1,174 research outputs found
Triple oxygen isotopic composition of the high-<sup>3</sup>He/<sup>4</sup>He mantle
Measurements of Xe isotope ratios in ocean island basalts (OIB) suggest that Earthâs mantle accreted heterogeneously, and that compositional remnants of accretion are sampled by modern, high-3He/4He OIB associated with the Icelandic and Samoan plumes. If so, the high-3He/4He source may also have a distinct oxygen isotopic composition from the rest of the mantle. Here, we test if the major elements of the high-3He/4He source preserve any evidence of heterogeneous accretion using measurements of three oxygen isotopes on olivine from a variety of high-3He/4He OIB locations. To high precision, the Î17O value of high-3He/4He olivines from Hawaii, Pitcairn, Baffin Island and Samoa, are indistinguishable from bulk mantle olivine (Î17OBulk Mantle â Î17OHigh 3He/4He olivine = â0.002 ± 0.004 (2 Ă SEM)â°). Thus, there is no resolvable oxygen isotope evidence for heterogeneous accretion in the high-3He/4He source. Modelling of mixing processes indicates that if an early-forming, oxygen-isotope distinct mantle did exist, either the anomaly was extremely small, or the anomaly was homogenised away by later mantle convection.
The ÎŽ18O values of olivine with the highest 3He/4He ratios from a variety of OIB locations have a relatively uniform composition (âŒ5â°). This composition is intermediate to values associated with the depleted MORB mantle and the average mantle. Similarly, ÎŽ18O values of olivine from high-3He/4He OIB correlate with radiogenic isotope ratios of He, Sr, and Nd. Combined, this suggests that magmatic oxygen is sourced from the same mantle as other, more incompatible elements and that the intermediate ÎŽ18O value is a feature of the high-3He/4He mantle source. The processes responsible for the ÎŽ18O signature of high-3He/4He mantle are not certain, but ÎŽ18Oâ87Sr/86Sr correlations indicate that it may be connected to a predominance of a HIMU-like (high U/Pb) component or other moderate ÎŽ18O components recycled into the high-3He/4He source
The accuracy of ADC measurements in liver is improved by a tailored and computationally efficient local-rigid registration algorithm
This study describes post-processing methodologies to reduce the effects of physiological motion in measurements of apparent diffusion coefficient (ADC) in the liver. The aims of the study are to improve the accuracy of ADC measurements in liver disease to support quantitative clinical characterisation and reduce the number of patients required for sequential studies of disease progression and therapeutic effects. Two motion correction methods are compared, one based on non-rigid registration (NRA) using freely available open source algorithms and the other a local-rigid registration (LRA) specifically designed for use with diffusion weighted magnetic resonance (DW-MR) data. Performance of these methods is evaluated using metrics computed from regional ADC histograms on abdominal image slices from healthy volunteers. While the non-rigid registration method has the advantages of being applicable on the whole volume and in a fully automatic fashion, the local-rigid registration method is faster while maintaining the integrity of the biological structures essential for analysis of tissue heterogeneity. Our findings also indicate that the averaging commonly applied to DW-MR images as part of the acquisition protocol should be avoided if possible
Information decomposition of symbolic sequences
We developed a non-parametric method of Information Decomposition (ID) of a
content of any symbolical sequence. The method is based on the calculation of
Shannon mutual information between analyzed and artificial symbolical
sequences, and allows the revealing of latent periodicity in any symbolical
sequence. We show the stability of the ID method in the case of a large number
of random letter changes in an analyzed symbolic sequence. We demonstrate the
possibilities of the method, analyzing both poems, and DNA and protein
sequences. In DNA and protein sequences we show the existence of many DNA and
amino acid sequences with different types and lengths of latent periodicity.
The possible origin of latent periodicity for different symbolical sequences is
discussed.Comment: 18 pages, 8 figure
Magnetohydrodynamics and Plasma Cosmology
We study the linear magnetohydrodynamic (MHD) equations, both in the
Newtonian and the general-relativistic limit, as regards a viscous magnetized
fluid of finite conductivity and discuss instability criteria. In addition, we
explore the excitation of cosmological perturbations in anisotropic spacetimes,
in the presence of an ambient magnetic field. Acoustic, electromagnetic (e/m)
and fast-magnetosonic modes, propagating normal to the magnetic field, can be
excited, resulting in several implications of cosmological significance.Comment: 9 pages, RevTeX, To appear in the Proceedings of the Peyresq X
Meeting, IJTP Conference Serie
Wormholes and Ringholes in a Dark-Energy Universe
The effects that the present accelerating expansion of the universe has on
the size and shape of Lorentzian wormholes and ringholes are considered. It is
shown that, quite similarly to how it occurs for inflating wormholes, relative
to the initial embedding-space coordinate system, whereas the shape of the
considered holes is always preserved with time, their size is driven by the
expansion to increase by a factor which is proportional to the scale factor of
the universe. In the case that dark energy is phantom energy, which is not
excluded by present constraints on the dark-energy equation of state, that size
increase with time becomes quite more remarkable, and a rather speculative
scenario is here presented where the big rip can be circumvented by future
advanced civilizations by utilizing sufficiently grown up wormholes and
ringholes as time machines that shortcut the big-rip singularity.Comment: 11 pages, RevTex, to appear in Phys. Rev.
