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951.
The interstitial water composition (
, alkalinity, Ca2+, Mg2+, Sr2+, Na+, K+) and the cation exchange capacity (CEC) were determined for the muddy sediments of the continental shelf off the Gironde Estuary (France), in the area where the sediment represents the deposit of the muddy suspension of the river. In comparison with seawater concentrations, the pore waters below 10 cm depth, show depletions of
and Ca2+ and below a 30 cm depth show depletions of Mg2+. Inversely, the upper 10 cm an enrichment of Ca2+ concentration, and an increase of K+ concentration to a 40 cm depth. High values of
are observed at the top 4 cm. Alkalinity enrichment is observed along the length of the core. Applying the alkalinity models for the sediment below a 10 cm depth demonstrates generally that calculated alkalinities are higher than the measured ones. Ca2+ dissolution occurs at the first 10 cm and authigenic carbonate precipitation starts beneath that level. Mg2+ depletion is accompanied by bicarbonate loss. This proves that Mg2+ depletion is due to a Mg-silicate reaction. The result of the CEC does not confirm the Mg2+ uptake by clay minerals in exchangeable site, under reducing conditions. Diffusion and bioturbation play an important role in the pore water concentration at the top of the core. 相似文献
952.
953.
Alpine olivine- and titanian clinohumite-bearing assemblages in the Erro-Tobbio peridotite (Voltri Massif, NW Italy) 总被引:3,自引:0,他引:3
M. SCAMBELLURI E. H. HOOGERDUIJN STRATING G. B. PICCARDO R. L. M. VISSERS E. RAMPONE 《Journal of Metamorphic Geology》1991,9(1):79-91
The microstructures in the Erro-Tobbio peridotite indicate several stages of recrystallization of olivine + titanian clinohumite-bearing assemblages. The development of these assemblages is closely associated with serpentinite mylonites, in which they occur in shear bands and foliations and are inferred to have grown synkinematically, in veins, and as post-kinematic radial aggregates. In the peridotite wall-rock adjacent to these mylonites, the same assemblages have recrystallized statically at the expense of original olivine and pyroxenes, mesh-textured chrysolite and antigorite veins. In addition, the olivine-bearing assemblage occurs in widespread vein systems. The brittle deformation of the peridotite resulting in the development of these vein systems is closely related to ductile deformation of metagabbroic dykes in the peridotite. Although early metasomatism resulted in extensive rodingitization of the gabbros, some dykes show an eclogitic assemblage of Na-clinopyroxene + garnet + chloritoid + chlorite ± talc. These observations, the microstructures and the mineral chemistry all suggest that the assemblages in the ultramafic rocks and metagabbros developed during a prograde evolution towards high pressures (>13–16 kbar, 450–550° C), and during subsequent decompression. This metamorphic evolution is considered to be related to Late Cretaceous intraoceanic subduction in the Alps-Apennine system and closure of the Piedmont-Ligurian basin. 相似文献
954.
Ole H. Caspersen MSc PhD. Cecil C. Konijnendijk Anton S. Olafsson 《Geografisk tidskrift / udgivet af Bestyrelsen for Det Kongelige danske geografiske selskab》2013,113(2):7-20
Geografisk Tidsskrift, Danish Journal of Geography 106(2):7–20, 2006 In its Regional Plan of 2005, The Greater Copenhagen Authority (abbreviated as “HUR” in Danish) places special emphasis on the future recreational values associated with the regional green structure. In this paper, the development of the urban green structure in Greater Copenhagen is elaborated upon, focusing on land use changes and the effectiveness of regionally coordinated planning measures. EU MOLAND data are used to analyse the development of the region's green structure during the period 1954 to 1998. Analysis of two “green wedges” within the green structure illustrates that the development of the green recreational areas is the result of both formal and more informal planning initiatives. Development has shown equal phases that correspond to the applied regional planning measures and the general economic conditions. However, local preferences in the involved municipalities likewise have played an important role and have resulted in different urbanisation pressure within the two wedges. Land use has transformed from an agricultural to a primarily recreational landscape. In some areas, however, urbanisation pressure has resulted in residential and green industrial areas instead of the planned recreational land use. Based on its historical development it is concluded that future preservation and development of the green structure in Greater Copenhagen requires regional planning measures to be incorporated into municipal plans. In this way the increasingly independent municipalities will comply to objectives of the Regional Plan 2005. 相似文献
955.
