Earth-orbit-approaching comets and their theoretical meteor radiants     

Jack D. Drummond 《Icarus》1981,47(3):500-517

Sixteen comets produce recognizable meteor showers that are found in A. F. Cook's (1973, In Evolutionary and Physical Properties of Meteoroids (C. L. Hemenway, P. M. Millman, and A. F. Cook, Eds.), pp. 183–191, U.S. Govt. Printing Office, Washington, D.C.), working list of meteor streams. Of these, five are long period, including one in a parabolic and one in a hyperbolic orbit. The largest Earth-comet orbit miss distance is 0.20 AU for P/Encke and the Northern and Southern Taurids. Using this is an upper limit for meteor showers from comets, all comets which approach the Earth's orbit to within 0.20 AU were extracted from the Catalogue of Cometary Orbits (B. G. Marsden, 1979. 3rd ed., Central Bureau of Astronomical Telegrams, IAU SAO, Cambridge, Mass.). A compilation of such comets is presented by date minimum approach, along with the distance of closest approach and the theoretical geocentric radiants and velocities of possible associated meteor showers. Both pre- and postpperihelion encounters with the Earth's orbit are considered. There are 240 entries for 178 long-period comets, and 36 for 28 short-period comets. It is noted that all short-period comets that have approached the Earth's orbit to within 0.08 AU have produced meteors, except P/Lexell, P/Finlay, P/Denning-Fujikawa, and P/Grigg-Skjellerup. Attention is called to the favorable observing conditions for detecting meteors from P/Grigg-Skjellerup in April 1982, and for the possibility of another great Draconid storm from P/Giacobini-Zinner in October 1985. A comparison is made between observed sporadic meteor rates and the distribution of theoretical radiants throughout the year, from which it is concluded that the currently known comets can account for sporadic meteors. A criterion is developed to test whether or not an observed meteor shower can be associated with a given theoretical radiant. Based on known examples, a qualitative model for comet/meteor relationships is also presented.  相似文献   

Phytoliths as indicators of pedogenesis and paleoenvironmental changes in the Brazilian cerrado     

Miriam Borba-Roschel  Anne Alexandre  Anglica Fortes Drummond Chicarino Varajo  Jean Dominique Meunier  Csar Augusto Chicarino Varajo  Fabrice Colin 《Journal of Geochemical Exploration》2006,88(1-3):172

This paper presents the preliminary results of phytolith analyses of a peat located in the cerrado of the Uberaba municipality, State of Minas Gerais, Brazil. The phytolith soil sequence is discussed by comparison with phytolith assemblages extracted from dominant plants (Cyperaceae and Poaceae). Increasing mean age of phytolith assemblages with depth is assumed. Poaceae Cuneiform bulliform cell, Parallelepiped bulliform cell and Elongate smooth long cell types dominate up to 80 cm, rapidly drop to 42% at 80 cm and regularly decrease from 42% to 2% upwards. Cyperaceae Rondel concave type shows the inverse trend, being dominant in the upper part of the profile. This pattern can be assigned to increasing selective dissolution of the Cyperaceae phytolith type with depth, or/and to a decrease of water stress suffered by the grasses, leading to a decrease of bulliform cell silicification. Soil processes and paleo-environmental changes hypotheses are discussed.  相似文献   

Preliminary 3–D geological model of the Kalgoorlie region,Yilgarn Craton,Western Australia,based on deep seismic‐reflection and potential‐field data     

B. R. Goleby  R. J. Korsch  T. Fomin  B. Bell  M. G. Nicoll  B. J. Drummond 《Australian Journal of Earth Sciences》2013,60(6):917-933

