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21.
A. Sarkar S. Sengupta J.M. McArthur P. Ravenscroft M.K. Bera Ravi Bhushan A. Samanta S. Agrawal 《Quaternary Science Reviews》2009,28(25-26):2564-2581
Sedimentology, carbon isotope and sequence stratigraphic analysis of subsurface sediments from western part of Ganges–Brahmaputra (GB) delta plain shows that a Late Quaternary marine clay and fluvial channel-overbank sediments of MIS 5 and 3 highstands are traceable below the Holocene strata. During the Last Glacial Maximum (LGM) sea-level lowering of >100 m produced a regional unconformity (type 1), represented by palaeosols and incised valley. C4 vegetation expanded on exposed lowstand surface in an ambient dry glacial climate. At 9 ka transgression inundated the lowstand surface pushing the coastline and mangrove front 100 km inland. Simultaneous intensification of monsoon and very high sediment discharge (4–8 times than modern) caused a rapid aggradation of both floodplain and estuarine valley fill deposits between 8 and 7 ka. The Hoogli River remaining along its present drainage possibly acted as the main conduit for transgression and sediment discharge that was subsequently abandoned. C3 vegetation dominated the delta plain during this time. From 7 ka onward progradation of delta plain started and continued till recent. This period experienced a mixed C3–C4 vegetation with localized mangroves in the mid-Holocene to dominant return of C4 vegetation in the late Holocene period. The study indicates that while the initiation of western part of GB delta occurred at least 1 ka earlier than the global mean delta formation age, the progradation started at 7 ka, at least 2 ka earlier than thought before. The terrestrial vegetation change was modulated by changes in depositional environment, specific ecological niches and climate rather than pCO2. 相似文献
22.
Samanta Manon Lax Eric Wade Peterson Stephen J. Van der Hoven 《Environmental Earth Sciences》2017,76(20):708
The concentration of chloride (Cl?) in streams in northern regions has increased as a result of applications of deicers. This study focused on quantifying the relationship between land use and stream Cl? concentrations. The study area comprises two adjacent watersheds in central Illinois, with similar geology and climate but different land uses (agricultural and urban). GIS analysis delineated watershed land use and calculated road surface areas. Stream water samples were collected and analyzed for anionic composition. During the winter months, streams dominated with urban land use experienced a 20-fold increase in Cl? concentrations (range between 36 and 1350 mg L?1); Cl? concentrations in agricultural dominated streams also increased, but the increase was smaller (3X) and concentrations remained low (between 11 and 58 mg L?1). As road salts are not the sole source of Cl? in a stream, Cl? and bromide (Br?) mass ratios (Cl/Br) and Cl? and sodium (Na) molar ratios ([Cl]/[Na]) were used to identify potential sources of Cl?. The ratios indicate urbanized watersheds were impacted by road salts; agricultural watersheds ratios indicate other anthropogenic sources. A nonlinear relationship between urban land use and stream Cl? concentrations indicates urban land use as low as 23% results in elevated Cl? concentrations (greater than 150 mg L?1) in stream waters. 相似文献
23.
In this paper, comparative evaluation of various local and global learning algorithms in neural network modeling was performed for ore grade estimation in three deposits: gold, bauxite, and iron ore. Four local learning algorithms, standard back-propagation, back-propagation with momentum, quickprop back-propagation, and Levenberg–Marquardt back-propagation, along with two global learning algorithms, NOVEL and simulated annealing, were investigated for this purpose. The study results revealed that no benefit was achieved using global learning algorithms over local learning algorithms. The reasons for showing equivalent performance of global and local learning algorithms was the smooth error surface of neural network training for these specific case studies. However, a separate exercise involving local and global learning algorithms on a nonlinear multimodal optimization of a Rastrigin function, containing many local minima, clearly demonstrated the superior performance of global learning algorithms over local learning algorithms. Although no benefit was found by using global learning algorithms of neural network training for these specific case studies, as a safeguard against getting trapped in local minima, it is better to apply global learning algorithms in neural network training since many real-life applications of neural network modeling show local minima problems in error surface. 相似文献
24.
Utpal Kumar Samanta Prasanta Chatterjee Manimala Mej 《Astrophysics and Space Science》2013,345(2):291-296
Solitons and shocks are addressed in a pair ion plasma in the presence of a kappa distribution. The dissipation is taken care of through the kinematic viscosity of both positive and negative ions in the plasma. The Kadomtsev–Petviashvili–Burger (KPB) equation is derived using the small amplitude expansion method. The Abel equation is obtained from the KPB equation and a solution is obtained by using the factorization method. The effect of the parameters κ and β (temperature ratio of ion species) is observed. Analytically we can find both solitons and shocks. The change of profile from soliton to shocks is shown in the figures. This study may be of wide relevance for the study of the formation of shocks and solitons in laboratory-produced pair ion plasmas. 相似文献
25.
