Eolian Transport of Geogenic Hexavalent Chromium to Ground Water     

by Warren W. Wood  David Clark  Jeffrey L. Imes  Terry B. Councell 《Ground water》2010,48(1):19-29

A conceptual model of eolian transport is proposed to address the widely distributed, high concentrations of hexavalent chromium (Cr⁺⁶) observed in ground water in the Emirate of Abu Dhabi, United Arab Emirates. Concentrations (30 to more than 1000 μg/L Cr⁺⁶) extend over thousands of square kilometers of ground water systems. It is hypothesized that the Cr is derived from weathering of chromium-rich pyroxenes and olivines present in ophiolite sequence of the adjacent Oman (Hajar) Mountains. Cr⁺³ in the minerals is oxidized to Cr⁺⁶ by reduction of manganese and is subsequently sorbed on iron and manganese oxide coatings of particles. When the surfaces of these particles are abraded in this arid environment, they release fine, micrometer-sized, coated particles that are easily transported over large distances by wind and subsequently deposited on the surface. During ground water recharge events, the readily soluble Cr⁺⁶ is mobilized by rain water and transported by advective flow into the underlying aquifer. Chromium analyses of ground water, rain, dust, and surface (soil) deposits are consistent with this model, as are electron probe analyses of clasts derived from the eroding Oman ophiolite sequence. Ground water recharge flux is proposed to exercise some control over Cr⁺⁶ concentration in the aquifer.  相似文献   

New insights into the origin and evolution of Lake Vida,McMurdo Dry Valleys,Antarctica — A noble gas study in ice and brines     

Jessica L. Malone  M. Clara Castro  Chris M. Hall  Peter T. Doran  Fabien Kenig  Chris P. McKay 《Earth and Planetary Science Letters》2010,289(1-2):112-122

Unlike other lakes in the McMurdo Dry Valleys, Antarctica, Lake Vida has a thick (~ 19 m) ice cover sealing a liquid brine body of unusually high salinity (~ 245 g/L) from the atmosphere. To constrain the conditions under which the atypical Lake Vida ice cover formed and evolved, 19 ice samples were collected down to a depth of ~ 14 m, together with three brine samples trapped in the ice at ~ 16 m for analysis of helium, neon, argon, krypton, and xenon concentrations. The broad pattern of noble gas concentrations for Lake Vida samples is fundamentally different from that of air saturated water (ASW) at 0 °C and an elevation of 340 m for salinities of 0 (ice) and 245 g/L (brine). Overall, ice samples are enriched in He and depleted in Ne with saturation relative to ASW averages of 1.38 and 0.82, respectively, and strongly depleted in Ar, Kr, and Xe with relative saturations of 0.10, 0.06, and 0.05, respectively. By contrast, brine samples are generally depleted in He and Ne (relative saturation averages of 0.33 and 0.27, respectively) but enriched in Ar, Kr, and Xe, with relative saturation averages of 1.45, 3.15, and 8.86, respectively. A three-phase freezing partitioning model generating brine, ice and bubble concentrations for all stable noble gases was tested and compared with our data. Measured brine values are best reproduced for a salinity value of 175 g/L, a pressure of 1.1 atm, and a bubble volume of 20 cm³ kg^?1. Sensitivity tests for ice + bubble samples show an ideal fit for bubble volumes of ~ 1–2 cm³ kg^?1. Our results show that the conditions under which ice and brine formed and evolved at Lake Vida are significantly different from other ice-covered lakes in the area. Our brine data suggest that Lake Vida may be transitioning from a wet to a dry-based lake, while the ice + bubble data suggest at least partial re-equilibration of residual liquid with the atmosphere as ice forms at the top of Lake Vida ice cover.  相似文献   

Mixed fluid sources involved in diamond growth constrained by Sr–Nd–Pb–C–N isotopes and trace elements     

Ofra Klein-BenDavid  D. Graham Pearson  Geoff M. Nowell  Chris Ottley  John C.R. McNeill  Pierre Cartigny 《Earth and Planetary Science Letters》2010,289(1-2):123-133