Longâterm trends in the distribution, abundance and impact of native âinjuriousâ weeds
Questions: How can we quantify changes in the distribution and abundance of injurious weed species (Senecio jacobaea, Cirsium vulgare, Cirsium arvense, Rumex obtusifolius, Rumex crispus and Urtica dioica), over long time periods at wide geographical scales? What impact do these species have on plant communities? To what extent are changes driven by anthropogenically induced drivers such as disturbance, eutrophication and management?
Location: Great Britain.
Methods: Data from national surveys were used to assess changes in the frequency and abundance of selected weed species between 1978 and 2007. This involved novel method development to create indices of change, and to relate changes in distribution and abundance of these species to plant community diversity and inferred changes in resource availability, disturbance and management.
Results: Three of the six weed species became more widespread in GB over this period and all of them increased in abundance (in grasslands, arable habitats, roadsides and streamsides). Patterns were complex and varied by landscape context and habitat type. For most of the species, there were negative relationships between abundance, total plant species richness, grassland, wetland and woodland indicators. Each individual species responds to a different combination of anthropogenic drivers but disturbance, fertility and livestock management significantly influenced most species.
Conclusions: The increase in frequency and abundance of weeds over decades has implications for landscapeâscale plant diversity, fodder yield and livestock health. This includes reductions in plant species richness, loss of valuable habitat specialists and homogenisation of vegetation communities. Increasing landâuse intensity, excessive nutrient input, overgrazing, sward damage, poaching and bare ground in fields and undermanagement or too frequent cutting on linear features may have led to increases in weeds. These weeds do have conservation value so we are not advocating eradication, rather coâexistence, without dominance. Land management policy needs to adapt to benefit biodiversity and agricultural productivity
Patient experiences and perceptions of Guillain-Barré Syndrome and associated inflammatory neuropathies following discharge from hospital
Noncommutative solitons on Kahler manifolds
We construct a new class of scalar noncommutative multi-solitons on an
arbitrary Kahler manifold by using Berezin's geometric approach to quantization
and its generalization to deformation quantization. We analyze the stability
condition which arises from the leading 1/hbar correction to the soliton energy
and for homogeneous Kahler manifolds obtain that the stable solitons are given
in terms of generalized coherent states. We apply this general formalism to a
number of examples, which include the sphere, hyperbolic plane, torus and
general symmetric bounded domains. As a general feature we notice that on
homogeneous manifolds of positive curvature, solitons tend to attract each
other, while if the curvature is negative they will repel each other.
Applications of these results are discussed.Comment: 26 pages, 3 figures, harvmac; references adde
Measurement of the B0-anti-B0-Oscillation Frequency with Inclusive Dilepton Events
The - oscillation frequency has been measured with a sample of
23 million \B\bar B pairs collected with the BABAR detector at the PEP-II
asymmetric B Factory at SLAC. In this sample, we select events in which both B
mesons decay semileptonically and use the charge of the leptons to identify the
flavor of each B meson. A simultaneous fit to the decay time difference
distributions for opposite- and same-sign dilepton events gives ps.Comment: 7 pages, 1 figure, submitted to Physical Review Letter
Structures of Rhodopseudomonas palustris RC-LH1 complexes with open or closed quinone channels
The reaction-center light-harvesting complex 1 (RC-LH1) is the core photosynthetic component in purple phototrophic bacteria. We present two cryoâelectron microscopy structures of RC-LH1 complexes from Rhodopseudomonas palustris. A 2.65-Ă
resolution structure of the RC-LH114-W complex consists of an open 14-subunit LH1 ring surrounding the RC interrupted by protein-W, whereas the complex without protein-W at 2.80-Ă
resolution comprises an RC completely encircled by a closed, 16-subunit LH1 ring. Comparison of these structures provides insights into quinone dynamics within RC-LH1 complexes, including a previously unidentified conformational change upon quinone binding at the RC QB site, and the locations of accessory quinone binding sites that aid their delivery to the RC. The structurally unique protein-W prevents LH1 ring closure, creating a channel for accelerated quinone/quinol exchange
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