Fractionation of carbon isotopes by phytoplankton and estimates of ancient CO2 levels

Reports of the 13C content of marine particulate organic carbon are compiled and on the basis of GEOSECS data and temperatures, concentrations, and isotopic compositions of dissolved CO2 in the waters in which the related phytoplankton grew are estimated. In this way, the fractionation of carbon isotopes during photosynthetic fixation of CO2 is found to be significantly correlated with concentrations of dissolved CO2. Because ancient carbon isotopic fractionations have been determined from analyses of sedimentary porphyrins [Popp et al., 1989], the relationship between isotopic fractionation and concentrations of dissolved CO2 developed here can be employed to estimate concentrations of CO2 dissolved in ancient oceans and, in turn, partial pressures of CO2 in ancient atmospheres. The calculations take into account the temperature dependence of chemical and isotopic equilibria in the dissolved-inorganic-carbon system and of air-sea equilibria. Paleoenvironmental temperatures for each sample are estimated from reconstructions of paleogeography, latitudinal temperature gradients, and secular changes in low-latitude sea surface temperature. It is estimated that atmospheric partial pressures of CO2 were over 1000 micro atm 160 - 100 Ma ago, then declined to values near 300 micro atm during the next 100 Ma. Analysis of a high-resolution record of carbon isotopic fractionation at the Cenomanian-Turonian boundary suggests that the partial pressure of CO2 in the atmosphere was drawn down from values near 840 micro atm to values near 700 micro atm during the anoxic event.     

Electrical resistivity structure of the eastern Moroccan Atlas System and its Tectonic implications

The electrical resistivity structure of the crust and upper mantle of the Atlas Mountain System was studied using magnetotelluric and geomagnetic deep soundings. Field experiments were done in eastern Morocco along a traverse from the Anti Atlas to the Rides Rifaines in two campaigns in 1983 and 1988.Zones of very low electrical resistivity could be identified in the various structural settings at different depth ranges, most likely connected directly to the tectonic evolution of the mountain belts. A mid-crustal low resistivity layer with total conductance (thickness-resistivity ratio) of about 2000 Siemens stretches from the southern border of the High Atlas towards the Middle Atlas. This layer seems to characterize the base of crustal detachment, e.g., the plane for large horizontal overthrusting, and supports the idea of thick- and thin-skinned tectonics involved in Atlasic mountain building. In the western Middle Atlas an upper-crustal low resistivity layer (at depth < 10 km) was found in the area where volcanic activity was present, pointing towards a direct relation between low electrical resistivity and volcanic or postvolcanic events. North of the Middle Atlas resistivity structures change totally: The Pre and parts of the Sub-Rif have a highly conductive cover, presumably connected to the molasse basin. Total conductance was calculated to reach 6000 Siemens. No further conductive structures, like, e.g., the ones found beneath the High and Middle Atlas, are seen within the resistive crust, but at much greater depth within the upper mantle.

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Zusammenfassung Im marokkanischen Atlas-System wurde der elektrische Widerstand der Kruste und des oberen Mantels mit magnetotellurischen und erdmagnetischen Tiefen-Sondierungen erkundet. Die Feldmessungen wurden in den Jahren 1983 und 1988 im östlichen Marokko entlang einer Traverse vom Anti Atlas bis in das Rif durchgeführt.In den einzelnen strukturellen Einheiten wurden in unterschiedlichen Krustentiefen Schichten sehr kleiner elektrischer Widerstände entdeckt, die direkt mit der tektonischen Entwicklung des Atlasgebirges verknüpft zu sein scheinen. Zwischen dem Südrand des Hohen Atlas und dem Mittleren Atlas im Norden erstreckt sich eine Zone sehr kleiner Widerstände (integrierte Leitfähigkeit etwa 2000 Siemens) in mittleren Krustentiefen. Diese Schicht scheint einen Bereich krustaler Ablösung zu kennzeichnen und kann als Basis einer weitspannigen Überschiebung angenommen werden. Diese Form der »thick- & thin-skinned« Tektonik scheint wesentlich am Prozess der Gebirgsbildung im Atlas beteiligt zu sein. Im westlichen Mittleren Atlas, im Bereich des jungen Vulkanismus, wurde eine gutleitende Zone in der oberen Kruste (Tiefe < 10 km) gefunden. Diese Beobachtung legt einen direkten Zusammenhang zwischen erhöhter elektrischer Leitfähigkeit und vulkanischen bzw. postvulkanischen Ereignissen nahe. Nördlich des Mittleren Atlas ändern sich die Widerstandsstrukturen sehr stark: Im Prä- und teilweise auch noch im Sub-Rif liegt eine sehr gut leitende Deckschicht (integrierte Leitfähigkeit etwa 6000 Siemens) über einem hochohmigen Basement. In der Kruste des Rif werden keine weiteren, tieferliegenden Schichten erhöhter Leitfähigkeit gefunden, wie sie z.B. unter dem Hohen Atlas vorliegen. Dagegen gibt es Hinweise auf Zonen hoher Leitfähigkeit im oberen Mantel.

