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151.
Spatial distribution patterns ofScirpus validus were studied in tidal marshes of the lower Savannah River. The hypothesis that changes in spatial pattern forS. validus would accompany differences in environmental parameters was tested by sampling densities and biomass along environmental gradients of salinity and elevation. Coefficients of dispersion were calculated forS. validus and used to compare spatial patterns among freshwater, midly oligohaline, strongly oligohaline, and mesohaline tidal marshes. Results indicated significantly greater clumping ofS. validus in mesohaline marsh than in freshwater marsh. Only the mildly oligohaline site supported a random population ofS. validus, while the strongly oligohaline marsh supported a uniform spatial distribution. Spatial pattern and relative importance ofS. validus, as well as composition of co-occurring species, changed significantly with changing salinity. The relations between changes in relative importance ofS. validus and differences in soil organic matter and elevation were also significant. 相似文献
152.
The ability of turbulent nuées ardentes (surges) to transport coarse pyroclasts has been questioned on the basis that settling velocities of coarse fragments in the deposits are much too high for them to have been supported by turbulence in a dilute gas suspension. A computer model is used to evaluate the settling velocity of pyroclasts in suspensions of varying concentration and temperature. Since suspension of grains in low-concentration surges occurs if the shear velocity exceeds the settling velocity, the shear velocities related to the 16th and 84th percentiles, and the mean of the grain-size distribution are compared in surge deposits of the Vulsini, with the shear velocity necessary to move the coarsest grain on the bed surface (the Shields criterion). The results show that the settling velocities do not vary significantly in gaseous suspensions having volume concentrations lower than 15%, and that an increase in concentration to 25% is not sufficient to decrease the settling velocity of the coarser fraction, if it represents flow shear velocity. It is shown that the settling velocity of the mean grain size (M
z
) best depicts the shear velocity of a dilute turbulent suspension. Applying the results to the May 1902 paroxysmal nuées ardentes of Mount Pelée shows that the estimated mean velocities are well within the observed velocities, and sufficient to support all the clasts in dilute, turbulent suspensions. 相似文献
153.
Leonard Guelke 《Geoforum》1985,16(2):131-137
While physical geographers are united in a commitment to the scientific method with its emphasis on quantitative evidence, human geographers face a dilemma in deciding whether to adopt a scientific or humanistic approach in their research. The scientific approach offers a more secure, objective knowledge, but limits the scholar to a relatively narrow range of topics. The humanistic approach allows the scholar to explore a wide range of human experiences, but it lacks rigorous procedures of objective verification. The difficulty of the application of theoretical ideas to human societies can, to some extent, be avoided by adopting an historical approach, with an emphasis on the empirical investigation of human activity as a reflection of ideas. As long as human geographers have a commitment to basing their interpretations of geographical phenomena on objective evidence the possibility of a profitable co-operation exists among proponents of different philosophical approaches. A unified human geography embracing scholars of diverse views depends for its success on the identification of geographical problems that transcend philosophical and theoretical points of view. 相似文献
154.
Leonard Robert Gardner 《Chemical Geology》1984,46(1):55-73
Stable carbon and oxygen isotope measurements have been obtained from ten calcareous soil profiles developed on noncalcareous parent materials in Nevada and New Mexico. Despite the number of variables that can affect the isotopic composition of soil carbonate, most of these profiles show remarkably little variation in δ 13C and δ 18O with depth. This tendency towards uniformity is attributed to the fact that pedogenic carbonate forms by the incremental accumulation of a large number of small aliquots of CaCO3. As the number of aliquots of CaCO3 increases, the composition of their sum, which is the calcareous horizon itself, tends to converge to the long-term average of the individual aliquots. A mathematical model is presented to show how the time required for convergence is affected by the amplitude and period of aliquot variation and the extent of dissolution and reprecipitation of previously deposited carbonate. Calculations further indicate that in some cases the variation in δ 18O of soil carbonates may be moderated by the opposing effects of temperature change on the H2OCaCO3 fractionation factor and on the extent of evapotranspiration.The data of this study indicate that soil carbonates from sites within a homogeneous climatic region show relatively little site-to-site variation in δ 18O as compared to geographical variation in δ 13C. This suggests that the δ 18O of soil carbonates is primarily controlled by broad climatic factors (rainfall composition and temperature) whereas site factors (e.g., soil CO2) have a strong effect on δ 13C. In Nevada and New Mexico the δ 18O of Pleistocene calcareous profiles is 2.0–4.0‰ lighter than that of nearby Holocene soil carbonate, suggesting that old well-developed profiles may have formed dominantly under cooler pluvial conditions. 相似文献
155.
