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  • 1
    Publication Date: 2022-05-26
    Description: © The Author(s), 2016. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Science Advances 2 (2016): e1600883, doi:10.1126/sciadv.1600883.
    Description: The formation of the Isthmus of Panama stands as one of the greatest natural events of the Cenozoic, driving profound biotic transformations on land and in the oceans. Some recent studies suggest that the Isthmus formed many millions of years earlier than the widely recognized age of approximately 3 million years ago (Ma), a result that if true would revolutionize our understanding of environmental, ecological, and evolutionary change across the Americas. To bring clarity to the question of when the Isthmus of Panama formed, we provide an exhaustive review and reanalysis of geological, paleontological, and molecular records. These independent lines of evidence converge upon a cohesive narrative of gradually emerging land and constricting seaways, with formation of the Isthmus of Panama sensu stricto around 2.8 Ma. The evidence used to support an older isthmus is inconclusive, and we caution against the uncritical acceptance of an isthmus before the Pliocene.
    Description: This study was supported by the Smithsonian Tropical Research Institute to A.O., J.B.C.J., N.K., and H.A.L.; the NSF (EAR 1325683) to A.O., P.G.R.-D., and E.L.G.; the National System of Investigators to A.O.; the Secretaría Nacional de Ciencia, Tecnología e Innovación (Panamá) to A.O., H.A.L., and S.E.C.; the U.S. Geological Survey to R.F.S.; and the Consejo Nacional de Investigaciones Científicas y Técnicas (Argentina) to A.L.C., G.M.G., E.S., and L.S.
    Repository Name: Woods Hole Open Access Server
    Type: Article
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  • 2
    Electronic Resource
    Electronic Resource
    [s.l.] : Macmillian Magazines Ltd.
    Nature 405 (2000), S. 775-778 
    ISSN: 1476-4687
    Source: Nature Archives 1869 - 2009
    Topics: Biology , Chemistry and Pharmacology , Medicine , Natural Sciences in General , Physics
    Notes: [Auszug] How and where the ocean tides dissipate their energy are long-standing questions that have consequences ranging from the history of the Moon to the mixing of the oceans. Historically, the principal sink of tidal energy has been thought to be bottom friction in shallow seas. There has long ...
    Type of Medium: Electronic Resource
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  • 3
    Electronic Resource
    Electronic Resource
    Oxford, UK : Blackwell Publishing Ltd
    Geophysical journal international 102 (1990), S. 0 
    ISSN: 1365-246X
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences
    Notes: Yee & Paulson (1988) derive an integral representation and some dispersion relations for a general magnetotelluric impedance tensor measured at a single point on the surface of the 3-D Earth. These results derive from the assumption that the impedance tensor defines a linear system which is causal and passive. We give a more careful discussion of the justification for causality and passivity and show that they do not follow from simple physical arguments; at the very least, there is a substantial gap in the mathematical derivation given by Yee & Paulson (1988). In fact, both conditions can almost certainly fail for physically realizable 3-D conductivities. We present a qualitative analysis of some simple examples which illustrate how this could occur, and give examples of estimated impedance tensors which are clearly not consistent with the passivity condition. We thus conclude that the relevance to real data of many of the results derived by Yee & Paulson must be seriously questioned.
    Type of Medium: Electronic Resource
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  • 4
    Electronic Resource
    Electronic Resource
    Oxford, UK : Blackwell Publishing Ltd
    Geophysical journal international 106 (1991), S. 0 
    ISSN: 1365-246X
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences
    Notes: We describe a general method for combining multiple station transfer function results (i.e., uniform source response space estimates) from a large number of overlapping small geomagnetic arrays, to produce an estimate of the uniform source response for a large synthetic array. The resulting synthesized response can be used to plot anomalous magnetic fields in a region covered by the non-simultaneous arrays. We show that this general array synthesis problem can be approximately reduced to a fairly simple non-linear least-squares problem. Because the number of parameters is very large, we develop a specialized and efficient estimation scheme which exploits the special structure of the problem. We also discuss the computation of a large sample approximation to the covariance matrix of the earth response parameters. While the method has been developed for combining non-simultaneous arrays, it can also be used to ‘patch up’ large simultaneous arrays with some missing data. Both aspects of the method are illustrated on an overlapping set of 19 three-five-station three-component magnetovariational arrays from southwestern Washington. Results from the synthetic 49 station three-component array are discussed briefly.
