Late Paleozoic Climatic Reconstruction of Western Argentina: Glacial Extent and Deglaciation of Southwestern Gondwana

Throughout its history Earth has experienced both icehouse and greenhouse conditions. Shifts and transitions from one end member to the other are driven by numerous driving mechanisms on global, orbital and more local scales. In particular, the late Paleozoic ice age (LPIA) is thought to have been driven by global drivers such as the drift of the Gondwanan continent across the South Pole, fluctuations in atmospheric CO2 concentrations, and Milankovitch cycles. It was also affected by more local and regional drivers such as active tectonism along accretionary margins and changes in atmospheric and oceanic circulation patterns. South American Gondwana provides an excellent opportunity to examine and evaluate the effects that global versus local driving mechanisms had on regional climates during the shift from icehouse to greenhouse conditions around the Carboniferous-Permian boundary. Of particular interest to this study are the margin and foreland basins of western Argentina in comparison to their paleolatitudinal counterparts of Brazil and eastern Argentina (i.e. the Chaco-Paraná and Paraná basins). This study focuses on determining the extent of glaciation during the Serpukhovian-Bashkirian of the Paganzo and Calingasta-Uspallata basins, the subsequent and relatively early deglaciation and shift in climate from humid conditions to extreme aridity, and the driving mechanisms for this change. This study tracks changes in facies, sediment dispersal, and climate indicators throughout the late Paleozoic strata in the Paganzo, Calingasta-Uspallata and Paraná basins, with special focus on the Paganzo Group strata. Here, we conclude that glaciation of the Paganzo and Calingasta-Uspallata basins was restricted to the Precordilleran region and nucleated on a significant uplift known as the Protoprecordillera and adjacent uplands. A paleoclimate reconstruction for the late Carboniferous using the Chemical Index of Alteration (CIA) indicates a shift from cold and arid to warm and humid following the deglaciation of the region, which is then succeeded by a drastic shift to an extremely arid environment. A provenance study using detrital zircon geochronology for selected units of the Paganzo Group strata indicates a restricted foreland basin setting in the early-middle Carboniferous that evolves and broadens through the Pennsylvanian and into the Permian as the active tectonic margin moves westward. With the accretion of a magmatic arc during the latest Carboniferous, the detrital zircon geochronology and the facies of the Paganzo Group record an enhancement, or an increase/expansion of the orographic effect originally created by the Protoprecordilleran range during the early-middle Carboniferous glaciation

Sedimentology and Paleoecology of Fossil-bearing, High-latitude Marine and Glacially Influenced Deposits in the Tepuel Basin, Patagonia, Argentina

The glacial and non-glacial intervals of the Late Paleozoic Ice Age (LPIA) are of great interest because they are our best deep time analogue for Pleistocene climate change. The changes and adaptations of the biota, as seen in the rock record, can serve as a proxy for understanding future trends in Earth\u27s climate system. Most of the known LPIA marine faunal data come from low-latitudinal regions, and thus have been used as a global proxy. However, modern organisms in the low-latitudes (far-field basins) respond differently to a changing climate relative to marine organisms in the polar regions (near-field basins). In high-paleolatitude regions, glacial and non-glacial communities were ecologically dissimilar and may have had a dissimilar response to climate change relative to contemporaneous fauna at low-paleolatitudes. It is important to understand the how different global climate regimes affected the adaptability of the fauna that lived within them. This study focuses on a high-latitude fauna from the Tepuel-Genoa Basin in Chubut Province in Patagonia, Argentina in order to better understand the responses of a high-latitude fauna to changing environmental conditions, and to develop a more robust understanding of climate change and its impacts on the biosphere. The Pampa de Tepuel Formation records Mississippian to Permian depositional history within the Tepuel Basin. Based on current age models for the basin, the analyzed section reported here occurs in the upper portion of the Lanipustula biozone, likely from the late Bashkirian to the early Moscovian. Field work consisted of counts of six fossil beds and a 276-meter stratigraphic section was measured and described in order to identify the lithofacies that indicate changes in depositional environments. The fossil count data was analyzed using various methods such as relative abundance comparisons, diversity indices and multivariate tests in order to determine and better define the paleoecology of the Lanipustula biozone and its fauna, which has not been accomplished to date. This particular section of the Pampa de Tepuel Formation in the Sierra de Tepuel is representative of at least two depositional sequences, with evidence of a late highstand and falling stage systems tract in the lower portion of the section followed by a transgressive systems tract at the top of the section. There is also the appearance of slump and slide blocks throughout the section and clinoforms in the middle of the section suggesting that clastics periodically made it into the deeper parts of the basin. Much of the strata described in this study can be related to normal marine processes acting on the outershelf and slope of the Tepuel Basin rather than having occurred in a glacial marine setting. There are only minor glacial signatures observed within this portion of the formation. However, other parts of the Pampa De Tepuel Formation did accumulate in a glaciomarine setting. The paleoecology data suggest that there may be a new faunal composition near the top of the section that does not fit into the Lanipustula biozone, although there is no statistically significant difference taxonomically based on the relative abundance values and diversity indices. Results of the multivariate analyses of the paleocommunities seem to reflect that changes in the local depositional and environmental settings may be the cause for any changes seen in the faunal assemblages within the previously established Lanipustula biozone. By continuing research on the LPIA, we may be better able to understand the fundamental factors of species and ecosystem instability because of the substantial environmental and climatic shifts that occurred