The granite‐greenstone terranes of the Eastern Goldfields Province, Yilgarn Craton, Western Australia, are a major Australian and world gold and nickel source. The Kalgoorlie region, in particular, hosts several world‐class gold deposits. To attempt to understand why these deposits occur where they do, it is important to understand the crustal architecture in the region and how the major mineral systems operate in this architecture. One way to understand these relationships is to develop a detailed 3–D geological model for the region. The best method to map the 3–D geometry of major geological structures is by acquisition and interpretation of seismic‐reflection profiles. To contribute to this aim, a grid of deep seismic‐reflection traverses was acquired in 1999 to examine the 3–D geometry of the region in an area including the Kalgoorlie mineral region and mineral fields to the north and west. This grid was tied to the 1991 regional deep seismic traverse and 1997 high‐resolution seismic profiles in the same region. The grid covers an area measuring approximately 50 km wide by 50 km long and extended to a depth of approximately 50 km (below the base of the crust in this region). The resulting 3–D geological model was further constrained by both surface geological data and geophysical interpretations, with the seismic interpretations themselves also constrained by gravity and magnetic modelling. The 3–D model was used to investigate the geometric relationships between the major faults and shear zones in the area, the relationship between the granite‐greenstone succession and the basement, and the spatial relationships between the greenstones and the granites. Interpretation of the grid of seismic lines and construction of the 3–D geological model confirmed the existence of the detachment surface and led to the recognition that the granite‐greenstone contact usually occurs at a much shallower level than the detachment. Also, west‐dipping faults in the vicinity of the Golden Mile, including the Abattoir Shear through to Boulder‐Lefroy Fault, appear to be more important than previously thought in controlling the structure of that area. An antiformal thrust stack occurs beneath a triangle zone centred on the Golden Mile. The Black Flag Group was deposited in a probable extensional setting, and late extension was also probably more important than previously thought. The granite‐gneiss domes were uplifted by the formation of antiformal thrust stacks at depth beneath them.  相似文献   

Dimensions of drought: South African case studies   总被引：1，自引：0，他引：1  

C. H. Vogel  J. H. Drummond 《GeoJournal》1993,30(1):93-98

The recent drought in southern Africa has underscored the need for detailed analysis of the phenomenon. While geographers have researched the causes and impacts of drought in many African contexts, South Africa and in particular its Bantustans have not received sufficient similar attention. This paper outlines firstly the dimensions of drought in South Africa, including the biophysical and socio-economic factors. Issues such as land-use management, drought planning and relief are interrogated in the South African context. The final section of the paper highlights these debates with specific reference to case studies of past and present drought initiatives in South Africa.  相似文献   

Andesite and dacite genesis via contrasting processes: the geology and geochemistry of El Valle Volcano,Panama   总被引：3，自引：0，他引：3  

Mare J. Defant  Lee F. Clark  Robert H. Stewart  Mark S. Drummond  Jelle Z. de Boer  René C. Maury  Hervé Bellon  Thomas E. Jackson  Juan F. Restrepo 《Contributions to Mineralogy and Petrology》1991,106(3):309-324

The easternmost stratovolcano along the Central American arc is El Valle volcano, Panama. Several andesitic and dacitic lava flows, which range in age 5–10 Ma, are termed the old group. After a long period of quiescence (approximately 3.4 Ma), volcanic activity resumed approximately 1.55 Ma with the emplacement of dacitic domes and the deposition of dacitic pyroclastic flows 0.9–0.2 Ma. These are referred to as the young group. All of the samples analyzed are calc-alkaline andesites and dacites. The mineralogy of the two groups is distinct; two pyroxenes occur in the old-group rocks but are commonly absent in the young group. In contrast, amphibole has been found only in the young-group samples. Several disequilibrium features have been observed in the minerals (e.g., oscillatory zoning within clinopyroxenes). These disequilibrium textures appear to be more prevalent among the old- as compared with the young-group samples and are most likely the result of magma-mixing, assimilation, and/or polybaric crystallization. Mass-balance fractionation models for major and trace elements were successful in relating samples from the old group but failed to show a relationship among the young-group rocks or between the old- and young-group volcanics. We believe that the old-group volcanics were derived through differentiation processes from basaltic magmas generated within the mantlewedge. The young group, however, does not appear to be related to more primitive magmas by differentiation. The young-group samples cannot be related by fractionation including realistic amounts of amphibole. Distinctive geochemical features of the young group, including La/Yb ratios〉15, Yb〈1, Sr/Y〉150, and Y〈6, suggest that these rocks were derived from the partial melting of the subducted lithosphere. These characteristics can be explained by the partial melting of a source with residual garnet and amphibole. Dacitic material with the geochemical characteristics of subducted-lithosphere melting is generated apparently only where relatively hot crust is subducted, based on recent work. The young dacite-genesis at El Valle volcano is related to the subduction of relatively hot lithosphere.  相似文献   