    Type of Medium: Electronic Resource
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  • 5
    ISSN: 1435-1528
    Keywords: Superimposed oscillations ; complex viscosity ; complex shear modulus ; nearly viscometric flow ; dynamic viscometer
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: Abstract A new instrument to carry out complex viscosity measurements in equilibrium and in a steady shear flow has been developed. A small amplitude harmonic excitation is superimposed orthogonally to the steady shear rate component. It is realized by a thin-walled cylinder, which oscillates in the axial direction in a rotating annular cup. The cylinder is suspended from a spring and guided by an air bearing. The spring, which itself is attached to an axially oscillating exciter, has been designed especially for a low linear and a large torsional spring constant. The motion of the cylinder and exciter is recorded and analysed. The applicable frequency ranges from 0.001 up to 50 Hz, the steady shear rate from 0.001 up to 100 s-1 and the oscillatory shear amplitude from 3 down to 0.003. The temperature can be set and controlled between 5° and 70°C. The instrument is suited for viscosities in the range from 0.3 mPas up to 1 Pas. Considering this, the range and sensitivity with respect to existing instruments has been extended significantly. Besides a description of the instrument much attention has been paid in this paper to the theoretical background of orthogonal super-position in frames of the simple fluid theory. Also the measurement range and calibration of the instrument as well as the accuracy of the obtainable results are discussed. At last some demonstration measurements on both a polymer solution and an ordering latex, dispersion are presented.
    Type of Medium: Electronic Resource
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  • 6
    Electronic Resource
    Electronic Resource
    Springer
    Hydrobiologia 32 (1968), S. 27-32 
    ISSN: 1573-5117
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology
    Notes: Summary Sixteen epifaunal species are identified encrusting dead valves of Mercenaria mercenaria, Mya arenaria, and Aequipecten irradians exposed for one year on the bottom of Buzzards Bay, Massachusetts. The three most common cheilostomatous bryozoans are Schizoporella unicormis, S. biaperta, and Parasmittina trispinosa. Balanus (Balanus) amphitrite neveus demonstrates a preferred orientation of the rostro-carinal axis parallel to the growth lines of Mercenaria mercenaria, Mya arenaria and Aequipecten irradians. No preference for difference bivalve shells is apparent in any of the epifauna examined. The lesser abundance of most species on Mercenaria mercenaria valves may be due to the more rapid burial of these heavier valves in shifting sediments of the study area.
    Type of Medium: Electronic Resource
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  • 7
    Electronic Resource
    Electronic Resource
    Springer
    Hydrobiologia 37 (1971), S. 347-359 
    ISSN: 1573-5117
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology
    Description / Table of Contents: Résumé La minéralogy, la configuration et le classement par ordre de grandeur des grains de sable qui forment le centre pour le grandissement colonial dans Discoporella umbellata depressa et Cupuladria doma ont été examinés et comparés à ces mêmes paramèters dans les alluvionnements d'où venaient les especès mentionnées. Ces bryozoaires, qui se trouvent sur le banc continental de Georgie encroûtent des grains de sable. Les diamèters des grains encroûtés dépassent de beaucoup le diametre moyen des particules dans les alluvionnements associés. D. umbellata depressa encroûte de plus grands grains que C. doma. Les grains encroûtés se composent de quartz et de fragments de rocker. On n'a trouvé aucun zoaria des deux especes qui avait encroute des grains de carbonate, malgré le fait que de tels grains font une importante fraction de l'alluvionnement dans Faire de notre spécialization. On suggère un choix larvaire des sites de fixation, basé sur la grandeur des grains et la minéralogy. De plus, on suggerè que ces parametrès sédimentaires pourraient expliquer en partie le fait que tous les deux espèces bryozoaires sont présentes en grand nombre sur le bane continental du sud-est des Etats-Unis, mais sont à peu près absentes du talus continental.
