Sub-fossil Chironomidae from East Africa. 2. Chironominae (Chironomini and Tanytarsini)     

Hilde Eggermont  Dirk Verschuren 《Journal of Paleolimnology》2004,32(4):413-455

This paper and a companion article present illustrated guides to the identification of sub-fossil chironomid larvae (Insecta: Diptera: Chironomidae) preserved in the sediments of low- and mid-elevation lakes in East Africa. They are based on analysis of surface-sediment death assemblages from 61 lakes located in the humid to semi-arid environments in equatorial East Africa (Uganda, Kenya, Tanzania), supplemented with similar surface-sediment samples from 12 lakes in the Horn of Africa (Ethiopia), and sub-recent core samples from six lakes in Kenya and two in Uganda. We analyzed about 11,000 specimens and identified 98.4% of these to species, species group, genus, or tribe level, depending on current -taxonomic knowledge of the various considered genera and the taxonomic resolution of preserved diagnostic features. We distinguished 90 different sub-fossil morphotypes, of which 16 are Tanypodinae, 19 are Orthocladiinae, and 55 are Chironominae. In this paper we focus on the subfamily Chironominae (tribes Chironomini and Tanytarsini). The diagnostic characters distinguishing these morphotypes from each other resemble differences at the species level in the better-known Holarctic fauna, hence we consider most of our morphotypes equivalent to morphological species or groups of closely related species. Given that core samples yielded only seven morphotypes not also found in the surface-sediment samples, the current inventory of 90 taxa likely represents the large majority of distinct sub-fossil chironomid larval types to be found in East African lakes, excluding the few very large Rift lakes, cold-water lakes above treeline, and special standing-water environments such as fens and bogs. Consistent use of a single set of morphological characters to identify both fossil and living chironomid larvae would ensure exchangeability of information between modem and paleoenvironmental studies on aquatic invertebrate communities in African lakes, and increase the relevance of paleoenvironmental reconstructions to water-quality evaluations aimed at sustainable management of scarce, fluctuating surface-water resources in tropical East Africa.  相似文献   

Retracing the development of raw water quality in water works applying reactive controlled material flux analyses     

Carsten?HansenEmail author  Wolfgang?van Berk 《Aquatic Sciences - Research Across Boundaries》2004,66(1):60-77

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Siltation by sediment-induced density currents     

Johan C.?WinterwerpEmail author  Thijis?van Kessel 《Ocean Dynamics》2003,53(3):186-196

This paper describes the results of numerical experiments with a three-dimensional hydrodynamic and mud transport model in which sediment–fluid interaction is taken into account through the effects of hindered settling, buoyancy destruction in the turbulence k– model and sediment-induced barocline pressure gradients in the momentum equations. The model was applied to a schematic case representing a coastal area with a tidal river, navigation channel and harbour basin, and a real-world case, viz. Rotterdam harbour area in The Netherlands. The results show that the sediment transport into the harbour area, and subsequent siltation rates, increase by a factor 3 to 5 due to the sediment–fluid interaction. It is shown that the larger contribution stems from an increase in vertical gradients in suspended sediment.Responsible Editor: Jens Kappenberg  相似文献   

Deep-sea fan pinch-out geometries and their relationship to fan architecture,Tanqua Karoo basin (South Africa)     

Wytze?van der WerffEmail author  Stephen?D.?Johnson 《International Journal of Earth Sciences》2003,92(5):728-742

