Hydromagnetic cylindrical shock in self-gravitating gas     

S. Kumar  D. K. Saxena 《Astrophysics and Space Science》1984,100(1-2):65-83

Chisnell-Chester-Whitham method has been used to study the propagation of diverging hydromagnetic cylindrical shock through an infinitely electrically conducting self-gravitating gas having an initial density distribution ₀= r^-w where is the density at the axis of symmetry andw is a constant, simultaneously for the two cases, viz.: (i) when the shock is weak and (ii) when it is strong. The magnetic field is taken to be axial and initially of constant strength. Analytical relations for shock velocity and shock strength have been obtained. the expressions for the pressure, the density and the particle velocity immediately behind the shock have also been derived.  相似文献   

Evolution and paleobiogeography of mangroves     

Jyoti Srivastava  Vandana Prasad 《Marine Ecology》2019,40(6)

We reviewed the geological record of mangroves based on fossil pollen, fruits, and wood evidence of Nypa, Avicennia, Sonneratia, Rhizophoraceae, and mangrove associates to trace the origin, distribution, extinction, and range contraction of paleo‐mangroves during the Late Cretaceous–Miocene time. Our study region covers paleocoastal areas of Indo‐West Pacific (IWP) and Atlantic East Pacific (AEP) region. First, we compiled the mangrove fossil records from the Late Cretaceous till Miocene and identified the migration pattern for Nypa, Avicennia, Sonneratia, Rhizophoraceae members, and mangrove associates such as Acrostichum, Wetherellia, Pelliciera, Aegiceras, Heritiera, Excoecaria, and Barringtonia. Second, we interpreted the paleoclimate shifts which caused the dispersal/extinction of this specialized ecosystem. Lastly, we proposed the future consequences of mangrove diversity for restoration and conservation strategies. First mangroves appeared during the Late Cretaceous, 100–65 Ma, since then their evolution is closely related to sea‐level changes in geological times. The oldest geological record of Nypa palm which prefers broad ecological tolerance is a good example for pantropical distribution of mangroves. High sea‐level and humid climate offered sufficient coastal regions and climate for the development of 12 genera of mangroves in nine families and subsequent proliferation into newer areas during early to middle Eocene (~50–40 Ma). The Eocene/Oligocene boundary crisis heralds the beginning of a biogeographical split between the present‐day eastern and western provinces of mangroves with records of Sonneratia, Rhizophora, Pelliciera, Barringtonia, and Acrostichum. However, during Oligocene and Middle Miocene mangroves occupied the present geographical position with addition of Nypa, Avicennia, and Excoecaria species. Re‐evaluation of Cenozoic records suggests that the climatic conditions of Late Paleocene, end of Eocene, and middle Pliocene were the driving force that led to the evolution and expansion of mangrove flora. During the Neogene, latitudinal contraction, extinction, and migration of mangroves led to the present bipartite distribution. The Himalayan uplift and establishment of Asian summer monsoon toward Late Neogene further affected the coastal dynamics which tailored the mangrove distribution of the Indian subcontinent. Loss of ecological habitats and local extinction forming disjunct distribution of mangroves during the Quaternary have also affected its overall biogeography.  相似文献   

Investigation of different biogeochemical cover configurations for mitigation of landfill gas emissions: laboratory column experiments     

Chetri  Jyoti K.  Reddy  Krishna R.  Grubb  Dennis G. 《Acta Geotechnica》2022,17(12):5481-5498

Acta Geotechnica - Municipal solid waste (MSW) landfills are a major source of anthropogenic methane (CH4) and carbon dioxide (CO2) emissions, which are also major greenhouse gases. Apart from...  相似文献   

Genesis and rapid weakening of tropical cyclone Lehar (2013)     

Rajasree  V. P. M.  Bhate  Jyoti N.  Kesarkar  Amit P.  Singh  Vikas 《Natural Hazards》2021,109(1):371-388

Natural Hazards - Tropical cyclogenesis and rapid weakening are subjects of considerable interest in the literature. This paper addresses the genesis and rapid weakening of a North Indian Ocean...  相似文献   

Arsenic-enriched aquifers: Occurrences and mobilization of arsenic in groundwater of Ganges Delta Plain, Barasat, West Bengal, India     

Sandeep Kar  Jyoti Prakash Maity  Jiin-Shuh Jean  Chia-Chuan Liu  Bibhash Nath  Huai-Jen Yang  Jochen Bundschuh   《Applied Geochemistry》2010,25(12):1805-1814