26.
Numerical modeling of heterogeneous flow fields around rigid objects with special reference to particle paths, strain shadows and foliation drag 总被引：2，自引：0，他引：2
With the help of two-dimensional numerical models this paper investigates three aspects of heterogeneous deformation around rigid objects: (1) the nature of particle paths; (2) the development of strain shadow zones; and (3) the drag patterns of passive markers. In simple shear, spherical objects develop typically a concentric vortex motion, showing particle paths with an eye (double-bulge)-shaped separatrix. The separatrix has no finite dimension along the central line, parallel to the shear direction. Under a combination of pure shear and simple shear, the particle paths assume a pattern with a bow-tie shaped separatrix. With increase in the ratio of pure shear to simple shear (Sr), the separatrix around the object shrinks in size. The axial ratio of the object (R) is another important factor that controls the geometry of particle paths. When R<1.5, the loci of a particle close to the object form an elliptical shell with the long axis lying along the central line. With increase in axial ratio R, the loci form a doublet elliptical shell structure. Objects with R>3 do not show closed particle paths, but give rise to elliptical or circular spiral particle paths.
The development of strain shadow zones against equant rigid bodies depends strongly on the strain ratio Sr. When Sr=0 (simple shear), they develop opposite to the extensional faces of the object, forming a typical σ-type tail. The structure has a tendency to die out with an increase in the pure shear component of the bulk deformation (Sr). The initial angle of the long axis of the object with the shear direction (φ) and the axial ratio of the object (R) determine the development of strain shadow zones near inequant rigid objects. Objects with large R and φ between 60 and 120° form pronounced zones of low finite strain, giving rise to strain shadow structures. A geometrical classification of diverse drag patterns of passive markers around rigid objects is presented along with their conditions of formation. 相似文献
27.
Dibakar Ghosh Tusar Dutta Susanta K. Samanta Dipak C. Pal 《Journal of the Geological Society of India》2013,81(1):101-112
The Singhbhum Shear Zone in eastern India is one of the largest repositories of uranium and copper in India. Besides uranium and copper, apatite-magnetite mineralization is widespread in this shear zone. This study aims at deciphering the physico-chemical evolution of magnetite mineralization in relation to progressive shearing integrating field relations, micro-textures, structures and compositions of magnetite in the Banduhurang uranium mine. Apatite-magnetite ores occur as discrete patches, tongues, and veins in the strongly deformed, fine grained quartzchlorite schist. Textures and microstructures of magnetite indicate at least three stages of magnetite formation. Coarsegrained magnetite (magnetite-1) with long, rotational, and complex strain fringes, defined by fibrous and elongate quartz, is assigned to a stage of pre-/early-shearing magnetite formation. Medium grained magnetite (magnetite-2), characterized by single non-rotational strain fringe equivalent to the youngest fringe of magnetite-1, grew likely at the mid-/late-stage of shearing. Fine grained magnetite (magnetite-3) is generally devoid of any pressure shadow. This indicates even a much later stage of formation of this magnetite, presumably towards the closing stage of shearing. Some of the magnetite-1 grains are optically heterogeneous with a dark, pitted Cr-Ti-bearing core overgrown by lighter, fresh rim locally containing pyrite, chalcopyrite, and chlorite inclusions. The cores are also locally characterized by high Al and Si content. Homogeneous magnetite-1 is optically and compositionally similar to the overgrowth of heterogeneous magnetite-1. This homogeneous magnetite-1 that grew as separate phase is contemporaneous with the overgrowth on pitted core of heterogeneous magnetite-1. Magnetite-2 is compositionally very similar to homogeneous magnetite-1, but is devoid of sulfide inclusion. Magnetite-3 is generally devoid of any silicate or sulfide inclusion and is most pure with least concentrations of trace/minor elements. The high Al and Si content in some magnetite can be explained by coupled substitution that involves substitution of Si4+ for Fe3+ in the tetrahedral sites and Fe2+ for Fe3+ in the octahedral sites, with a simple substitution of Al3+ for Fe3+ in the octahedral sites. The mode of occurrences of apatite-magnetite ores indicates a predominantly hydrothermal origin of most magnetite. However, the Cr-Ti-bearing magnetite-1 cores and inferred mafic nature of the original protolith indicates that some magnetite was inherited from the original igneous rock. We propose that the pre-/early-shearing hydrothermal event of magnetite formation was associated with sulfide mineralization and alteration of existing magmatic magnetite. The second stage of magnetite formation at the mid-/late-stage of shearing was not associated with sulfide formation. Finally, fine-grained compositionally pure magnetite formed at the closing stage of shearing likely due to metamorphism of Fe-rich protolith. 相似文献
28.
