125 research outputs found
Early Cambrian archaeocyathan limestone blocks in low-grade meta-conglomerate from El Jagüelito Formation (Sierra Grande, Río Negro, Argentina
Massive grey limestone blocks containing a fairly diverse but poorly preserved archaeocyath fauna were recovered from a meta-conglomerate bed in the El Jagüelito Formation (Sierra Grande area, Eastern North Patagonian Massif, Río Negro, Argentina). This is the first documented reference of the presence of archaeocyaths in continental Argentina. Seven different taxa were identified, preliminary described and figured. Recrystallization of the skeletons due to regional low-grade metamorphism and deformation of the unit does not allow observation of key detailed features and prevents identification to genera and species. Nevertheless, the specimens studied show general affinities with archaeocyathan assemblages from the Australia-Antarctica palaeobiogeographic province and indicate a middle Early Cambrian (Atdabanian-Botomian) maximum age for the deposition of the El Jagüelito Formation protoliths. The similarities between the North Patagonian Early Paleozoic El Jagüelito Formation and those rocks from Antarctica suggest a geologic and biologic common history of these regions on the same southwest margin of Gondwana during that time
Optimal atomic detection by control of detuning and spatial dependence of laser intensity
Atomic detection by fluorescence may fail because of reflection from the
laser or transmission without excitation. The detection probability for a given
velocity range may be improved by controlling the detuning and the spatial
dependence of the laser intensity. A simple optimization method is discussed
and exemplified
Bohmian transmission and reflection dwell times without trajectory sampling
Within the framework of Bohmian mechanics dwell times find a straightforward
formulation. The computation of associated probabilities and distributions
however needs the explicit knowledge of a relevant sample of trajectories and
therefore implies formidable numerical effort. Here a trajectory free
formulation for the average transmission and reflection dwell times within
static spatial intervals [a,b] is given for one-dimensional scattering
problems. This formulation reduces the computation time to less than 5% of the
computation time by means of trajectory sampling.Comment: 14 pages, 7 figures; v2: published version, significantly revised and
shortened (former sections 2 and 3 omitted, appendix A added, simplified
mathematics
Symbionts and diseases associated with invasive apple snails
This contribution summarizes knowledge of organisms associated with apple snails, mainly Pomacea spp., either in a facultative or obligate manner, paying special attention to diseases transmitted via these snails to humans. A wide spectrum of epibionts on the shell and operculum of snails are discussed. Among them algae, ciliates, rotifers, nematodes, flatworms, oligochaetes, dipterans, bryozoans and leeches are facultative, benefitting from the provision of substrate, transport, access to food and protection. Among obligate symbionts, five turbellarian species of the genus Temnocephala are known from the branchial cavity, with T. iheringi the most common and abundant. The leech Helobdella ampullariae also spends its entire life cycle inside the branchial cavity; two copepod species and one mite are found in different sites inside the snails. Details of the nature of the relationships of these specific obligate symbionts are poorly known. Also, extensive studies of an intracellular endosymbiosis are summarized. Apple snails are the first or second hosts of several digenean species, including some bird parasites. A number of human diseases are transmitted by apple snails, angiostrongyliasis being the most important because of the potential seriousness of the disease.Facultad de Ciencias Naturales y Muse
Symbionts and diseases associated with invasive apple snails
This contribution summarizes knowledge of organisms associated with apple snails, mainly Pomacea spp., either in a facultative or obligate manner, paying special attention to diseases transmitted via these snails to humans. A wide spectrum of epibionts on the shell and operculum of snails are discussed. Among them algae, ciliates, rotifers, nematodes, flatworms, oligochaetes, dipterans, bryozoans and leeches are facultative, benefitting from the provision of substrate, transport, access to food and protection. Among obligate symbionts, five turbellarian species of the genus Temnocephala are known from the branchial cavity, with T. iheringi the most common and abundant. The leech Helobdella ampullariae also spends its entire life cycle inside the branchial cavity; two copepod species and one mite are found in different sites inside the snails. Details of the nature of the relationships of these specific obligate symbionts are poorly known. Also, extensive studies of an intracellular endosymbiosis are summarized. Apple snails are the first or second hosts of several digenean species, including some bird parasites. A number of human diseases are transmitted by apple snails, angiostrongyliasis being the most important because of the potential seriousness of the disease.Facultad de Ciencias Naturales y Muse
Ultra-fast propagation of Schr\"odinger waves in absorbing media
We identify the characteristic times of the evolution of a quantum wave
generated by a point source with a sharp onset in an absorbing medium. The
"traversal'' or "B\"uttiker-Landauer'' time (which grows linearly with the
distance to the source) for the Hermitian, non-absorbing case is substituted by
three different characteristic quantities. One of them describes the arrival of
a maximum of the density calculated with respect to position, but the maximum
with respect to time for a given position becomes independent of the distance
to the source and is given by the particle's ``survival time'' in the medium.
