26Al/10Be ratios reveal the source of river sediments in the Kimberley,NW Australia     

Gaël Cazes  David Fink  Alexandru T. Codilean  Réka-Hajnalka Fülöp  Toshiyuki Fujioka  Klaus M. Wilcken 《地球表面变化过程与地形》2020,45(2):424-439

We use cosmogenic ¹⁰Be and ²⁶Al in both bedrock and fluvial sediments to investigate controls on erosion rates and sediment supply to river basins at the regional scale in the Kimberley, NW Australia. The area is characterised by lithologically controlled morphologies such as cuestas, isolated mesas and extensive plateaus made of slightly dipping, extensively jointed sandstones. All sampled bedrock surfaces at plateau tops, ridgelines, and in the broader floodplain of major rivers over the region show similar slow lowering rates between 0.17 and 4.88 m.Myr^-1, with a mean value of 1.0 ± 0.6 m.Myr^-1 (n=15), whilst two bedrock samples collected directly within river-beds record rates that are one to two orders of magnitude higher (14.4 ± 1.5 and 20.9 ± 2.5 m.Myr^-1, respectively). Bedrock ²⁶Al/¹⁰Be ratios are all compatible with simple, continuous sub-aerial exposure histories. Modern river sediment yield lower ¹⁰Be and ²⁶Al concentrations, apparent ¹⁰Be basin-wide denudation rates ranging between 1.8 and 7.7 m.Myr^-1, with a median value of 2.6 m.Myr^-1, more than double the magnitude of bedrock erosion rates. ²⁶Al/¹⁰Be ratios of the sediment samples are lower than those obtained for bedrock samples. We propose that these depleted ²⁶Al/¹⁰Be ratios can largely be explained by the supply of sediment to river basins from the slab fragmentation and chemical weathering of channel gorge walls and plateau escarpments that result in diluting the cosmogenic nuclide concentration in river sediments measured at the basin outlets. The results of a mass-balance model suggest that ~60–90% of river sediment in the Kimberley results from the breakdown and chemical weathering of retreating vertical sandstone rock-walls in contrast to sediment generated by bedrock weathering and erosion on the plateau tops. This study emphasises the value of analysing two or more isotopes in basin-scale studies using cosmogenic nuclides, especially in slowly eroding post-orogenic settings. © 2019 John Wiley & Sons, Ltd.  相似文献   

On decadal-scale ocean-atmosphere interactions in the extended ECHAM1/LSG climate simulation     

C. Frankignoul  E. Kestenare  N. Sennéchael  G. de Coëtlogon  F. D'Andrea 《Climate Dynamics》2000,16(5):333-354

 The last 810 years of a control integration with the ECHAM1/LSG coupled model are used to clarify the nature of the ocean-atmosphere interactions at low frequencies in the North Atlantic and the North Pacific. To a first approximation, the atmosphere acts as a white noise forcing and the ocean responds as a passive integrator. The sea surface temperature (SST) variability primarily results from short time scale fluctuations in surface heat exchanges and Ekman currents, and the former also damp the SST anomalies after they are generated. The thermocline variability is primarily driven by Ekman pumping. Because the heat, momentum, and vorticity fluxes at the sea surface are correlated in space and time, the SST variability is directly linked to that in the ocean interior. The SST is also modulated by the wind-driven geostrophic fluctuations, resulting in persistent correlation with the thermocline changes and a slight low-frequency redness of the SST spectra. The main dynamics are similar in the two oceans, although in the North Pacific the SST variability is more strongly influenced by advection changes and the oceanic time scales are larger. A maximum covariance analysis based on singular value decomposition in lead and lag conditions indicates that some of the main modes of atmospheric variability in the two oceans are sustained by a very weak positive feedback between the atmosphere, SST, and the strength of the subtropical and subpolar gyres. In addition, in the North Atlantic the main surface pressure mode has a small quasi-oscillatory component at 6-year period, and advective resonance occurs for SST around 10-year period, both periods being also singled out by multichannel singular spectrum analysis. The ocean-atmosphere coupling is however much too weak to redden the tropospheric spectra or create anything more than tiny spectral peaks, so that the atmospheric and oceanic variability is dominated in both ocean sectors by the one-way interactions. Received: 2 April 1999 / Accepted: 14 October 1999  相似文献   

