Climate and CO2 saturation in an alpine lake throughout the Holocene

This study shows that diatom sediment records can be used to investigate the long-term inorganic carbon dynamics in oligotrophic and poorly acid-buffered lakes. Using a training set of 115 high-mountain lakes in the Pyrenees, we found that both alkalinity and potential hydrogen (pH) independently explained some of the variability in diatom assemblages. Transfer functions for both variables were developed and applied to a Holocene record from Lake Redon and CO2 changes calculated. CO2 saturation broadly followed alkalinity, which in turn was related to summer and autumn air-temperature fluctuations. In general, warmer climate during the ice-free period led to higher supersaturation, due to increased alkalinity, which facilitated retention of CO2 from respiration, and decreased primary production (assessed by diatom fluxes). Only during the early Holocene, there were periods of extreme undersaturation, corresponding to cold periods of low alkalinity (<20 microequivalents per liter [meq L21]), and suggesting carbon limitation of primary production. The winter and spring climate, which determines the ice cover duration, appears to be relevant for CO2 saturation only during periods when the organic-matter content of the sediments was low (,22%). Longer periods of ice cover led to lower lake CO2 saturation, suggesting that the ice cover influence on internal nutrient loading may regulate lake productivity fluctuations under low allocthonous nutrient and organic-matter inputs. Alkalinity ,20 meq L21 and sediment organic matter ,22% appear as critical thresholds in the way lake CO2 levels respond to climate fluctuations.Peer ReviewedPostprint (published version

An agent-based simulator applied to teaching-learning process to predict sociometric indices in higher education

Most novice teachers and even some experienced teachers can lack appropriate tools for designing teaching strategies that ensure the quality of education. The ability of working in teams is crucial in educating professionals. The literature proves that social relations influence the performance of teams. For instance, the team cohesion is directly related with its performance. In the current work, we have developed an agent-based tool for assisting teachers in simulating their teaching strategies to estimate their influence on the group sociometrics like cohesion, coherence of reciprocal relations, dissociation and density of relations. The experiments with nine scenarios in disciplines of computer science, electronic, psychology, business, tourism and renewal energies with 239 students and six teachers including experienced and novice ones show that its underlying agent-based framework can adapt to different disciplines obtaining similar outcomes to the real ones. We learned that the tool was especially reliable in predicting the density of relations and the cohesion, being the latter one probably the most relevant due to its known relation with academic performance. In addition, we also learned that it was difficult to assess the prediction quality of the dissociation in higher education, due to the usual low amounts or absence of reciprocal rejections in the students' groups in this educational stage. The presented agent-based tool is publicly distributed as open source for facilitating other researchers in following this research line

Axions and the luminosity function of white dwarfs : the thin and thick discs, and the halo

The evolution of white dwarfs is a simple gravothermal process of cooling. Since the shape of their luminosity function is sensitive to the characteristic cooling time, it is possible to use its slope to test the existence of additional sources or sinks of energy, such as those predicted by alternative physical theories. The aim of this paper is to study if the changes in the slope of the white dwarf luminosity function around bolometric magnitudes ranging from 8 to 10 and previously attributed to axion emission are, effectively, a consequence of the existence of axions and not an artefact introduced by the star formation rate. We compute theoretical luminosity functions of the thin and thick disc, and of the stellar halo including axion emission and we compare them with the existing observed luminosity functions. Since these stellar populations have different star formation histories, the slope change should be present in all of them at the same place if it is due to axions or any other intrinsic cooling mechanism. The signature of an unexpected cooling seems to be present in the luminosity functions of the thin and thick discs, as well as in the halo luminosity function. This additional cooling is compatible with axion emission, thus supporting to the idea that DFSZ axions, with a mass in the range of 4–10 meV, could exist. If this were the case, these axions could be detected by the future solar axioscope IAXO.Peer ReviewedPostprint (author's final draft

The initial–final mass relationship of white dwarfs in common proper motion pairs and open clusters

