Sharp depletion of atmospheric refractive index associated with Nor��wester over Gangetic West Bengal: a possible method of forecasting Nor��wester     

S. K. Midya  H. Sarkar  U. Saha 《Meteorology and Atmospheric Physics》2011,111(3-4):149-152

The paper presents the nature of the variation of refractive index of atmospheric medium with time and altitude before, during and after the onset of thunderstorms over Gangetic West Bengal during the pre-monsoon period. A critical analysis shows that sharp depletion of the refractive index takes place before the onset of Nor??westers and possible explanations are also offered for the said occurrence.  相似文献   

Saddle-shaped reticulate <Emphasis Type="Italic">Nummulites</Emphasis> from Early Oligocene rocks of Khari area,SW Kutch,India     

S Sengupta  Sampa Sarkar  S Mukhopadhyay 《Journal of Earth System Science》2011,120(2):263-268

Saddle-shaped reticulate Nummulites from the Early Oligocene rocks of Khari area, SW Kutch, India is reported here for the first time. Unusual shape of this Nummulites is due to the curved nature of the coiling plane, indicating space constrained postembryonic test growth. With regular development of chambers, septa and septal filaments, the saddle-shaped Nummulites constitutes the third morphotype of N. cf. fichteli Michelotti form A. Other morphotypes of the species reported earlier include inflated lenticular and conical tests. Multiple morphotypes of N. cf. fichteli form A indicates varied test growth in response to substrate conditions. Morphological variability exhibited by N. cf. fichteli form A from Kutch and some Early Oligocene reticulate Nummulites from the Far East are comparable. This faunal suite is morphologically distinct from the contemporary reticulate Nummulites of the European localities.  相似文献   

Experiment on imaging and visualising for assessing late-tertiary microstructural evidences of depositional changes in Gajalia fold,south Tripura     

Sudip Dey  Sushmita Paul  Chandrani Debbarma  Prasamita Sarkar 《Journal of the Geological Society of India》2011,77(4):367-376

The present work concentrates on microstructure imaging for visualising the changes of depositional environment during Tertiary period. For that purpose Tipam layers of Gajalia fold area in southern Tripura was selected which was deposited during late Tertiary period under marine-coastal environment. Thin sections of eleven selected samples of the study area were prepared in the laboratory for microstructure analysis. For visualising the crystalline particles or quartz brightness and contrast of the image was increased up to maximum level. Surface conditions of the samples were analysed within RGB combination. The grain size and shapes of the eleven samples tested prove that the depositional environment remained very dynamic through Tertiary period in the study area. The microstructure and surface conditions also strongly support this view.  相似文献   

Occurrence of well-crystallized nontronite in the gas vesicles of a megaporphyritic basalt flow near Nighoj, Ahmednagar district, Maharashtra     

P. K. Sarkar  D. V. Upasani  A. C. Soman 《Journal of the Geological Society of India》2013,82(4):363-367

Occurrence of well crystallized nontronite in a megaporphyritic basalt flow from Nighoj area, Maharashtra, India is reported. Thin section studies of the vesicles from periphery to the centre reveal the presence of greenish palagonite, probably indicating the stages of devitrification of glass and the weathering of palagonite to the formation of nontronite. The centre of the vesicles is marked by the presence of dark greenish-brown, radiating and fibrous sheets of nontronite. X-ray diffraction (XRD) analysis reveals the dominant presence of smectite (nontronite) as the major phase with traces of pyroxene. Scanning Electron Microscopy (SEM) shows well developed sheets of nontronite within the gas vesicles of the megaporphyritic basalt flows. In the present investigation, the nontronite within the gas vesicles of the megaporphyritic basalts at Nighoj was formed in-situ due to the devitrification of glass containing palagonite. Palagonite is an intermediate step in the alteration that may be accomplished by the steam generated in quenching of the lava. Weathering of palagonite under conditions of poor drainage is essential to the genesis of nontronite as has been suggested by Victor and Vernon (1946).  相似文献   

Thermal metamorphism of the Arunachal Himalaya, India: Raman thermometry and thermochronological constraints on the tectono-thermal evolution   总被引：1，自引：0，他引：1  

George Mathew  Sharmistha De Sarkar  Kanchan Pande  Suryendu Dutta  Shakir Ali  Apritam Rai  Shilpa Netrawali 《International Journal of Earth Sciences》2013,102(7):1911-1936

Determination of the peak thermal condition is vital in order to understand tectono-thermal evolution of the Himalayan belt. The Lesser Himalayan Sequence (LHS) in the Western Arunachal Pradesh, being rich in carbonaceous material (CM), facilitates the determination of peak metamorphic temperature based on Raman spectroscopy of carbonaceous material (RSCM). In this study, we have used RSCM method of Beyssac et al. (J Metamorph Geol 20:859–871, 2002a) and Rahl et al. (Earth Planet Sci Lett 240:339–354, 2005) to estimate the thermal history of LHS and Siwalik foreland from the western Arunachal Pradesh. The study indicates that the temperature of 700–800 °C in the Greater Himalayan Sequence (GHS) decreases to 650–700 °C in the main central thrust zone (MCTZ) and decreases further to <200 °C in the Mio-Pliocene sequence of Siwaliks. The work demonstrates greater reliability of Rahl et al.’s (Earth Planet Sci Lett 240:339–354, 2005) RSCM method for temperatures >600 and <340 °C. We show that the higher and lower zones of Bomdila Gneiss (BG) experienced temperature of ~600 °C and exhumed at different stages along the Bomdila Thrust (BT) and Upper Main Boundary Thrust (U.MBT). Pyrolysis analysis of the CM together with the Fission Track ages from upper Siwaliks corroborates the RSCM thermometry estimate of ~240 °C. The results indicate that the Permian sequence north of Lower MBT was deposited at greater depths (>12 km) than the upper Siwalik sediments to its south at depths <8 km before they were exhumed. The ⁴⁰Ar/³⁹Ar ages suggest that the upper zones of Se La evolved ~13–15 Ma. The middle zone exhumed at ~11 Ma and lower zone close to ~8 Ma indicating erosional unroofing of the MCT sheet. The footwall of MCTZ cooled between 6 and 8 Ma. Analyses of P–T path imply that LHS between MCT and U.MBT zone falls within the kyanite stability field with near isobaric condition. At higher structural level, the temperatures increase gradually with P–T conditions in the sillimanite stability field. The near isothermal (700–800 °C) condition in the GHS, isobaric condition in the MCTZ together with T–t path evidence of GHS that experienced relatively longer duration of near peak temperatures and rapid cooling towards MCTZ, compares the evolution of GHS and inverted metamorphic gradient closely to channel flow predictions.  相似文献   