Pulsar bow-shock nebulae. II. Hydrodynamical simulation

We present hydrodynamical simulations, using a 2-D two component model (ambient medium and pul sar wind have different specific heat ratios), of bow shocks in a representative regime for pu lsar wind driven bow-shock nebulae. We also investigate the behaviour of a passive toroidal ma gnetic field wound around the pulsar velocity direction. Moreover we estimate the opacity of t he bow-shock to penetration of ISM neutral hydrogen: this quantity affects observable properti es of the nebula, like its size, shape, velocity and surface brightness distribution. Finally we compare these numerical results with those from an analytical model. The development of mor e realistic models is needed in order to tune the criteria for searches of new such objects, a s well as to interpret data on the known objects.Comment: 17 pages, Latex, 6 Encapsulated PostScript figures, accepted for publication in A&

ISO observations of the Galactic center Interstellar Medium: neutral gas and dust

The 500 central pc of the Galaxy (hereafter GC) exhibit a widespread gas component with a kinetic temperature of 100-200 K. The bulk of this gas is not associated to the well-known thermal radio continuum or far infrared sources like Sgr A or Sgr B. How this gas is heated has been a longstanding problem. With the aim of studying the thermal balance of the neutral gas and dust in the GC, we have observed 18 molecular clouds located at projected distances far from thermal continuum sources with the Infrared Space Observatory (ISO). In this paper we present observations of several fine structure lines and the full continuum spectra of the dust between 40 and 190 microns. A warm dust component with a temperature between 27 and 42 K is needed to fit the spectra. We have compared the gas and the dust emission with the predictions from J-type and C-type shocks and photodissociation region (PDRs) models. We conclude that the dust and the fine structure lines observations are best explained by a PDR with a density of 10$^3$ cm^-3 and an incident far-ultraviolet field 10$^3$ times higher than the local interstellar radiation field. PDRs can naturally explain the discrepancy between the gas and the dust temperatures. However, these PDRs can only account for 10-30% of the total H2 column density with a temperature of ~ 150 K. We discuss other possible heating mechanisms (short version).Comment: Accepted for publication by A&

The Ellipticity and Orientation of Clusters of Galaxies from N-Body Experiments

In this study we use simulations of 128$^3$ particles to study the ellipticity and orientation of clusters of galaxies in N-body simulations of differing power-law initial spectra (P(k) \propto k^n ,n = +1, 0, -1, -2$), and density parameters ($\Omega_0 = 0.2$to 1.0). Furthermore, unlike most theoretical studies we mimic most observers by removing all particles which lie at distances greater than 2 1/h Mpc from the cluster center of mass. We computed the axial ratio and the principal axes using the inertia tensor of each cluster. The mean ellipticity of clusters increases strongly with increasing$n$. We also find that clusters tend to become more spherical at smaller radii. We compared the orientation of a cluster to the orientation of neighboring clusters as a function of distance (correlation). In addition, we considered whether a cluster's major axis tends to lie along the line connecting it to a neighboring cluster, as a function of distance (alignment). Both alignments and correlations were computed in three dimensions and in projection to mimic observational surveys. Our results show that significant alignments exist for all spectra at small separations ($D < 15 h^{-1}$Mpc) but drops off at larger distance in a strongly$n-$dependent way.Comment: 22 pages, requires aaspp4.sty, flushrt.sty, and epsf.sty Revised manuscript, accepted for publication in Ap