Hydrogeochemical characteristics and elemental features of groundwater and core sediments have been studied to better understand the sources and mobilization process responsible for As-enrichment in part of the Gangetic plain (Barasat, West Bengal, India). Analysis of water samples from shallow tubewells (depth 24.3–48.5 m) and piezometer wells (depth 12.2–79.2 m) demonstrate that the groundwater is mostly the Ca-HCO₃ type and anoxic in nature (mean Eh_SHE = 34 mV). Arsenic concentrations ranged from <10–538 μg/L, with high concentrations only present in the shallow to medium depth (30–50 m) of the aquifer along with high Fe (0.07–9.8 mg/L) and relatively low Mn (0.15–3.38 mg/L) as also evidenced in core sediments. Most groundwater samples contained both As(III) and As(V) species in which the concentration of As(III) was generally higher than that of As(V), exhibiting the reducing condition. Results show lower concentrations of NO₃, SO₄ and NO₂ along with higher values of DOC and HCO₃, indicating the reducing nature of the aquifer with abundant organic matter that can promote the release of As from sediments into groundwater. Positive correlations of As with Fe and DOC were also observed. The presence of DOC may actively drive the redox processes. This study revealed that reduction processes of FeOOH was the dominant mechanism for the release of As into the groundwater in this part of the Ganges Delta plain.  相似文献   

On the Possibility of Gly and Ala Amino Acids on Titan’s Surface     

P. P. Saxena 《Earth, Moon, and Planets》2010,106(2-4):113-118

In view of the possible production of formaldehyde and acetaldehyde in Titan’s atmosphere, the production of α-amino nitriles, the precursors of Glycine and Alanine amino acids, is explored in the upper atmosphere of Titan. The presence of Glycine and Alanine amino acids or their precursor amino nitriles can be used as a diagnostic, respectively for the presence or absence of water locally on the surface of Titan.  相似文献   

Problems in Himalayan geology     

M. N. Saxena Ph.D. 《International Journal of Earth Sciences》1973,62(2):563-581

The Central Crystalline Axis of the Great Himalaya holds clues to the solution of many problems dealing with the crystallines, metamorphics and geological contrasts encountered in the areas to the north and south of the Axis. The problems discussed include criteria for deciphering the stratigraphy in the Himalayan zone to the south of the Crystalline Axis, age of Jutogh and Chail groups, their relationship with each other and with the Crystalline Axis, nature of the Simla Slates, Jaunsar, Blaini, Infra-Krol, and Krol formations, unfossiliferous nature of the Palaeozoic and Mesozoic sediments, relationship between the Lower, Middle and Upper Tertiaries and the age, presence and absence, and also the cause of metamorphism. It has been found that high grade metamorphism is not a Tertiary phenomenon. Very low grade metamorphism in the sedimentaries and the absence of plutonic intrusives from the Palaeozoic, Mesozoic and Tertiary sedimentary rocks indicate that magmatism, regional metamorphism and orogeny are not always mutually associated in space and time. Orogenic and epeirogenic episodes other than Cretaceous-Tertiary hold clues to many problems of Himalayan Geology.

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Zusammenfassung Die zentrale kristalline Achse des Gro\en Himalaya bietet den Schlüssel für die Lösung vieler Probleme für die im Norden und Süden gelegenen Metamorphite und die geologischen GegensÄtze. Neue Daten für die Deutung der Stratigraphie in der Himalaya-Zone südlich der kristallinen Achse, das Alter der Jutogh- und der Chail-Gruppe, ihre Beziehung untereinander und zur kristallinen Achse, die Natur der Simla Slates sowie der Jaunsar-, Blaini-, Infra-Krol- und Krol-Formation, die Fossilleere der palÄozoischen und mesozoischen Sedimente, die Beziehung zwischen unterem, mittlerem und oberem TertiÄr und schlie\lich Alter, Vorkommen und Fehlen sowie die Ursache der Metamorphose werden gegeben. Hochgradige Metamorphose ist nicht an das TertiÄr gebunden. Niedriggradige Metamorphose in den Sedimenten und das Fehlen plutonischer Intrusiva aus palÄozoischen, mesozoischen und tertiÄren Sedimentgesteinen weisen darauf hin, da\ Magmatismus, regionale Metamorphose und Orogenese nicht immer zeitlich und rÄumlich miteinander verknüpft sind. Orogene und epirogene Episoden au\erhalb von Kreide und TertiÄr können wichtig für die Himalaya-Geologie sein.