Inhomogeneous distribution of REE in scheelite and dynamics of Archaean hydrothermal systems (Mt. Charlotte and Drysdale gold deposits, Western Australia)   总被引：5，自引：0，他引：5  

Joël Brugger  Yann Lahaye  Sylvie Costa  David Lambert  Roger Bateman 《Contributions to Mineralogy and Petrology》2000,139(3):251-264

Scheelite is a widespread accessory mineral in hydrothermal gold deposits, and its rare earth element (REE) patterns and Nd and Sr isotopic compositions can be used to constrain the path and origin of the mineralising fluids and the age of the hydrothermal activity. Micro-analyses by laser ablation high resolution inductively coupled mass spectroscopy and cathodoluminescence imaging reveal a very inhomogeneous distribution of the REE in single scheelite grains from the Mt. Charlotte and Drysdale Archaean gold deposits in Western Australia. Two end-member REE patterns are distinguished: type I is middle REE (MREE)-enriched, with no or minor positive Eu-anomaly, whereas type II is flat or MREE-depleted with a strong positive Eu-anomaly. The chemical inhomogeneity of these scheelites is related to oscillatory zoning involving type I and type II patterns, with zone widths varying from below 1 to 200 μm. Intra-sectorial growth discontinuities, syn-crystallisation brittle deformation, and variations in the relative growth velocities of crystallographically equivalent faces suggest a complex crystallisation history under dynamic hydraulic conditions. The co-existence of MREE-enriched and MREE-depleted patterns within single scheelite crystals can be explained by the precipitation of a mineral which strongly partitions MREE relative to light and heavy REE. Scheelite itself has such characteristics, as does fluorapatite, which is locally abundant and has REE contents similar to that of scheelite. In this context, the systematic increase of the Eu-anomaly between type I and type II patterns is produced by the difference between the partition coefficients of Eu²⁺ and Eu³⁺, and not by fluid mixing or redox reactions. Consequently, the high positive Eu-anomaly typical of scheelite from gold ores may not necessarily be inherited from the hydrothermal fluid, but may reflect processes occurring during ore deposition. This case study demonstrates that in hydrothermal systems characterised by low REE concentrations in the fluid, and by the precipitation of a REE-rich mineral which strongly fractionates the REE, the REE patterns of such a mineral will be highly sensitive to the dynamics of the hydrothermal system. Received: 1 November 1999 / Accepted: 4 February 2000  相似文献   

Detailed Study of Three Contiguous Segments of the Mid-Atlantic Ridge,South of the Azores (37° N to 38°30′ N), Using Acoustic Imaging Coupled with Submersible Observations     

Ondréas  Hélène  Fouquet  Yves  Voisset  Michel  Radford-Knoery  Joël 《Marine Geophysical Researches》1997,19(3):231-255

Using a new tool of seafloor characterisation (sonar images from FARA-SIGMA cruise; Needham et al., 1992), coupled with submersible observations (DIVA1 cruise) we compare, at different scales of observation, three contiguous segments of the Mid-Atlantic Ridge, South of the Azores Triple Junction, between 37° N and 38°30 N.The two northernmost segments (38°20 N and Menez-Gwen) show unusual morphological features for the MAR; the rift valley is absent and the present-day magmatism is focused on shallow axial volcanoes. On the third segment (Lucky Strike), the morphology is the one usually found on the MAR. On the Menez-Gwen and 38°20 N segments, volcanic constructional activity can obliterate, during periods of high magmatic supply, the morphology inherited from tectonic activity. The dive results constrain the recent evolution of each segment and show that a temporal variability in volcanic dynamics exists. On the three segments, outcrops of eruptive lavas alternate with large areas of explosive volcanic ejecta. This cycle in volcanic activity is influenced by changes in water depth, both spatially (i.e. between segments) and temporally (i.e. for the same segment through time).Each segment has known a specific history in its accretionary processes with a succession of tectonic and volcanic predominance and changes in its volcanic phases between volcanic ejecta and effusive dynamics.The hydrothermal activity is focused at the central part of each segment and is controlled by the presence of fresh lava and major tectonic features.  相似文献   

