Correction factors for estimating suspended sediment export from loess catchments

This study focuses on problems related to estimating the exact exported suspended sediment quantity and quality from flow‐proportional samples taken at a fixed height at the outlet of a small, agricultural catchment in central Belgium. First, a correction factor is introduced to estimate the real suspended sediment concentration. This factor depends on the stream depth and the grain size distribution of the sediment and is necessary because of the presence of a concentration gradient in the stream. Secondly, water depth is corrected for the sediment volume, and thirdly, the grain size distribution of the exported sediment is corrected. The application of these correction factors leads to a reduction of the estimated total sediment export by 18 per cent (from 4810 Mg over a period of three years to 3930 Mg). Furthermore, the importance of small grain size fractions increases, leading to higher enrichment ratios for the clay fraction and lower ratios for the sand fraction. Suspended sediment export is thus more selective than was previously thought. Due to the overestimation of the sediment export, total phosphorus losses from the catchment are also overestimated by 17 per cent. This paper therefore emphasizes that point sampling of suspended sediment is biased and that a method should be incorporated to correct the sediment export in terms of quantity and quality. Copyright © 2001 John Wiley & Sons, Ltd.     

Laboratory Studies of the Hydrogen Kinetic Isotope Effects (KIES) of the Reaction of Non-Methane Hydrocarbons with the OH Radical in the Gas Phase

The hydrogen kinetic isotope effects (KIEs) of the reactions of 15 non-methane hydrocarbons (NMHCs) with the OH radical were measured at 298 ± 2 K. The measurements were made using NMHCs without artificial isotopic labeling or enrichment. The following average hydrogen KIE values, in per mil (), were obtained: 29.8 ± 2.1 (toluene),51.6 ± 2.1 (n-butane), 97.3± 12.5 (i-butane), 63.2 ± 5.9 (cyclopentane), 10.5 (p-xylene), 26.8 ± 3.5 (ethylbenzene), 65.9± 7.0 (n-pentane), 79.5 ± 9.6 (cyclohexane), 52.8 ± 5.0(n-hexane), 38.9 ± 7.8 (n-heptane), 33.4 ± 3.1 (n-octane), 29.6 ± 1.6(n-nonane), and 29.0 ± 5.3 (n-decane). The KIEs for reactions of two alkenes (cyclohexene and 1-heptene) could not be determined accurately due to interference from reaction with ozone, but nevertheless the results clearly show that the KIEs for reaction of alkenes with OH are significantly lower than those for saturated hydrocarbons. The KIEs for reaction of alkanes are smaller than isotope effects reported in literature for the reactions of NMHCs artificially labeled with deuterium. The main reason for this difference is the reduced probability for reaction at a labeled site for compounds with close to natural deuterium abundance, although some impact of secondary isotope effects cannot be ruled out. Still, the KIEs for NMHCs with natural or close to natural abundance of deuterium are of sufficient magnitude to allow determination of the extent of chemical processing of hydrocarbons in the atmosphere using methods analogous to stable carbon KIE studies. Furthermore, it is shown that combining stable hydrogen and stable carbon isotope ratio data has the potential to also provide valuable information regarding the stable isotope ratios of emissions, and specifically to test one of the key assumptions of the stable isotope hydrocarbon clock, the absence of significant variations of the stable isotope ratio for the emitted NMHCs.     

Strong magnetic linear dichroism in Fe <Emphasis Type="Italic">L</Emphasis><Subscript>23</Subscript> and O <Emphasis Type="Italic">K</Emphasis> electron energy-loss near-edge spectra of antiferromagnetic hematite α-Fe<Subscript>2</Subscript>O<Subscript>3</Subscript>

We report strong magnetic linear dichroism (MLD) at the Fe L₂₃ and O K edges of the antiferromagnetic compound hematite -Fe₂ O₃ in high-resolution orientation- and temperature-dependent electron energy-loss spectroscopy (EELS). Large intensity differences of corresponding spectral features are observed when the Fe L₂₃ and O K edges are measured with momentum transfer either parallel or perpendicular to the magnetization. The resultant difference spectra for the Fe L₂₃ edges is consistent with the MLD observed in X-ray absorption spectroscopy. For the first time we have observed MLD at the O K edge, where the magnetic origin of this dichroism is demonstrated by temperature-dependent investigations across the Morin transition temperature T_M= 263 K, at which the Fe electron spins, i.e. the magnetic moments, rotate by 90°. The O K edge MLD is interpreted in terms of superexchange between the spins of the Fe 3d and O 2p electrons through overlapping Fe 3d and O 2p orbitals. The experiments were performed in a transmission electron microscope (TEM), yielding information about the anisotropic electronic structure at nanoscale spatial resolution when operated with a focused electron probe. The effects of MLD at the Fe L₂₃ edges on the determination of Fe³⁺/Fe in hematite at submicrometre scale using different independent quantification methods are discussed.     

