• Volume 96, Issue 2

      September 1987,   pages  81-202

    • Variable190Os/184Os ratio in acid residues of iron meteorites

      P S Goel

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      In residual materials obtained on dissolution of iron meteorites in 2M H2SO4, the ratio of190Os/184Os has been measured by radiochemical neutron activation analysis. Most residues have a normal isotopic ratio (to within ±2%). However, in some residues both positive and negative deviations in the isotopic ratio are seen. The most spectacular deviations are in the insoluble fragments (nuggets) from Sikhote Alin iron meteorite where the190Os/184Os ratio is about 50% of the normal value. The new results confirm our earlier observations that iron meteorites contain pre-solar grains.

    • Gold-bearing rocks of Mangalur greenstone belt, Gulbarga District, Karnataka

      S G Tenginkai A G Ugarkar

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      The Mangalur greenstone belt of Dharwar Craton, South India, is an Archaean schist belt dominated by metavolcanic rocks. The gold mineralization occurs within the metavolcanics and the fabric, mineralogy and geochemistry of these host rocks indicate that they were tholeiitic basalts regionally metamorphosed under medium to low-grade greenschist facies. The basic metavolcanic rocks occur as tholeiitic metabasalts and amphibolites. The rocks have undergone some fractionation and appear to be derived from melts generated by 10 to 25% melting of the mantle at depths 30 to 35 km around temperature 1200°C and pressure 12 kb. The source of gold is mainly in the basalts and not in the surrounding granites.

    • 146-day signal in the geomagnetic field—a probable association with the periodicity of the solar flare index

      D R K Rao B D Kadam

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      Results on the spectral analysis using geomagnetic field at three low latitude stations and the planetary magnetic activity index have shown peaks in the power densities in a broad band centred around 146-day period. This periodic behaviour appears to be close to that shown by the solar flare activity index for the same interval. It is suggested that the geoeffectiveness of the flare activity signal in different phases of the solar cycle can be better worked out using long series of ground-based geomagnetic data.

    • Turbulent fluxes over east-central Arabian sea during MONEX

      K K Varma N N Raman

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      Fluxes of momentum, latent heat and sensible heat at fixed stations in the east-central Arabian sea during MONEX were studied. Observations at the same locations at different periods as well as simultaneous observation at different locations were compared. During the advance of monsoon, momentum flux showed remarkable increase. Latent heat loss from sea also increased while sensible heat flux, in general, changed direction to become a gain by the sea. SST decreased by about 1.5°C and air temperature decreased by about 1°C during the advance phase. A north-south difference in SST in the study region seemed to be favourable for the genesis of onset vortex of monsoon. The possible differential effect of this storm at two different locations, depending upon the SST before the storm, is also discussed.

    • Geochemical studies on Kashmir loess profiles

      G S Lodha K J S Sawhney H Razdan D P Agrawal N Juyal

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      Major and trace element concentrations of a few representative loess profiles in Kashmir valley are studied using EDXRF spectroscopy. Principal component analysis for pattern recognition has been employed to study association between elemental concentration variations in various horizons. The present study reveals that elements related to clay illuviation (K, Rb, Fe, Ti, Mn) and plant activity (Cu, Zn) are generally higher in B horizon of the palaeosols whereas Ca and Sr have a high concentration in the loess layers. Such an elemental concentration variation provides us with an objective index for identification and confirmation of palaeosols. There is further a close association in the movement of (i) K, Rb, (ii) Fe, Ti, Mn, (iii) Cu, Zn and (iv) Ca, Sr. These clusters of elements are also present in modern soils, suggesting that the soil characteristics in palaeosols have been preserved. Concentration of iron and titanium is also observed to be higher in loess profiles on Pir Panjal mountain flank which is dominated by basic rocks, compared to profiles on Himalayan flank, suggesting that the loess in Kashmir might have been derived from the nearby rocks in a localized way.

    • On global summer monsoonal drought mechanics

      M Sankar-Rao S V Sachidananda

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      It is shown that within the framework of a linear five-level quasi-geostrophic steady state global model the middle latitude systems can always have significant influence on the Asian summer monsoonal system through the lower tropospheric monsoonal westerly window region around 80°E. It is hypothesized that quasistationarity of the middle latitude longwave systems results in stronger teleconnections through this window and the consequent monsoon breaks when the phase is right.

    • Characteristics of air flow over Andaman islands including precipitation

      Sarat C Kar N Ramanathan

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      The characteristics of simulated air flow over Andaman Islands are studied with a two-dimensional version of the University of Virginia meso scale model (UVMM). Using the observed synoptic data as initial conditions, 24 hr simulations are obtained for a day each in April and November. These days are chosen to study the variations in the simulated flow pattern under different synoptic conditions including precipitation effects. A large scale condensation scheme is employed to consider the effect of latent heat release on the perturbations. The results show that the latent heat released by condensation strengthens the intensity of perturbations and the topography accelerates the arrival of sea breeze by about an hour. The model-simulated results, given in graphical form, are discussed and compared with available observations.

    • Distribution of particulate organic carbon in the central Bay of Bengal

      K Nandakumar K Venkat N B Bhosle

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      Particulate organic carbon (POC) was measured for 77 water samples collected over a 3000 m water column along 88° E in the central Bay of Bengal. The POC values varied from 80 to 895 μg per litre at the surface and 171 to 261 μg per litre at 2000 m. The POC decreased with increasing water depth at all the stations. Deep water concentrations of POC were higher than those reported from other oceanic waters. Distribution of POC was not influenced by water masses. The POC was not significantly correlated with chlorophylla.

    • North-south asymmetry in response of geomagnetic activity to different solar events

      G K Rangarajan

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      Study of the response of geomagnetic activity to five different kinds of solar events reveals that an average north-south asymmetry of about 15% exists which diminishes with enhanced geomagnetic activity. The response of the geomagnetic field is quite significant only when high speed solar wind stream in association with sector boundary of interplanetary magnetic field (IMF) or solar proton streams near sector boundary sweeps past the earth. When the frequency of occurrence of indices of geomagnetic activity is considered, the index zero shows a marked difference in its response characteristics for the southern hemisphere. This appears to be a real feature and not attributable to any artefact of the index or its derivation.

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