1,838 research outputs found
Investigation of the Stock Structure of Atlantic Walrus (Odobenus rosmarus rosmarus) in Canada and Greenland Using Dental Pb Isotopes Derived from Local Geochemical Environments
The chemical composition of animal tissues such as teeth appears to reflect an individual's exposure to its geochemical environment. In this study, the lead (Pb) isotope composition of dental cementum was used to investigate the stock structure of Atlantic walrus (Odobenus rosmarus rosmarus) in the Canadian Arctic and Greenland. The 12 communities providing walrus samples for this study represent most of the Canadian and Greenlandic villages where walrus still form an important part of the traditional Inuit diet. Significant differences between locations in mean Pb isotope ratios and the limited overlap of the ranges of values indicate that each village harvested walrus herds that exploited substantially different geological/geographical habitats. This geographic segregation based on isotopic signatures suggests that most walrus stocks (i.e., the groups of walrus that interact with hunters at each community) are more localized in their range than previously thought. 208Pb/207Pb and 208Pb/204Pb were the most important stock discriminators, reflecting the influence of local geological Th/U composition (i.e., 208Pb) on Pb isotope composition in walrus teeth. 204Pb-based isotope ratios in walrus were consistently higher (more radiogenic) and more homogeneous than those in regional terrestrial bedrock, a difference probably due to selective leaching of radiogenic Pb from mineral phases into seawater and mixing during weathering and transport. Dental Pb isotope signatures may have widespread application to stock discrimination of other coastal marine mammal species.La composition chimique de tissus animaux tels que les dents semble reflĂ©ter l'exposition d'un individu Ă son milieu gĂ©ochimique. Pour la prĂ©sente Ă©tude, on a utilisĂ© la composition isotopique du plomb (Pb) contenu dans le cĂ©ment pour examiner la structure du stock du morse de l'Atlantique (Odobenus rosmarus rosmarus) dans l'Arctique canadien et le Groenland. Les 12 communautĂ©s qui ont fourni les Ă©chantillons de morse pour ce projet reprĂ©sentent la majoritĂ© des villages canadiens et groenlandais oĂč le morse constitue toujours une grande partie du rĂ©gime alimentaire traditionnel des Inuits. Des diffĂ©rences marquĂ©es entre les sites dans la moyenne des rapports isotopiques du Pb et le faible recoupement des gammes de valeurs rĂ©vĂšlent que chaque village prĂ©levait des morses au sein de troupeaux qui exploitaient des habitats gĂ©ologiques/gĂ©ographiques bien distincts. Cette sĂ©grĂ©gation gĂ©ographique fondĂ©e sur des signatures isotopiques suggĂšre que la plupart des stocks de morses (c.-Ă -d. le groupe de morses qui interagit avec les chasseurs dans chaque communautĂ©) sont plus localisĂ©s dans leur territoire qu'on ne le pensait auparavant. 208Pb/207Pb et 208Pb/204Pb Ă©taient les grands caractĂšres discriminants des stocks, reflĂ©tant l'influence de la composition gĂ©ologique locale Th/U (c-Ă -d. 208Pb) sur la composition isotopique du Pb dans les dents du morse. Les rapports isotopiques fondĂ©s sur 204Pb Ă©taient constamment plus Ă©levĂ©s (plus radiogĂ©niques) et plus homogĂšnes que ceux du substratum terrestre, la diffĂ©rence Ă©tant probablement due Ă la lixiviation sĂ©lective du Pb radiogĂ©nique passant des phases minĂ©rales dans l'eau de mer et Ă son mĂ©lange durant la mĂ©tĂ©orisation et le transport. Les signatures isotopiques du plomb dentaire peuvent avoir de vastes applications dans la discrimination des stocks d'autres espĂšces de mammifĂšres marins cĂŽtiers
The Evolution and Long-Term Results of Laparoscopic Antireflux Surgery for the Treatment of Gastroesophageal Reflux Disease
When carefully performed, laparoscopic antireflux surgery is an effective, durable procedure for the control of gastroesophageal reflux disease
Extensive Noachian fluvial systems in Arabia Terra: Implications for early Martian climate
Valley networks are some of the strongest lines of evidence for extensive fluvial activity on early (Noachian; >3.7 Ga) Mars. However, their purported absence on certain ancient terrains, such as Arabia Terra, is at variance with patterns of precipitation as predicted by "warm and wet" climate models. This disagreement has contributed to the development of an alternative "icy highlands" scenario, whereby valley networks were formed by the melting of highland ice sheets. Here, we show through regional mapping that Arabia Terra shows evidence for extensive networks of sinuous ridges. We interpret these ridge features as inverted fluvial channels that formed in the Noachian, before being subject to burial and exhumation. The inverted channels developed on extensive aggrading flood plains. As the inverted channels are both sourced in, and traverse across, Arabia Terra, their formation is inconsistent with discrete, localized sources of water, such as meltwater from highland ice sheets. Our results are instead more consistent with an early Mars that supported widespread precipitation and runoff
The stringy nature of the 2d type-0A black hole
We investigate the thermodynamics of the RR charged two-dimensional type-0A
black hole background at finite temperature, and compare with known 0A matrix
model results. It has been claimed that there is a disagreement for the free
energy between the spacetime and the dual matrix model. Here we find that this
discrepancy is sensitive to how the cutoff is implemented on the spacetime
side. In particular, the disagreement is resolved once we put the cutoff at a
fixed distance away from the horizon, as opposed to a fixed position in space.
