Springs of Great Britain     

J. B. W. Day 《Environmental Geology》1996,27(2):77-78

Predictably, in a country such as Britain, with its preponderance of consolidated, sedimentary, mainly fissure-flow aquifers, there is a very large number of springs, many of which are, or have been, used for public supply. Migratory springs are a feature of the British (Ur. Cretaceous) Chalk, the most important British aquifer. The Chalk's low specific yield and high capillary moisture retention together give rise to very considerable fluctuations (more than 33 m in some areas) of the unconfined water table. Along the gentle dip slopes of the Chalk (North and South Downs of southern and southeastern England) springs may migrate laterally for several miles, giving rise to seasonal streams locally known as bournes or lavants. However, springs such as at Duncton, West Sussex, at the base of the much steeper scarp slopes of the Chalk, form point sources, the flows from which tend to be relatively steady; such springs commonly supply and are the original reason for the existence of many of the small towns and villages which nestle along the bases of the chalk scarps of Sussex and Kent.Where the Chalk forms coastal cliffs, a number of springs break out at the base of the cliff between high and low tide levels; there are major chalk coastal springs, for instance, at St. Margaret's Bay (Kent) and at Arish Mells, east of Lulworth Cove, Dorset. Such springs are not used for direct supply (their salinity is usually too high) but are indicators of the presence of local reserves of groundwater for possible future development.  相似文献   

Integrated chronostratigraphy of Proterozoic-Cambrian boundary beds in the western Anabar region, northern Siberia   总被引：2，自引：0，他引：2  

Kaufman AJ  Knoll AH  Semikhatov MA  Grotzinger JP  Jacobsen SB  Adams W 《Geological magazine》1996,133(5):509-533

Carbonate-rich sedimentary rocks of the western Anabar region, northern Siberia, preserve an exceptional record of evolutionary and biogeochemical events near the Proterozoic/Cambrian boundary. Sedimentologically, the boundary succession can be divided into three sequences representing successive episodes of late transgressive to early highstand deposition; four parasequences are recognized in the sequence corresponding lithostratigraphically to the Manykal Formation. Small shelly fossils are abundant and include many taxa that also occur in standard sections of southeastern Siberia. Despite this coincidence of faunal elements, biostratigraphic correlations between the two regions have been controversial because numerous species that first appear at or immediately above the basal Tommotian boundary in southeastern sections have first appearances scattered through more than thirty metres of section in the western Anabar. Carbon- and Sr-isotopic data on petrographically and geochemically screened samples collected at one- to two-metre intervals in a section along the Kotuikan River, favour correlation of the Staraya Reckha Formation and most of the overlying Manykai Formation with sub-Tommotian carbonates in southeastern Siberia. In contrast, isotopic data suggest that the uppermost Manykai Formation and the basal 26 m of the unconformably overlying Medvezhya Formation may have no equivalent in the southeast; they appear to provide a sedimentary and palaeontological record of an evolutionarily significant time interval represented in southeastern Siberia only by the sub-Tommotian unconformity. Correlations with radiometrically dated horizons in the Olenek and Kharaulakh regions of northern Siberia suggest that this interval lasted approximately three to six million years, during which essentially all 'basal Tommotian' small shelly fossils evolved.  相似文献   

Derivation of A-type Granites from a Dehydrated Charnockitic Lower Crust: Evidence from the Chaelundi Complex, Eastern Australia   总被引：28，自引：3，他引：25  

LANDENBERGER  B.; COLLINS  W. J. 《Journal of Petrology》1996,37(1):145-170

Triassic I- and A-type granites of the Chaelundi Complex, NewEngland Fold Belt, eastern Australia, were generated in a subduction-relatedtectonic setting. Although isotopic ages of the suites are indistinguishable,field relations indicate that the A-type is younger. The mostmafic granitoids from each suite have similar silica contents(66–68% SiO₂), slightly LREE enriched patterns withoutEu anomalies, low Rb/Sr and K/Ba ratios, and high K/Rb ratios,suggesting that both represent parental magmas. The A-type isdistinguished mineralogically by abundant orthoclase and sodicplagioclase (total >60%), ferro-hornblende, annite and allanite.In contrast, the I-type has more hornblende and biotite, whichare more magnesian in composition, and less feldspar. The parentalmagmas of both suites have many similar geochemical characteristics,although the A-type has slightly higher alkalis, Zr, Hf, Znand LREE, and lower CaO, MgO, Sr, V, Cr, Ni and Fe³⁺/Fe. Thegeochemical properties characteristic of leucocratic A-typegranites, such as high Ga/Al, Nb, , HREE and F contents, areonly manifest in the more felsic members of the A-type suite.These features were produced by 70% fractional crystallizationof feldspar, hornblende, quartz and biotite. Both granite suites were generated by water-undersaturated partialmelting of a similar source, but the A-type parent magma resultedfrom lower aH₂O conditions during partial melting. Generationand rapid ascent of the earlier 1-type magma during disequilibriumpartial melting produced a relatively anhydrous, but not refractory,charnockitic lower crust. Continued thermal input from mantle-derivedmagmas, during continuing subduction, partially melted the ‘charnockitized’lower crust at temperatures in excess of 900C, to produce A-typemagmas. Charnockitic magmas (C-type) form in a similar way toA-type magmas, although their different composition reflectsvariations in the anhydrous lower-crustal mineral assemblagesthat remain after the previous (1-type) granite-forming event. The New England Fold Belt was a subduction—accretion complexuntil the late Carboniferous, when the deeper parts underwentpartial melting to produce S-type granites. As the I-and A-typegranites intruded penecontemporaneously, a tonalitic sourcemodel for genesis of the Chaelundi A-type is untenable. KEY WORDS: A-type; charnockitization; eastern Australia ^*Corresponding author.  相似文献   