Portions of the Gunflint Iron Formation, originally a ferruginoussediment, were metamorphosed by the intrusion of the DuluthComplex to assemblages containing: pigeonite (Wo10En24Fs66)+olivine(Fo13Fa37)+Fe-Ti oxide (Mt62Usp34Hc4)+plagioclase (An94Ab6)+vapor+augite (Wo40En20Fs40) or cummingtonite Fe/(Fe+Mg) {smalltilde} 0.69; quartz was present but probably was not in equilibriumwith olivine. Comparison with synthetic phase-equilibrium studiesindicate conditions of initial recrystallization of T 800 °C,Ptotal 2kb, fo2 slightly below that of the pure fayalite-magnetite-quartzassemblage, and PH2O < Ptotal. During the slow cooling process following initial recrystallization,the phases present underwent a complex series of exsolution,inversion, oxidation, and hydration reactions. Pigeonite initiallyexsolved augite along (001), then inverted to orthopyroxene,which then exsolved augite along (100). The augite exsolvedonly pigeonite on (001) during its cooling history. The Fe-Tioxide for the most part oxidized to an intergrowth of magnetiteand ilmenite, although unoxidized portions later exsolved ulvöspinel.Cummingtonite exsolved actinolite, forming irregular patchesof the latter. Olivine, orthopyroxene, and augite reacted withplagioclase to form retrograde amphiboles. Orthopyroxene had difficulty nucleating during this slow coolingprocess, forming only at widely spaced points in mosaics ofprimary pigeonite grains, and never nucleating within primaryaugite grains. The resulting orthopyroxene grains are much largerthan the original pigeonite grains. 相似文献
956.
957.
We examined the contrasting, effects of floods and droughts produced by large changes in local climatology on vegetation patterns
in Nueces marsh, a semi-arid subtropical salt marsh in south Texas from 1995 to 2005. Climate variations during the study
included an initial 4-yr period of moderate conditions, followed by a 2-yr interval of drought, and a recent 4-yr wet period
that included large-scale floods. Variation in freshwater inflow, rainfall, and potential evapotranspiration were used in
conjunction with field measurements of salinity, inorganic nitrogen, and vegetation structure collected at sites located at
varying distances from Nueces Bay. Tidal creek salinities varied with Nueces Bay salinity, with strength of effect inversely
related to distance from the bay. Mean (±standard deviation) pore water salinities ranged from 59±54‰ at two high, marsh stations
farthest from the bay (10.1 km distant) to 30±21‰ in soil at a low marsh site closest to the bay (0.5 km distant). Mean pore
water ammonium was also higher at stations most distant from the bay; nitrate + nitrite did not exhibit a high marsh to low
marsh gradient. Nueces Bay salinity decreased substantially when the 10-d cumulative mean daily Nueces River flows exceeded
10 m3 s−1. During periods of low and moderate flood frequency (flows mostly below 10 m3 s−1), vegetation assemblages were dominated by stress-tolerant clonal plants. A catastrophic flood, which immersed vegetation
for several weeks between July and September 2002, resulted in extensive plant mortality, but within months, unvegetated areas
were rapidly colonized by the obligate annualSalicornia bigelovii. With the end of major flooding by late 2004, plant community structure began a return to pre-drought assemblages at high
and middle marsh stations by summer 2005. At the low marsh station, new conditions favored clonal dominants (Spartina alterniflora andBorrichia frutescens), with the latter replacingSalicornia virginica as the dominant species. Our results support the theory that the importance of competition and abiotic stress in determining
community composition are inversely related. 相似文献
958.
