Species pool versus site limitations of macrophytes in urban waters     

K. Vermonden  R. S. E. W. Leuven  G. van der Velde  A. J. Hendriks  M. M. van Katwijk  J. G. M. Roelofs  E. C. H. E. T. Lucassen  O. Pedersen  K. Sand-Jensen 《Aquatic Sciences - Research Across Boundaries》2010,72(3):379-389

Biodiversity in urban areas is affected by a multitude of stressors. In addition to physico-chemical stress factors, the native regional species pool can be greatly reduced in highly urbanized landscapes due to area loss and fragmentation. In this study, we investigated how macrophyte composition and diversity in urban water systems are limited by the regional species pool and local environmental conditions. Canonical correspondence analysis of the macrophyte species composition revealed that urban and semi-natural water systems differed and differences could be related to local abiotic variables such as pH and iron concentrations. Macrophytes in the semi-natural area were typical for slightly acid and oligotrophic conditions. In urban water systems, exotic species characteristic of eutrophic conditions were present. In the semi-natural areas, the number of macrophyte species exceeded the number of species expected from species–area relationships of artificial water bodies in rural areas. In urban areas, the number of macrophyte species was similar to artificial water systems in rural areas. Macrophyte species present in the study areas also were generally found within 20–30 km distance to the study area. Macrophyte species composition in urban water systems and semi-natural water systems appeared to be influenced by the regional species pool within approximately 30 km of the locations. Nevertheless, site limitation ultimately determined the local macrophyte species composition and diversity in urban water systems and in semi-natural water systems.  相似文献   

Ionospheric temperature and density measurements by means of spherical double probes     

U. Fahleson  C.-G. Fälthammar  A. Pedersen 《Planetary and Space Science》1974,22(1):41-66

Rocket-borne double probes for electric field measurements can be intermittently operated in special, diagnostic modes involving current bias and low-impedance shunts to obtain information on the properties of the ambient ionospheric plasma along the flight path. Several such modes, and the information that they can provide, are analyzed. For example, in a low-impedance mode with asymmetric bias, the attenuation ratio (i.e. signal amplitude in this mode over the signal amplitude in the electric-field measuring mode) is in a simple way related to the electron temperature of the ambient plasma. The special surface coatings (Aquadag or vitreous carbon) normally used for electric field probes provide very homogeneous surface properties, a feature which also contributes to the reliability of the electron temperature measurements. In addition to electron temperature, the modes analyzed can be used to measure electron density and to give some information on ion temperature. The data from four rocket flights from ESRANGE are discussed in the light of these results. Electron temperature was measured in three of these flights. In all cases the temperature profile is in good agreement with theoretically predicted profiles based on the CIRA 1965 reference atmosphere and the solar illumination prevailing during the respective flights (twilight). Electron density profiles obtained by means of the double probe are in good agreement with the density measured by the Langmuir probe in the two flights for which both kinds of data are available. They are also in agreement with the electron density data available from ionosondes. Finally, pulses occurring when one of the probes passed through the rocket's shadow, are used to determine the photoelectron yield of the probe coatings (Aquadag or vitreous carbon). The values obtained, (7 ± 3) × 10^?6 A/m² for Aquadag and (4 ± 2) × 10^?6 A/m² for vitreous carbon are in good agreement with expectations based on laboratory data and solar Lyman α radiation.  相似文献   

Vaca Muerta mesosiderite strewnfield     

Holger Pedersen  Claudio Canut de Bon  Harri Lindgren 《Meteoritics & planetary science》1992,27(2):126-135

Abstract A field investigation is presented of the strewnfield of the mesosiderite Vaca Muerta, originally found in 1861. The area, 11.5 km long, 2.1 km wide, is located about 60 km southeast of Taltal, Chile, in the Atacama Desert. It has yielded 80 meteorites with a total mass exceeding 3782 kg. Most fragments were found in an undisturbed state, but some had been broken by prospectors. The present studies, in connection with historical records, indicate that the original mass of Vaca Muerta exceeded 6 metric tons. One impact feature, somewhat modified by man, consists of a 10.5-m diameter, 1.7-m deep hole, without an uplifted rim. Small masses were scattered up to 85 m from the hole.  相似文献   

