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51.
A Sr isotope study on fluorite and siderite from post-orogenic mineral veins in the eastern Harz Mountains,Germany 总被引:2,自引:0,他引:2
Rb–Sr isotope data for siderite and fluorite from sediment-hosted epithermal mineral veins in the eastern Harz Mountains (Germany)
are presented. Several fluorite and siderite-bearing paragenetic stages have been proposed for these veins, with the most
important mineralization being related to a quartz–sulfide and a subsequent calcite–fluorite–quartz stage, which occurred
at 226±1 and 209±2 Ma, respectively. Our Rb–Sr data do not permit the identification of distinct generations of siderite and
fluorite, but rather reveal straight internal mixing relations, reflecting mixing of fluids or differential fluid–rock interaction
processes. This indicates merely two significant phases of mineral deposition related to the quartz–sulfide and calcite–fluorite–quartz
stages. It is shown that the Paleozoic sedimentary host rocks of the veins are the most likely source for the siderite Sr,
whereas fluorite displays a two-component mixture between sedimentary Sr and radiogenic Sr derived from locally occurring
Permian metavolcanic rocks.
Editorial handling: B. Lehmann 相似文献
52.
Werner?Schneider Frank?MatternEmail author PuJun?Wang Cai?Li 《International Journal of Earth Sciences》2003,92(2):228-254
The ophiolite-bearing Bangong-Nujiang zone (BNZ) traversing central Tibet from east to west separates the Qiangtang block in the north from the Lhasa block in the south. Their stratigraphic development indicates that both blocks once formed a continuous continental platform until the Late Triassic. Following Late Paleozoic-Triassic rifting, ocean crust formed between both blocks during the Late Triassic creating the Dongqiao-Naqu basin (DNB) among other basins (Yu et al. 1991). The analysis of the rift flank sequences reveals that rifting was dominated by transtension. The basin was shortened by post-Mid-Cretaceous transpression. Thus, the overall basin evolution represents a Reading cycle despite some active margin processes which gave this cycle a special imprint. Major basin parts were preserved despite transpressional shortening suggesting that the eastern BNZ represents a remnant basin. Our understanding of the DNB solves the prior problem of viewing the BNZ as a Mid-Late Jurassic collisional suture although typical collision-related deformation, thickening, mountain building, as well as related molasse formation are lacking. Our model also explains the scattered linear ophiolite distribution by local transpression of remnant oceanic basin floor without having to consider problematic long range ophiolite thrusting. 相似文献
53.
Sabine Hilt Elisabeth M. Gross Michael Hupfer Harald Morscheid Jens Mhlmann Arnulf Melzer Jens Poltz Stefan Sandrock Eva-Maria Scharf Susanne Schneider Klaus van de Weyer 《Limnologica》2006,36(3):155-171
One of the most serious problems caused by eutrophication of shallow lakes is the disappearance of submerged macrophytes and the switch to a turbid, phytoplankton-dominated state. The reduction of external nutrient loads often does not result in a change back to the macrophyte-dominated state because stabilising mechanisms that cause resilience may delay a response. Additional internal lake restoration measures may therefore be needed to decrease the concentration of total phosphorus and increase water clarity. The re-establishment of submerged macrophytes required for a long-term stability of clear water conditions, however, may still fail, or mass developments of tall-growing species may cause nuisance for recreational use. Both cases are often not taken into account when restoration measures are planned in Germany, and existing schemes to reduce eutrophication consider the topic inadequately. Here we develop a step-by-step guideline to assess the chances of submerged macrophyte re-establishment in shallow lakes. We reviewed and rated the existing literature and case studies with special regard on (1) the impact of different internal lake restoration methods on the development of submerged macrophytes, (2) methods for the assessment of natural re-establishment, (3) requirements and methods for artificial support of submerged macrophyte development and (4) management options of macrophyte species diversity and abundance in Germany. This guideline is intended to help lake managers aiming to restore shallow lakes in Germany to critically asses and predict the potential development of submerged vegetation, taking into account the complex factors and interrelations that determine their occurrence, abundance and diversity. 相似文献
54.
The formal stereological transformation equation for particle sieve size distribution from measurements in lower dimensional spaces is applied to laser diffractometer measurements. The transformation function for iron ore particles is measured experimentally, and modeled. The solution is tested against the measured transformation function data as well as synthetic composite distributions of the original sample. The natural size distribution of a sample taken from a grinding circuit stream was measured by a combination of standard sieving and cyclosizer, and the result is compared to the transformed size distribution calculated from laser diffractometer measurements. The stereological transformation technique performed well in all cases. 相似文献
55.