Objective measures of the information density of satellite data     

H. -L. Huang  R. J. Purser 《Meteorology and Atmospheric Physics》1996,60(1-3):105-117

Summary We propose and objective method whereby the density of Shannon's information associated with the retrieval of a profile of atmospheric variables from satellite-derived infrared radiance measurements may be estimated. The technique is a natural extension of one we previously proposed to estimate the effective data density in a profile. We test the method in a comparison of simulated satellite instruments to show that the method does indeed provide an objective summary of the spatial distribution of each instrument's information content. We propose that further extensions of the method be developed to include other more traditional data sources in a fully three-dimensional scheme. We also note that analogous and compatible methods may be used to diagnose the information content of meteorological analysis and forecast fields relative to the information contained in the covariance, at the appropriate season, of the corresponding climate fields.With 8 Figures  相似文献   

Geochemical framework for water quality studies in sub-Saharan Africa     

W.M. Edmunds 《Journal of African Earth Sciences》1996,22(4):385-389

The scope for using hydrogeochemical techniques in water quality studies in Africa is reviewed as a background to a set of thematic papers. Water quality problems are emerging as a key issue in Africa either:

• 1. 

  i) in view of the pressures of man-made pollution on finite resources; or

• 2. 

  ii) the existence of regions with naturally induced geological problems, for example fluoride endemic areas.

Such natural problems are the focus of this paper and the following topics were emphasised during a workshop in Sodere, Ethiopia: the need to determine natural baselines as a means of recognizing anthropogenic pollution; the need for high quality samples and field data, but relatively basic analytical data; the use of chloride to assist in recharge estimation and water-balance studies; an understanding of depth stratification of water quality as part of the design of well/borehole drilling programmes; the use of hydrogeochemistry in geothermal studies; the recognition of health and/or acceptability problems, especially for F, Fe, Mn, As, I and Al. Water quality standards for African countries need to be considered in the context of local geochemical environments and some of the WHO limits, especially for major ions, may be unattainable owing to naturally high total mineralization.  相似文献   

Trace metals in suspended particulate matter and in sediment trap material from a permanently anoxic fjord — Framvaren, South Norway     

Jens M. Skei  Douglas H. Loring  Reijo T. T. Rantala 《Aquatic Geochemistry》1996,2(2):131-147

Geochemical studies of the trace metal concentrations in suspended particulate matter (SPM) and sediment trap material from a permanently anoxic fjord, Framvaren, South Norway in 1989 and 1993 indicate that extremely high concentrations of zinc (max = 183920 mg/kg), copper (max = 4130 mg/kg), lead (max = 2752 mg/kg), and cadmium (max= 8.1 mg/kg) sometimes (1993) occur in the SPM collected in the anoxic water layer. The highest concentrations of Zn occur just below the redoxcline at 22 m water depth (in 1993), and copper, lead and cadmium have maximum concentrations between 30 and 80 m depth, where the amount of total SPM is at a minimum (about 0.3 mg/L). On a mass per volume (g/L) basis, the maximum concentrations of Cd, Cu and Fe occur at the interface (21m) and those of Zn occur just below the redoxcline (22 m depth). The SPM and sediment trap data suggest that the metals are precipitated as sulfide minerals in the anoxic water. The presence of particulate sulfides was confirmed by SEM studies that show the occurrence of discrete metal (Cu, Fe, Pb, and Zn) sulfide particles in size from 10–20 m as well as framboidal pyrites (1–5 m in size). Higher levels of metal sulfides at intermediate depths rather than in the deep water of Framvaren (> 100 m), may be due to input of trace metals by water exchange over the sill in the upper part of the water column. In the deep water, less metal sulfide precipitation takes place due to depletion of trace metals, and the dilution of particulate metal concentrations by organic matter and by the chemogenic formation of calcite.  相似文献   

Overbreak and underbreak in underground openings Part 2: causes and implications   总被引：7，自引：0，他引：7  

