10,432 research outputs found
Simulating anthropogenic impacts to bird communities in tropical rain forests
We used an aggregated modelling approach to simulate the impacts ofanthropogenic disturbances on the long-term dynamics of faunal diversityin tropical rain forests. We restricted our study to bird communities eventhough the approach is more general. We developed a model calledBIODIV which simulated the establishment of hypothetical bird speciesin a forest. Our model was based on the results of a simple matrix modelwhich calculated the spatio-temporal dynamics of a tropical rain forest inMalaysia. We analysed the establishment of bird species in a secondaryforest succession and the impacts of 60 different logging scenarios on thediversity of the bird community. Of the three logging parameters(cycle length, method, intensity), logging intensity had the most servereimpact on the bird community. In the worst case the number of bird specieswas reduced to 23% of the species richness found in a primary forest
Relaxation of curvature induced elastic stress by the Asaro-Tiller-Grinfeld instability
A two-dimensional crystal on the surface of a sphere experiences elastic
stress due to the incompatibility of the crystal axes and the curvature. A
common mechanism to relax elastic stress is the Asaro-Tiller-Grinfeld (ATG)
instability. With a combined numerical and analytical approach we demonstrate,
that also curvature induced stress in surface crystals can be relaxed by the
long wave length ATG instability. The numerical results are obtained using a
surface phase-field crystal (PFC) model, from which we determine the
characteristic wave numbers of the ATG instability for various surface
coverages corresponding to different curvature induced compressions. The
results are compared with an analytic expression for the characteristic wave
number, obtained from a continuum approach which accounts for hexagonal
crystals and intrinsic PFC symmetries. We find our numerical results in
accordance with the analytical predictions.Comment: 6 pages, 5 figure
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Infrared spectroscopy of circumstellar dust: signs of differentiated materials?
Mid-infrared absorption spectra of powdered achondrites are compared with the astronomical spectra of dust around young, evolving stars, to find evidence (or not) of dust formed in collisional cascades of material from planetesimals
Magnetic field enhanced structural instability in EuTiO_{3}
EuTiO_{3} undergoes a structural phase transition from cubic to tetragonal at
T_S = 282 K which is not accompanied by any long range magnetic order. However,
it is related to the oxygen ocathedra rotation driven by a zone boundary
acoustic mode softening. Here we show that this displacive second order
structural phase transition can be shifted to higher temperatures by the
application of an external magnetic field (increased by 4 K for mu_{0}H = 9 T).
This observed field dependence is in agreement with theoretical predictions
based on a coupled spin-anharmonic-phonon interaction model.Comment: 4 pages, 4 figure
Ions in glass forming glycerol: Close correlation of alpha and fast beta relaxation
We provide broadband dielectric loss spectra of glass-forming glycerol with
varying additions of LiCl. The measurements covering frequencies up to 10 THz
extend well into the region of the fast beta process, commonly ascribed to
caged molecule dynamics. Aside of the known variation of the structural alpha
relaxation time and a modification of the excess wing with ion content, we also
find a clear influence on the shallow loss minimum arising from the fast beta
relaxation. Within the framework of mode-coupling theory, the detected
significant broadening of this minimum is in reasonable accord with the found
variation of the alpha-relaxation dynamics. A correlation between
alpha-relaxation rate and minimum position holds for all ion concentrations and
temperatures, even below the critical temperature defined by mode-coupling
theory.Comment: 5 pages, 5 figure
Climate-Vegetation-Feedbacks as a Mechanism for Accelerated Climate Change: The onset of the African Humid Period
Paleo-environmental records and models indicate that the African Humid Period (AHPabruptly ended about 5000-4000 years before present (BP). Some proxies indicate alsan abrupt onset of the AHP between 14,000 and 11,000 BP. How important are local orbitaforcing, ice-sheet forcing, greenhouse gas forcing, and the reorganization of the AtlantiMeridional Overturning Circulation (AMOC) for changes in the African Monsoon/vegetatiosystem? Here we use transient simulations with climate-vegetation models of differencomplexity to identify the factors that control the onset of the African Monsoon/VegetationWe test the following hypothesis:(1) There is no indication for insolation-thresholds for the onset/break of the AHP.(2) Forcing from CO2/ice-sheets significantly controls the climate of North Africa.(3) CO2 fertilization contributes to the vegetation changes over North Africa.(4) A shutdown of the AMOC is as important as orbital insolation for the African Monsoon
Residual entanglement of accelerated fermions is not nonlocal
We analyze the operational meaning of the residual entanglement in
non-inertial fermionic systems in terms of the achievable violation of the
Clauser-Horne-Shimony-Holt (CHSH) inequality. We demonstrate that the quantum
correlations of fermions, which were previously found to survive in the
infinite acceleration limit, cannot be considered to be non-local. The
entanglement shared by an inertial and an accelerated observer cannot be
utilized for the violation of the CHSH inequality in case of high
accelerations. Our results are shown to extend beyond the single mode
approximation commonly used in the literature.Comment: 5 pages, 3 figures; v2: minor changes, reference and section headers
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Rapid changes in ice core gas records Part 2: Understanding the rapid rise in atmospheric CO2 at the onset of the Bølling/Allerød
During the last glacial/interglacial transition the Earth's climate underwent rapid changes around 14.6 kyr ago. Temperature proxies from ice cores revealed the onset of the Bølling/Allerød (B/A) warm period in the north and the start of the Antarctic Cold Reversal in the south. Furthermore, the B/A is accompanied by a rapid sea level rise of about 20 m during meltwater pulse (MWP) 1A, whose exact timing is matter of current debate. In situ measured CO<sub>2</sub> in the EPICA Dome C (EDC) ice core also revealed a remarkable jump of 10±1 ppmv in 230 yr at the same time. Allowing for the age distribution of CO<sub>2</sub> in firn we here show, that atmospheric CO<sub>2</sub> rose by 20–35 ppmv in less than 200 yr, which is a factor of 2–3.5 larger than the CO<sub>2</sub> signal recorded in situ in EDC. Based on the estimated airborne fraction of 0.17 of CO<sub>2</sub> we infer that 125 Pg of carbon need to be released to the atmosphere to produce such a peak. Most of the carbon might have been activated as consequence of continental shelf flooding during MWP-1A. This impact of rapid sea level rise on atmospheric CO<sub>2</sub> distinguishes the B/A from other Dansgaard/Oeschger events of the last 60 kyr, potentially defining the point of no return during the last deglaciation
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