Measurements of tropospheric OH concentrations: A comparison of field data with model predictions     

D. Perner  U. Platt  M. Trainer  G. Hübler  J. Drummond  W. Junkermann  J. Rudolph  B. Schubert  A. Volz  D. H. Ehhalt  K. J. Rumpel  G. Helas 《Journal of Atmospheric Chemistry》1987,5(2):185-216

Using long path UV absorption spectroscopy we have measured OH concentrations close to the earth's surface. The OH values observed at two locations in Germany during 1980 through 1983 range from 0.7×10⁶ to 3.2×10⁶ cm^-3. Simultaneously we measured the concentrations of O₃, H₂O, NO, NO₂, CH₄, CO, and the light non methane hydrocarbons. We also determined the photolysis rates of O₃ and NO₂. This allows calculations of OH using a zero dimensional time depdendent model. The modelled OH concentrations significantly exceed the measured values for low NO _x concentrations. It is argued that additional, so far unidentified. HO _x loss reactions must be responsible for that discrepancy.  相似文献   

Actinometric measurements and the radiation climate of South-West Africa     

A. J. Drummond  J. D. Wentzel 《Theoretical and Applied Climatology》1955,6(3):236-273

Summary In the paper two records of solar radiation obtained on the high plateau of South-West Africa are analysed. One of them contains about 6000 observations (until recently unpublished) of the intensity of direct radiation made, at Brukkaros, over the five-year period from 1927 to 1931, by the solar-constant observing-team of the Smithsonian Institution. The other is a double record of total and of diffuse hemispherical radiation at Windhoek. This latter station has been operating since August 1951 and is one of a number maintained by the Weather Bureau of South Africa. An account is included of the techniques which have been adopted for the operation of such radiation stations in the Union's network.For Brukkaros not only are various aspects of the basic intensity record discussed but an attempt has been made to build up a set of representative data for the total radiation which would be received on a horizontal plane at all times of the year from a cloudless sky. Sections of the paper are devoted to a preliminary investigation of the turbidity and the absorptive and reflective properties of the atmosphere over this semi-desert sub-tropical highland. The Windhoek material, especially, has been subjected to statistical analysis and a number of summaries are presented of the salient features of the record. A comparison is drawn between the two assemblages of values and generalizations are made regarding the solar radiation climate of the territory.As this would appear to be the first comprehensive study to be undertaken of the distribution of solar radiation in an arid zone of Africa, detail which has been considered of importance, particularly from the viewpoint of the future utilization of solar energy, has been included. The perfect integration of the two distinctly different records places a very high degree of confidence upon their interpretation.