The application of road salts as deicing agents contributes to the anthropogenic loading of chloride (Cl−) on the environment. Using a 2D solute model, the potential of the unsaturated zone to serve as a reservoir and the mechanisms
controlling the movement of Cl− were examined. Physical properties and initial pore-water Cl− concentrations were derived from an array of soil borings. Initial pore-water concentrations show the presence of a Cl− “slug” approximately 0.75 m below the surface. Simulations show that within the unsaturated zone, Cl− transport is predominantly vertical, driven by molecular diffusion. After a 1-year simulation, a Cl− slug similar to the background observation was noted. While Cl− is retained in the unsaturated zone, a net loss of Cl− from the unsaturated zone was simulated for the first 10 years. In year 11, an equilibrium between the Cl− input and output is achieved. The presence of Cl− in the unsaturated zone becomes a long-term source of chloride to the groundwater, which eventually discharges into the surface
water. Historically, surface water chemistry data support the continual discharge of chloride to the surface water in the
area, further supporting the hypothesis that the unsaturated zone serves as a Cl− reservoir. 相似文献
29.
Dipankar Chakraborti Mohammad Mahmudur Rahman Bhaskar Das Amit Chatterjee Dipankar Das Biswajit Nayak Arup Pal Uttam Kumar Chowdhury Sad Ahmed Bhajan Kumar Biswas Mrinal Kumar Sengupta Md. Amir Hossain Gautam Samanta M. M. Roy Rathindra Nath Dutta Khitish Chandra Saha Subhas Chandra Mukherjee Shyamapada Pati Probir Bijoy Kar Adreesh Mukherjee Manoj Kumar 《Hydrogeology Journal》2017,25(4):1165-1181
During a 28-year field survey in India (1988–2016), groundwater arsenic contamination and its health effects were registered in the states of West Bengal, Jharkhand, Bihar and Uttar Pradesh in the Ganga River flood plain, and the states of Assam and Manipur in the flood plain of Brahamaputra and Imphal rivers. Groundwater of Rajnandgaon village in Chhattisgarh state, which is not in a flood plain, is also arsenic contaminated. More than 170,000 tubewell water samples from the affected states were analyzed and half of the samples had arsenic >10 μg/L (maximum concentration 3,700 μg/L). Chronic exposure to arsenic through drinking water causes various health problems, like dermal, neurological, reproductive and pregnancy effects, cardiovascular effects, diabetes mellitus, diseases of the respiratory and gastrointestinal systems, and cancers, typically involving the skin, lungs, liver, bladder, etc. About 4.5% of the 8,000 children from arsenic-affected villages of affected states were registered with mild to moderate arsenical skin lesions. In the preliminary survey, more than 10,000 patients were registered with different types of arsenic-related signs and symptoms, out of more than 100,000 people screened from affected states. Elevated levels of arsenic were also found in biological samples (urine, hair, nails) of the people living in affected states. The study reveals that the population who had severe arsenical skin lesions may suffer from multiple Bowens/cancers in the long term. Some unusual symptoms, such as burning sensation, skin itching and watering of eyes in the presence of sun light, were also noticed in arsenicosis patients. 相似文献
30.
Subir Sarkar Santanu Banerjee Pradip Samanta Silambuchelvan Jeevankumar 《Journal of Earth System Science》2006,115(1):49-60
This paper addresses macroscopic signatures of microbial mat-related structures within the 1.6Ga-old Chorhat Sandstone of
the Semri Group — the basal stratigraphic unit of the Vindhyan succession in Son valley. The Chorhat Sandstone broadly represents
a prograding succession of three depositional facies ranging from shallow shelf to coastal margin with aeolian sandsheet.
The mat-mediated structures were generated because of plastic or brittle deformation of sand, turned cohesive and even thixotropic
because of microbial mat growth. Mat growth also favoured abundant preservation of structures that usually have low preservation
potential. Prolific growth of microbial mat in the subtidal to intertidal zone of the Chorhat sea was facilitated due to lack
of grazing and burrowing activities of organisms in the Precambrian. It further indicates low rate of sedimentation between
the storms, as also attested by frequent superposition of storm-beds, even near the storm wave base. It also reduces erosion
and that, in turn, would imply low sediment concentration in flows leading to development of bedforms that are likely to be
smaller in size and isolated from each other in a single train in contrast to those that form in mat-free sands. 相似文献