A strontium isotope evolution model for cenozoic magma genesis,Estern Great Basin,U.S.A.

Cenozoic volcanism in the Great Basin is characterized by an outward migration of volcanic centers with time from a centrally located core region, a gradational decrease in the initial Sr⁸⁷/Sr⁸⁶ ratio with decreasing age and increasing distance from the core, and a progressive change from calc-alkalic core rocks to more alkalic basin margin rocks. Generally each volcanic center erupted copious silicic ignimbrites followed by small amounts of basalt and andesite. The Sr⁸²/Sr⁸⁶ ratio for old core rocks is about 0.709 and the ratio for young basin margin rocks is about 0.705. Spatially and temporally related silicic and mafic suites have essentially the same Sr⁸⁷/Sr⁸⁶ ratios. The locus of older volcanism of the core region was the intersection of a north-south trending axis of crustal extension and high heat flow with the northeast trending relic thermal ridge of the Mesozoic metamorphic hinterland of the Sevier Orogenic Belt. Derivation of the Great Basin magmas directly from mantle with modification by crustal contamination seems unlikely. Initial melting of lower crustal rocks probably occurred as a response to decrease in confining pressure related to crustal extension. Volcanism was probably also a consequence of the regional increase in the geothermal gradient that is now responsible for the high heat flow of the Basin and Range Province. High Sr isotopic ratios of the older core volcanic rocks suggests that conditions suitable for the production of silicic magmas by partial fusion of the crust reached higher levels within the crust during initial volcanism than during production of later magmas with lower isotopic ratios and more alkaline chemistry. As the Great Basin became increasingly attenuated, progressively lower portions of the crust along basin margins were exposed to conditions suitable for magma genesis. The core region became exhausted in low temperature melting components, and volcanism ceased in the core before nearby areas had completed the silicic-mafic eruption cycle leading to their own exhaustion of crustal magma sources.     

The Amazon River headstream area in the Cordillera Chila,Peru: hydrographic,hydrological and glaciological conditions

Abstract

The source of the world's largest river has fascinated scientists and adventurers for a long time. Extensive studies have been undertaken in the unexplored Llogueta River valley, Cordillera Chila, to identify the main stream of the Amazon River. Analysis of the Lloqueta River network and measurements of its hydrographic and hydrometric characteristics are presented in this study. On the basis of the acquired data, the northern hillside of the Cordillera Chila massif, concretely the basins of four mountainous courses—the Carhuasanta, Apacheta, Ccaccansa and Sillanque rivers—should be regarded as the headwaters territory of the Amazon River. Factors influencing the river system—glaciers and soils—were examined for each catchment. Glacier retreat in the last 50 years has left perennial snowfields only in the highest part of the study area, resulting in modification of the headwater runoff regimes. Preliminary results are afforded by the continual automatic water-level monitoring of the Lloqueta River since June 2008. Our investigations have determined that all types of soil in the area could be classified into two main categories: hydromorphic soils or poorly developed cryic soils.

Citation Janský, B., Engel, Z., Kocum, J., ?efrna, L. & ?esák, J. (2011) The Amazon River headstream area in the Cordillera Chila, Peru: hydrographical, hydrological and glaciological conditions. Hydrol. Sci. J. 56(1), 138–151.     

On the interpretation of seasonal variations of stratospheric ozone

Insight into the causes of the annual and semi-annual ozone oscillations may be gained from the analysis of photochemical model behavior. In this paper, the monthly variations of the ozone mixing ratio computed by the two-dimensional photochemical model of Garcia and Solomon (1983, J. geophys. Res. 88, 1379) are Fourier-analyzed and compared with SBUV observations of ozone mixing ratio. Remarkably good qualitative agreement between the model calculations and the observations is found. Analysis of computed transport and chemical production and destruction rates reveals the causes of the modelled seasonal ozone variations.

It will be shown that at high latitudes and low altitudes, modelled ozone abundances increase in the winter due to transport and decrease in the summer due to chemical destruction. In the middle stratosphere, the calculated annual ozone variation is largely due to the annual variation in the odd-oxygen production rate, and in the upper stratosphere, the computed annual ozone variation is caused by the large calculated annual oscillation in temperature. Comparison between the model and observations suggests that the equatorial semi-annual oscillation above 10 mb is caused mainly by the semi-annual temperature and wind oscillation (SAO). Below 10 mb the computed equatorial ozone variation is caused by the increased rates of odd-oxygen production associated with the semi-annual zenith crossings of the Sun. Finally, the calculated polar semi-annual ozone oscillations are found to be caused by modulation of the radiatively driven middle-stratospheric ozone variation by temperature dependent chemical destruction processes.     

Wo ist das äquivalentbarotrope Niveau zu suchen?

