U-Sr isotopic speedometer: Fluid flow and chemical weathering rates in aquifers   总被引：2，自引：0，他引：2  

Kate Maher  Donald J. DePaolo  John N. Christensen 《Geochimica et cosmochimica acta》2006,70(17):4417-4435

Both chemical weathering rates and fluid flow are difficult to measure in natural systems. However, these parameters are critical for understanding the hydrochemical evolution of aquifers, predicting the fate and transport of contaminants, and for water resources/water quality considerations. ⁸⁷Sr/⁸⁶Sr and (²³⁴U/²³⁸U) activity ratios are sensitive indicators of water-rock interaction, and thus provide a means of quantifying both flow and reactivity. The ⁸⁷Sr/⁸⁶Sr values in ground waters are controlled by the ratio of the dissolution rate to the flow rate. Similarly, the (²³⁴U/²³⁸U) ratio of natural ground waters is a balance between the flow rate and the dissolution of solids, and α-recoil loss of ²³⁴U from the solids. By coupling these two isotope systems it is possible to constrain both the long-term (ca. 100’s to 1000’s of years) flow rate and bulk dissolution rate along the flow path. Previous estimates of the ratio of the dissolution rate to the infiltration flux from Sr isotopes (⁸⁷Sr/⁸⁶Sr) are combined with a model for (²³⁴U/²³⁸U) to constrain the infiltration flux and dissolution rate for a 70-m deep vadose zone core from Hanford, Washington. The coupled model for both (²³⁴U/²³⁸U) ratios and the ⁸⁷Sr/⁸⁶Sr data suggests an infiltration flux of 5 ± 2 mm/yr, and bulk silicate dissolution rates between 10^−15.7 and 10^−16.5 mol/m²/s. The process of α-recoil enrichment, while primarily responsible for the observed variation in (²³⁴U/²³⁸U) of natural systems, is difficult to quantify. However, the rate of this process in natural systems affects the interpretation of most U-series data. Models for quantifying the α-recoil loss fraction based on geometric predictions, surface area constraints, and chemical methods are also presented. The agreement between the chemical and theoretical methods, such as direct measurement of (²³⁴U/²³⁸U) of the small grain size fraction and geometric calculations for that size fraction, is quite good.  相似文献   

Rates of silicate dissolution in deep-sea sediment: In situ measurement using U/U of pore fluids     

Katharine Maher  Donald J. DePaolo  Jo Chiu-Fang Lin 《Geochimica et cosmochimica acta》2004,68(22):4629-4648

Bulk dissolution rates for sediment from ODP Site 984A in the North Atlantic are determined using the ²³⁴U/²³⁸U activity ratios of pore water, bulk sediment, and leachates. Site 984A is one of only several sites where closely spaced pore water samples were obtained from the upper 60 meters of the core; the sedimentation rate is high (11-15 cm/ka), hence the sediments in the upper 60 meters are less than 500 ka old. The sediment is clayey silt and composed mostly of detritus derived from Iceland with a significant component of biogenic carbonate (up to 30%).The pore water ²³⁴U/²³⁸U activity ratios are higher than seawater values, in the range of 1.2 to 1.6, while the bulk sediment ²³⁴U/²³⁸U activity ratios are close to 1.0. The ²³⁴U/²³⁸U of the pore water reflects a balance between the mineral dissolution rate and the supply rate of excess ²³⁴U to the pore fluid by α-recoil injection of ²³⁴Th. The fraction of ²³⁸U decays that result in α-recoil injection of ²³⁴U to pore fluid is estimated to be 0.10 to 0.20 based on the ²³⁴U/²³⁸U of insoluble residue fractions. The calculated bulk dissolution rates, in units of g/g/yr are in the range of 4 × 10⁻⁷ to 2 × 10⁻⁶ yr⁻¹. There is significant down-hole variability in pore water ²³⁴U/²³⁸U activity ratios (and hence dissolution rates) on a scale of ca. 10 m. The inferred bulk dissolution rate constants are 100 to 10⁴ times slower than laboratory-determined rates, 100 times faster than rates inferred for older sediments based on Sr isotopes, and similar to weathering rates determined for terrestrial soils of similar age. The results of this study suggest that U isotopes can be used to measure in situ dissolution rates in fine-grained clastic materials.The rate estimates for sediments from ODP Site 984 confirm the strong dependence of reactivity on the age of the solid material: the bulk dissolution rate (R_d) of soils and deep-sea sediments can be approximately described by the expression R_d ≈ 0.1 Age⁻¹ for ages spanning 1000 to 5 × 10⁸ yr. The age of the material, which encompasses the grain size, surface area, and other chemical factors that contribute to the rate of dissolution, appears to be a much stronger determinant of dissolution rate than any single physical or chemical property of the system.  相似文献   