Element Mobility During Incipient Granulite Formation at Kabbaldurga, Southern India 总被引:7,自引:2,他引:7
At Kabbaldurga, infiltration of carbonic fluids along a systemof ductile shears and foliation planes has led to partial transformationof Archaean grey biotitehornblende gneiss to coarse-grainedmassive charnockite at about 2.5 b.y. ago. The dehydration ofthe gneiss assemblage was induced by a marked metasomatic changeof the reacting system from granodioritic to granitic, and obviouslytook place under conditions of an open system at 700750?C and 57 kb. Extensive replacement of plagioclase (An1630)by K-feldspar through Na, CaK exchange reactions withthe ascending carbonic fluids led to strong enrichment in K,Rb, Ba, and SiO2, and to a depletion in Ca. Progressive dissolutionof hornblende, biotite, magnetite, and the accessory mineralsapatite and zircon resulted in a marked depletion in Fe, Mg,Ti, Zn, V, P, and Zr. Most important is the recognition of REEmobility: with advancing charnockitization, the moderately fractionatedREE distribution patterns of the grey gneisses (LaN270; LaN/YbN= 520; EuN27; Eu/Eu* = 0.60.3) give way to stronglyfractionated REE patterns with a positive Eu-anomaly (LaN200;LaN/YbN = 2080; EuN22; Eu/Eu* = 0.61.8). The systematicdepletion especially in the HREE is due to the progressive dissolutionof zircon, apatite (and monazite), which strongly concentratethe REE. Stable isotope data (18O of 6.98.0 per mille for gneissesand charnockites; 13C of 8.5 and 6.5 per millefor late carbonate) indicate a magmatogenic source for the carbonicfluids. In contrast to the currently favoured derivation ofcarbonic fluids by decarbonation of the upper mantle or degassingof underplated basaltic intrusions, it is discussed here thatabundant fluid inclusions in lower crustal charnockites providedan extensive reservoir of fossil carbonic fluids.Shear deformation has tapped this reservoir and generated thechannel-ways for fluid ascent. Charnockitization of the Kabbaldurgatypethus appears to be a metasomatic process which is tectonicallycontrolled and restricted to the crustal level of the amphiboliteto granulite transition. 相似文献
156.
157.
Tectono-thermal modeling of the Yinggehai Basin,South China Sea 总被引:1,自引:0,他引:1
Lijuan?HeEmail author Liangping?Xiong Jiyang?Wang Jihai?Yang Weiliang?Dong 《中国科学D辑(英文版)》2001,44(1):7-13
Based on the observed data, the average value of surface heat flow in the Yinggehai Basin is calculated and it turns out to
be 84.1 mW/m2. The thermal evolution of the basin since the Cenozoic era has been attempted by tectono-thermal modeling. Three-phase extension
made the basin become hotter and hotter, reaching its climax in paleo-temperature history since 5.2 Ma. And nowadays, the
basin is in the heat flow decreasing period. During the Cenozoic era, the basement heat flow remained at 50–70 mW/m2 all the time. This is related to the degree of each extension phase, stretching rate mode and also the limited basin scale.
Modeling results also show that, the surface heat flow is controlled mainly by the basement heat flow, and less than 20% comes
from radiogenic heat production in the sediments of the basin 相似文献
158.
Iodine contents of soils developed over the major rock formations of the northern zone of the Eastern Pontide Tectonic Belt
(Northeastern Turkey) have been investigated with respect to soil-parent rock relationship, effect of topography, elevation,
and climate to construe its effect on the health of the local population. Samples were collected from the A and B horizons
of the soils developed over the major stratigraphic units constituting the eastern Pontides, including the Lower Basic Complex
of Jurassic-Lower Cretaceous age, the Berdiga limestone (Jurassic-lower Cretaceous), the Dagbasi granitoid (Upper Cretaceous),
volcano-sedimentary sequence of Upper Cretaceous age, ore-bearing and barren dacites of Upper Cretaceous age, and Neogene
alkaline basalts. Chemical analyses of soil samples indicate significantly lower iodine abundances for all the soils studied
(5–28 ppm) in comparison to the average abundance of iodine in analogous soils of other parts of the world (22–93 ppm). The
concentration of iodine in soils developed over the same geologic formation decrease with increasing elevation. In certain
cases, this decrease may reach up to 70%. Goiter is highly common throughout this region in Turkey. The results of this study
suggest that the iodine deficiency of region’s soils may be a principal underlying cause for this area of Turkey being an
endemic goiter region. 相似文献
159.
Environmental tracers are used to estimate groundwater ages and travel times, but the strongly heterogeneous nature of many subsurface environments can cause mixing between waters of highly disparate ages, adding additional complexity to the age-estimation process. Mixing may be exacerbated by the presence of wells because long open intervals or long screens with openings at multiple depths can transport water and solutes rapidly over a large vertical distance. The effect of intraborehole flow on groundwater age was examined numerically using direct age transport simulation coupled with the Multi-Node Well Package of MODFLOW. Ages in a homogeneous, anisotropic aquifer reached a predevelopment steady state possessing strong depth dependence. A nonpumping multi-node well was then introduced in one of three locations within the system. In all three cases, vertical transport along the well resulted in substantial changes in age distributions within the system. After a pumping well was added near the nonpumping multi-node well, ages were further perturbed by a flow reversal in the nonpumping multi-node well. Results indicated that intraborehole flow can substantially alter groundwater ages, but the effects are highly dependent on local or regional flow conditions and may change with time. 相似文献
160.