Petrogenesis of acapulcoites and lodranites: A shock-melting model

Acapulcoites are modeled as having formed by shock melting CR-like carbonaceous chondrite precursors; the degree of melting of some acapulcoites was low enough to allow the preservation of 3-6 vol % relict chondrules. Shock effects in acapulcoites include veins of metallic Fe-Ni and troilite, polycrystalline kamacite, fine-grained metal-troilite assemblages, metallic Cu, and irregularly shaped troilite grains within metallic Fe-Ni. While at elevated temperatures, acapulcoites experienced appreciable reduction. Because graphite is present in some acapulcoites and lodranites, it seems likely that carbon was the principal reducing agent. Reduction is responsible for the low contents of olivine Fa (4-14 mol %) and low-Ca pyroxene Fs (3-13 mol %) in the acapulcoites, the observation that, in more than two-thirds of the acapulcoites, the Fa value is lower than the Fs value (in contrast to the case for equilibrated ordinary chondrites), the low FeO/MnO ratios in acapulcoite olivine (16-18, compared to 32-38 in equilibrated H chondrites), the relatively high modal orthopyroxene/olivine ratios (e.g., 1.7 in Monument Draw compared to 0.74 in H chondrites), and reverse zoning in some mafic silicate grains. Lodranites formed in a similar manner to acapulcoites but suffered more extensive heating, loss of plagioclase, and loss of an Fe-Ni-S melt.Acapulcoites and lodranites experienced moderate post-shock annealing, presumably resulting from burial beneath material of low thermal diffusivity. The annealing process repaired damaged olivine crystal lattices, lending acapulcoites and lodranites the appearance of unshocked (i.e., shock-stage S1) rocks. Any high-pressure phases that may have formed during initial shock reverted to their low-pressure polymorphs during annealing. Some samples were subsequently shocked again; several acapulcoites reached shock-stage S2 levels, ALH 84190 reached S3, and the lodranite MAC 88177 reached S5.     

Klimatologische Untersuchung über die geographische Verteilung der Globalstrahlung und der diffusen Himmelsstrahlung

Zusammenfassung Die vorliegende strahlungsklimatologische Studie verfolgt den Zweck, ausschließlich auf Grund von Meßergebnissen anhand ausgewählter Beispiele gegensätzlicher Großaumklimate zunächst die typischen Eigenschaften des Jahresganges der Globalstrahlung (T) und der diffusen Himmelsstrahlung (D) zu untersuchen. In Ergänzung hierzu werden häufigkeitsstatistische Betrachtungen über die Eigenheiten im Verhalten dieser Strahlungselemente im Bereich verschieden beschaffener geographischer Räume angestellt. Dabei zeigt sich, daß neben dem jahreszeitlichen Ablauf auch die Häufigkeitsverteilungen vonT undD sich zur Klimatypisierung heranziehen lassen. Ein besonderes Interesse beansprucht der jahreszeitliche Verlauf des VerhältnissesD/T in den Hauptklimazonen der Erde. Ferner wird die Breitenabhängigkeit der Globalstrahlung (T), der diffusen Himmelsstrahlung (D) und des VerhältnissesD/T behandelt. Hinsichtlich der Höhenabhängigkeit ergibt sich auf Grund des Verhaltens vonT undD für die gemäßigten Breiten eine Abnahme des QuotientenD/T. Den Abschluß bilden strahlungsklimatologische Überlegungen zur Frage der Abhängigkeit des VerhältnissesD/T von der Sonnenscheindauer im Bereich charakteristischer Klimazonen der Erde.

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Summary The present study of radiation climatology, exclusively basing on results of measurements, presents selected examples of contrary large-scale climatic conditions and examines the typical features of the annual variation of global radiation (T) and diffuse sky radiation (D). Supplementary considerations on frequency statistics refer to the peculiarities in the behaviour of these two radiation elements in different geographical areas. It results that beside the seasonal variation frequency distributions ofT andD can be used for a characterization of radiation climates. The seasonal variation of the characteristic ratioD/T in the main climatic zones of the earth is of particular interest. Furthermore, the latitudinal dependency of global radiation (T), diffuse sky radiation (D) and of the ratioD/T is discussed. As to the dependence on altitude the ratioD/T shows a decrease in temperate latitudes on account of the behaviour ofD andT. Finally climatological reflections on the dependence of the ratioD/T on the duration of sunshine in characteristic climatic regions of the earth are discussed.