Patterns of Holocene environmental change in the midwestern United States     

Richard G. Baker  Louis J. Maher  Craig A. Chumbley  Kent L. Van Zant 《Quaternary Research》1992,37(3)

Four pollen sequences along a transect from north-central Iowa to southeast Wisconsin reveal the distribution of prairie and forest during the Holocene and test the use of pollen isopolls in locating the Holocene prairie-forest border. Prairie was dominant in central Iowa and climate was drier than present from about 8000 to 3000 yr B.P. During the driest part of this period in central Iowa (6500-5500 yr B.P.), mesic forest prevailed in eastern Iowa and Wisconsin, suggesting conditions wetter than at present. Prairie replaced the mesic forest about 5400 yr B.P. in eastern Iowa but did not extend much farther east; mesic forests were replaced in southern Wisconsin and northern Illinois about 5400 yr B.P. by xeric oak forests. This change from mesic to xeric conditions at 5400 yr B.P. was widespread and suggests that the intrusion of drier Pacific air was blocked by maritime tropical air from the Gulf of Mexico until the late Holocene in this area.  相似文献   

The occurrence of arsenic in selected marine macroalgae from two coastal areas of South Australia     

W.A. Maher  S.M. Clarke 《Marine pollution bulletin》1984,15(3):111-112

Total arsenic concentrations have been measured in macroalgae specimens from two coastal areas of South Australia. Phaeophyta in both areas were found to contain elevated arsenic concentrations (42.2–179 μg g^?1 and 26.3–65.3 μg g^?1) relative to Rhodophyta (17.6–31.3 μg g^?1 and 12.5–16.2 μg g^?1) and Chlorophyta (6.3–16.3 μg g^?1 and 9.9–10.8 μg g^?1).  相似文献   

中国黄土堆积的磁性记录与古降雨量重建   总被引：8，自引：0，他引：8  

胡蒙育  B.Maher 《地球科学进展》2002,17(3):320-325

概述了中国黄土和古土壤堆积的磁性特征，进而介绍成壤作用磁铁矿成因模式，讨论控制黄土—古土壤层矿物磁性特征的气候参量及其利用土壤成因磁化率重建古降雨量。  相似文献   

Stress Analysis for Rock Mass Failure with Offset Joints     

Omer Mughieda  Maher T. Omar 《Geotechnical and Geological Engineering》2008,26(5):543-552

Two-dimensional finite element model was created in this work to investigate the stress distribution within rock-like samples with offset open non-persistent joints under uniaxial loading. The results of this study have explained the fracture mechanisms observed in tests on rock-like material with open non-persistent offset joints (Mughieda and Alzo’ubi, Geotech Geol Eng J 22:545–562, 2004). Finite element code SAP2000 was used to study the stresses distribution within the specimens. Four-noded isoperimetric plain strain element with two degrees of freedom per node, and the three-noded constant strain triangular element with two degree of freedom per node were used in the present study. The results of the present study showed that the tensile stress in the bridge area caused coalescence for specimens with overlapped preexisting cracks (joints) while the coalescence of the secondary cracks, due to shear stress, caused the failure of specimens with non-overlapping cracks.  相似文献   

Process and response in contemporary Urban development: Melbourne in the 1980s     

Chris Maher 《The Australian geographer》1988,19(1):162-181

In the current climate of economic restructuring, technological change and demographic transition, cities such as Melbourne are experiencing new and different pressures. That these pressures are exerted upon an already developed urban structure, and within a particular planning and administrative structure, means that outcomes vary from city to city. However, Melbourne, in common with a number of cities, is facing substantial changes in many aspects including the nature and location of economic enterprise, the structure and composition of the population, and the relative competitive strength of the region. The result is that Melbourne has experienced problems both in its ability to hold and continue to attract activities which provide a basis for further growth and development, and in the exacerbation of internal variations in the level of opportunity and quality of life available to different groups in the population.

The role of government has traditionally been relatively minor in guiding the process of urban development in Australia. However, since 1982 the state government in Victoria has made a concerted effort to reorganise and coordinate urban management and planning, integrating these with other strategic initiatives relating to economic development and social justice. While there is evidence that many of the initiatives are proving effective, there are still a number of problem areas. Some of these, such as global economic trends, are beyond the scope of government to control. Others, such as measures to facilitate the development of new economic activities, are within the power of government, but require careful formulation and management. The extent to which such measures are successful depends upon both the correct identification of problems and solutions, and the ability of government to gain a wide degree of acceptance of its goals. It is these that will prove the most difficult to achieve in the long run.  相似文献   

Estimation of bearing capacity of open-ended model piles in sand     

Mohammed Y. Fattah  Wissam H. Al-Soudani  Maher Omar 《Arabian Journal of Geosciences》2016,9(3):242

