Geochronological and isotopical review of pre-Devonian crustal basement of the Colombian Andes

Under the flatlands east of the Andes, the crustal basement is exposed in a few places, composed mainly of the Mitú migmatitic complex and the Parguaza granite, whose ages range between 1.78 and 1.45 Ga. Extensive outcrops of high-grade metamorphic rocks are found in several places. Two metamorphisms are dated between 1.2–1.1 and 1.0–0.9 Ga. They are considered blocks that formed during the Grenville orogeny and have Sm–Nd T_DM model ages of 1.87–1.47. The Andaquí terrane is formed mainly by the Garzón Massif, composed of granulites, migmatites, and granites, and the metamorphic rocks of the Sierra de la Macarena, which are covered by undeformed Cambrian sediments. It is believed that after the Grenville orogeny, this unit remained attached to the Amazonic Craton. All the other areas grouped in the Chibcha terrane, though they formed during the Grenville orogeny, are believed to have remained either as part of another continental block or dispersed islands to be amalgamated to the Amazonic Craton during the Lower Paleozoic orogeny, which in the Quetame Massif is dated between the Silurian and Devonian and is named the Quetame orogenic event.     

Correction to: Microphysics and Optical Attenuation in Fog: Observations from Two Coastal Sites

Boundary-Layer Meteorology -     

Analysis of natural and beneficiated ferruginous bauxites by both X-ray diffraction and X-ray fluorescence

The quantitative determination of the main compounds Al (OH)₃ (gibbsite) as Al₂O₃ and hydrated Fe-oxides as Fe₂O₃ in natural and beneficiated ferruginous bauxites (low-grade bauxites) by X-ray diffraction (XRD) is presented employing the external standard method. The calibration parameters for the analysis by XRD were ascertained by using ten synthetic mixtures of the pure oxides Al₂O₃ and Fe₂O₃ which were calcined at 1200°C. The concentration ranges for Fe₂O₃ were 10–70 wt.% and those for alumina 30–90 wt.%. The standard deviation for the calibrations of the two oxides was 3.33 wt.%. The results obtained by XRD for both oxides were compared with those attained by X-ray fluorescence. The precision of the determinations by XRD was ascertained by analysing a natural sample consecutively 5 times. The accuracy was verified by the analysis of two international standard reference materials with low silica contents.     

Seed magnetic Fields Generated by Primordial Supernova Explosions

The origin of the magnetic field in galaxies is an open question in astrophysics. Several mechanisms have been proposed related, in general, to the generation of small seed fields amplified by a dynamo mechanism. In general, these mechanisms have difficulty in satisfying both the requirements of a sufficiently high strength for the magnetic field and the necessary large coherent scales. We show that the formation of dense and turbulent shells of matter, in the multiple explosion scenario of Miranda &38; Opher for the formation of the large-scale structures of the Universe, can naturally act as a seed for the generation of a magnetic field. During the collapse and explosion of Population III objects, a temperature gradient not parallel to a density gradient can naturally be established, producing a seed magnetic field through the Biermann battery mechanism. We show that seed magnetic fields ∼ 10⁻¹²–10⁻¹⁴ G can be produced in this multiple explosion scenario on scales of the order of clusters of galaxies (with coherence length L  ∼ 1.8 Mpc) and up to ∼ 4.5 × 10⁻¹⁰ G on scales of galaxies ( L  ∼ 100 kpc).     

Study of magnetic contrasts applied to hydrocarbon exploration in the Maturín Sub-Basin (eastern Venezuela)

We have studied near-surface magnetic contrasts in nine oil wells from an oil prospective area in eastern Venezuela (compressive deformation front of the northeastern Maturín Sub-Basin). Samples are drill cuttings taken at intervals of about 15 m within approximately the first 1200 m of most of these wells. Rock magnetic experiments, electron paramagnetic resonance (EPR), extractable organic matter (EOM), X-ray diffraction and scanning electron microscopy (SEM) are used to discriminate between two anomalies in magnetic susceptibility (MS), related to either a reducing environment caused by the underlying reservoir (Type A), or to primary lithological contrasts (Type B). Contour maps of MS, S-ratio and organic-matter-free radical concentrations (OMFRC), for the A-like depth levels, show a major central zone of anomalous high values. This area is probably associated with either maximum accumulation of hydrocarbons in the reservoir or with their southerly migration from the northern petroleum source towards the deformation front. We argue that such a result could be used, for future exploration and production ventures in the region, as a preliminary characterization of the reservoir. We also show in a Day plot, that saturation remanence — saturation magnetization ratio (M_rs/M_s) and coercivity of remanence — coercive force ratio (H_cr/H_c) stand as additional criteria to discriminate between these two types of magnetic contrasts. In fact, for the area of study, hysteresis ratios show three distinct trends corresponding to samples from A-like, B-like and non-anomalous depth levels.     