We have studied white dwarfs in common proper motion pairs (CPMPs) to improve the semi-empirical initial–final mass relationship of white dwarfs. In this contribution, we re- port new results obtained from spectroscopic observations of both members of several CPMPs composed of a F, G or K type star and a DA white dwarf.Peer Reviewe

A white dwarf cooling age of 8 Gyr for NGC 6791 from physical separation processes

NGC 6791 is a well studied open cluster1 that it is so close to us that can be imaged down to very faint luminosities. The main sequence turn-off age (~8 Gyr) and the age derived from the termination of the white dwarf cooling sequence (~6 Gyr) are significantly different. One possible explanation is that as white dwarfs cool, one of the ashes of helium burning, 22Ne, sinks in the deep interior of these stars. At lower temperatures, white dwarfs are expected to crystallise and phase separation of the main constituents of the core of a typical white dwarf, 12C and 16O, is expected to occur. This sequence of events is expected to introduce significant delays in the cooling times, but has not hitherto been proven. Here we report that, as theoretically anticipated, physical separation processes occur in the cores of white dwarfs, solving the age discrepancy for NGC 6791.Comment: 3 pages, 2 figures, published in Natur

Mutations in the voltage-gated sodium channel gene associated with deltamethrin resistance in commercially sourced Phytoseiulus persimilis

The implementation of Integrated Pest Management (IPM) in current agricultural practice is a convenient and very effective strategy to maintain pest populations under control. The use of biological control agents, like Phytoseiulus persimilis, is key for the success of such an approach. This predatory mite is widely used since it is very effective for controlling Tetranychus urticae, one of the most devastating crop pests. Here we identify several mutations located in the Voltage Gated Sodium Channel (VGSC) of commercially sourced P. persimilis that correlate with a reduced susceptibility to the pyrethroid deltamethrin. We found that the mites sourced from two different biocontrol product companies have intrinsic genotypic differences that correlate with their phenotype when tested with different concentrations of deltamethrin. Mites from Syngenta Bioline, carrying the mutations M918L and A1536T, were able to survive deltamethrin concentrations of up to 10 ppm, while the mites from Koppert Biological Systems, with the combination M918L, L925V and S1539T, survived treatment with 40 ppm. All of the point mutations identified in the predatory mite samples are located in a particular region of the VGSC, previously proposed as the binding site for this family of pesticides and identified as a 'hot spot' for resistance

The white dwarf cooling sequence of NGC 6791: a unique tool for stellar evolution

NGC 6791 is a well-studied, metal-rich open cluster that is so close to us that can be imaged down to luminosities fainter than that of the termination of its white dwarf cooling sequence, thus allowing for an in-depth study of its white dwarf population. We use a Monte Carlo simulator that employs up-to-date evolutionary cooling sequences for white dwarfs with hydrogen-rich and hydrogen-deficient atmospheres, with carbon-oxygen and helium cores. The cooling sequences for carbon-oxygen cores account for the delays introduced by both Ne^22 sedimentation in the liquid phase and by carbon-oxygen phase separation upon crystallization. We do not find evidence for a substantial fraction of helium-core white dwarfs, and hence our results support the suggestion that the origin of the bright peak of the white dwarf luminosity function can only be attributed to a population of unresolved binary white dwarfs. Moreover, our results indicate that the number distribution of secondary masses of the population of unresolved binaries has to increase with increasing mass ratio between the secondary and primary components of the progenitor system. We also find that the observed cooling sequence appears to be able to constrain the presence of progenitor sub-populations with different chemical compositions and the fraction of non-DA white dwarfs. Our simulations place interesting constraints on important characteristics of the stellar populations of NGC 6791. In particular, we find that the fraction of single helium-core white dwarfs must be smaller than 5%, that a sub-population of stars with zero metallicity must be <12%, while if the adopted metallicity of the sub-population is solar the upper limit is ~8%. Finally, we also find that the fraction of non-DA white dwarfs in this particular cluster is surprinsingly small <6%.Comment: 9 pages, 14 figures, accepted for publication in Astronomy & Astrophysic