The plugging of pipe piles is an important phenomenon, which is not adequately accounted for in the current design recommendations. An open-ended pipe pile is said to be plugged when the soil inside the pile moves down with the pile, resulting in the pile becoming effectively closed-ended. Plugging is believed to result in an increase in the horizontal stresses between the pile and the surrounding soil, which results in an increase in skin friction. A total number of 60 model pile tests are carried out to investigate the behavior of plugs on the pile load capacity and the effects of plug removal. Different parameters are considered, such as pile diameter–to–length ratio, types of installation in sands of different densities, and removal of the plug in three stages (50, 75, and 100 %) with respect to the length of plug. The changes in the soil plug length and incremental filling ratio (IFR) with the penetration depth during pile driving show that the open-ended piles are partially plugged from the outset of the pile driving. The pile reached a fully plugged state for pressed piles in loose and medium sand and partially plugged (IFR = 10 %) in dense sand. For driven piles, the IFR is about 30 % in loose sand, 20 % in medium sand, and 30 % in dense sand. The pile load capacity increases with increases in the length of the plug length ratio (PLR). The rate of increase in the value of the pile load capacity with PLR is greater in dense sand than in medium and loose sand. Based on test results, new empirical relation for the estimation of the load carrying capacity of open-ended piles based on the IFR is proposed.  相似文献   

Evaluation of Methods Used for Reducing Heavy Metal Leaching from Sandy Soil   总被引：1，自引：0，他引：1  

Maher Omar  Abdallah Shanableh 《Geotechnical and Geological Engineering》2016,34(5):1413-1423

Excessive heavy metal content in sandy soils poses risk to human health and the environment. The rapid expansion of urban areas makes it imperative to manage contaminated sites so that land can be reclaimed for beneficial purposes. Several methods have been proposed to control the leaching of heavy metals from contaminated soils. In this study, four techniques for mobilization and immobilization of metals in sandy soil were compared. The assessed mobilization techniques included chemical extraction using aqueous solutions of acids and chelating agents as well as biochemical extraction using sulfur-oxidizing microorganisms. The evaluated immobilization techniques included lime-cement-pozzolan stabilization and natural-zeolite stabilization. The immobilization techniques do not involve removing metals from soil and instead focus on addition of substances to the soil that alter its composition, volume, and properties. On the other hand, mobilization techniques entail the removal of metals from soil and changes in the soil properties. The findings confirmed that both mobilization and immobilization are effective in controlling the leaching of metals from sandy soils and thereby minimize the risk to the environment and human health. However, the appropriate technique for application at a given site should be chosen on a case-by-case basis, while accounting for the economic and technical feasibility, the necessary level of cleanup, and effect of residual metals on human health and the environment.  相似文献   

Tidal amplification and along‐strike process variability in a mixed‐energy paralic system prograding onto a low accommodation shelf,Edgeøya,Svalbard     

Ingrid Anell  Valentin Zuchuat  Anna Daniela Rhnert  Aleksandra Smyrak‐Sikora  Simon Buckley  Gareth Lord  Harmon Maher  Ivar Midtkandal  Kei Ogata  Snorre Olaussen  Per T. Osmundsen  Alvar Braathen 《Basin Research》2021,33(1):478-512

The study describes the depositional development and sediment partitioning in a prograding paralic Triassic succession. The deposits are associated with the advance of large prism‐scale clinoforms across a shallower platform area. Approaching the platform, the limited accommodation and associated relative higher rates of deposition generated straighter clinoforms with lower foreset angles. The vertical restriction across the platform is interpreted to have amplified the tidal signature. Sediment was redistributed from the coast into increasingly sandy delta‐front deposits, compared to offshore equivalents. The deposits comprise extensive compound dune fields of amalgamated and increasingly clean sandbodies up‐section. Rapid deposition of significant amounts of sand led to differential subsidence and growth‐faulting in the delta front, with downthrown fault blocks further amplifying the tidal energy through funnelling. A mixed‐energy environment created along‐strike variability along the delta front with sedimentation governing process‐regime. Areas of lower sedimentation were reworked by wave and storm‐action, whereas high sedimentation rates preserved fluvially dominated mouth bars. A major transgression, however, favoured tidally dominated deposits also in these areas, attributed to increasing rugosity of the coastline. Formation of an extensive subaqueous platform between the coast and delta front dampened incoming wave energy, and tidally dominated deposits dominate the near‐shore successions. Meanwhile formation of wave‐built sand‐bars atop the platform attest to continued wave influence. The strong tidal regime led to the development of a heterolithic near‐shore tidally dominated channel system, and sandier fluvial channels up‐river. The highly meandering tidal channels incising the subaqueous platform form kilometre wide successions of inclined heterolithic stratification. The fluvially dominated channels which govern deposition on the delta plain are narrower and slightly less deep, straighter, generally symmetric and filled with cleaner sands. This study provides important insight into tidal amplification and sand redistribution during shallowing on a wide shelf, along with along‐strike process‐regime variability resulting from variations in sediment influx.  相似文献