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Zusammenfassung In der vorliegenden Arbeit werden zwei Registrierreihen der Sonnenstrahlung untersucht, die auf dem Hochplateau von Südwestafrika gewonnen wurden. Die eine davon umfaßt etwa 6000 erst kürzlich veröffentlichte Beobachtungen der Intensität der direkten Sonnenstrahlung, die während der fünfjährigen Periode 1927 bis 1931 von der Solarkonstanten-Beobachtungsgruppe der Smithsonian Institution in Brukkaros angestellt wurden. Das andere ist eine Parallelregistrierung der globalen und der diffusen Strahlung auf Horizontalfläche in Windhoek, wo im Strahlungsnetz des Weather Bureau von Südafrika seit August 1951 eine Station in Betrieb ist. Es wird auch über die in diesem Strahlungsnetz angewandten instrumentellen Einrichtungen berichtet.Für Brukkaros werden nicht nur verschiedene Gesichtspunkte der grundlegenden Intensitätsmessungen erörtert, sondern es wurde auch der Versuch gemacht, eine Serie der repräsentativen Größen für die Globalstrahlung zu berechnen, die von einer horizontalen Ebene zu jeder Zeit des Jahres von einem wolkenlosen Himmel erhalten werden. Weiterhin wurde eine vorläufige Untersuchung über die Trübung und die Absorptions- und Reflexionseigenschaften der Atmosphäre über diesem subtropischen Steppenhochland durchgeführt. Speziell das Material von Windhoek wurde einer statistischen Analyse unterzogen und in einigen zusammenfassenden Darstellungen wurden die wesentlichen Resultate der Registrierungen herausgearbeitet. Zwischen den Ergebnissen der beiden Stationen wird eine Vergleichung durchgeführt und es werden die allgemeinen Züge des Strahlungsklimas des Landes abgeleitet.Da es sich hierbei offenbar um eine erste zusammenfassende Untersuchung über die Verteilung der Sonnenstrahlung in einer Trockenzone von Afrika handelt, werden Einzelheiten erörtert, denen speziell im Hinblick auf eine zukünftige Ausnützung der Sonnenenergie eine besondere Bedeutung zugeschrieben werden muß. Die gute Übereinstimmung der zwei deutlich verschiedenen Beobachtungsreihen verleiht ihrer Deutung einen hohen Grad von Zuverlässigkeit.

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Résumé L'auteur analyse deux séries d'enregistrements du rayonnement solaire obtenues sur le haut plateau du Sud-Ouest africain; l'une comprend 6000 mesures de l'intensité du rayonnement solaire direct faites de 1927 à 1931 par un groupe d'observateurs de la constante solaire de la Smithsonian Institution à Brukkaros et publiées récemment; l'autre comprend un enregistrement parallèle du rayonnement global et diffus sur une surface horizontale à Windhoek où se trouve une station de rayonnement du Weather Bureau de l'Afrique du Sud depuis août 1951. Il décrit l'appareillage utilisé dans ce réseau pour l'étude du rayonnement.Pour Brukkaros, l'auteur ne se borne pas à discuter les mesures d'intensité de base, mais il tente de calculer une série de grandeurs représentatives du rayonnement global reçu par une surface horizontale à tout moment de l'année par ciel sans nuages. Il étudie provisoirement le trouble atmosphérique et les propriétés absorbantes et réfléchissantes de l'atmosphère de ce haut plateau semi-désertique subtropical. Le matériel de Windhoek spécialement a été soumis à l'examen statistique et les résultats essentiels des enregistrements mis en évidence par quelques condensés. Il compare ensuite les résultats des deux stations et esquisse le climat radiatif du pays.Dans cette première synthèse de la distribution du rayonnement dans une région sèche d'Afrique on mentionne aussi des détails importants pour l'utilisation future de l'énergie solaire. Le bon accord de ces deux séries différentes de mesures leur confère un degré élevé de crédibilité.

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With 9 Figures.  相似文献   

From subduction to transform motion: a seabeam survey of the Hellenic trench system     

Xavier Le Pichon  Jacques Angelier  Jean Aubouin  Nicolas Lyberis  Serge Monti  Vincent Renard  Henri Got  Ken Hsu¨  Yossi Mart  Jean Mascle  Drummond Matthews  Dimitri Mitropoulos  Pandelis Tsoflias  Georges Chronis 《Earth and Planetary Science Letters》1979,44(3):441-450

Preliminary results of a multi-narrow beam survey of the Hellenic trench system, in the Eastern Mediterranean, are presented. The southwestern Ionian branch is divided in small basins, partly filled with Pleistocene sediments. The morphology suggests that the basins are deformed by a compressional stress acting roughly perpendicularly to the trench along N50°E. This direction is the direction of the regional slip vector of the shallow thrust-type earthquakes. The structure of the southeastern Pliny-Strabo branch is quite different. Narrow en-e´chelon slots, oriented N40°E, have been mapped within the main troughs oriented N60°E. The regional earthquake slip vector is also oriented along N40°E. We conclude that the Hellenic trench system is an active subduction system, dominated by thrust along the Ionian branch and by transform motion along the Pliny-Strabo branch.  相似文献   

