Origin of Cretaceous phosphorites from the onshore of Tamil Nadu,India     

V. Purnachandra Rao  Pratima M. Kessarkar  R. Nagendra  E. V. S. S. K. Babu 《Journal of Earth System Science》2007,116(6):525-536

Cretaceous phosphorites from the onshore of Tamil Nadu have been investigated for their origin and compared with those in the offshore. Cretaceous phosphorites occur as light brown to yellowish brown or white nodules in Karai Shale of the Uttatur Group in the onshore Cauvery basin. Nodules exhibit phosphatic nucleus encrusted by a chalky shell of carbonate. The nucleus of the nodules consists of light and dark coloured laminae, phosphate peloids/coated grains and detrital particles interspersed between the laminae. Scanning electron microscope (SEM) studies reveal trapping and binding activity of microbial filaments. A mat structure with linearly arranged microbial filaments and hollow, cell-based coccoid cyanobacterial mat are present. Nodules contain abundant carbonate fluorapatite, followed by minor calcite, quartz and feldspar. The P₂O₅ content of the phosphorites ranges from 18 to 26%. The CaO/P₂O₅, Sr and F contents are higher than that of pure carbonate fluorapatite. Concentrations of Si, Al, K, Fe, and Ti are low. We suggest that the nuclei of the nodules represent phosphate clasts related to phosphate stromatolites formed at intertidal conditions. At high energy levels the microbial mats were disintegrated into phosphate clasts, coated with carbonate and then reworked into Karai Shale. On the other hand, Quaternary phosphorites occur as irregular to rounded, grey coloured phosphate clasts at water depths between 180 and 320m on the continental shelf of Tamil Nadu. They exhibit grain-supported texture. Despite Quaternary in age, they also resemble phosphate stromatolites of intertidal origin and reworked as phosphate clasts onto the shelf margin depressions. Benthic microbial mats probably supplied high phosphorus to the sediments. Availability of excess phosphorus seems to be a pre-requisite for the formation of phosphate stromatolites.  相似文献   

On selection of kernel parametes in relevance vector machines for hydrologic applications     

Shivam Tripathi  Rao S. Govindaraju 《Stochastic Environmental Research and Risk Assessment (SERRA)》2007,21(6):747-764

Recent advances in statistical learning theory have yielded tools that are improving our capabilities for analyzing large and complex datasets. Among such tools, relevance vector machines (RVMs) are finding increasing applications in hydrology because of (1) their excellent generalization properties, and (2) the probabilistic interpretation associated with this technique that yields prediction uncertainty. RVMs combine the strengths of kernel-based methods and Bayesian theory to establish relationships between a set of input vectors and a desired output. However, a bias–variance analysis of RVM estimates revealed that a careful selection of kernel parameters is of paramount importance for achieving good performance from RVMs. In this study, several analytic methods are presented for selection of kernel parameters. These methods rely on structural properties of the data rather than expensive re-sampling approaches commonly used in RVM applications. An analytical expression for prediction risk in leave-one-out cross validation is derived. For brevity, the effectiveness of the proposed methods is assessed first by data generated from the benchmark sinc function, followed by an example involving estimation of hydraulic conductivity values over a field based on observations. It is shown that a straightforward maximization of likelihood function can lead to misleading results. The proposed methods are found to yield robust estimates of parameters for kernel functions.  相似文献   

Respiration and denitrification in permeable continental shelf deposits on the South Atlantic Bight: Rates of carbon and nitrogen cycling from sediment column experiments     

Alexandra M.F. Rao  Mark J. McCarthy  Wayne S. Gardner  Richard A. Jahnke 《Continental Shelf Research》2007

Nitrogen (N) cycling and respiration rates were measured in sediment columns packed with southeastern United States continental shelf sands, with high permeability (4.66×10⁻¹¹ m²) and low organic carbon (0.05%) and nitrogen (0.008%). To simulate porewater advection, natural shelf seawater was pumped through columns of different lengths to achieve fluid residence times of approximately 3, 6, and 12 h. Experiments were conducted seasonally at in situ temperature. Fluid flow was uniform in nearly all columns, with minimal dead zones and channeling. Significant respiration (O₂ consumption and ∑CO₂ production) occurred in all columns, with highest respiration rates in summer. Most (78–100%) remineralized N was released as N₂ in the majority of cases, including columns with oxic porewater throughout, with only a small fraction released as NO₃⁻ from some oxic columns. A rate of 0.84–4.83×10¹⁰ mol N yr⁻¹, equivalent to 1.06–6.09×10⁻⁶ mmol N cm⁻² h⁻¹, was calculated for benthic N₂ production in the South Atlantic Bight, which can account for a large fraction of new N inputs to this shelf region. Metal and sulfate reduction occurred in long residence time columns with anoxic outflow in summer and fall, when respiration rates were highest. Because permeable sediments dominate continental shelves, N₂ production in high permeability coastal sediments may play an important role in the global N cycle.  相似文献   

Modulation of Geomagnetic Lunar Daily Variation in H at Alibag with Lunar Distance     