Exceptional exposures of Permian basin floor fans (fans 3, 4) and a slope fan (fan 5) in the Tanqua Karoo foreland basin of South Africa have enabled an investigation of the relation between the pinch-out geometries and fan architecture. The pinch-out geometry of fan 3 is characterized by the down dip transition from thin to medium bedded sheet deposits to pinch-out fingers, which are overlain by younger prograding sheet deposits. This geometry reflects the progradational stacking pattern of the fan. In contrast, the fan 4 pinch-out fingers consist of stacked channel fills in the same conduit. This pinch-out configuration relates to the dominant aggradational style observed on the mid and distal parts of fan 4. Fan 5 represents a slope fan comprising an axial channel conduit, which branches down slope into three distributary channels. The distal fan is characterized by larger channel fills, which may represent bypass channels to other basin floor fans. The very thick-bedded nature of the youngest channel fill unit suggests early bypass followed by retrogradation as indicated by the presence of thinner bedded heterolithic channel fill deposits along the axial conduit. Although some of the massive pinch-out channels exhibit basal scour, their depositional morphology suggests that they mainly originated due to the infill of subtle topographic depressions by low concentration turbidity currents. Instead of describing these features as channel fills, the use of the term pinch-out fingers is preferred.  相似文献   

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>     

P. A.?van AkenEmail author  S.?Lauterbach 《Physics and Chemistry of Minerals》2003,30(8):469-477

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.  相似文献   

Correlation between microbiological and chemical parameters of some hydrothermal springs in New Mexico, USA     

Bart omiej Rzonca  Dirk Schulze-Makuch 《Journal of Hydrology》2003,280(1-4):272-284

Discharge areas of hydrothermal springs are known to be inhabited by diverse types of microorganisms including archaea, prokaryotes and eukaryotes. A total of 11 hydrothermal samples from the Rio Grande rift and the Valles caldera in New Mexico were analyzed to investigate the correlation between chemical and microbiological parameters of hydrothermal waters. The sampled fluids are categorized into three chemical groups: (I) steam-condensing acid sulfate waters, (II) deep geothermal and derivative waters and (III) thermal meteoric waters. Analyses of the microbial phospholipid fatty acids and denaturing gradient gel electrophoresis of DNA show that acid sulfate waters were populated by thermoacidophilic organisms and had high biomass content. Mineralized deep geothermal and derivative waters exhibited a high degree of microbial diversity, but had low biomass content. Thermal meteoric waters are low in total dissolved solids, and exhibit very low biomass content and microbial diversity. DNA sequences from several previously unknown microbial species were detected. The results of this study support the hypothesis that microbes can be used as tracers for specific types of subsurface environments.  相似文献   

On the formation of continental silicic melts in thermochemical mantle convection models: implications for early Earth     

P. van Thienen  A.P. van den Berg  N.J. Vlaar 《Tectonophysics》2004,394(1-2):111-124

Seafloor magnetotelluric (MT) data were collected at seven sites across the Hawaiian hot spot swell, spread approximately evenly between 120 and 800 km southwest of the Hawaiian-Emperor island chain. All data are consistent with an electrical strike direction of 300°, aligned along the seamount chain, and are well fit using two-dimensional (2D) inversion. The major features of the 2D electrical model are a resistive lithosphere underlain by a conductive lower mantle, and a narrow, conductive, ‘plume’ connecting the surface of the islands to the lower mantle. This plume is required; without it the swell bathymetry produces a large divergence of the along-strike and across-strike components of the MT fields, which is not seen in the data. The plume radius appears to be less than 100 km, and its resistivity of around 10 Ωm, extending to a depth of 150 km, is consistent with a bulk melt fraction of 5–10%.A seismic low velocity region (LVR) observed by Laske et al. [Laske, G., Phipp Morgan, J., Orcutt, J.A., 1999. First results from the Hawaiian SWELL experiment, Geophys. Res. Lett. 26, 3397–3400] at depths centered around 60 km and extending 300 km from the islands is not reflected in our inverse model, which extends high lithospheric resistivities to the edge of the conductive plume. Forward modeling shows that resistivities in the seismic LVR can be lowered at most to 30 Ωm, suggesting a maximum of 1% connected melt and probably less. However, a model of hot subsolidus lithosphere of 10² Ωm (1450–1500 °C) within the seismic LVR increasing to an off-swell resistivity of >10³ Ωm (<1300 °C) fits the MT data adequately and is also consistent with the 5% drop in seismic velocities within the LVR. This suggests a ‘hot, dry lithosphere’ model of thermal rejuvination, or possibly underplated lithosphere depleted in volatiles due to melt extraction, either of which is derived from a relatively narrow mantle plume source of about 100 km radius. A simple thermal buoyancy calculation shows that the temperature structure implied by the electrical and seismic measurements is in quantitative agreement with the swell bathymetry.  相似文献   