Site amplification at Avcılar, Istanbul

Avcılar is the suburb of Istanbul that was most heavily damaged during the August 17, 1999 M_w 7.4 Izmit earthquake. Strong ground motion caused fatalities and damage in Avcılar despite being 90 km from the epicenter. We deployed five portable seismograph stations equipped with Reftek 24-bit recorders and L4C-3D seismometers for 2 months, in order to understand why the local site response was different from elsewhere in Istanbul. A reference station was placed on a hard rock site, and the remaining four stations were placed on other geological units, in areas that had experienced varying levels of damage. We calculated frequency-dependent ground amplification curves by taking the ratios of the spectra at soft and hard rock sites. We obtained similar site response curves for most earthquakes at each site in the frequency range of 0.3–1.6 Hz, and observed no significant site amplification beyond 2.0 Hz at any site. The overall characteristics of the recorded S-waveforms and our modeling of the calculated site amplification curves are consistent with amplification as a result of trapping of seismic energy within a 100–150 m thick, low-velocity subsurface layer. We also review the applicability of microtremor measurements to estimate local site effects at Avcılar. For these data, we used ratios of spectra of horizontal to vertical components to obtain each site response. These results are compared with standard spectral ratios. These microtremor measurements provide consistent estimates of the amplification at most sites at the higher end of the frequency band, namely above 1 Hz. The results from both methods indeed agree well in this part of the frequency band. However, the microtremor method fails to detect amplification at lower frequencies, namely <1.0 Hz.     

The role of population change in widening the mortality gap in Scotland

The social gradient in health and mortality has been acknowledged for some time and, more recently, there is some evidence that the gap between the richer and poorer is widening in Britain. Other studies have also shown that health and mortality outcomes are worse in places that are experiencing population decline and better in places that are growing. This analysis examines, first, whether there is a mortality gap between the least and most deprived people in Scotland, second, whether this gap has widened since the 1980s and, third, whether the gap can be explained by patterns of population change. Our results demonstrate that the widening mortality gap in Scotland cannot be explained simply as an artefact of population change, even though it is widening most in those places that are experiencing population decline. This type of research is made possible by the reliable denominator population information provided in the 1981 and 2001 censuses.     

Mantle Preconditioning by Melt Extraction during Flow: Theory and Petrogenetic Implications

Mantle preconditioning may be defined as the extraction of smallmelt fractions from mantle asthenosphere during its flow tothe site of magma generation. Equations may be written for mantlepreconditioning, assuming that the mantle comprises enriched‘plums’ in a depleted matrix. The equations takeinto account variations in mass fraction of plums, the relativerate of melting of plums and matrix, the temperature and pressureof melt extraction, the mass fraction of melt extracted, theextent of chemical exchange between plums and matrix, and theefficiency of melt extraction. Monitoring mineralogical changesand variations in partition coefficients along the inferredP–T–t path of the mantle asthenosphere allows theequations to be correctly applied to the conditions under whichmelt extraction takes place. Numerical experiments demonstratethe influence of petrogenetic variables on the shape of meltextraction trajectories and provide new criteria for distinguishingbetween melt extraction and mixing as the cause of regionalgeochemical gradients. Representative examples of arc–back-arcsystems (Scotia), continental break-up (Afar) and plume–ridgeinteraction (Azores) indicate that the compositions of the mantlesources of mid-ocean ridge basalts and island arc basalts maybe determined, at least in part, by the melt extraction historiesof their asthenospheric sources. KEY WORDS: geochemical modelling; mantle flow; isotope ratios; trace elements     

Constraints from geochemistry and Sr–Nd isotopes for the origin of albitite deposits from Central Sardinia (Italy)

