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Rhabdosargus holubi is a small (maximum weight=2.4?kg) yet important fishery species in the estuaries of the south-east coast of South Africa. Little is known of its biology and specifically its growth rate, which is essential for sustainable management of the fishery. We examined and counted the opaque zones in the sectioned otoliths of 134 R. holubi to determine its age and growth parameters. The otoliths from two recaptured fish marked with oxytetracycline confirmed that one opaque zone was deposited annually. The species reached a maximum age of 18 years and growth was adequately described by a von Bertalanffy growth function of the form: Lt = 358.1 (1 – e?0.24(t+0.77)) mm fork length. There were no significant differences between any of the male and female growth parameters (likelihood ratio test: p = 0.3). The growth was slow (omega index: ω = 86.56); however, despite this, the unique life history of R. holubi may provide a degree of resilience to heavy fishing pressure in estuaries. 相似文献
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A riparian ecosystem downstream of a small dam in central Texas was instrumented for sap flow, soil moisture content, and stream level from 2001. Stable isotopes in water (D and 18O) were analysed from rainfall, stream, lake, and cored sapwood cellulose from cedar elm (Ulmus crassifolia). The isotope signature of water source to cedar elm was identified by back calculation starting with the water isotopes in cellulose, and accounting for leaf‐water evaporation and biological fractionation during cellulose synthesis. The estimated mean isotope of the source water to cedar elm was enriched above rainfall in similarity to stream water during 2002. Flow paths that may have contributed to estimated variability from regional base flow and recharge water were identified using the variably saturated HYDRUS‐2D model. Copyright © 2004 John Wiley & Sons, Ltd. 相似文献
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J. M. Duke 《Mathematical Geosciences》1983,15(1):219-220
Sulfide ores that formed by magmatic segregation processes currently account for about 60% of mankind's supply of nickel, 98% of the platinum group metals, and substantial quantities of copper, cobalt, and other elements. The first stage in the generation of such an ore is the separation of an immiscible sulfide liquid from a mafic or ultramafic silicate magma. The sulfide acts as a collector of the ore metals and its tenor is determined by the concentrations of the metals in the parent magma, the degree to which the metals partition into the sulfide phase, and the relative proportions of sulfide, silicate liquid, and crystalline phases that equilibrate. Deposits of an economically exploitable grade represent metal concentrations of at least an order of magnitude greater than those in the parent magmas, and so sulfide saturation in itself does not lead to the formation of an ore. It is necessary that some process such as gravitative settling or flowage differentiation operate in the magma to accumulate the exsolved sulfide. Ultimately the ore grade reflects both the metal tenor of the sulfide phase and the efficiency of the accumulation process. If a magma is to give rise to a magmatic sulfide deposit, it must be at least locally saturated with sulfide at some stage during its differentiation history. It follows that it would be most advantageous in the exploration for magmatic sulfide ores to be able to identify bodies of igneous rock that crystallized from magmas that were once sulfide saturated. To this end, a computer model has been formulated that simulates the differentiation of mafic and ultramafic magmas under sulfide-saturated and sulfide-undersaturated conditions (Duke and Naldrett, 1978; Duke, 1979).The calculation is based on equations expressing the conservation of mass and the equilibrium partitioning of elements between silicate liquid and each of the other phases. The input to the program includes the bulk composition of the magma and the proportions in which the phases olivine, clinopyroxene, orthopyroxene, plagioclase, and sulfide are to be fractionally segregated from the magma. The procedure is a reiterative one involving the calculation of the partition coefficients for each element as a function of the magma composition, calculation of the compositions of each of the fractionating phases, and subtraction of these in the desired proportions from the magma. These steps are repeated using the final liquid composition from one increment as the initial composition for the subsequent increment. Perfect fractionation is simulated as long as the increment size is sufficiently small. The output of the program includes the compositions of the residual liquid and each of the fractionating phases as a function of the amount of fractionation. The computer model has been used to simulate ore-forming processes in a variety of mafic and ultramafic magma suites but its application to the differentiation of Archean komatiitic magmas has been particularly instructive. Comparison of the model results with analyses of natural rocks indicates that the liquid equivalent members of komatiitic suites from a number of Archean greenstone sequences could be derivative liquids produced by fractional crystallization of olivine from sulfide-undersaturated parent magmas initially containing about 32% MgO. Analogous samples from ore-bearing komatiitic sequences, however, are in many cases depleted in chalcophile elements to the degree predicted by the computer model for sulfide-saturated fractionation. Examples of such chalcophile element depletion have been documented in the ultramafic lavas of the