Sub-micrometer inclusions in diamonds carry high-density fluids (HDF) from which the host diamonds have precipitated. The chemistry of these fluids is our best opportunity of characterizing the diamond-forming environment. The trace element patterns of diamond fluids vary within a limited range and are similar to those of carbonatitic/kimberlitic melts that originate from beneath the lithospheric mantle. A convecting mantle origin for the fluid is also implied by C isotopic compositions and by a preliminary Sr isotopic study (Akagi, T., Masuda, A., 1988. Isotopic and elemental evidence for a relationship between kimberlite and Zaire cubic diamonds. Nature 336, 665–667.). Nevertheless, the major element chemistry of HDFs is very different from that of kimberlites and carbonatites, varying widely and being characterized by extreme K enrichment (up to ～ 39 wt.% on a water and carbonate free basis) and high volatile contents. The broad spectrum of major element compositions in diamond-forming fluids has been related to fluid–rock interaction and to immiscibility processes.Elemental signatures can be easily modified by a variety of mantle processes whereas radiogenic isotopes give a clear fingerprint of the time-integrated evolution of the fluid source region. Here we present the results of the first multi radiogenic-isotope (Sr, Nd, Pb) and trace element study on fluid-rich diamonds, implemented using a newly developed off-line laser sampling technique. The data are combined with N and C isotope analysis of the diamond matrix to better understand the possible sources of fluid involved in the formation of these diamonds. Sr isotope ratios vary significantly within single diamonds. The highly varied but unsupported Sr isotope ratios cannot be explained by immiscibility processes or fluid-mineral elemental fractionations occurring at the time of diamond growth. Our results demonstrate the clear involvement of a mixed fluid, with one component originating from ancient incompatible element-enriched parts of the lithospheric mantle while the trigger for releasing this fluid source was probably carbonatitic/kimberlitic melts derived from greater depths. We suggest that phlogopite mica was an integral part of the enriched lithospheric fluid source and that breakdown of this mica releases K and radiogenic Sr into a fluid phase. The resulting fluids operate as a major metasomatic agent in the sub-continental lithospheric mantle as reflected by the isotopic composition and trace element patterns of G10 garnets.  相似文献   

Spatial analysis of sand dunes with a new global topographic dataset: new approaches and opportunities   总被引：2，自引：0，他引：2  

Chris H. Hugenholtz  Tom E. Barchyn 《地球表面变化过程与地形》2010,35(8):986-992

On June 29, 2009, version 1 of the ASTER GDEM (Advanced Spaceborne Thermal Emission and Reflection Radiometer Global Digital Elevation Model) was made available at no cost or restrictions to users worldwide via electronic download. The ASTER GDEM provides expanded spatial coverage and better resolution than other global digital elevation models (DEMs). In this paper we demonstrate how the ASTER GDEM provides new opportunities for investigating large aeolian sand dunes in three‐dimensions. Two dune‐specific spatial analysis methods are presented to illustrate potential applications of these data for discriminating dune generations and quantifying spatial variations of sediment supply. Moreover, we review how existing and emerging fields of dune pattern analysis and simulation modeling will be able to make significant advances through application of these data, potentially leading to future progress in studies of dune morphodynamics, environmental controls, and paleoenvironmental reconstructions. Copyright © 2010 John Wiley & Sons, Ltd.  相似文献   

中国大城市低收入邻里及其居民的贫困集聚度和贫困决定因素   总被引：11，自引：3，他引：8  

何深静  刘玉亭  吴缚龙  Chris Webster 《地理学报》2010,65(12):1464-1475

基于2007 年在中国6 个大城市开展的大规模家庭户调查,采用FGT指数法和逻辑回归分析方法,探讨中国大城市低收入邻里及其居民的贫困集聚程度和决定因素。研究发现,老城衰败邻里的贫困集聚度最高,同时承受多重不利状况的下岗/失业人员的贫困集聚度也是最高;事实贫困、被剥夺感与社会福利分配之间存在不匹配现象。在贫困的决定因素方面,制度保障和市场薪酬对城市贫困的发生具有同等重要的作用,但对不同低收入邻里和不同社会群体又表现出不同的关联性。随着市场作用的加强,市场薪酬机制正在成为贫困产生的更为重要的决定性因素。  相似文献   

Practical Use of Computationally Frugal Model Analysis Methods          下载免费PDF全文

Mary C. Hill  Dmitri Kavetski  Martyn Clark  Ming Ye  Mazdak Arabi  Dan Lu  Laura Foglia  Steffen Mehl 《Ground water》2016,54(2):159-170

Three challenges compromise the utility of mathematical models of groundwater and other environmental systems: (1) a dizzying array of model analysis methods and metrics make it difficult to compare evaluations of model adequacy, sensitivity, and uncertainty; (2) the high computational demands of many popular model analysis methods (requiring 1000's, 10,000 s, or more model runs) make them difficult to apply to complex models; and (3) many models are plagued by unrealistic nonlinearities arising from the numerical model formulation and implementation. This study proposes a strategy to address these challenges through a careful combination of model analysis and implementation methods. In this strategy, computationally frugal model analysis methods (often requiring a few dozen parallelizable model runs) play a major role, and computationally demanding methods are used for problems where (relatively) inexpensive diagnostics suggest the frugal methods are unreliable. We also argue in favor of detecting and, where possible, eliminating unrealistic model nonlinearities—this increases the realism of the model itself and facilitates the application of frugal methods. Literature examples are used to demonstrate the use of frugal methods and associated diagnostics. We suggest that the strategy proposed in this paper would allow the environmental sciences community to achieve greater transparency and falsifiability of environmental models, and obtain greater scientific insight from ongoing and future modeling efforts.  相似文献   