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Résumé La zone axiale cristalline du Grand Himalaya apporte la clé pour la solution de nombreux problèmes concernant les métamorphites situées au Nord et au Sud, et les contrastes géologiques qu'elles présentent. De nouvelles données sont apportées sur la stratigraphie dans la zone de l'Himalaya au sud de l'axe cristallin, sur l'âge des groupes de Jutogh et de Chail, sur leurs relations mutuelles et avec l'axe cristallin, sur la nature des formations des phyllades de Simla, de Jaunsar, de Blaini, de l'Infrala nature des formations des phyllades de Simla, de Jaunsar, de Blaini, de l'Infra-Krol et de Krol, sur l'absence de fossiles dans les sédiments du PaléozoÏque et du MésozoÏque sur la présence et l'absence du métamorphisme, ainsi que sur sa cause. Le métamorphisme de degré élevé n'est pas lié au Tertiaire. Le métamorphisme de degré faible dans les sédiments et l'absence d'intrusions plutoniques dans les roches sédimentaires paléozoÏques, mésozoÏques et tertiaires montrent que le magmatisme, le métamorphisme régional et l'orogenèse ne sont pas toujours associés dans l'espace et le temps. Les épisodes orogéniques et épeirogéniques survenant en dehors du Crétacique et du Tertiaire peuvent Être importants pour la géologie himalayenne.

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. , , Jutogh Chail, , Simla Jaunsar, Blaini, Infra-Kroll & Kroll, , , , . . , , , . .

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Semi-empirical method for calculation of maximum subsidence in coal mines     

P. P. Bahuguna  Bhawani Singh  A. M. C. Srivastava  N. C. Saxena 《Geotechnical and Geological Engineering》1993,11(4):249-261

Summary Most empirical methods of subsidence prediction are based on the prior knowledge of the maximum possible subsidence, S_max, for a particular coalfield. The subsidence profile and other associated parameters along a desired line may be derived, in most cases, as a function of S _max. These methods are site specific and are not based upon the rational concepts of mechanics. Purely mechanistic methods, on the other hand, could not find wide application because of their limitations in representing the complex behaviour of a rockmass.Therefore a semi-empirical method of calculation of S _max or S, which is an improvement over the empirical method reported elsewhere (Bahuguna et al., 1991a), has been derived from combining the mechanistic and empirical approach. The effects of various parameters are studied by numerical modelling. These qualitative results are then used to influence the development of a more general semiempirical method. The method has been tested for 125 coal mine workings in India and 22 mines of the North Appalachian basin. The method is easily adaptable to other countries.  相似文献   

Laser-heated diamond anvil cell experiments at high pressure: Melting curve of nickel up to 700 kbar     

P. Lazor  G. Shen  S. K. Saxena 《Physics and Chemistry of Minerals》1993,20(2):86-90

Laser-heated experiments in a diamond anvil cell have been performed on high pressure melting of nickel up to 700 kbar. The laser heating system consists of diamond anvil cell, Nd:YAG and argon lasers, spectrograph with diode array, computer with software, CCD camera with monitor and optics. Experiments on melting of tungsten, nickel and platinum at 1 bar outside the diamond anvil cell and melting of nickel below 80 kbar in the cell were carried out to check the system for pressure and temperature measurements. The results show that for solid pressure medium the uncertainties in measurements of pressure at the experimental spot vary between ±5 kbar at 100 kbar and ±25 kbar at 660 kbar. Spectroradiometrically determined temperature is reliable within ±70 K. Melting was detected in situ by visual observation. The melting point of nickel at 660 kbar has been found to be 2557 ±66 K.  相似文献   

Influence of Temperature on the Toxicity of Phenol and its Chloro- and Nitro-derivatives to the Fish Notopterus notopterus (PALLAS)     

S. Gupta  R. C. Dalela  P. K. Saxena 《洁净——土壤、空气、水》1983,11(2):187-192

There is determined the LC₅₀ for 24 … 96 h of phenol, 2,4-dinitrophenol and pentachlorophenol as sodium salt by static assay at water temperatures of 16, 23 and 36°C. In addition, the combinations of the three substances were tested in the same way. With decreasing temperature, the toxicity of the individual substances decreased by 6.5 … 22%, that of the mixtures by 10 … 87%; with rising temperature, the toxicity increased by 38 … 747% and 75 … 744%, respectively. The safety limits at 23°C lie between 0.3 μg/l for a synergistically acting mixture and 0.9 mg/l for an antagonistically acting one; for the individual substances this range is 3.5 μg/l to 0.5 mg/l.  相似文献