The accountability imperative for quantifying the uncertainty of emission forecasts: evidence from Mexico

Governmental climate change mitigation targets are typically developed with the aid of forecasts of greenhouse-gas (GHG) emissions. The robustness and credibility of such forecasts depends, among other issues, on the extent to which forecasting approaches can reflect prevailing uncertainties. We apply a transparent and replicable method to quantify the uncertainty associated with projections of gross domestic product growth rates for Mexico, a key driver of GHG emissions in the country. We use those projections to produce probabilistic forecasts of GHG emissions for Mexico. We contrast our probabilistic forecasts with Mexico’s governmental deterministic forecasts. We show that, because they fail to reflect such key uncertainty, deterministic forecasts are ill-suited for use in target-setting processes. We argue that (i) guidelines should be agreed upon, to ensure that governmental forecasts meet certain minimum transparency and quality standards, and (ii) governments should be held accountable for the appropriateness of the forecasting approach applied to prepare governmental forecasts, especially when those forecasts are used to derive climate change mitigation targets.

POLICY INSIGHTS

• No minimum transparency and quality standards exist to guide the development of GHG emission scenario forecasts, not even when these forecasts are used to set national climate change mitigation targets.

• No accountability mechanisms appear to be in place at the national level to ensure that national governments rely on scientifically sound processes to develop GHG emission scenarios.

• Using probabilistic forecasts to underpin emission reduction targets represents a scientifically sound option for reflecting in the target the uncertainty to which those forecasts are subject, thus increasing the validity of the target.

• Setting up minimum transparency and quality standards, and holding governments accountable for their choice of forecasting methods could lead to more robust emission reduction targets nationally and, by extension, internationally.

     

Gravitational wave background from Population III black hole formation

We study the generation of a stochastic gravitational wave (GW) background produced from a population of core-collapse supernovae, which form black holes in scenarios of structure formation. We obtain, for example, that the formation of a population (Population III) of black holes, in cold dark matter scenarios, could generate a stochastic GW background with a maximum amplitude of   h _BG≃10⁻²⁴  and corresponding closure energy density of  Ω_GW∼10⁻⁷  , in the frequency band   ν _obs≃30–470 Hz  (assuming a maximum efficiency of generation of GWs, namely,   ɛ _GWmax=7×10⁻⁴)  for stars forming at redshifts   z ≃30–10  . We show that it will be possible in the future to detect this isotropic GW background by correlating the signals of a pair of 'advanced' LIGO observatories (LIGO III) at a signal-to-noise ratio of ≃40. We discuss what astrophysical information could be obtained from a positive (or even a negative) detection of such a GW background generated in scenarios such as those studied here. One of them is the possibility of obtaining the initial and final redshifts of the emission period from the observed spectrum of GWs.     

Accretion history of active black holes from type 1 AGN

Almost all galaxies have massive central black holes in their centers with masses typically ranging from ～10⁵ to ～10⁹ M_⊙. However, the origin and evolution of these objects and their connection with the hosting galaxies are not completely understood yet. In this work we analyze the mass accretion rate of supermassive black holes (SMBH’s) and the mean Eddington ratio (MER) of type 1 AGN using data from the Sloan Sky Survey. For this purpose we improve the method for constructing the subsample of SMBH, taking into account the survey flux limit and the bias of the sample. It was observed that the mean bolometric luminosity of the active black holes can be represented by a function composed by a power law in mass and a like-Schechter function in redshift. Our results also show that both the mean Eddington ratio and the mass accretion rate are proportional to this function.