This later effect, unlike the Hartman effect, occurs for injection frequencies
under or above the cut-off, and for arbitrarily large distances. A possible
physical realization is proposed by illuminating a two-level atom with a
detuned laser
Improvement by laser quenching of an "atom diode": a one-way barrier for ultra-cold atoms
Different laser devices working as ``atom diodes'' or ``one-way barriers''
for ultra-cold atoms have been proposed recently. They transmit ground state
level atoms coming from one side, say from the left, but reflect them when they
come from the other side. We combine a previous model, consisting of the
stimulated Raman adiabatic passage (STIRAP) from the ground to an excited state
and a state-selective mirror potential, with a localized quenching laser which
produces spontaneous decay back to the ground state. This avoids backwards
motion, provides more control of the decay process and therefore a more compact
and useful device.Comment: 6 page
Bohmian arrival time without trajectories
The computation of detection probabilities and arrival time distributions
within Bohmian mechanics in general needs the explicit knowledge of a relevant
sample of trajectories. Here it is shown how for one-dimensional systems and
rigid inertial detectors these quantities can be computed without calculating
any trajectories. An expression in terms of the wave function and its spatial
derivative, both restricted to the boundary of the detector's spacetime volume,
is derived for the general case, where the probability current at the
detector's boundary may vary its sign.Comment: 20 pages, 12 figures; v2: reference added, extended introduction,
published versio
Biofacies analysis of Hettangian-Sinemurian bivalve/brachiopod associations from the Neuquén Basin (Argentina)
Based upon extensive sampling with strict stratigraphical control on Hettangian and Sinemurian deposits cropping out at the river Atuel region (southwest Mendoza Province), the presence of all bivalve and brachiopod genera was recorded. Data gathered from the analysed interval were processed by multivariate analysis and the resulting Q-mode dendrogram was used to discriminate five associations (i.e. Entolium-limoids, Pholadomya-Pinna-Pleuromya, Weyla-Gryphaea, Lywea and Cardinioides). These are not in simple stratigraphical succession but are recurrent, reflecting palaeosynecological and biofacies influences. The palaeoenvironmental implications of these associations are analysed, as well as their geographical and stratigraphical distributions in the studied sections (Arroyo Malo, El Pedrero and Las Chilcas). The Cardinioides association is controlled by palaeosalinity and corresponds to a paralic/estuarine environment, of restricted geographical and stratigraphical distribution, whereas the remaining associations correspond to normal marine shelf to littoral environments. The Lywea association is interpreted as allochthonous and probably an impoverished subset derived from the diverse Weyla-Gryphaea association. On the basis of the palaeoautecological characteristics of the taxa involved in terms of guilds represented in these benthonic associations, it is concluded that most of the associations were dominated by the epifauna, whereas in the Pholadomya-Pinna-Pleuromya association the infauna and semi-infauna were particularly conspicuous and diverse, and the Cardinioides association was characterized by shallow infauna. Typical basinal biofacies from deeper, off-shore environments, and fresh-water biofacies, are both missing
Quantum optical time-of-arrival model in three dimensions
We investigate the three-dimensional formulation of a recently proposed
operational arrival-time model. It is shown that within typical conditions for
optical transitions the results of the simple one-dimensional version are
generally valid. Differences that may occur are consequences of Doppler and
momentum-transfer effects. Ways to minimize these are discussed.Comment: 14 pages, 5 figure
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