Climatic impacts of historical wetland drainage in Switzerland 总被引:1,自引:0,他引:1
The effects of historical land-use and land-cover changes on the climate of the Swiss Plateau in the different seasons were
investigated. In the 19th century, a civil engineering project was initiated to reshape the lake and river system on the Swiss
Plateau in order to ban the frequent flooding during extreme weather events. The landscape modifications consisted primarily
of a conversion of wetlands with extended peat soils into a highly productive agricultural landscape. Historical maps (1800–1850)
served as a basis for the reconstruction of the past land use. The “Lokal-Modell” of the Consortium for Small-Scale Modelling
was used to conduct eight one-month long high-resolution simulations (1.5 × 1.5 km2) with present and past landscape conditions. The modified soil and surface properties led to distinctly altered energy and
moisture exchanges at the surface and as a consequence affected the local and regional climate. The climatic changes show
different characteristics and magnitudes in the cold (October – March) as compared to the warm season (April – September).
The landscape modifications led to an average daytime cooling between −0.12 °C (January) and −0.61 °C (April) and a night-time
warming of 0.19 °C−0.34 °C. The differences in the mean monthly temperatures show a warming of 0.1 °C−0.2 °C in the cold season
and a cooling of similar magnitude in most of the study area in the warm season. The modification of the radiation budget
and the surface energy balance distinctly affected the convective activity in the study area in the warm season, but had only
a weak effect on convectivity in the cold season. The cloud coverage in the warm season is therefore distinctly reduced compared
to the past. 相似文献
56.
57.
Bernhard Geiger & Peter Schneider 《Monthly notices of the Royal Astronomical Society》1998,295(3):497-510
Analysing the weak lensing distortions of the images of faint background galaxies provides a means to constrain the average mass distribution of cluster galaxies and potentially to test the extent of their dark matter haloes as a function of the density of their environment. The observable image distortions are a consequence of the interplay between the effects of a global cluster mass distribution and the perturbations resulting from individual cluster galaxies. Starting from a reconstruction of the cluster mass distribution with conventional techniques, we apply a maximum likelihood method to infer the average properties of an ensemble of cluster galaxies. From simulations this approach is found to be reliable as long as the galaxies including their dark matter haloes only contribute a small fraction to the total mass of the system. If their haloes are extended, the galaxies contain a substantial mass fraction. In this case our method is still applicable in the outer regions of clusters, where the surface mass density is low, but yields biased estimates of the parameters describing the mass profiles of the cluster galaxies in the central part of the cluster. In that case it will be necessary to resort to more sophisticated strategies by modelling cluster galaxies and an underlying global mass distribution simultaneously. We conclude that galaxy–galaxy lensing in clusters provides a unique means to probe the presence and extent of dark haloes of cluster galaxies. 相似文献
58.
An integrated wavelet concept of physical geodesy 总被引:4,自引:1,他引:3
For the determination of the earth's gravity field many types of observations are nowadays available, including terrestrial
gravimetry, airborne gravimetry, satellite-to-satellite tracking, satellite gradio-metry, etc. The mathematical connection
between these observables on the one hand and gravity field and shape of the earth on the other is called the integrated concept
of physical geodesy. In this paper harmonic wavelets are introduced by which the gravitational part of the gravity field can
be approximated progressively better and better, reflecting an increasing flow of observations. An integrated concept of physical
geodesy in terms of harmonic wavelets is presented. Essential tools for approximation are integration formulas relating an
integral over an internal sphere to suitable linear combinations of observation functionals, i.e. linear functionals representing
the geodetic observables. A scale discrete version of multiresolution is described for approximating the gravitational potential
outside and on the earth's surface. Furthermore, an exact fully discrete wavelet approximation is developed for the case of
band-limited wavelets. A method for combined global outer harmonic and local harmonic wavelet modelling is proposed corresponding
to realistic earth's models. As examples, the role of wavelets is discussed for the classical Stokes problem, the oblique
derivative problem, satellite-to-satellite tracking, satellite gravity gradiometry and combined satellite-to-satellite tracking
and gradiometry.
Received: 28 February 1997 / Accepted: 17 November 1997 相似文献
59.
Our literature studies show that the thermal regime along continental strike-slip rifts is inconspicuous and that they are "low-volcanicity rifts" at best. Along with that, young continental strike-slip rifts exhibit no signs of major thermally controlled doming. We suggest that the larger the strike-slip component of a rift is, the less likely major thermal doming is causally associated with the rift zone. Since vertical lithosphere movements are reflected in the stratigraphic record of a rifted area, different rift modes (strike-slip, dip-slip) may be distinguished by analyzing the relevant sequences. Two ancient and especially suitable strike-slip rift margins in Tethyan mountain belts, the Bangong/Nujiang zone of Tibet and the South Penninic zone of the Alps, were analyzed with regard to their uplift history. The results confirm recent regional rift models which indicate in both cases that rifting was dominated by strike-slip. The stratigraphic approach may provide significant clues as to the mode of paleorifting when structural data are unavailable. 相似文献
60.