B. R. Arora  D. R. K. Rao 《Geophysical Journal International》1975,43(2):627-633

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Spherical radiation-driven shock waves in a non-uniform atmosphere     

G. Narasimhulu Naidu  M. P. Ranga Rao 《Astrophysics and Space Science》1983,95(2):383-391

Similarity solutions are obtained for spherical radiation-driven shock waves propagating in a non-uniform atmosphere at rest obeying a density power law. Approximate analytical solutions are also obtained and found to be in good agreement with the numerical solutions. The effect of the parameter characterizing the initial density distribution of the gas on solutions of the flow field is studied in detail. It is also shown analytically that the shock wave propagates as an overdriven detonation.  相似文献   

Collinear equilibria and their characteristic exponents in the restricted three-body problem when the primaries are oblate spheroids     

Sharma  Ram Krishan  Rao  P. V. Subba 《Celestial Mechanics and Dynamical Astronomy》1975,12(2):189-201

Celestial Mechanics and Dynamical Astronomy - In this paper location of the collinear libration points is investigated numerically, by taking the oblateness of the primaries into consideration, for...  相似文献   

Earthquake detection capability of North Indian seismological stations     

B. S. R. Rao  T. K. S. Prakasa Rao  A. S. Krishnamurthy 《Pure and Applied Geophysics》1976,114(3):461-467

Summary The relative detection capability of nine seismological stations situated north of 20°N in India has been studied. The best stations were the WWSSN stations NDI and SHL, followed by CHA, DDI, BOK and BHK. The remaining stations CAL, SEH and TOC are inferior in their detection capability.  相似文献   

Stationary solutions and their characteristic exponents in the restricted three-body problem when the more massive primary is an oblate spheroid     

Ram Krishan Sharma  P. V. Subba Rao 《Celestial Mechanics and Dynamical Astronomy》1976,13(2):137-149

This paper deals with the numerical investigations of the locations of the five equilibrium points by taking into consideration the effect of oblateness of the more massive primary for some systems of astronomical interest. This note is further concerned with the periodic solutions of the linearized equations of motion around the five equilibrium points. Interesting differences in the trends of the angular frequencies of these motions have been noticed.  相似文献   

Impacts of recent El Niño Modoki on dry/wet conditions in the Pacific rim during boreal summer     

Hengyi Weng  Karumuri Ashok  Swadhin K. Behera  Suryachandra A. Rao  Toshio Yamagata 《Climate Dynamics》2007,29(2-3):113-129

Present work uses 1979–2005 monthly observational data to study the impacts of El Niño Modoki on dry/wet conditions in the Pacific rim during boreal summer. The El Niño Modoki phenomenon is characterized by the anomalously warm central equatorial Pacific flanked by anomalously cool regions in both west and east. Such zonal SST gradients result in anomalous two-cell Walker Circulation over the tropical Pacific, with a wet region in the central Pacific. There are two mid-tropospheric wave trains passing over the extratropical and subtropical North Pacific. They contain a positive phase of a Pacific-Japan pattern in the northwestern Pacific, and a positive phase of a summertime Pacific-North American pattern in the northeastern Pacific/North America region. The western North Pacific summer monsoon is enhanced, while the East Asian summer monsoon is weakened. In the South Pacific, there is a basin-wide low in the mid-latitude with enhanced Australian high and the eastern South Pacific subtropical high. Such an atmospheric circulation pattern favors a dry rim surrounding the wet central tropical Pacific. The El Niño Modoki and its climate impacts are very different from those of El Niño. Possible geographical regions for dry/wet conditions influenced by El Niño Modoki and El Niño are compared. The two phenomena also have very different temporal features. El Niño Modoki has a large decadal background while El Niño is predominated by interannual variability. Mixing-up the two different phenomena may increase the difficulty in understanding their mechanisms, climate impacts, and uncertainty in their predictions.  相似文献   

Characterization of channel planform features and sinuosity indices in parts of Yamuna River flood plain using remote sensing and GIS techniques     

Armugha Khan  Liaqat A. K. Rao  Ali P. Yunus  Himanshu Govil 《Arabian Journal of Geosciences》2018,11(17):525

River planform features and sinuosity are widely acknowledged as important geomorphological indices that control the channel hydraulics and stream power which determines the flow velocity and sediment supply to downstream reaches. Despite their significance, there has been little study about these indices in mighty Yamuna River for understanding the topographic control and hydraulic regime. This paper investigates the channel planform features and sinuosity by employing topographic map, digital elevation model (DEM), and satellite imageries to elucidate the morphological, sedimentological, and hydrological characteristics of the channel. Based on sinuosity index (varies between 1.02 and 1.41), the river segments are categorized into straight (segments 1, 3, 4, 5, 6), sinuous (segments 2, 7, 8, 9, and 11), and meandering (segment 10). The parameters analyzed to find out the influencing factors on sinuosity indicate geomorphological and anthropologic control in their development. Tectonic control of sinuosity in the studied stretch is ruled out as it is flowing through an alluvial plain with low rate of discharge.  相似文献