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Résumé La présente étude de la climatologie du rayonnement a pour but de déterminer les propriétés typiques de l'évolution annuelle du rayonnement global (T) et du rayonnement diffus du ciel (D) et cela exclusivement sur la base de mesures faites et d'exemples choisis parmi des climats très différenciés. En complément, on prend en considération des statistiques de la fréquence des particularités de ces deux éléments à l'intérieur d'aires géographiques de climats divers. On en déduit que, à côté de l'évolution annuelle, la répartition des fréquences deT et deD permettent d'établir le type du climat. L'évolution saisonnière du rapportD/T est tout spécialement intéressant dans les principales zones climatiques de la terre. On considère ensuite la dépendance du rayonnement global (T), du rayonnement diffus du ciel (D) et du rapportD/T de la latitude du lieu. Dans les zones tempérées, le quotientD/T diminue avec l'altitude. L'auteur termine par des réflexions concernant la climatologie du rayonnement en relation avec la dépendance du rapportD/T de la durée d'insolation dans des zones climatiques caractéristiques de notre planète.

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Mit 12 Textabbildungen     

Das weltweite Verhalten der Höhe der F2-Schicht

Summary Maps of the factor (M 3000)F2 which is a parameter corresponding to the inverse of the altitude of theF2-layer have been prepared for America between the magnetic equator and high magnetic latitudes. Common features of all maps are a daily minimum on the magnetic equator, a sunrise-maximum and a daily maximum at a geographic latitude of about 40° N. This latter is less pronounced in summer than in winter. With increasing solar activity we find a general decrease ofM-values the equator daily minimum becoming relatively deeper.

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Résumé On a construit des cartes du facteur (M3000)F2 qui est un paramètre correspondant à l’inverse de l’altitude du centre de la coucheF2. Ces cartes sont valables pour l’Amérique entre l’équateur magnétique et une latitude magnétique assez élevée. Il y a des résultats qui se reproduisent sur l’ensemble des cartes: Un maximum se produit vers le lever du soleil, il ne dépend guère de la latitude. De jour il y a un minimum sur l’équateur magnétique et un maximum auprès de la latitude géographique de 40°N. Ce dernier maximum est moin net en eté que durant l’hiver. Avec une activité solaire croissante les valeursM diminuent en même temps, le minimum sur l’équateur devient de plus en plus profond.

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ProfessorJonathan Zenneck zum fünfundachtzigsten Geburtstag in Dankbarkeitgewidmet.

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Vertrags-Ingenieure der franz?sischen Marine beim Service de Prévision Ionosphérique Militaire.     

Main results of recent investigations into the physical mechanisms of the interaction of tropical cyclones and the ionosphere

Izvestiya, Atmospheric and Oceanic Physics - International investigations into the correlation between tropical cyclones (TCs) and the ionosphere are associated with great difficulties in proving...     

Interaction between atmospheric flows and obstacles: Experiments in a rotating channel

We performed an experimental study using scale models in a hydrodynamic rotating channel, concerning the interactions between fluid flows and obstacles of different shapes. The study was meant to analyze the characteristics of the wakes observed on the lee side of quasi-bidimensional obstacles, in a neutral atmosphere.The obstacles were half-cylinders (with aspect ratio 0.87), placed transversally on the channel bottom and totally submerged in the fluid. We call them quasi-bidimensional since their width was a little smaller than the channel width, thus allowing the flow to partially go round their edges.The simulations were performed in the rotating hydraulic channel of ICG-CNR in Turin, and included various conditions of rotation period and flow speed. An interesting behaviour of the wakes was found on the lee side of subsynoptic-scale obstacles, modelled in conditions of Reynolds-Rossby similitude. More precisely, if a given threshold of flow velocity is exceeded, wake size is constant and is fully determined by the height of the obstacle.     

Temperature trends through urbanization in Metropolitan Washington,D.C., 1945–1979

Summary Annual and seasonal changes of the urban heat island magnitude in Washington, D.C. and a close-in suburb were analyzed for the period of 1945 through 1979. Monthly and annual mean temperature data, corrected for elevation, from three stations (National Airport, D.C.—downtown, Silver Spring, MD—suburban, and Beltsville, MD—rural) were evaluated by testing the statistical significance of temperature differences between the different data sets.The analysis indicated that the absolute heat island magnitudes have increased at the rate of 0.020 and 0.019 C/yr at Washington National Airport and Silver Spring, respectively during the study period. The differences between these two rates were not statistically significant at the 0.95 confidence level. However, the increase of the heat island magnitude from 1945 to 1979 at the suburban station was 20% greater than that at the downtown station. Those results indicate that as urbanization progresses, the suburban site, Silver Spring, has become incorporated into the Washington, D.C. urban heat island. The data also indicate that the magnitude of the heat island near the city center has continued to increase despite a significant population decrease since 1950.With 5 Figures     

