1,238 research outputs found
Drumlin sedimentology in a hard-bed, lowland setting, Connemara, western Ireland: implications for subglacial bedform generation in areas of sparse till cover
Cores of coastal drumlins in Connemara contain stratified diamictons that interdigitate with gravelly clinoforms and finer grained rhythmites. The diamictons are interpreted as subaqueous mud apron deposits delivered by subglacial till advection to continuously failing subaqueous ice-contact fans, whose strata were being syn-depositionally over-steepened by glacitectonic deformation. The localized nature of the stratified sediments reflects the emergence of subglacial deforming tills and meltwater deposits in a glacilacustrine environment to produce interdigitated mass flow diamictons and grounding line fans/wedges. These depo-centres became glacitectonized and subglacially streamlined during glacier overriding and hence regional drumlin sedimentology reflects the varying degrees of inheritance of pre-existing glacigenic deposits and suggests that drumlin production relates more to the position of localized sediment accumulations at the glacier bed than full-depth till deformation processes (e.g. instability mechanisms) within the same drumlin field. Till cored drumlins give way down ice to stratified cored drumlins with till caps and then to stratified drumlins. This zonation is compatible with the increased lateral variability in drumlin composition that would arise from the occurrence of linear assemblages of glacifluvial (esker) and subaqueous (grounding line) sediments in an otherwise marginal-thickening till sheet
Controls upon the Last Glacial Maximum deglaciation of the northern Uummannaq Ice Stream System, West Greenland
The Uummannaq Ice Stream System (UISS) was a convergent cross-shelf ice stream system that operated in West Greenland during the Last Glacial Maximum (LGM). This paper presents new evidence constraining the geometry and evolution of the northern sector of the UISS and considers the factors controlling its dynamic behaviour. Geomorphological mapping, 21 new terrestrial cosmogenic nuclide (TCN) exposure ages, and radiocarbon dating constrain LGM warm-based ice stream activity in the north of the system up to 1400 m a.s.l. Intervening plateaux areas either remained ice free, or were covered by cold-based icefields. Beyond the inner fjords, topography and bathymetry forced ice flow southwards into the Uummannaq Trough, where it coalesced with ice from the south, and formed the trunk zone of the UISS. Deglaciation of the UISS began at 14.9 cal. ka BP. Rapid retreat from the LGM limit was forced by an increase in air temperatures and rising sea level, enhanced by the bathymetric over-deepening of the Uummannaq and Igdlorssuit Sund troughs. Ice reached the inner fjord confines in the northern Uummannaq area by 11.6 ka and experienced an ice marginal stabilisation in Rink–Karrat Fjord for up to 5 ka. This was a function of topographic constriction and bathymetric shallowing, and occurred despite continued climatic forcing. In the neighbouring Ingia Fjord this did not occur. Following this period of stability, ice within Rink–Karrat Fjord retreated, reaching the present ice margin or beyond after 5 ka. The presence of a major ice stream within a mid-fjord setting, during the mid-Holocene and the Holocene Thermal Maximum (∼11–5 ka) is in direct contrast to records of other ice streams throughout West Greenland, which suggest ice had retreated beyond its present margin by 9–7 ka. This demonstrates the potential importance of topographic control on calving margin stability, and its ability to override climatic forcing
Vertical transport and electroluminescence in InAs/GaSb/InAs structures: GaSb thickness and hydrostatic pressure studies
We have measured the current-voltage (I-V) of type II InAs/GaSb/InAs double
heterojunctions (DHETs) with 'GaAs like' interface bonding and GaSb thickness
between 0-1200 \AA. A negative differential resistance (NDR) is observed for
all DHETs with GaSb thickness 60 \AA below which a dramatic change in the
shape of the I-V and a marked hysteresis is observed. The temperature
dependence of the I-V is found to be very strong below this critical GaSb
thickness. The I-V characteristics of selected DHETs are also presented under
hydrostatic pressures up to 11 kbar. Finally, a mid infra-red
electroluminescence is observed at 1 bar with a threshold at the NDR valley
bias. The band profile calculations presented in the analysis are markedly
different to those given in the literature, and arise due to the positive
charge that it is argued will build up in the GaSb layer under bias. We
conclude that the dominant conduction mechanism in DHETs is most likely to
arise out of an inelastic electron-heavy-hole interaction similar to that
observed in single heterojunctions (SHETs) with 'GaAs like' interface bonding,
and not out of resonant electron-light-hole tunnelling as proposed by Yu et al.