Malartic Group in northwestern Quebec which hosts the Marbridge deposit, in the komatiitic sequence at Kambalda in Western Australia which hosts at least 20 deposits (Lesher, Lee, Groves, Bickle, and Donaldson, 1981),and in the komatiitic lavas in the vicinity of the Scotia deposit in Western Australia (Stolz, 1981).These results demonstrate the potential utility of the modeling procedure in mineral exploration. The model calculations may also be of use in the evaluation of mineral resources. The degree of chalcophile element depletion that occurs during the fractionating of a sulfide-saturated magma is related to the relative amount of sulfide that is segregated. Thus, if the initial quantity of magma is known, the quantity of sulfide that was segregated from the magma may be estimated from the chalcophile element trends. For example, Lesher, Lee, Groves, Bickle, and Donaldson (1981)have concluded that the amount of ore represented by past production and current reserves of the Kambalda camp is only a fraction of the total quantity of sulfide segregated from the magma that gave rise to the ultramafic sequence there. 相似文献
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Duke U. Ophori 《Hydrogeology Journal》1998,6(2):193-203
Flow of groundwater with variable density and viscosity was simulated at the Atikokan Research Area (ARA) in northwestern
Ontario, Canada. An empirical viscosity–concentration equation was modified to include total-dissolved-solids (TDS) data from
the ARA. The resulting equation was used successfully to estimate reasonably accurate viscosity values over the expected range
of temperature and concentration, in comparison with experimental values derived for sodium chloride solutions. A three-dimensional
finite-element code, MOTIF, developed by Atomic Energy of Canada Limited, was used in the simulations. The inclusion of the
effects of depth-increasing temperature and TDS-dependent fluid-density distribution, while maintaining only a temperature-dependent
viscosity relationship in a simulation, resulted in a more penetrative flow against expected buoyancy effects (i.e., the physics
of the system was not honored). Accounting for concentration in the viscosity equation caused water to be less penetrative
and more in accordance with the expected physics of the system. A conclusion is that fluid concentration should be considered
simultaneously in calculating the density and viscosity of a fluid during modeling of variable-density flow in areas underlain
by fluids with high TDS. Results of simulations suggest that both flow directions and magnitudes should be employed simultaneously
during the calibration of a model. Large-scale groundwater movement in the ARA may be analyzed with carefully selected vertical
no-flow boundaries. By incorporating the geothermal temperature gradient, groundwater recharge increases by 12%; thus, this
gradient plays a significant role in groundwater flow at the ARA. Variability in the fluid concentration at the ARA neither
decreases nor increases recharge into the groundwater system. The hypothesis that an isolated continuous regional flow system
may exist at depth in the ARA is not supported by these simulations.
Received, September 1996 Revised, September 1997, February 1998 Accepted, February 1998 相似文献
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Water quality in the Great Barrier Reef region: responses of mangrove, seagrass and macroalgal communities 总被引：1，自引：0，他引：1
Marine plants colonise several interconnected ecosystems in the Great Barrier Reef region including tidal wetlands, seagrass meadows and coral reefs. Water quality in some coastal areas is declining from human activities. Losses of mangrove and other tidal wetland communities are mostly the result of reclamation for coastal development of estuaries, e.g. for residential use, port infrastructure or marina development, and result in river bank destabilisation, deterioration of water clarity and loss of key coastal marine habitat. Coastal seagrass meadows are characterized by small ephemeral species. They are disturbed by increased turbidity after extreme flood events, but generally recover. There is no evidence of an overall seagrass decline or expansion. High nutrient and substrate availability and low grazing pressure on nearshore reefs have lead to changed benthic communities with high macroalgal abundance. Conservation and management of GBR macrophytes and their ecosystems is hampered by scarce ecological knowledge across macrophyte community types. 相似文献
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Eric?Rodriguez Kevin?A.?McGuinness Duke?U.?OphoriEmail author 《Environmental Geology》2004,45(5):623-632
An initial assessment of an old manufacturing site with groundwater impacted by trichloroethene (TCE) contamination in the metropolitan New York area showed that the TCE was being removed naturally by reductive dechlorination. However, complete dechlorination was not expected at the site because the process was progressing too slowly under transitional to aerobic conditions at a degradation constant of –0.0013 and a TCE half life of 533 days. A pilot test was conducted at the site in which a carbohydrate substrate (molasses) was injected into the groundwater to create an In-Situ Reactive Zone (IRZ). Post-pilot test groundwater sampling and analysis indicated that an IRZ was created successfully as the total organic carbon (TOC) content and conductivity increased significantly while oxidation-reduction (REDOX) potential and dissolved oxygen (DO) decreased. The created IRZ caused enhanced reductive dechlorination of TCE at the site, found to proceed with a degradation constant of –0.0158 and a TCE half life of 44 days. 相似文献