Short-Term Correlation of Solar Activity and Sunspot: Evidence of Lifetime Increase     

E. M. Blanter  J.-L. Le Mouël  F. Perrier  M. G. Shnirman 《Solar physics》2006,237(2):329-350

We perform a nonlinear study of the short-term correlation properties of the solar activity (daily range) in order to reveal their long-life variations. We estimate the lifetime of the high-frequency component of a Markov-type signal when the high-frequency component is modulated by a slowly varying multiplicative factor. This treatment is applied to different series of solar activity: Wolf Sunspot numbers (WSN), Sunspot Group numbers (SGN), and Royal Greenwich Observatory (RGO) sunspot group series. We obtain that all the lifetime estimates exhibit similar temporal variations that agree with the variations of the sunspot lifetimes directly measured from the RGO data and those of the sunspot areas. An increase of lifetimes by a factor 1.4 is observed from 1915 to 1940. At the same time, a stable ratio is observed between the sunspot group’s maximal area and the lifetime, confirming the Gnevyshev–Waldmeier-type relationship. The analysis identifies also time intervals where the homogeneity of the different time series may be questioned.  相似文献   

Comparative study of Langmuir probe characteristics in different ionospheric conditions     

Joëlle Margot-Chaker  A.G. McNamara 《Planetary and Space Science》1984,32(11):1427-1437

This paper examines the role played by the high energy tail of the electron distribution function on Langmuir probe characteristics. A model is developed to derive the mean energy and the density of the hyperthermal electrons from probe characteristics for two ionospheric rocket flights involving different plasma conditions. The hyperthermal electrons are shown to influence the electron temperature measurement even if they constitute only a small fraction of the total electron concentration. The influence of the geomagnetic field, the collisions and the velocity of the vehicle on the probe data are also examined.  相似文献   

Systematique U-Pb et Evolution du Gisement d'Uranium de Lodève (France)     

Joël R. Lancelot  Bruno de Saint André  Henri de la Boisse 《Mineralium Deposita》1984,19(1):44-53

Résumé Considérée comme syngénétique ou diagénétique précoce, la minéralisation uranifère de la couche 0 de l'Autunien du bassin de Lodève a été étudiée par la méthode U-Pb sur roches totales. Les données U-Pb démontrent l'existence de perte en radon, principalement dans la chaine de désintégration de ²³⁸U, et l'intérêt de l'utilisation du couple ²⁰⁷Pb-²³⁵U pour la détermination des âges de cristallisation des concentrations uranifères. Pour la couche 0, deux phases de remobilisation de l'uranium et du plomb ont pu être déterminées respectivement à 173±6 Ma et 108±5 Ma. La plus ancienne de ces deux phases est la plus marquée dans les échantillons étudiés, dont les systèmes U-Pb ne montrent pas la mémoire d'une concentration uranifère permienne. La première mobilisation de l'uranium et du plomb s'est faite lors d'une phase de distension à 160–170 Ma, affectant la croûte continentale du Sud du Massif Central. Cette phase a provoqué la circulation de fluides minéralisés et est marquée par une recristallisation des illites des pélites permiennes entre 100 et 200 °C (du fait de l'élévation du gradient géothermique) et par la mise en place d'un volcanisme d'origine mantellique daté à 155±6 Ma. La composition isotopique en plomb d'une galène de l'Autunien du Lodévois, est analogue à celles des feldspaths des granitoïdes du Massif Central français et à celles des galènes des minéralisations stratiformes de Pb-Zn des Causses, ce qui fournit un argument pour faire dériver au moins une partie des métaux en traces dans les pélites autuniennes de l'altération de la croûte continentale hercynienne environnante.