Evaluation of Horner plot-corrected log-derived temperatures in the Danish Central Graben,North Sea     

Douglas?W.?WaplesEmail author  May?Ritt?Pedersen 《Natural Resources Research》2004,13(4):223-227

A small dataset comprising all temperature available data from reliable Horner plots from the Danish Central Graben was examined. Temperatures obtained by extrapolation using standard Horner plots were determined to be lower than true formation temperatures, as interpreted from DST data. Excellent agreement between true formation temperatures and Horner plot temperatures was achieved when the Horner plot temperatures (T_HP) were corrected upward by an amount proportional to the slope (A) of the Horner plot using the equation where the temperatures and the slope are in degrees Celsius. The standard deviation of the error in the corrected Horner plot temperatures was 2.1°C, indicating that this method is consistent. Further studies using larger numbers of Horner plots from a variety of geographic areas should be carried out to test and refine the hypotheses presented here. Efforts also should be made to understand the causes of variability in slopes of Horner plots.  相似文献   

Ordovician faunas, island arcs and ophiolites in the Scandinavian Caledonides     

R.B. Pedersen  D.L. Bruton  H. Furnes 《地学学报》1992,4(2):217-222

Ordovician faunal data from the Scandinavian Caledonides is tested with new geochemical information from zircons to give U/Pb ages and source origins of volcanic arc and ophiolite sequences. Early Ordovician (Arenig-Llanvirn), low latitude, Toquima-Table Head faunas from the upper Upper Allochthon are associated with an island arc system formed adjacent to Laurentia. Contemporaneous mafic magmas were contaminated by crustal material during subduction and associated granites contain inherited zircons of Archaean age. The nearest source for such rocks is on the Laurentian rather than the Baltic side. Higher latitude Celtic province faunas from the upper Upper Allochthon are from one insular site accessible to forms from both Laurentia and Baltica.
The late Ordovician low-latitude Holorhynchus and subtropical Hirnantia faunas occur in overstep sequences above deeply eroded early Ordovician arc complexes. The transgression appears to be coeval with a second generation of spreading-related complexes. Single detrital zircons from sediments show sources from Archaean, Proterozoic and early Ordovician terranes. This suggests that deposition was in a basin situated along the same continental margin (Laurentia) to which the early Ordovician ophiolite/arc sequences had already become accreted. The late Ordovician faunas link both Laurentia and Baltica at a time of narrowing of lapetus.
The new geochemical data together with the faunal information is supported by recent palaeomagnetic studies.  相似文献   

Magnetotellurics along the Fennoscandian Long Range profile   总被引：2，自引：0，他引：2  

T. M. Rasmussen  R. G. Roberts  L. B. Pedersen 《Geophysical Journal International》1987,89(3):799-820

Summary. Estimates of magnetotelluric transfer functions along the Fennoscandian Long Range (FENNOLORA) profile are presented and discussed in relation to the major lithotectonic subdivisions of the Swedish part of the Baltic shield. None of the transfer functions are found to be consistent with a one-dimensional earth structure, thereby making a quantitative interpretation difficult. However, the geological units are found to display characteristic differences in the obtained transfer functions, and in some areas a quantitative interpretation can be justified. The most interesting structure indicated is a low resistive (4 μm) crustal structure of lateral extension more than 150 km centred around the Skellefte ore district. A minimum thickness for this structure of 15 km is inferred.  相似文献   

Memory-Efficient Categorical Multi-point Statistics Algorithms Based on Compact Search Trees   总被引：1，自引：1，他引：0  

