Las costras calcáreas del Pleistoceno en el registro de cambios de polaridad magnética de la tierra

La presente contribución se refiere a los registros paleomagnéticos obtenidos en costras calcáreas y en sedimentos loessoides de la localidad de Camet, al norte de Mar del Plata. Los resultados obtenidos confirman la existencia del límite Brunhes/Matuyama (0,78 Ma) en los acantilados de la zona. Las muestras de las unidades U1 y U2 en la base del perfil presentan polaridad reversa y se asignan al cron de magnetopolaridad Matuyama (>0,78 Ma), las muestras de las unidades U3 a U7 son de polaridad normal y se asignan a Brunhes (>0,78 Ma). El patrón de susceptibilidad magnética es similar al obtenido en sedimentos loéssicos del norte de la provincia de Buenos Aires. Los sedimentos menos pedogenizados presentan valores relativamente más elevados (390 x 10⁻⁸ m³kg), los paleosuelos valores intermedios (70x10⁻⁸ m³kg ) y los horizontes hidromórficos (17x10⁻⁸ m³kg) los valores más bajos. El valor de susceptibilidad dependiente de frecuencia, que se utiliza para estimar la contribución superparamagnética, es bajo a lo largo de todo el perfil, aunque se incrementa en los horizontes pedogenizados. Estimamos que la presente contribución puede ser de utilidad a la paleontología y particularmente a la bioestratigrafía debido a la existencia de fauna de vertebrados fósiles.The present contribution refers to one sedimentary sequence exposed at Camet (37º 53´34´´ S. Lat. and 57º 31´16´´ W. Long.), north of Mar del Plata. Paleomagnetic directions as well as magnetic parameters were measured on samples collected from seven units separated by discontinuities. The obtained results confirm the existence of the BBM in sediments and calcrete layers assigned to the Ensenadean at the base of the cliffs. The samples from the units labelled U1 and U2 at the base of the profile show reverse polarity while the samples collected from units U3 to U7 show normal polarity. The pattern of magnetic susceptibility follows that of the north of the Buenos Aires province. The less pedogenized sediments show the higher LF susceptibility (390x10⁻⁸ m³kg) while the paleosols (70x10⁻⁸ m³kg) and gley (17x10⁻⁸ m³kg ) horizons show the lower values Frequency dependent susceptibility (χfd) employed, to estimate the superparamagnetic contribution, is low along the profile but it increases in the pedogenized horizons and decreases markedly in loess. Due to the findings of mammal fossils in the Pleistocene cliffs of Camet, the present contribution may probably be of value for paleontological and particularly for the biostratigraphical research

Magnetoestratigrafía en sedimentos del cenozoico tardío de la localidad de Villa Cacique, provincia de Buenos Aires

La presente contribución se refiere a una sucesión sedimentaria del Cenozoico tardío expuesta en la localidad de Villa Cacique (37° 41’ 19.70” S y 59° 21’ 46.78” W). Las muestras paleomagnéticas se obtuvieron de una sección de 9.10 m de potencia integrada por las formaciones Barker y Las Animas. Una interpretación preliminar permite suponer que las muestras extraídas de la formación Las Animas pertenecen al cron Brunhes (< 0.78 Ma) y que las muestras extraídas de la formación Barker pertenecen al cron Matuyama (0.78 – 2.588 Ma).The present contribution refers to a Late Cenozoic sedimentary sequence exposed in the locality of Villa Cacique (37° 41’ 19.70” S. y 59° 21’ 46.78” W.). Paleomagnetic samples were collected from a section of 9.10 m that was assigned to the Barker and Las Animas formations. A preliminary interpretation of results allows us to consider that the samples collected from Las Animas formation correspond to the Brunhes chron (< 0.78 Ma) while the samples of the Barker formation correspond to the Matuyama chron (0.78 - 2.588 Ma).Fil: Gómez Samus, Mauro Leandro. Provincia de Buenos Aires. Gobernacion. Comision de Investigaciones Cientificas. Laboratorio de Entrenamiento Multidisciplinario para la Investigación Tecnológica; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; ArgentinaFil: Bidegain, Juan Carlos. Provincia de Buenos Aires. Gobernacion. Comision de Investigaciones Cientificas. Laboratorio de Entrenamiento Multidisciplinario para la Investigación Tecnológica; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - la Plata; Argentin

Nova Sco 2001 (V1178 Sco)