Zusammenfassung Fjörtofts graphische Integration der geostrophisch approximierten, divergenzfreien barotropen Wirbelgleichung weist einen Weg, wie man ohne kostspielige Rechenmaschinen der objektiven Wettervorhersage näherkommen kann; sie ist deshalb besonders beachtenswert. Ein wesentlicher Punkt dieser Methode ist das Steuerungsprinzip, das einer eingehenden Untersuchung bedarf. Erfahrungsberichte liegen kaum vor. Es wird zu einer einschlägigen Arbeit vonSanders undKessler [3] kritisch Stellung genommen. Da in dieser Arbeit die Abweichungen von der ArbeitsweiseFjörtofts in tragbaren Grenzen bleiben, die Steuerung aber nicht im Sinne der graphischen Integrationsmethode funktioniert, wird der Schluß gezogen, daß die fundamentale Voraussetzung der Barotropie in den untersuchten Fällen nicht erfüllt war. Wegen der im natürlichen Wetterablauf vorherrschenden Windzunahme mit der Höhe wäre das äquivalentbarotrope Niveau in tieferen Schichten als in 500 mb zu suchen, was den Erfahrungen vonRiehl entspräche. Bevor ein abschließendes Urteil abgegeben wird, soll deshalb die MethodeFjörtofts noch für die Vorhersage von Topographien tieferer Druckflächen herangezogen werden.

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Summary Fjörtoft's graphic integration of the geostrophically approximated, nondivergent, and barotropic vorticity equation shows a way of approaching objective weather forecasting without costly calculating-machines. For this reason it is of particular interest. An essential point of the method is the steering-principle which calls for a thorough investigation. A paper bySanders andKessler concerning this problem is discussed. As, in this paper, differences fromFjörtoft's results remain within tolerable limits, while steering does not work in the sense of the graphic integration method, it is concluded that the basic assumption of barotropy was not fulfilled in the investigated cases. On account of the increase of wind velocity with height predominating under normal weather conditions the equivalent-barotropic level should be found in layers lower than 500 mb which would correspond toRiehl's experiences. Therefore, before definitely judging in this matter,Fjörtoft's method should be applied also in forecasting topographies of lower pressure levels.

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Résumé Le procédé deFjörtoft d'intégration graphique de l'équation tourbillonnaire (géostrophisme approché, sans divergence et avec barotropie) montre comment on peut espérer une prévision objective du temps sans machine à calculer coûteuse; elle est de ce fait intéressante. Un point essentiel de la méthode réside dans le principe de «l'action directrice» qui mérite un examen attentif. On ne connaît pas de résultats d'expérience. L'auteur prend position à l'égard d'une étude deSanders etKessler. Comme dans cette dernière les divergences par rapport à la méthode deFjörtoft restent petites, tandis que l'action directrice ne fonctionne pas dans le sens de la méthode graphique d'intégration, il en résulte que l'hypothèse fondamentale de la barotropie n'a pas été réalisée dans les cas étudiés. Vu qu'en général le vent augmente avec l'altitude, le niveau équivalent-barotrope devrait être recherché dans les basses couches plutôt qu'à 500 mb., ce qui correspondrait aux expériences deRiehl. Avant de conclure définitivement, il convient donc d'appliquer la méthode deFjörtoft aussi à la prévision de la topographie des basses couches.

     

Ergänzung zur Arbeit: „Zur Physik der Taubildung an der Erdoberfläche”

Zusammenfassung In einer früheren Arbeit wurde auf Grund der Voraussetzung eines höhenkonstanten Austauschkoeffizienten eine Tauformel abgeleitet und diskutiert, die für größere Turbulenz Abweichungen von der Wirklichkeit zeigt. Durch zusätzliche Überlegungen werden nun die nachteiligen Folgen des Rechnens mitA = const. behoben.     

Disequilibrium of strontium isotopes between mineral phases of parental rocks during magma genesis — a discussion

Consideration of available diffusion rate data at high temperatures and pressures casts serious doubt on disequilibrium melting as a mechanism by which variable ⁸⁷Sr/⁸⁶Sr ratios can be generated in basaltic melts whose parental rocks were at or below the low-velocity layer for more than a million years. Diffusion rates are orders of magnitude faster in these regions than in the upper lithosphere and lower crust. Ultramafic fragments with minerals in isotopic disequilibrium are pieces of the upper lithosphere, where elapsed time and comparatively low temperatures have resulted in the observed disequilibrium. Regional isotopic heterogeneity in the mantle is the most reasonable explanation for isotopic variability found in abyssal basalts.     

Application of general circulation models to the study of stratospheric ozone

Summary Observations of the ozone distribution indicate that modifications are required to the photochemical theory. These modifications involve ozone destruction by hydrogen and nitrogen products and ozone transport (both vertical and horizontal) due to atmospheric motions in the stratosphere. If the photochemical terms in the ozone continuity equation are omitted, changes due to atmospheric transport alone can be evaluated.Numerical computations were made of the three-dimensional wind structure as derived from the 12-layer (0–36 km) General Circulation Model developed by NCAR. The results showed that ozone is transported from the equatorial stratosphere poleward and downward in both hemispheres. The horizontal transport is primarily by the Hadley Cell in the tropics and by large-scale eddies in mid and high latitudes. The dominant mechanism for ozone transport are found to be similar to those derived for the horizontal heat and momentum transport found in other general circulation studies.     

On the Eulerian source-receptor relationship

A new approach is proposed to delineate the source-receptor relationship in an Eulerian model. A small, unique oscillatory signal is superimposed on each emission source in the airshed. At receptor sites the concentration time series are analyzed by a Fourier transform to give the amplitudefrequency spectrum. Distinct peaks found in the spectrum are identified at the emission frequencies. The amplitude of the spectrum at the source frequency represents the individual contribution from that emitter. Various atmospheric transport and diffusion models are solved to show the effects of different physical and chemical processes on the oscillatory signals. Both analytical and numerical solutions are used to demonstrate the performance of the method.The National Center for Atmospheric Research is sponsored by the National Science Foundation.