Trend analysis and ARIMA modelling of pre-monsoon rainfall data for western India   总被引：1，自引：0，他引：1  

Priya Narayanan  Ashoke Basistha  Sumana Sarkar  Sachdeva Kamna 《Comptes Rendus Geoscience》2013,345(1):22-27

Spatial and temporal variability of rainfall over different seasons influence physical, social and economic parameters. Pre-monsoon (March, April and May – MAM) rainfall over the country is highly variable. Since heat lows and convective rainfall in MAM have an impact on the intensity of the ensuing monsoons, hence the pre-monsoon period was chosen for the study. The pre-whitened Mann Kendall test was used to explore presence of rainfall trend during MAM. The results indicate presence of significant (at 10% level) increasing trend in two stations (Ajmer, Bikaner). The practical significance of the change in rainfall was also explored as percentage changes over long term mean, using Theil and Sen's median slope estimator. Forecast using univariate ARIMA model for pre-monsoon months indicates that there is a significant rise in the pre-monsoon rainfall over the northwest part of the country.  相似文献   

Bottomside profile shape parameters during low solar activity and comparisons with IRI-2007 model     

N.K. Sethi  R.S. Dabas  Purshottam Bhawre  S.K. Sarkar 《Journal of Atmospheric and Solar》2009,71(17-18):1935-1942

Modern digital ionosonde measurements at low–middle latitude station, New Delhi, India, are used to assess the IRI-2007 model for the bottomside profile shape parameters B0 and B1 during solar minimum. Comparative analysis shows that in general, the IRI (B0 Table) option reveals better agreement with the B0 observations during daytime in all the seasons, while outside this time period, the IRI (Gulyaeva) predicted B0 values are closer to the observations. For B1 parameter, both the options in the IRI reproduce similar diurnal variations in all the seasons and are closer to observed values except during pre-sunrise and post-sunset hours.  相似文献   

Rice Husk Ash as a Low Cost Adsorbent for the Removal of Methylene Blue and Congo Red in Aqueous Phases     

Anirban K. Chowdhury  Anupam D. Sarkar  Amitava Bandyopadhyay 《洁净——土壤、空气、水》2009,37(7):581-591

Adsorption is of significant importance for effluent treatment, especially for the treatment of colored effluent generated from the dyeing and bleaching industries. Low cost adsorbents have gained attention over the decades as a means of achieving very high removal efficiencies to meet effluent discharge standards. The present article reports on batch investigations for color removal from aqueous solutions of Methylene Blue (MB) and Congo Red (CR) using Rice Husk Ash (RHA) as an alternative low cost adsorbent. The performance analysis was carried out as a function of various operating parameters, such as initial concentration of dye, adsorbent dose, contact time, shaker speed, interruption of shaking and ionic concentration. Performance studies revealed that a very high percentage removal of color was achievable for both dyes. The maximum percentage removal of MB was 99.939%, while 98.835% removal was observed for CR. These percentage removals were better than existing systems. Detailed data analysis indicated that adsorption of MB followed the Temkin isotherm, while CR followed the Freundlich isotherm. These isotherms were feasible within the framework of experimentation. Batch kinetic data, on the other hand, indicated that pseudo second order kinetics governed adsorption in both cases. Sensitivity analysis further indicated that the effects of initial dye concentration, shaker speed, pH and ionic strength had no noticeable effect on the percentage dye removal at equilibrium. Batch desorption studies revealed that 50% acetone solution was optimum for CR, while desorption of MB varied directly with acetone concentration.  相似文献   

High-resolution profile of stable isotopes and iridium across a K/T boundary section from Koshak Hill, Mangyshlak, Kazakhstan     

A. Sarkar  S.K. Bhattacharya  P.N. Shukla  N. Bhandari  D.P. Naidin 《地学学报》1992,4(5):585-590

Oxygen and carbon isotopes were measured to a high depth resolution in a nannoplankton carbonate sequence spanning the Cretaceous-Tertiary boundary at Koshak Hill, Mangyshlak Peninsula, Kazakhstan. The boundary is characterized by the presence of a ∼ 1 cm thick clay layer having a sharp peak in iridium concentration with a maximum value of 3.7 ng g^-1. The δ¹⁸O data reveal rapid (∼10³ years) excursion of sea-surface temperature at the boundary where an initial cold pulse is followed by a persistent warm period. The δ¹³C data, in contrast, indicate only a gradual change in productivity across the boundary. The observations suggest a biogeochemical scenario for the boundary event pertaining to shallow epicontinental seas.  相似文献   

    

S. C. Sarkar  S. Kabiraj  S. Bhattacharya  A. B. Pal 《Mineralium Deposita》1997,32(6):622

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