We present intermediate resolution spectroscopy and near infrared photometry of NOVA Sco 2001 (V1178 Sco), which was first detected May 13th 2001 and reported June 21th 2001, and obtained by us the same day. We also retrieved very accurate astrometry of the target in this very crowded field. This is needed to be able to do follow up observations of the postnova during the next years. The spectrum shows an overall expansion of 2100 km/s and has clearly complex, and most likely nonsymmetric, outflow substructures. We clearly identify this object as classical nova, "Fe II" subclass.Comment: 4 pages, TeX, accepted for Publication in A&A (Letter

Magnetostratigraphy in the Vela and Barker Formations, Tandil, Buenos Aires Province, Argentina

La presente contribución se refiere a una sucesión sedimentaria del Cenozoico tardío expuesta en la localidad de María Ignacia, Argentina. Las muestras paleomagnéticas se obtuvieron de una sección asignada principalmente a las Formaciones Vela y Barker. El análisis de los datos paleomagnéticos indica que la Formación Vela es de polaridad normal, y se atribuye, con seguridad, al Cron Brunhes (< 0,781Ma). La porción expuesta de la Formación Barker es, en cambio, de polaridad inversa y puede ser asignada al Cron Gilbert (3,596 – 6,033 Ma). Los estudios magnetoestratigráficos constituyen una vía apropiada para conocer la edad de la Formación Barker ya que se carece de dataciones radiométricas y de registros bioestratigráficos hasta el presente. Los parámetros de magnetismo de rocas indican diferencias entre las unidades estudiadas. Se estudiarán exhaustivamente en futuras contribuciones con el fin de poder realizar interpretaciones de las condiciones paleo-ambientales en el lapso de tiempo considerado.The present contribution refers to paleomagnetic studies carried out in a late Cenozoic sedimentary sequence exposed in the locality of María Ignacia, Argentina. Paleomagnetic samples were collected from a section assigned mainly to the Vela Formation (the youngest) and the Barker Formation (the oldest). The analysis of Paleomagnetic data indicates that the Vela Formation presents samples of normal polarity while the Barker Formation presents samples of reverse polarity. A preliminary interpretation allows us to assign the samples of normal polarity to the Brunhes Normal Polarity Chron (< 0.781 Ma) while the reverse polarity samples might be assigned to the Gilbert Reverse Polarity Chron (3.596 – 6.033 Ma). Magnetostratigraphic studies provide a suitable way for estimate the age of the Barker Formation, since there are a lack of radiometric datable materials and fossils remains up to now. The rock magnetic parameters indicate difference between geological units; such behavior will be studied carefully in future contributions for estimating the paleoenvironmental conditions during the considered time span.Fil: Gómez Samus, Mauro Leandro. Consejo Nacional de Investigaciones Científicas y Técnicas; ArgentinaFil: Bidegain, Juan Carlos. Provincia de Buenos Aires. Gobernación. Comisión de Investigaciones Científicas; Argentin

The outskirts of globular clusters as modified gravity probes

In the context of theories of gravity modified to account for the observed dynamics of galactic systems without the need to invoke the existence of dark matter, a prediction often appears regarding low acceleration systems: wherever $a$ falls below $a_{0}$ one should expect a transition from the classical to the modified gravity regime.This modified gravity regime will be characterised by equilibrium velocities which become independent of distance, and which scale with the fourth root of the total baryonic mass, $V^{4} \propto M$. The two above conditions are the well known flat rotation curves and Tully-Fisher relations of the galactic regime. Recently however, a similar phenomenology has been hinted at, at the outskirts of Galactic globular clusters, precisely in the region where $a<a_{0}$. Radial profiles of the projected velocity dispersion have been observed to stop decreasing along Keplerian expectations, and to level off at constant values beyond the radii where $a<a_{0}$. We have constructed gravitational equilibrium dynamical models for a number of globular clusters for which the above gravitational anomaly has been reported, using a modified Newtonian force law which yields equilibrium velocities equivalent to MOND. We find models can be easily constructed having an inner Newtonian region and an outer modified gravity regime, which reproduce all observational constraints, surface brightness profiles, total masses and line of sight velocity dispersion profiles. Through the use of detailed single stellar population models tuned individually to each of the globular clusters in question, we derive estimates of the total masses for these systems. Interestingly, we find that the asymptotic values of the velocity dispersion profiles are consistent with scaling with the fourth root of the total masses, as expected under modified gravity scenarios.Comment: Accepted in ApJ, 13 pages, 7 figure