J. A. Ibarra  N. H. Maerz  J. A. Franklin 《Geotechnical and Geological Engineering》1996,14(4):325-340

Summary The newly developed light sectioning method has been used to investigate some of the causes and costs of overbreak and underbreak. Investigations at the Aquamilpa Hydroelectric Project in Mexico have shown decreased overbreak and increased underbreak as a result of increased rock quality and decreased explosive energy. A new measure of explosive energy, the perimeter powder factor (PPF), has been defined and shown to be useful in the context of tunnel-wall rock damage. Tentative results indicate that explosive energy (PPF) may be a more important factor in producing underbreak, whereas rock quality may be a greater factor in producing overbreak. A site-specific equation is given for predicting overbreak or underbreak as a function of rock quality and explosive energy, with an evaluation of the cost of underbreak and overbreak.  相似文献   

Far-infrared contribution to interstellar extinction from graphite whiskers     

N. C. Wickramasinghe  Daryl H. Wallis 《Astrophysics and Space Science》1996,240(1):157-160

A mass contribution of graphite whiskers as small as 0.1% to the population of interstellar grains could dramatically change their far-infrared extinction properties. With varying mass fractions of graphite whiskers, and for different size parameters, the infrared extinction could vary from a ^-2 dependence to ⁰, consistent with the requirements of some astronomical observations.  相似文献   

Climate change and snow-cover duration in the Australian Alps   总被引：2，自引：0，他引：2  

P. H. Whetton  M. R. Haylock  R. Galloway 《Climatic change》1996,32(4):447-479

This study uses a model of snow-cover duration, an observed climate data set for the Australian alpine area, and a set of regional climate-change scenarios to assess quantitatively how changes in climate may affect snow cover in the Australian Alps. To begin, a regional interannual climate data set of high spatial resolution is prepared for input to the snow model and the resulting simulated interannual and spatial variations in snow-cover duration are assessed and compared with observations. The model provides a reasonable simulation of the sensitivities of snow-cover duration to changes in temperature and precipitation in the Australian Alps, although its performance is poorer at sites highly marginal for snow cover. (In a separate comparison, the model also performs well for sites in the European Alps.) The input climate data are then modified in line with scenarios of regional climate change based on the results of five global climate models run in enhanced greenhouse experiments. The scenarios are for the years 2030 and 2070 and allow for uncertainty associated with projecting future emissions of greenhouse gases and with estimating the sensitivity of the global climate system to enhanced greenhouse forcing. Attention focuses on the climate changes most favourable (best-case scenario) and least favourable (worst-case scenario) for snow cover amongst the range of climate changes in the scenarios. Under the best case scenario for 2030, simulated average snow-cover duration and the frequency of years of more than 60 days cover decline at all sites considered. However, at the higher sites (e.g., more than 1700 m) the effect is not very marked. For the worst case scenario, a much more dramatic decline in snow conditions is simulated. At higher sites, simulated average snow cover duration roughly halves by 2030 and approaches zero by 2070. At lower sites (around 1400 m), near zero average values are simulated by 2030 (compared to durations of around 60 days for current climate).These simulated changes, ranging between the best and worst case, are likely to be indicative of how climate change will affect natural snow-cover duration in the Australian Alps. However, note that the model does not allow directly for changes in the frequency and intensity of snow-bearing circulation systems, nor do the climate-change scenarios allow possible changes in interannual variability (particularly that due to the El Niño-Southern Oscillation) and local topographical effects not resolved by global climate models. The simulated changes in snow cover are worthy of further consideration in terms of their implications for the ski industry and tourism, water resources and hydroelectric power, and land-use management and planning.68 Barada Crescent, Aranda ACT 2614, Australia.  相似文献   

Global comparison of the regional rainfall results of enhanced greenhouse coupled and mixed layer ocean experiments: Implications for climate change scenario development     

Peter H. Whetton  Matthew H. England  Siobhan P. O'Farrell  Ian G. Watterson  A. Barrie Pittock 《Climatic change》1996,33(4):497-519

The extent of agreement amongst current global climate models (GCMs) on the global pattern of rainfall change simulated under enhanced greenhouse conditions is assessed. We consider the results of five experiments which use a simple mixed layer ocean formulation and five which use a fully dynamic ocean model (coupled experiments). For many regions of the northern hemisphere there is strong agreement amongst both mixed layer and coupled experiments on the sign of simulated rainfall change. However, in the southern hemisphere there are large, and apparently systematic, differences between the coupled and mixed layer experiments. In particular, whereas the mixed layer experiments agree on simulated rainfall increase in summer in the tropics and subtropics of the Australian sector, the coupled experiments agree (although more weakly) on rainfall decreases. These differences appear to relate to the much reduced warming simulated by the coupled experiments in the high latitudes of the southern hemisphere. However, recent oceanographie evidence suggests that this suppressed warming may be considerably overestimated. We conclude therefore that despite the in-principle advantages of coupled models, it may be too soon to base some regionally specific climate change scenarios solely on the results of coupled experiments.  相似文献