaporation (α^evap) coefficients that are only dependent on temperature. Two versions of the PCR model, Cases 1 and 2, are explored to try to resolve apparently contradictory conclusions about the composition of the evaporating species based on Mg and Si isotope fractionation during evaporation (species are not in thermodynamic equilibrium proportions) and direct measurements of gas species in Langmuir experiments (species are in roughly equilibrium proportions). The Case 2 and EQR models cannot explain the observed isotope fractionations unless evaporation occurred under non‐Rayleigh conditions, either because there was significant recondensation during the experiments or because diffusion was playing a limiting role. Whether or not the role of diffusion is included, the PCR and EQR models are able to reproduce the elemental results of evaporation experiments of “chondritic” melts from temperatures of 1700 to 2000 °C, and up to mass losses of about 95%. However, the models underestimate absolute evaporation rates in very Ca‐ and Al‐rich melts. This may reflect errors in the model used to estimate oxide activities. The EQR model can only reproduce the observed evaporation behavior of Na if, unlike the other oxides, its α^evap coefficient is close to unity. Based on available diffusion data, diffusion is not slow enough in “chondritic” or forsteritic melts to explain the isotopic fractionations of Mg and O in the evaporation experiments, but it may play a role in limiting Si isotope fractionation. Provided recondensation was not a significant factor in the experiments, at present PCR Case 1 appears to be the best model if both the Langmuir and the isotopic fractionation experiments are to be explained.  相似文献   

A STEADY‐STATE MASS‐BALANCE MODEL FOR THE FRANZ JOSEF GLACIER,NEW ZEALAND: TESTING AND APPLICATION     

DAVID J. ALEXANDER  TIM R. DAVIES  JAMES SHULMEISTER 《Geografiska Annaler: Series A, Physical Geography》2011,93(1):41-54

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The insoluble carbonaceous material of CM chondrites: A possible source of discrete organic compounds under hydrothermal conditions     

Hikaru YABUTA  Lynda B. WILLIAMS  George D. CODY  Conel M. O'D. ALEXANDER  Sandra PIZZARELLO 《Meteoritics & planetary science》2007,42(1):37-48

Abstract— We report on the molecular analyses of the water‐ and solvent‐soluble organic compounds released from the insoluble organic material (IOM) of the Murray meteorite upon treatment with weight‐equivalent amounts of water and under conditions of elevated temperature and pressure. A varied suite of compounds was identified by gas chromatography‐mass spectrometry (GC‐MS). C₃‐C₁₇ alkyl dicarboxylic acids and N‐ and O‐containing hydroaromatic and aromatic compounds were found in the water extracts. The solvent extracts contained N‐, O‐, and S‐containing aromatic compounds, a large number of their isomers and homologs, and a series of polycyclic aromatic hydrocarbons (PAHs) of up to five rings, together with noncondensed aromatic species such as substituted benzenes, biphenyl, and terphenyls as well as their substituted homologs, and hydrated PAHs. Isotopic analyses showed that residue IOMs after hydrothermal treatment had lower deuterium and ¹⁵N content than the untreated material (ΔD = ?833‰ and Δ¹⁵N = ?24.1) but did not differ from it in ¹³C composition. The effect of the hydrothermolytic release was recorded in significant differences between the NMR spectra of untreated and residue IOM. A possible relation to common precursors for the dicarboxylic acids found in the IOM and bulk extracts is discussed.  相似文献   

The role of the West Spitsbergen Current in the formation of ice edge position     

ALEXANDER MAKSHTAS  IGOR PODGORNY 《Polar research》1991,9(2):207-210

The intensity of sea-ice interaction in the Fram Strait is estimated from hydrographic data by use of three different methods. Preliminary results concerning fresh water formation rate are reported.  相似文献   

Iron isotopes in chondrules: Implications for the role of evaporation during chondrule formation     