Effect of soil crusting on the emission and transport of wind-eroded sediment: field measurements on loamy sandy soil   总被引：3，自引：0，他引：3  

Dirk Goossens   《Geomorphology》2004,58(1-4):145-160

Field data are reported for the horizontal and vertical flux of wind-eroded sediment on an agricultural field in northern Germany. Measurements were made during a windstorm that hit the region on 18 May 1999. The magnitude of both fluxes was significantly affected by the presence of a surface crust covering the test field. Measuring the physical crust strength at 45 locations with a torvane, the relationships between crust strength (τ) and the horizontal (F_h) and vertical (F_v) sediment fluxes were investigated. Both fluxes decreased as the surface crust became stronger. The decay behaved as an exponential function for both types of flux. The horizontal sediment flux over a crusted surface can be accurately predicted by completing Marticorena and Bergametti's [Journal of Geophysical Research 100 (1995) 16415] erosion model with a crust function. The vertical particle flux over crusted soil can be calculated by adding a similar function to Alfaro and Gomes's [Journal of Geophysical Research 106D (2001) 18075] dust production model. The study also suggests that the gradual bombardment of a surface crust by impacting particles does not immediately result in a decay of the crust's protective effect, provided that the crust has a minimum thickness. However, once the crust becomes perforated, its protective effect disappears very quickly, leading to much higher horizontal and vertical sediment fluxes than predicted for undamaged crusted soil.  相似文献   

Sonic anemometers in aeolian sediment transport research   总被引：3，自引：0，他引：3  

J. H. van Boxel  G. Sterk  S. M. Arens   《Geomorphology》2004,59(1-4):131

Fast-response wind and turbulence instruments, including sonic anemometers, are used more and more in aeolian sediment transport research. These instruments give information on mean wind, but also on fluctuations and turbulent statistics, such as the uw covariance, which is a direct measure of Reynolds' stress (RS) and friction velocity. This paper discusses the interpretation of sonic anemometer data, the transformations needed to get proper results and turbulence spectra, and how they are influenced by instrument size, sampling frequency, and measurement height.Turbulence spectra characterize how much the different frequencies in the turbulent signals contribute to the variance of wind speed, or to the covariance of horizontal and vertical wind speed. They are important in determining the measurement strategy when working with fast-response instruments, such as sonic anemometers, and are useful for interpreting the measurement results. Choices on the type of sonic anemometer, observation height, sampling period, sampling frequency, and filtering can be made on the basis of expected high and low-frequency losses in turbulent signals, which are affected by those variables, as well as wind speed and atmospheric stability.Friction velocity and RS, important variables in aeolian sediment transport research, are very sensitive to tilt or slope errors. During a field experiment, the slope sensitivity of the RS was established as 9% per degree of slope, which is 1.5 times the value reported in literature on the basis of theoretical considerations. An important reason for the difference probably is the large influence of streamline curvature on turbulence statistics and thereby on the slope sensitivity of the RS. An error of 9% per degree of slope in the RS will translate into an error of approximately 4% per degree of slope in the calculated friction velocity.Space–time correlation of the horizontal wind speed is much larger than that of the vertical wind speed and the instantaneous RS. This largely explains why, in previous studies, a poor correlation was found between instantaneous RS measured at 3 m height and saltation flux near the surface, whereas the correlation between wind speed at some height and saltation flux was much better. Therefore, the poor correlation between RS away from the surface and saltation flux does not contradict that saltation flux is caused by RS.  相似文献   

Three-dimensional upper-mantle S-velocity model for the Eurasia–Africa plate boundary region     

Federica Marone  Suzan van der Lee†  Domenico Giardini 《Geophysical Journal International》2004,158(1):109-130

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