Holocene sea-level changes along the North Carolina Coastline and their implications for glacial isostatic adjustment models     

B.P. Horton  W.R. Peltier  S.J. Culver  R. Drummond  S.E. Engelhart  A.C. Kemp  D. Mallinson  E.R. Thieler  S.R. Riggs  D.V. Ames  K.H. Thomson 《Quaternary Science Reviews》2009,28(17-18):1725-1736

We have synthesized new and existing relative sea-level (RSL) data to produce a quality-controlled, spatially comprehensive database from the North Carolina coastline. The RSL database consists of 54 sea-level index points that are quantitatively related to an appropriate tide level and assigned an error estimate, and a further 33 limiting dates that confine the maximum and minimum elevations of RSL. The temporal distribution of the index points is very uneven with only five index points older than 4000 cal a BP, but the form of the Holocene sea-level trend is constrained by both terrestrial and marine limiting dates. The data illustrate RSL rapidly rising during the early and mid Holocene from an observed elevation of ?35.7 ± 1.1 m MSL at 11062–10576 cal a BP to ?4.2 m ± 0.4 m MSL at 4240–3592 cal a BP.We restricted comparisons between observations and predictions from the ICE-5G(VM2) with rotational feedback Glacial Isostatic Adjustment (GIA) model to the Late Holocene RSL (last 4000 cal a BP) because of the wealth of sea-level data during this time interval. The ICE-5G(VM2) model predicts significant spatial variations in RSL across North Carolina, thus we subdivided the observations into two regions. The model forecasts an increase in the rate of sea-level rise in Region 1 (Albemarle, Currituck, Roanoke, Croatan, and northern Pamlico sounds) compared to Region 2 (southern Pamlico, Core and Bogue sounds, and farther south to Wilmington). The observations show Late Holocene sea-level rising at 1.14 ± 0.03 mm year^?1 and 0.82 ± 0.02 mm year^?1 in Regions 1 and 2, respectively. The ICE-5G(VM2) predictions capture the general temporal trend of the observations, although there is an apparent misfit for index points older than 2000 cal a BP. It is presently unknown whether these misfits are caused by possible tectonic uplift associated with the mid-Carolina Platform High or a flaw in the GIA model. A comparison of local tide gauge data with the Late Holocene RSL trends from Regions 1 and 2 support the spatial variation in RSL across North Carolina, and imply an additional increase of mean sea level of greater than 2 mm year^?1 during the latter half of the 20th century; this is in general agreement with historical tide gauge and satellite altimetry data.  相似文献   

Modelling mechanical behaviour of limestone under reservoir conditions     

Lúcia Carvalho Coelho  Antonio Claudio Soares  Nelson Francisco F. Ebecken  Jos Luis Drummond Alves  Luiz Landau 《国际地质力学数值与分析法杂志》2006,30(14):1477-1500

High porosity and low permeability limestone has presented pore collapse. As fluid is withdrawn from these reservoirs, the effective stresses acting on the rock increase. If the strength of the rock is overcome, pore collapse may occur, leading to irreversible compaction of porous media with permeability and porosity reduction. It impacts on fluid withdrawal. Most of reservoirs have been discovered in weak formations, which are susceptible to this phenomenon. This work presents a study on the mechanical behaviour of a porous limestone from a reservoir located in Campos Basin, offshore Brazil. An experimental program was undergone in order to define its elastic plastic behaviour. The tests reproduced the loading path conditions expected in a reservoir under production. Parameters of the cap model were fitted to these tests and numerical simulations were run. The numerical simulations presented a good agreement with the experimental tests. Copyright © 2006 John Wiley & Sons, Ltd.  相似文献