Environmental isotopic and hydrochemical study of groundwater in the Ejina Basin,northwest China 总被引:5,自引:0,他引:5
The study investigates the groundwater evolution and its residence time in the Ejina Basin, northwest China according to isotope
and hydrochemical analyses results. The groundwater chemistry is mainly controlled by the dissolution of halite, Glauber’s
salt, gypsum, dolomite and calcite, also influenced by other processes such as evaporation, ion exchange, and deposition.
Based on tritium content in atmospheric precipitation and by adopting a model with exponential time distribution function,
the mean residence time of the unconfined aquifer groundwater with fairly high tritium activities (21–49 TU) is evaluated.
The results show that these groundwaters have low residence time (5–120 years) and are renewable. In contrast, the deep confined
groundwaters are tritium-free and radiocarbon values range from 18.3 to 26.7 pmc. According to the most commonly used 14C correction models, the radiocarbon groundwater ages were calculated which yield ages of approximately 4,087–9,364 years
BP. Isotopic signatures indicate formation of deep confined groundwaters in a colder and wetter climate during the late Pleistocene
and Holocene. It is suggested that long-term, rational water usage guide should be set up for the Heihe River Basin as a whole
to permit a considerable discharge to the Ejina Basin. 相似文献
959.
The stratigraphy of the Pedwardine inlier of Precambrian (Longmyndian), Tremadoc, and what are now shown to be upper Llandovery rocks is described. The inlier is surrounded by Silurian rocks, including upper Llandovery (two very small areas), Wenlock, Ludlow and Přídolí (Red Downton Formation). The Ludlow rocks provide a distal record of the mass movement recorded more fully in the Wigmore Rolls area to the east. The Pedwardine area lies along the Church Stretton Fault Zone. Though ill-exposed, it shows evidence of the history of faulting, including thrusting. © 1997 John Wiley & Sons, Ltd. 相似文献
960.
Numerical simulation of a South China Sea typhoon Leo (1999) 总被引:6,自引:0,他引:6
K.-H. Lau Z.-F. Zhang H.-Y. Lam S.-J. Chen 《Meteorology and Atmospheric Physics》2003,83(3-4):147-161
Summary ?A South China Sea typhoon, Leo (1999), was simulated using the Penn State/NCAR mesoscale model MM5 with the Betts-Miller
convective parameterization scheme (BMEX). The simulation had two nested domains with resolutions at 54 and 18 km, and the
forecast duration was 36 hours. The model was quite successful in predicting the track, the rapid deepening, the central pressure,
and the maximum wind speed of typhoon Leo as verified with reports from the Hong Kong Observatory (HKO). The structure of
the eye, the eye wall, and the spiral convective cloud band simulated in the model are found to be comparable to corresponding
features identified in satellite images for the storm, and also with those reported by other authors.
A trajectory analysis was performed. Three kinds of trajectory were found: (1) spirally rising trajectories near the eye wall;
(2) spirally rising/descending trajectories in the convective/cloud free belt; (3) straight and fast rising trajectories in
a heavy convection zone along one of the cloud bands on the periphery of the tropical cyclone.
Both the HKO and the U.S. Joint Typhoon Warning Center (JTWC) reported the rapid deepening of Leo started around 00 UTC 29
April. In the model, the eye was first formed in the lower troposphere, and it extended to the upper troposphere within a
few hours. We speculate that the spin-up of cyclonic rotation in the low-level eye enhanced the positive vorticity along the
low-level eye wall. The positive vorticity was then transported to the upper troposphere by convection, leading to an extension
and growth of the eye into the upper troposphere.
To examine the impact of convective parameterization scheme (CPS) on the simulation, the Grell scheme (GLEX) was also tested.
The GLEX predicted a weaker typhoon with a wilder eye that extended not as high up in the upper troposphere as BMEX. The different
structures of the eye between the BMEX and GLEX suggest that the mesoscale features of the eye are dependent on the convection.
In other words, the vertical and horizontal distribution of convective heating is essential to the development and structure
of the eye.
Received December 18, 2001; accepted May 7, 2002
Published online: March 20, 2003 相似文献