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In the Lodeve basin, the uraniferous mineralization associated with the autunian pelitic layer 0 is usually considered as syngenetic or early diagenetic. U-Pb isotopic data performed on U bearing whole rocks demonstrate occurrence of radon losses, mainly in the ²³⁸U decay series; the ²⁰⁷Pb-²³⁵U geochronometer is particularly suitable to date U-Pb systems disturbed by such radon losses. For the autunian layer 0, two U-Pb mobilization phases have been respectively recognized at 173±6 Ma and 108±5 Ma. The oldest phase is the most clearly expressed in the studied samples, no memory of a permian age could be recognized in the U-Pb systems. The first U-Pb mobilization occurred 170 Ma ago, during a distension phase of the continental crust. This phase induced circulations of mineralized fluids, illite recristallization in the permian pelites at temperatures ranging from 100 to 200 °C and emplacement of a mantellic volcanism recently dated at 155±6 Ma. A galena from the Autunian of the Lodeve basin, the feldspars of the surrounding variscan granitoïds and galenas of mesozoïc stratiform deposits in the Causses, present similar Pb isotopic composition, which is in agreement with the hypothesis that some metals of the autunian pelites originated in the surrounding weathered variscan continental crust.

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Luftelektrische Tagesgänge und Massenaustausch im Hochgebirge der Alpen     

H. Israël  H. W. Kasemir  K. Wienert 《Meteorology and Atmospheric Physics》1955,8(1-2):72-94

Zusammenfassung Die luftelektrischen Untersuchungen auf dem Jungfraujoch im Sommer und Herbst 1950, über die vor einiger Zeit in dieser Zeitschrift berichtet worden war, wurden während einer dritten Meßperiode im Winter fortgesetzt und damit zu einer Jahresreihe ergänzt. Trotz ungünstiger Witterung, die den Umfang der Ergebnisse stark einschränkte, konnten die schon früher gezogenen Schlüsse voll bestätigt werden: Die im Sommer starke, im Herbst noch schwach vorhandene systematische Leitfähigkeitsvariation in dieser Alpenhöhe ist im Winter ganz verschwunden; Potentialgefälle und Vertikalstrom verlaufen in ihrem Tagesgang parallel zueinander; im Winter stellt sich somit in der Höhe des Alpenkammes, der jetzt von der Tagesvariation des Massenaustausches offenbar nicht mehr erreicht wird, der ozeanische Typ des luftelektrischen Verhaltens ein.Im Herbst 1950 wurden an einigen Tagen gleichzeitige Registrierungen des Potentialgefälles und des Vertikalstromes auf dem Jungfraujoch und dem 400 km entfernten Sonnblick durchgeführt. Die beiden Elemente zeigen sowohl im mittleren Tagesgang wie auch im einzelnen gute Übereinstimmung. Daraus ist zu entnehmen, daß die auf dem Jungfraujoch gewonnenen Ergebnisse, soweit sie nicht lokal gestört sind, repräsentative Bedeutung für die Kammhöhe der Alpen haben.Eine Studie über Feld und Strom während Niederschlages auf dem Jungfraujoch zeigt, daß trockener Schnee, d. h. Schnee als reines Sublimationsprodukt, stets positiv geladen ist und ein positives Feld (Feld normaler Richtung) erzeugt. Dagegen zeigen feste Niederschlagsteilchen, deren Zustandekommen über den Vergraupelungsprozeß geführt hat (nasser Schnee, Grieseln, Reif- und Frostgraupeln) im allgemeinen negative elektrische Ladung und negative Felder (Feldrichtungsumkehr). Besonders bemerkenswert ist dabei, daß im Sommer in der Regel das Auftreten von Graupeln und Schnee bzw. von Regen und Schnee hintereinander als Zeichen eines sich bildenden Gewitters gewertet werden kann.