Major- and trace-element contents and Sr–Nd isotope ratios were determined in albitite, albitized and unaltered late-Variscan granitoid samples from the world-class Na-feldspar deposits of central Sardinia, Italy. The albite deposit of high economic grade has geological, textural, and chemical features typical of metasomatic alteration affecting the host granitoids. Albitization, locally accompanied by chloritization and epidotization, was characterized by strong leaching of Mg, Fe, K, and geochemically similar trace elements, and by a significant increase of Na. Ca, and P were moderately leached in the most metasomatized rocks. Other major (Si, Ti, Ca) and trace elements (U, Th, Y, and Zr), along with light (LREE) and middle (MREE) rare-earth elements, behaved essentially immobile at the deposit scale. The Nd-isotope ratios (0.512098 to 0.512248) do not provide information on the emplacement age of the unaltered late-Variscan granitoids. On the other hand, their Sr-isotope ratios fit an errorchron of 274±29 Ma (1σ error), in fair agreement with all published ages of Sardinian Variscan granitoids. The very low Rb content of albitized rocks precludes application of the Rb–Sr radiometric system to determine the age of albitization. The Sm–Nd system is not applicable either, because the ¹⁴³Nd/¹⁴⁴Nd ratios of albitized rocks and unaltered granitoids overlap. The overlap confirms that Sm and Nd were substantially immobile during albitization. On the other hand, the measured ⁸⁷Sr/⁸⁶Sr ratios of the albitized rocks are appreciably lower than those of the unaltered host granitoids, whereas, their initial Sr-isotope ratios are higher. This seems to suggest that a) albitization was induced by non-magmatic fluids rich in radiogenic Sr, and b) albitization occurred shortly after the granitoid emplacement. This conclusion is supported by Nd isotopes, because unaltered granitoids and albitites fit the same reference isochron at 274 Ma. The fluids acquired radiogenic Sr by circulation through the Lower Paleozoic metasedimentary basement. Specifically, it is estimated that Sr supplied by the non-carbonatic basement represents about 22 wt% of total Sr in albitite.     

An assessment of soil contamination due to heavy metals around a coal-fired thermal power plant in India

Combustion of coals in thermal power plants is one of the major sources of environmental pollution due to generation of huge amounts of ashes, which are disposed off in large ponds in the vicinity of the thermal power plants. This problem is of particular significance in India, which utilizes coals of very high ash content (∼55 wt%). Since the thermal power plants and the ash ponds are located in densely populated areas, there is potential chance for contamination of soil and groundwater of the surrounding areas from the toxic trace elements in the ash. An attempt has been made to study the extent of soil contamination around one of the largest thermal power plants of India located at Kolaghat, West Bengal India. Chemical analysis of the top soils and the soils collected from the different depth profiles surrounding the ash ponds, show that the top soils are enriched in the trace elements Mo, As, Cr, Mn, Cu, Ni, Co, Pb, Be, V, Zn, which show maximum enrichment (2–5) in the top soils collected from all the soil profiles. These elements are also enriched in the pond ash. Since there are no other sources of industrial effluents, it can be said that the enrichment of the trace elements (Mn, Co, Mo, Cr, Cu, Pb, Zn, As, Ni, Be, V) is attributed to their input from ash from the disposal pond. The study has been further strengthened by log-normal distribution pattern of the elements.     

Crustal structure of Tushka Region, Abu-Simbel, Egypt, inferred from spectral ratios of P waves of local earthquakes

The crustal structure of North Abu-Simbel area was studied using spectral ratios of short-period P waves. Three-component short period seismograms from the Masmas seismic station of the Egyptian National Seismic Network Stations were used. The Thomson-Haskell matrix formulation was applied for linearly elastic, homogeneous crustal layers. The obtained model suggests that the crust under the study region consists of a thin (0.8 km) superficial top layer with a P-wave velocity of 3.8±0.7 km/s and three distinct layers with a mean P-wave velocity of 6.6 km/s, overlaying the upper mantle with a P-wave velocity of 8.3 km/s (fixed). The results were obtained for 14 different earthquakes. The P-wave velocities of the three layers are: 5.8±0.6 km/s, 6.5±0.4 km/s and 7.2±0.3 km/s. The total depth to the Moho interface is 32±2 km. The crustal velocity model estimated using observations is relatively simple, being characterized by smooth velocity variations through the middle and lower crust and normal crustal thickness. The resultant crustal model is consistent with the model obtained from previous deep seismic soundings along the northern part of Aswan lake zone.     

安徽蒙城尉迟寺遗址地层的磁化率与元素地球化学记录研究

通过磁化率和地球化学元素的因子分析法和比值法分析提取出尉迟寺遗址地层记录的环境演变信息。本区5050aBP以前的新石器时期为暖湿的气候环境;大汶口文化阶段气候总体偏干冷,存在数次波动;从大汶口至龙山文化期间,本区气候发生了较为明显的变化,由干冷转向温湿,这为水稻生产提供了有利条件,也促进了龙山文化的繁荣。在5050aBP之前及4620aBP、4700aBP前后可能都发生过冷干事件。