C. M. O'D. ALEXANDER  J. WANG 《Meteoritics & planetary science》2001,36(3):419-428

Abstract— We have measured the δ⁵⁷Fe of olivines in nine Chainpur chondrules. All are within error of normal (typically 2σ ≤ 1–2%0). Most of the chondules could not have lost more than ～20% of their FeO by Rayleigh evaporation and none can have lost more than ～61%. Yet, the range of Fo contents in these chondrules is Fo_78–99.9. The isotopic compositions of the chondrules clearly demonstrate that, for instance, type I chondrules cannot form from type II chondrules by evaporation of FeO under Rayleigh conditions. The isotopic compositions also place constraints on the minimum cooling rates these chondrules could have experienced. These cooling rates must also be equal to or slower than those required to produce the chondrule textures. Assuming flash heating and evaporation rates like those measured in vacuum, the minimum cooling rates necessary to prevent detectable Fe isotopic fractionation via Rayleigh evaporation approach those needed to produce barred and porphyritic textures. The presence of hydrogen in the nebula, non‐linear cooling and other effects will all tend to increase the cooling rates required to prevent δ⁵⁷Fe > 1–2%0, perhaps by as much as 1–2 orders of magnitude. The two most likely ways that the cooling rates required to prevent δ⁵⁷Fe >1–2%0 can be kept below those needed to produce barred and porphyritic textures are (1) the pH₂ in the nebula was low enough to keep evaporation rates close to those in vacuum, or (2) back reaction of chondrules with Fe in the gas suppressed isotopic fractionation.  相似文献   

Questions,questions: Can the contradictions between the petrologic,isotopic, thermodynamic,and astrophysical constraints on chondrule formation be resolved?     

Conel M. O’D. ALEXANDER  Denton S. EBEL 《Meteoritics & planetary science》2012,47(7):1157-1175

Abstract– Here, we show that several geochemical indicators point to number densities during chondrule formation that were far higher than can be accounted for by known nebula processes. The number densities implied by compound chondrules and nonspherical chondrules are shown to be significantly higher than estimated in previous studies. At the implied chondrule number densities, if a chondrule formation region survived a formation event it would have been gravitationally bound and would have collapsed quite rapidly to form an asteroidal‐sized body. The diversity of chondrule compositions and textures in a chondrite group could have formed in a single event in subvolumes of a formation region that were chemically isolated from one another because of slow diffusion in the gas. Within these subvolumes, equilibration between chondrules with different compositions would have been fairly rapid, although small isotopic mass fractionations in elements like Fe, Si, Mg, and O may persist. This could explain the existence of the small isotopic mass fractionations in these elements that have been observed in chondrules. However, the evidence for recycling of chondrules requires that there was more than one chondrule formation event prior to formation of a parent asteroid. Finally, we argue that OC and CO chondrule Mg‐Al systematics are both consistent with single ages or narrow ranges of ages, and that the CO, and possibly the OC, ages date parent body alteration. This would resolve the conundrum of needing to preserve in a turbulent nebula physically and chemically distinct CO and OC chondrule populations for 1–2 Myr.  相似文献   

POLITICAL GEOGRAPHY IN THE GEOGRAPHICAL REVIEW: VOLUMES 1–99*     

ALEXANDER B. MURPHY 《Geographical review》2010,100(4):443-450

Editors' Note: This essay by Alexander Murphy begins the final set of retrospective works commissioned to celebrate the centennial volume of the Geographical Review. In this group, American Geographical Society Councilors undertake the challenge of reviewing major conceptual approaches or topical fields as presented in the journal's first ninety‐nine volumes and to muse about how those approaches will contribute to the field in its second century. In addition to Murphy, the contributors comprise Councilors Marie Price and Douglas Sherman, as well as a review of fieldwork by Associate Editor Dydia DeLyser and her coauthor, Paul Karolczyk. As much as retrospective views, we hope these essays point the way for scholarship in the coming years.  相似文献