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Summary The investigations on atmospheric electricity of summer and autumn 1950 on the Jungfraujoch have been continued with a third recordingperiod in the winter. By these investigations the former results which are already reported in this review are completed to a full-year-research. In spite of bad weather, which strongly limited the extent of the results, the conclusions drawn from earlier investigations could be entirely verified. In this Alpine altitude the systematic variation of the conductivity, that has been found high in summer and scarcely remarkable in autumn, has completely disappeared in winter. The daily variations of the potential gradient and the air-earth-current are essentially parallel; it accounts for that in winter at the ridge of the Alps the oceanic type of atmospheric electricity is predominant. Obviously, the daily variations of the mass exchange do not reach these high levels in the winter months.In autumn 1950 simultaneous records of the potential gradient and the air-earth-current at Jungfraujoch and at Sonnblick, 400 km distant, have been made during some days. The two elements are in good agreement both with regard to the mean daily variations as to particulars. Hence it follows that the results received at Jungfraujoch are representative for the summit of the Alps except for cases with local disturbances.Furthermore an investigation has been carried out at Jungfraujoch on the electric field and the current during precipitation. It resulted that dry snow, i. e. snow originating by sublimation only, always carries a positive charge and generates an electric field of positive direction. On the contrary, the solid precipitation particles which are formed by graupel formation (wet snow, soft hail, granular snow) carry generally a negative charge and generate an electric field of negative direction. In summer the occurence of snow after soft hail or rain can be taken as a remarkable symptom for developing a thunderstorm.

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Résumé Les recherches d'électricité atmosphérique effectuées au Jungfraujoch pendant l'été et l'automne 1950 et dont il a été question précédemment dans cette revue ont été complétées par une série hivernale. Malgré un temps défavorable qui a limité les recherches, les résultats précédents ont pu être confirmés. La variation systématique de la conductibilité observée à haute altitude, forte en été et encore faiblement présente en automne, disparaît complètement en hiver. Le gradient de potentiel et le courant vertical présentent des marches diurnes parallèles, c'est-à-dire qu'en hiver, à l'altitude du faîte alpin qui n'est apparemment plus affecté par la variation diurne de l'échange de masse, s'établit le type océanique de l'état électrique de l'atmosphère.En automne 1950 les enregistrements du gradient de potentiel et du courant vertical furent exécultés pendant quelques jours simultanément au Jungfraujoch et au Sonnblick, distant de 400 km. du premier; il y a bon accord entre les mesures des deux éléments, tant pour la variation diurne moyenne que pour les cas isolés. Il s'ensuit que les résultats obtenus au Jungfraujoch, pour autant qu'ils ne sont pas troublés localement, sont représentatifs de la crête des Alpes.Une étude du champ et du courant au Jungfraujoch pendant des précipitations montre que la neige sèche, c'est à dire comme produit pur de sublimation, est toujours chargée positivement et engendre un champ positif (direction normale). Par contre les particules solides issues du grésil (neige mouillée, neige en grains, grésil) sont en général négativement chargées, dans des champs négatifs (direction inversée). Il est en particulier remarquable qu'en été la neige succédant au grésil ou à la pluie soit dans la règle le signe d'un orage en formation.

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Mit 4 Textabbildungen.  相似文献   

Le Capelinhos (Faïal,Açores) vingt ans après son éruption: le modèle éruptif «surtseyen» et les anneaux de tufs hyaloclastiques     

G. Camus  P. Boivin  A. De Goër De Herve  A. Gourgaud  G. Kieffer  J. Mergoil  P. M. Vincent 《Bulletin of Volcanology》1981,44(1):31-42

Phreatomagmatic structures are of two kinds: maars and tuff-rings. Data given by records of Capelinhos activity (Faïal, Açores, 1957–1958), by structures at west point of Faïal island, and by palagonitic breccias of Velay and Cantal areas (France) lead to relate hyaloclastic tuff-rings and shallow subaquatic («surtseyan») eruptions. It is possible to precise causes, characteristics, and mechanism of formation of tuff-rings.  相似文献   

Identification of a magma chamber in the Ghoubbet-Asal rift (Djibouti) from a magnetotelluric experiment     

Daniele Boyer  Jean-Louis Le Mouël  Vincent Courtillot 《Earth and Planetary Science Letters》1981,52(2):372-380

A very broad band (10^?3 to 10⁴ Hz) magnetotelluric investigation of the axial zone of the Ghoubbet-Asal rift (Djibouti) has revealed a shallow (2–4 km) magma chamber which can be mapped in some detail. The suggested roof of the chamber is shallowest very close to the Ardoukoba volcano which was built during the November 1978 rifting episode.  相似文献