Tuanfeng Zhang  Stein Inge Pedersen  Christen Knudby  David McCormick 《Mathematical Geosciences》2012,44(7):863-879

Multi-point statistics (MPS) has emerged as an advanced geomodeling approach. A practical MPS algorithm named snesim (simple normal equations simulation), which uses categorical-variable training images, was proposed in 2001. The snesim algorithm generates a search tree to store the occurrence statistics of all patterns in the training image within a given set of search templates before the simulation proceeds. The snesim search tree concept makes MPS simulation central processing unit efficient but consumes large amounts of memory, particularly when three-dimensional training images contain complex patterns and when a large search template is required to ensure optimal reproduction of the image patterns. To crack the memory-restriction bottleneck, we have developed a compact search tree that contains the same information but reduces memory cost by one order of magnitude. Furthermore, the compact structure also accelerates MPS simulation significantly. Such remarkable improvement makes MPS a more practical tool to use in building the large and complex three-dimensional facies models required in the oil and gas industry.  相似文献   

The pseudometeorite Angara     

Holger Pedersen 《Meteoritics & planetary science》2013,48(6):1091-1095

Several pallasites are known to have formed strewn fields with multiple fragments. Therefore, it seems possible that the famous Krasnojarsk pallasite—the Pallas Iron of 687 kg—could have been accompanied by one or more additional fragments that had not been recovered due to incomplete observations from the overgrown and remote place of fall. During a survey for literature accounts of distant fragments in such a hypothetical strewn field, a report of native iron was found, dating to 1847. The fragments of nickel‐free iron—amounting to at least a few kg—had been recovered 1836–1843 during placer gold mining north of the Angara River. The position of these finds is coincident with Burovaya, Murozhna, and Uderei, the three known fragments of the pseudometeorite Angara, all collected in 1885.  相似文献   

Determination of the crustal structure in southern Tibet by dispersion and amplitude analysis of Rayleigh waves   总被引：2，自引：0，他引：2  

N. Cotte  H. Pedersen  M. Campillo  J. Mars  J. F. Ni  R. Kind  E. Sandvol  & W. Zhao 《Geophysical Journal International》1999,138(3):809-819

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Concerning the generation of geomagnetic giant pulsations by drift-bounce resonance ring current instabilities     

K.-H. Glassmeier  S. Buchert  U. Motschmann  A. Korth  A. Pedersen 《Annales Geophysicae》1999,17(3):338-350

Giant pulsations are nearly monochromatic ULF-pulsations of the Earth’s magnetic field with periods of about 100 s and amplitudes of up to 40 nT. For one such event ground-magnetic observations as well as simultaneous GEOS-2 magnetic and electric field data and proton flux measurements made in the geostationary orbit have been analysed. The observations of the electromagnetic field indicate the excitation of an odd-mode type fundamental field line oscillation. A clear correlation between variations of the proton flux in the energy range 30–90 keV with the giant pulsation event observed at the ground is found. Furthermore, the proton phase space density exhibits a bump-on-the-tail signature at about 60 keV. Assuming a drift-bounce resonance instability as a possible generation mechanism, the azimuthal wave number of the pulsation wave field may be determined using a generalized resonance condition. The value determined in this way, m = −21±4, is in accord with the value m = −27±6 determined from ground-magnetic measurements. A more detailed examination of the observed ring current plasma distribution function f shows that odd-mode type eigenoscillations are expected for the case ∂f/∂W ≥ 0, much as observed. This result is different from previous theoretical studies as we not only consider local gradients of the distribution function in real space, but also in velocity space. It is therefore concluded that the observed giant pulsation is the result of a drift-bounce resonance instability of the ring current plasma coupling to an odd-mode fundamental standing wave. The generation of the bump-on-the-tail distribution causing ≥f/≥W ≥ 0 can be explained due to velocity dispersion of protons injected into the ring current. Both this velocity dispersion and the necessary substorm activity causing the injection of protons into the nightside magnetosphere are observed.  相似文献