Furthermore, the mass and the entropy of the black hole itself add up to an
analytic contribution to the free energy, which is precisely reproduced by the
0A matrix model. We also use results from the 0A matrix model to predict the
next to leading order contribution to the entropy of the black hole. Finally,
we note that the black hole is characterized by a Hagedorn growth in its
density of states below the Hagedorn temperature. This, together with other
results, suggests there is a phase transition at this temperature.Comment: 1+21 pages; v2: Substantial changes in the body of the paper, main
results the same. Clarified discussion on the thermodynamics, added section
on a phase transition, references added. v3: Typos corrected. v4: Final
version, to appear in JHE
Applying Laser Doppler Anemometry inside a Taylor-Couette geometry - Using a ray-tracer to correct for curvature effects
In the present work it will be shown how the curvature of the outer cylinder
affects Laser Doppler anemometry measurements inside a Taylor-Couette
apparatus. The measurement position and the measured velocity are altered by
curved surfaces. Conventional methods for curvature correction are not
applicable to our setup, and it will be shown how a ray-tracer can be used to
solve this complication.
By using a ray-tracer the focal position can be calculated, and the velocity
can be corrected. The results of the ray-tracer are verified by measuring an a
priori known velocity field, and after applying refractive corrections good
agreement with theoretical predictions are found. The methods described in this
paper are applied to measure the azimuthal velocity profiles in high Reynolds
number Taylor-Couette flow for the case of outer cylinder rotation
The fundamental cycle of concept construction underlying various theoretical frameworks
In this paper, the development of mathematical concepts over time is considered. Particular reference is given to the shifting of attention from step-by-step procedures that are performed in time, to symbolism that can be manipulated as mental entities on paper and in the mind. The development is analysed using different theoretical perspectives, including the SOLO model and various theories of concept construction to reveal a fundamental cycle underlying the building of concepts that features widely in different ways of thinking that occurs throughout mathematical learning
Effect of Chaotic Noise on Multistable Systems
In a recent letter [Phys.Rev.Lett. {\bf 30}, 3269 (1995), chao-dyn/9510011],
we reported that a macroscopic chaotic determinism emerges in a multistable
system: the unidirectional motion of a dissipative particle subject to an
apparently symmetric chaotic noise occurs even if the particle is in a
spatially symmetric potential. In this paper, we study the global dynamics of a
dissipative particle by investigating the barrier crossing probability of the
particle between two basins of the multistable potential. We derive
analytically an expression of the barrier crossing probability of the particle
subject to a chaotic noise generated by a general piecewise linear map. We also
show that the obtained analytical barrier crossing probability is applicable to
a chaotic noise generated not only by a piecewise linear map with a uniform
invariant density but also by a non-piecewise linear map with non-uniform
invariant density. We claim, from the viewpoint of the noise induced motion in
a multistable system, that chaotic noise is a first realization of the effect
of {\em dynamical asymmetry} of general noise which induces the symmetry
breaking dynamics.Comment: 14 pages, 9 figures, to appear in Phys.Rev.
CBR Anisotropy from Primordial Gravitational Waves in Two-Component Inflationary Cosmology
We examine stochastic temperature fluctuations of the cosmic background
radiation (CBR) arising via the Sachs-Wolfe effect from gravitational wave
perturbations produced in the early universe. We consider spatially flat,
perturbed FRW models that begin with an inflationary phase, followed by a mixed
phase containing both radiation and dust. The scale factor during the mixed
phase takes the form , where are
constants. During the mixed phase the universe smoothly transforms from being
radiation to dust dominated. We find analytic expressions for the graviton mode
function during the mixed phase in terms of spheroidal wave functions. This
mode function is used to find an analytic expression for the multipole moments
of the two-point angular correlation function
for the CBR anisotropy. The analytic expression for the multipole
moments is written in terms of two integrals, which are evaluated numerically.
The results are compared to multipoles calculated for models that are {\it
completely} dust dominated at last-scattering. We find that the multipoles
of the CBR temperature perturbations for are
significantly larger for a universe that contains both radiation and dust at
last-scattering. We compare our results with recent, similar numerical work and
find good agreement. The spheroidal wave functions may have applications to
other problems of cosmological interest.Comment: 28 pgs + 6 postscript figures, RevTe
The Shapes of Dirichlet Defects
If the vacuum manifold of a field theory has the appropriate topological
structure, the theory admits topological structures analogous to the D-branes
of string theory, in which defects of one dimension terminate on other defects
of higher dimension. The shapes of such defects are analyzed numerically, with
special attention paid to the intersection regions. Walls (co-dimension 1
branes) terminating on other walls, global strings (co-dimension 2 branes) and
local strings (including gauge fields) terminating on walls are all considered.
Connections to supersymmetric field theories, string theory and condensed
matter systems are pointed out.Comment: 24 pages, RevTeX, 21 eps figure
Loop-Generated Bounds on Changes to the Graviton Dispersion Relation
We identify the effective theory appropriate to the propagation of massless
bulk fields in brane-world scenarios, to show that the dominant low-energy
effect of asymmetric warping in the bulk is to modify the dispersion relation
of the effective 4-dimensional modes. We show how such changes to the graviton
dispersion relation may be bounded through the effects they imply, through
loops, for the propagation of standard model particles. We compute these bounds
and show that they provide, in some cases, the strongest constraints on
nonstandard gravitational dispersions. The bounds obtained in this way are the
strongest for the fewest extra dimensions and when the extra-dimensional Planck
mass is the smallest. Although the best bounds come for warped 5-D scenarios,
for which the 5D Planck Mass is O(TeV), even in 4 dimensions the graviton loop
can lead to a bound on the graviton speed which is comparable with other
constraints.Comment: 18 pages, LaTeX, 4 figures, uses revte
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