A Zener tunnelling mechanism is shown to contribute to the background current
beyond NDR.Comment: 8 pages 12 fig
The geomorphological record of an ice stream to ice shelf transition in Northeast Greenland
ACKNOWLEDGEMENTS This work was funded through NERC Standard Grant NE/N011228/1. We thank the Alfred Wegner Institute, and particularly Angelika Humbert and Hicham Rafiq, for their logistic support through the iGRIFF project. Further support was provided from Station Nord (Jorgen Skafte), Nordland Air, Air Greenland, and the Joint Arctic Command. Naalakkersuisut, Government of Greenland, provided Scientific Survey (VU-00121) and Export (046/2017) licences for this work. We thank Chris Orton for help with production of figures. Finally, we would like to thank our Field Ranger Isak (after which Isakdalen is informally named) and dog Ooni for keeping us safe in the field. We thank Rob Storrar and an anonymous reviewer for their comments which helped improve the manuscript.Peer reviewedPublisher PD
HI in the Outskirts of Nearby Galaxies
The HI in disk galaxies frequently extends beyond the optical image, and can
trace the dark matter there. I briefly highlight the history of high spatial
resolution HI imaging, the contribution it made to the dark matter problem, and
the current tension between several dynamical methods to break the disk-halo
degeneracy. I then turn to the flaring problem, which could in principle probe
the shape of the dark halo. Instead, however, a lot of attention is now devoted
to understanding the role of gas accretion via galactic fountains. The current
cold dark matter theory has problems on galactic scales, such as
the core-cusp problem, which can be addressed with HI observations of dwarf
galaxies. For a similar range in rotation velocities, galaxies of type Sd have
thin disks, while those of type Im are much thicker. After a few comments on
modified Newtonian dynamics and on irregular galaxies, I close with statistics
on the HI extent of galaxies.Comment: 38 pages, 17 figures, invited review, book chapter in "Outskirts of
Galaxies", Eds. J. H. Knapen, J. C. Lee and A. Gil de Paz, Astrophysics and
Space Science Library, Springer, in pres
New AdS non Abelian black holes with superconducting horizons
We present arguments for the existence of higher dimensional asymptotically
AdS non Abelian black holes with a Ricci flat event horizon and analyze their
basic properties. Unlike higher dimensional black holes with a curved horizon,
of the usual Einstein-Yang-Mills system, these solutions have finite
mass-energy. Below some non-zero critical temperature, they are
thermodynamically preferred over the Abelian configurations.Comment: 11 pages, 4 figure
Brane inflation and the fine-tuning problem
Brane inflation can provide a promissing framework for solving the
fine-tuning problem in standard inflationary models. The aim of this paper is
to illustrate the mechanism by which this can be achieved. By considering the
supersymmetric two-stage inflation model it is shown that the initial
fine-tuning of the coupling parameter can be considerably relaxed. SubPlanckian
values of the inflaton during inflation can also be obtained.Comment: 04 pages (Revtex
A new analysis of the GJ581 extrasolar planetary system
We have done a new analysis of the available observations for the GJ581
exoplanetary system. Today this system is controversial due to choices that can
be done in the orbital determination. The main ones are the ocurrence of
aliases and the additional bodies - the planets f and g - announced in Vogt et
al. 2010. Any dynamical study of exoplanets requires the good knowledge of the
orbital elements and the investigations involving the planet g are particularly
interesting, since this body would lie in the Habitable Zone (HZ) of the star
GJ581. This region,for this system, is very attractive of the dynamical point
of view due to several resonances of two and three bodies present there. In
this work, we investigate the conditions under which the planet g may exist. We
stress the fact that the planet g is intimately related with the orbital
elements of the planet d; more precisely, we conclude that it is not possible
to disconnect its existence from the determination of the eccentricity of the
planet d. Concerning the planet f, we have found one solution with period
days, but we are judicious about any affirmation concernig this
body because its signal is in the threshold of detection and the high period is
in a spectral region where the ocorruence of aliases is very common. Besides,
we outline some dynamical features of the habitable zone with the dynamical map
and point out the role played by some resonances laying there.Comment: 12 pages, 9 figure
Verbal working memory and functional large-scale networks in schizophrenia
The aim of this study was to test whether bilinear and nonlinear effective connectivity (EC) measures of working memory fMRI data can differentiate between patients with schizophrenia (SZ) and healthy controls (HC). We applied bilinear and nonlinear Dynamic Causal Modeling (DCM) for the analysis of verbal working memory in 16 SZ and 21 HC. The connection strengths with nonlinear modulation between the dorsolateral prefrontal cortex (DLPFC) and the ventral tegmental area/substantia nigra (VTA/SN) were evaluated. We used Bayesian Model Selection at the group and family levels to compare the optimal bilinear and nonlinear models. Bayesian Model Averaging was used to assess the connection strengths with nonlinear modulation. The DCM analyses revealed that SZ and HC used different bilinear networks despite comparable behavioral performance. In addition, the connection strengths with nonlinear modulation between the DLPFC and the VTA/SN area showed differences between SZ and HC. The adoption of different functional networks in SZ and HC indicated neurobiological alterations underlying working memory performance, including different connection strengths with nonlinear modulation between the DLPFC and the VTA/SN area. These novel findings may increase our understanding of connectivity in working memory in schizophrenia
Meridional Circulation and Global Solar Oscillations
We investigate the influence of large-scale meridional circulation on solar
p-modes by quasi-degenerate perturbation theory, as proposed by
\cite{lavely92}. As an input flow we use various models of stationary
meridional circulation obeying the continuity equation. This flow perturbs the
eigenmodes of an equilibrium model of the Sun. We derive the signatures of the
meridional circulation in the frequency multiplets of solar p-modes. In most
cases the meridional circulation leads to negative average frequency shifts of
the multiplets. Further possible observable effects are briefly discussed.Comment: 14 pages, 5 figures, submittted to Solar Physics Topical Issue
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