Fault-slip Triggering, Healing, and Viscoelastic Afterworking in Sediments in the Mexicali-Imperial Valley

—Crackmeter data from the southern tip of the Imperial fault in the Mexicali-Imperial Valley in northern Baja California, Mexico, show slip events that tend to group in episodes, or suites, that last over a few days, with months of quiescence between them. The events in these suites are apparently triggered and stopped by ground strain related to temperature changes. A characteristic decrease in peak extension velocities during the slip event suites indicates the presence of viscoelastic afterworking which contributes the strain necessary for succeeding events with diminishing driving stresses, and which can be roughly modeled as a series of Kelvin-Voigt viscoelastic solids. Occurrence of very small events towards the end of the suites plus gaps between suites indicate that for the sediments significant fault healing requires a minimum of several days without slip.     

Kīlauea summit overflows: their ages and distribution in the Puna District, Hawai'i

 The tube-fed pāhoehoe lava flows covering much of the northeast flank of Kīlauea Volcano are named the 'Ailā'au flows. Their eruption age, based on published and six new radiocarbon dates, is approximately AD 1445. The flows have distinctive paleomagnetic directions with steep inclinations (40°–50°) and easterly declinations (0°–10°E). The lava was transported ∼40 km from the vent to the coast in long, large-diameter lava tubes; the longest tube (Kazumura Cave) reaches from near the summit to within several kilometers of the coast near Kaloli Point. The estimated volume of the 'Ailā'au flow field is 5.2±0.8 km³, and the eruption that formed it probably lasted for approximately 50 years. Summit overflows from Kīlauea may have been nearly continuous between approximately AD 1290 and 1470, during which time a series of shields formed at and around the summit. The 'Ailā'au shield was either the youngest or the next to youngest in this series of shields. Site-mean paleomagnetic directions for lava flows underlying the 'Ailā'au flows form only six groups. These older pāhoehoe flows range in age from 2750 to <18,000 BP, and the region was inundated by lava flows only three times in the past 5000 years. The known intervals between eruptive events average ∼1600 years and range from ∼1250 years to >2200 years. Lava flows from most of these summit eruptions also reached the coast, but none appears as extensive as the 'Ailā'au flow field. The chemistry of the melts erupted during each of these summit overflow events is remarkably similar, averaging approximately 6.3 wt.% MgO near the coast and 6.8 wt.% MgO near the summit. The present-day caldera probably formed more recently than the eruption that formed the 'Ailā'au flows (estimated termination ca. AD 1470). The earliest explosive eruptions that formed the Keanakāko'i Ash, which is stratigraphically above the 'Ailā'au flows, cannot be older than this age. Received: 10 October 1998 / Accepted: 12 May 1999     

Observing and Modeling Earth’s Energy Flows

This article reviews, from the authors’ perspective, progress in observing and modeling energy flows in Earth’s climate system. Emphasis is placed on the state of understanding of Earth’s energy flows and their susceptibility to perturbations, with particular emphasis on the roles of clouds and aerosols. More accurate measurements of the total solar irradiance and the rate of change of ocean enthalpy help constrain individual components of the energy budget at the top of the atmosphere to within ±2 W m^?2. The measurements demonstrate that Earth reflects substantially less solar radiation and emits more terrestrial radiation than was believed even a decade ago. Active remote sensing is helping to constrain the surface energy budget, but new estimates of downwelling surface irradiance that benefit from such methods are proving difficult to reconcile with existing precipitation climatologies. Overall, the energy budget at the surface is much more uncertain than at the top of the atmosphere. A decade of high-precision measurements of the energy budget at the top of the atmosphere is providing new opportunities to track Earth’s energy flows on timescales ranging from days to years, and at very high spatial resolution. The measurements show that the principal limitation in the estimate of secular trends now lies in the natural variability of the Earth system itself. The forcing-feedback-response framework, which has developed to understand how changes in Earth’s energy flows affect surface temperature, is reviewed in light of recent work that shows fast responses (adjustments) of the system are central to the definition of the effective forcing that results from a change in atmospheric composition. In many cases, the adjustment, rather than the characterization of the compositional perturbation (associated, for instance, with changing greenhouse gas concentrations, or aerosol burdens), limits accurate determination of the radiative forcing. Changes in clouds contribute importantly to this adjustment and thus contribute both to uncertainty in estimates of radiative forcing and to uncertainty in the response. Models are indispensable to calculation of the adjustment of the system to a compositional change but are known to be flawed in their representation of clouds. Advances in tracking Earth’s energy flows and compositional changes on daily through decadal timescales are shown to provide both a critical and constructive framework for advancing model development and evaluation.