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Résumé Dans l'Atlas marocain la résistivité électrique de la croûte et du manteau supérieur a été étudiée à l'aide des sondages magnétotelluriques et géomagnétiques. Ces travaux ont été effectués le long d'une traverse menant de l'Anti-Atlas jusqu'au Rif durant les années 1983 et 1988.Dans différentes profondeurs de la croûte terrestre des zones de résistances électriques très basses ont pu être découvertes ce qui peut être attribué à l'évolution tectonique de l'Atlas. Une couche de résistances très basses (conductance vers 2000 Siemens) s'étend entre le bord au sud du Haut Atlas et l'Atlas Moyen au nord. Cette couche peut être le signe d'un détachement de l'écorce et d'une poussée très vaste. Cette forme tectonique »thick- & thin-skinned« semble être essentiellement responsable du processus de la formation orogénique de l'Atlas. Une zone de bonne conductance se trouve dans la croûte terrestre supérieure (profondeur < 10 km) dans l'ouest de l'Atlas Moyen aux environs de l'activité volcanique caractérisant une relation directe entre une haute conductance électrique et des événements volcaniques et post-volcaniques. Vers le nord de l'Atlas Moyen les structures de résistance subissent un changement total: Au Pré- et partiellement aussi au Subrif se trouve une bonne couche guide (conductance vers 6000 Siemens). Aucune autre couche de conductance supérieure — comme au-dessous du Haut Atlas — n'a été découverte dans la croûte du Rif. On en décèle, par contre, dans le manteau supérieur.

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Solar flares,microflares, nanoflares,and coronal heating

Solar flare occurrence follows a power-law distribution against total flare energy W: dN/dW W ^– with an index 1.8 as determined by several studies. This implies (a) that microflares must have a different, steeper distribution if they are energetically significant, and (b) there must be a high-energy cutoff of the observed distribution. We identify the distinct soft distribution needed for coronal heating, if such a distribution exists, with Parker's nanoflares.This paper considers a microflare to be a member of the normal X-ray burst population, with comparable physical parameters except for a smaller total energy.     

A new Bolivia-USSR astronomical observatory and its astrometric activities

In 1982 a Bolivia-USSR astronomical observatory began its activities near Tarija, Bolivia. Observations of 200 000 faint stars, 2822 bright stars (m6) and Halley's comet have been made with the expedition's astrograph.     

Irregular behaviour and eccentricity increase in the trajectories of a dynamical system modelling asteroidal motions

Many trajectories of the third body are integrated numerically in a modified elliptical restricted three body problem (ERTBP), in which the eccentricity, e, of the orbit of the second primary varies sinusoidally with time. It is found that, in the case of the 2:1 resonance, the introduction of the time variability of e modifies significantly the behaviour of the trajectories of the third body. In particular their osculating eccentricity e, present the following two notable features: (a) In all cases it shows a definite chaotic variation, which appears at significantly shorter time-scales than the one found by Wisdom in the e = constant case. (b) In many cases it shows a significant increase, up and beyond the (critical) value e _crit = 0.52. As a result the third body approaches the first primary at distances smaller than 0.29 (where by we denote the semi-major axis of the trajectory of the second primary around the first), which in the actual Sun-Jupiter-asteroid problem corresponds to the semi-major axis of Mars. Our result might be of interest in the context of explaining the Kirkwood gaps at the resonances where the osculating eccentricity of asteroid trajectories calculated in the classical (e = constant) ERTBP does not reach Mars crosser values.     

Resistive ballooning line-tied boundary conditions

Ideal and resistive ballooning modes are investigated for different ratios of a two-layer stratified density region representing a model for the photospheric/coronal boundary. Construction of the ballooning equations using a WKB approach is justified by comparison between the values of the growth rate obtained using Hain-Lüst and ballooning equations together with a WKB integral relation. Different values of the density ratio, radius, and resistivity are considered. Sausage-type and kink-type instabilities are found. One of these, depending on the value of r remained unstable for large density ratios. The other instability tended to marginal stability as the density ratio was increased, and allowed parallel and perpendicular flows across the boundary. This is contrary to the predictions of both the rigid-wall and flow-through conditions.     