Early volcanic history of the Rabaul area     

Chris O. McKee  Robert A. Duncan 《Bulletin of Volcanology》2016,78(4):24

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Incorporation of in situ exposure and biomarkers response in clams Ruditapes philippinarum for assessment of metal pollution in coastal areas from the Maluan Bay of China   总被引：1，自引：0，他引：1  

Wang Z  Yan C  Vulpe CD  Yan Y  Chi Q 《Marine pollution bulletin》2012,64(1):90-98

The clams Ruditapes philippinarum were used to assess the impact of metal contaminants when transplanted to seven study sites along the Maluan Bay (China). Metal concentrations in digestive gland tissues of clams after 7-day in situ cage exposure were determined in conjunction with antioxidant enzyme activities. The results showed the importance of specific antioxidant biomarkers to assess complex pollutant mixtures and their good correlations to the pollutant compositions of deployment sites. Multivariate analysis indicated causal relationship between the chemicals at each study site and the biochemical "response" of the caged clams at these sites and demonstrated the potential presence of two different contaminant sources. This study suggested that the incorporation of tissue residue analysis with biomarkers response in caged clams together with factor analysis can be a useful biomonitoring tool for the identification of causal toxic pollutants and the assessment of complex metal pollutions in marine coastal environment.  相似文献   

Topographic,pedologic and climatic interactions influencing streamflow generation at multiple catchment scales   总被引：1，自引：0，他引：1  

Geneviève Ali  Doerthe Tetzlaff  Chris Soulsby  Jeffrey J. McDonnell 《水文研究》2012,26(25):3858-3874

Dominant flow pathways (DFPs) in mesoscale watersheds are poorly characterized and understood. Here, we make use of a conservative tracer (Gran alkalinity) and detailed information about climatic conditions and physical properties to examine how temporally and spatially variable factors interact to determine DFPs in 12 catchments draining areas from 3.4 to 1829.5 km² (Cairngorms, Scotland). After end‐member mixing was applied to discriminate between near surface and deep groundwater flow pathways, variation partitioning, canonical redundancy analyses and regression models were used to resolve: (i) What is the temporal variability of DFPs in each catchment?; (ii) How do DFPs change across spatial scales and what factors control the differences in hydrological responses?; and (iii) Can a conceptual model be developed to explain the spatiotemporal variability of DFPs as a function of climatic, topographic and soil characteristics? Overall, catchment characteristics were only useful to explain the temporal variability of DFPs but not their spatial variation across scale. The temporal variability of DFPs was influenced most by prevailing hydroclimatic conditions and secondarily soil drainability. The predictability of active DFPs was better in catchments with soils supporting fast runoff generation on the basis of factors such as the cumulative precipitation from the seven previous days, mean daily air temperature and the fractional area covered by Rankers. The best regression model R² was 0.54, thus suggesting that the catchments’ internal complexity was not fully captured by the factors included in the analysis. Nevertheless, this study highlights the utility of combining tracer studies with digital landscape analysis and multivariate statistical techniques to gain insights into the temporal (climatic) and spatial (topographic and pedologic) controls on DFPs. Copyright © 2011 John Wiley & Sons, Ltd.  相似文献   

Seismic swarms, fault plane solutions, and stress tensors for S?o Miguel Island central region (Azores)     

Rita Silva  Jens Havskov  Chris Bean  Nicolau Wallenstein 《Journal of Seismology》2012,16(3):389-407

The central region of S?o Miguel Island is one of the most seismically active areas of the Azores archipelago. A revised analysis of the seismicity distribution at this region has, for the first time, shown that the seismicity is clustered in two distinct areas: the area around Fogo Volcano (Fogo) and the area around Congro maar (Congro), with each area having a highly localized swarm activity. From a total of about 15,000 events in the period from 2002 to 2010, 78 best located events were selected to make fault plane solutions using P-wave polarities and amplitude ratios. This set of fault plane solutions, and another six subsets derived from it, were inverted for the best fitting stress tensor. The stress tensor using all the 78 fault plane solutions is characterized by a subhorizontal ??1 striking WNW-ESE and a ??3 striking NNE-SSW, consistent with the regional stress field for this region. A similar result, using only the fault plane solutions located in the Fogo area, was obtained. On the other hand, for the Congro area, a local stress field seems to be superimposed on the regional field: subhorizontal ??3, striking NNE-SSW, and a near-vertical ??1. The same stress regime persists in the first 5?km depth, probably related to the upwelling of thermal fluids. The rising fluids generate horizontal extension at shallow depths, which favour the opening of cracks and the circulation and ascension of hydrothermal fluids. The stress regime deeper than 5?km is more uncertain; however, it is indicative of a compressional regime. Thus, it can be conclude that the smaller Fogo area appears to be dominated by the normal regional stress field while the high active Congro area seems to have a different, highly heterogeneous stress field dominated by local conditions.  相似文献