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排序方式: 共有1714条查询结果,搜索用时 15 毫秒
791.
Organic matter (OM) features, degradation and remineralisation were studied in the seawater of a natural coastal site (Marine Protected Area of Portofino) and at an urbanised coastal site (Quarto, Genoa city) of the Ligurian Sea. The accumulation of low trophic value OM and the persistence of phytoplanktonic biomass throughout the year were observed only at the urbanised site. Efficient OM degradation and remineralisation via high-activity (two times higher than at the Portofino site) hydrolytic enzymatic activities (alkaline phosphatase and leucine aminopeptidase) were recorded. This active degradation indicated a generally good resistance of the Quarto system to environmental pressures (coastal inputs and anthropogenic influence). However, the high potential release of recycled inorganic nutrients and the favourable environmental conditions (i.e. higher seawater temperature at the Quarto site and allochthonous inputs), could encourage unpredictable development of the autotrophic fraction, including an already observed dystrophic blooming of toxic microalgae. 相似文献
792.
793.
794.
Cristina E. Panaiotu Iuliana Vasiliev Cristian G. Panaiotu Wout Krijgsman Cor G. Langereis 《地学学报》2007,19(2):120-126
Provenance analysis of the sediments from foredeep basins is crucial in understanding the contemporaneous orogenic exhumation processes. We report in this paper complex sediment provenance analysis using sandstone petrography and mudstone geochemistry, combined with magnetic susceptibility of the Upper Miocene to Pliocene deposits from Focşani foredeep basin (Romania). Data show a change of source area between 5 and 6 Ma, from an active volcanic arc towards a recycled orogenic belt, concurrent with an important increase of accumulation rate. This change was triggered by exhumation and erosion of the outer nappes from East Carpathians. 相似文献
795.
M. A. Armienta G. Villaseñor R. Rodriguez L. K. Ongley H. Mango 《Environmental Geology》2001,40(4-5):571-581
Interaction of groundwater with As-bearing rocks has been proposed as one of three main sources of arsenic at Zimapán valley,
México. The complexity of the geology and hydrogeology of the valley make it difficult to identify the natural causes of arsenic
poisoning. Samples from the different rock outcrops and water from wells tapping various rock formations were analyzed. The
rocks from mineralized areas contained higher concentrations of arsenic with respect to the same formations in non-mineralized
areas. The arsenic minerals arsenopyrite, scorodite, and tennantite were identified in some rock samples. Higher temperature
and lower Eh values were found for those wells containing more arsenic. The physicochemical characteristics of these naturally
polluted well waters could be produced by arsenopyrite oxidation. The geochemical model PHREEQCI was used to perform the inverse
modeling of two wells located along the same fault. Arsenopyrite oxidation and scorodite dissolution appear to be the geochemical
processes producing the natural pollution according to the model. The release and transport of arsenic mainly occur through
fractures within the cretaceous limestones where the most productive wells are drilled. The presence of arsenic should be
expected also in other formations near mineralized zones in the Zimapán Valley. Field determinations of Eh and T could be
used to detect potentially polluted wells.
Received: 29 April 1999 / Accepted: 18 July 2000 相似文献
796.
Tomás Morales-Juberías Martín Olazar Jose María Arandes Pedro Zafra Iñaki Antigüedad Félix Basauri 《Environmental Geology》1997,30(3-4):143-151
A model based on numerical solutions, which allows for solving the dispersion equation under variable recharge and velocity conditions, is developed to simulate solute transport in conduit flow aquifers during flow recession periods. As an example, the evolution of a tracer in the little known karst conduit that links the sinking stream of Oma valley to the Olalde spring is investigated in the karstic region of Santa Eufemia-Ereñozar (Basque Country, Spain). The model, with different hypothetical structures, allows for obtaining series of tracer breakthrough curves, which are fitted to experimental data using an optimization algorithm. These results, although they can be used to simulate the tracer evolution between the two points considered, do not allow for determining the internal structure and spatial disposition of contributions in the aquifer. 相似文献
797.
A semi-numeric method by Herrera, Jiménez and Ruggeri is used to study the evolution of general relativistic spheres in the free streaming radiation approximation when specific contraction schemes are imposed and for particular oscillatory radiation profiles. On one hand, the emerging radiation is studied for oscillating contracting spheres, on the other, the variation of the boundary surface is analyzed when a pulsating profile is provided. We considered three particular equations of state previously studied. It is found that the oscillatory frequency of the surface for the equations of state considered coincide with the frequency of the oscillation for the radiation profile. 相似文献
798.
M.H. Ibáñez S 《Astrophysics and Space Science》1997,256(1-2):377-380
The effects of the thermal conduction on the linear stability analysis of an optically thin plasma with metallicity Z = 1 and including chemical relaxation time effects, is carried out. Generally, a further stabilizing effect on the unstable modes and an increasing of the damping of stable modes appear for disturbances with values of wave number large enough. However, the oscillating mode appearing at low temperature remain unaffected by the thermal conduction. 相似文献
799.
800.
We present 190 astrometric positions of comet Halley and 46 positions of comet Giacobini-Zinner obtained at Fabra Observatory during the 1985/1986 campaign by the astrometric network of the International Halley Watch. 相似文献