Results of TV imaging of Phobos (Experiment VSK-Fregat)

From February to March 1989 the Phobos 2 spacecraft took 37 TV images of Phobos at a distance of 190-1100 km. These images complement Mariner-9 and Viking data by providing higher-resolution coverage of a large region West of the crater Stickney (40-160 degrees W) and by providing disk-resolved measurements of surface brightness at a greater range of wavelengths and additional phase angles. These images have supported updated mapping and characterization of large craters and grooves, and have provided additional observations of craters' and grooves' bright rims. Variations in surface visible/near-infrared color ratio of almost a factor of 2 have been recognized; these variations appear to be associated with the ejecta of specific large impact craters. Updated determinations of satellite mass and volume allow calculation of a more accurate value of bulk density, 1.90 +/- 0.1 g cm-3. This is significantly lower than the density of meteoritic analogs to Phobos' surface, suggesting a porous interior perhaps containing interstitial ice.     

Shear-induced instability and arch filament eruption: A magnetohydrodynamic (MHD) numerical simulation

We investigate, via a two-dimensional (nonplanar) MHD simulation, a situation wherein a bipolar magnetic field embedded in a stratified solar atmosphere (i.e., arch-filament-like structure) undergoes symmetrical shear motion at the footpoints. It was found that the vertical plasma flow velocities grow exponentially leading to a new type of global MHD-instability that could be characterized as a Dynamic Shearing Instability, with a growth rate of about 8{ovV} _A a, where {ovV} _A is the average Alfvén speed and a ^–1 is the characteristic length scale. The growth rate grows almost linearly until it reaches the same order of magnitude as the Alfvén speed. Then a nonlinear MHD instability occurs beyond this point. This simulation indicates the following physical consequences: the central loops are pinched by opposing Lorentz forces, and the outer closed loops stretch upward with the vertically-rising mass flow. This instability may apply to arch filament eruptions (AFE) and coronal mass ejections (CMEs).To illustrate the nonlinear dynamical shearing instability, a numerical example is given for three different values of the plasma beta that span several orders of magnitude. The numerical results were analyzed using a linearized asymptotic approach in which an analytical approximate solution for velocity growth is presented. Finally, this theoretical model is applied to describe the arch filament eruption as well as CMEs.     

Earthquake hazard of the northern parts of the Bohemian Massif and Western Carpathians

The new procedure of earthquake hazard evaluation developed by Kijko and Sellevoll is tested and applied for the border region of Czechoslovakia and Poland. The new method differs from the conventional approach. It incorporates the uncertainty of earthquake magnitudes, and accepts mixed data containing only large historical events and recent, complete catalogues. Seismic hazard has been calculated for nine regions determined in the border area. In the investigated area, data of historical catalogues are uncertain or, in many cases, the epicentral intensities are unknown. Thus, a number of assumptions have to be adopted in data preparation of catalogues since the year 1200. The calculated values of parameters b in the Gutenberg-Richter frequency-intensity relation as well as the return periods, seem to be reasonable and are generally confirmed by the results obtained from catalogues for the last 80–130 years.     

Catastrophic landslip activity in two hard rock terrains of peninsular India

This paper presents the case histories of two catastrophic landslips in hard rock terrains with varied climatic and geological environments. The first slip is associated with a power project in very close proximity (200 m) of the Porthimund Dam (11°22N, 76°3430E), in a charnockitic terrain in the Nilgiri hills (Tamil Nadu), and the second is associated with a railroad structure (19°525N, 78°1720E), in Adilabad district (Andhra Pradesh), in a basaltic terrain.The landslip in the charnockites is attributable to: (1) a high degree of saprolitization in the charnockites, with maximum intensity in the crest portion; (2) the coincidence of a major joint pattern in a NE-SW direction, with the strike of the foliation; and (3) the poor-to-fair physical rock quality in the crest and scarp portions.The slips in the basaltic terrain are due to: (1) the partially altered, highly jointed nature of the regional trap rocks with boulder sizes varying from 20 cm to 250 cm in diameter and the debris accumulating in a precarious condition on the northeast side of the rail track, with unfavorable alignment direction; and (2) the instability created in the weak rock mass by the vibrational forces of heavily loaded running trains.The weathered state of the rock masses in both the cases, showing good agreement with their physical state, accounts for the landslips. The remedial measures suggested are also discussed.