    Notes: Summary The mineralogy, shape, and size distribution of sand grains which form the nucleus for colonial growth in Discoporella umbellata depressa and Cupuladria doma have been examined and compared to these same parameters in the sediment from which the species were collected. These bryozoa, occurring on the continental shelf of Georgia, encrust sand grains. The diameters of the encrusted grains greatly exceed the mean diameter of particles in the associated sediment. D. umbellata depressa encrusts larger grains than does C. doma. Encrusted grains are composed of quartz and rock fragments. No zoaria of either species were found that had encrusted carbonate grains despite the fact that such grains form an important fraction of the sediment in the study area. Larval selection of fixation sites on the basis of grain size and mineralogy is suggested. It is further suggested that these sedimentary parameters may partially explain the fact that both bryozoan species are abundantly present on the continental shelf of the southeastern United States but are nearly absent from the continental slope.
    Type of Medium: Electronic Resource
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  • 8
    Publication Date: 2015-12-07
    Print ISSN: 0967-3334
    Electronic ISSN: 1361-6579
    Topics: Medicine , Physics
    Published by Institute of Physics
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  • 9
    Publication Date: 2013-02-15
    Description: Dickman argues that our model of the Mf ocean tide is unrealistic and that this invalidates conclusions regarding mantle anelasticity. His evidence is based on comparison with his own ocean modelling experiments and on his physical intuition regarding near-equilibrium tides. That evidence is unconvincing. Simple physical arguments alone are enough to rebut his main points. Furthermore, we test here his suggestion of increasing bottom-friction dissipation by a factor of 100 over our preferred solution, and we show that this results in unacceptable polar motion and poor agreement with independent tide-gauge data. In contrast, tests against independent data lend support to the realism of our Mf tide model and give confidence that our estimates of mantle anelasticity are reliable within stated error bounds.
    Print ISSN: 0956-540X
    Electronic ISSN: 1365-246X
    Topics: Geosciences
    Published by Oxford University Press on behalf of The Deutsche Geophysikalische Gesellschaft (DGG) and the Royal Astronomical Society (RAS).
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  • 10
    Publication Date: 2015-10-03
    Description: With advancing computational resources, 3-D inversion techniques have become feasible in recent years and are now a more widely used tool for magnetotelluric (MT) data interpretation. Galvanic distortion caused by small-scale near-surface inhomogeneities remains an obstacle for 3-D MT inversion which so far has experienced little attention. If not considered properly, the effect on 3-D inversion can be immense and result in erroneous subsurface models and interpretations. To tackle the problem we implemented inversion of the distortion-free phase tensor into the ModEM inversion package. The dimensionless phase tensor components describe only variations of the conductivity structure. When inverting these data, particular care has to be taken of the conductivity structure in the a priori model, which provides the reference frame when transferring the information from phase tensors into absolute conductivity values. Our results obtained with synthetic data show that phase tensor inversion can recover the regional conductivity structure in presence of galvanic distortion if the a priori model provides a reasonable assumption for the regional resistivity average. Joint inversion of phase tensor data and vertical magnetic transfer functions improves recovery of the absolute resistivity structure and is less dependent on the prior model. We also used phase tensor inversion for a data set of more than 250 MT sites from the central San Andreas fault, California, where a number of sites showed significant galvanic distortion. We find the regional structure of the phase tensor inversion results compatible with previously obtained models from impedance inversion. In the vicinity of distorted sites, phase tensor inversion models exhibit more homogeneous/smoother conductivity structures.
    Keywords: Geomagnetism, Rock Magnetism and Palaeomagnetism
    Print ISSN: 0956-540X
    Electronic ISSN: 1365-246X
    Topics: Geosciences
    Published by Oxford University Press on behalf of The Deutsche Geophysikalische Gesellschaft (DGG) and the Royal Astronomical Society (RAS).
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