Thermal dissociation of dipositronium: path integral Monte Carlo approach

Path integral Monte Carlo simulation of the dipositronium "molecule" Ps$_2$ reveals its surprising thermal instability. Although, the binding energy is $\sim 0.4$ eV, due to the strong temperature dependence of its free energy Ps$_2$ dissociates, or does not form, above $\sim 1000$ K, except for high densities where a small fraction of molecules are in equilibrium with Ps atoms. This prediction is consistent with the recently reported first observation of stable Ps$_2$ molecules by Cassidy & Mills Jr., Nature {\bf 449}, 195 (07), and Phys.Rev.Lett. {\bf 100}, 013401 (08); at temperatures below 1000 K. The relatively sharp transition from molecular to atomic equilibrium, that we find, remains to be experimentally verified. To shed light on the origin of the large entropy factor in free energy we analyze the nature of interatomic interactions of these strongly correlated quantum particles. The conventional diatomic potential curve is given by the van der Waals interaction at large distances, but due to the correlations and high delocalization of constituent particles the concept of potential curve becomes ambiguous at short atomic distances.Comment: Submitted to the Physical Review Letter

Finite temperature quantum statistics of H3+_3^+ molecular ion

Full quantum statistical $NVT$ simulation of the five-particle system H$_3^+$ has been carried out using the path integral Monte Carlo method. Structure and energetics is evaluated as a function of temperature up to the thermal dissociation limit. The weakly density dependent dissociation temperature is found to be around $4000$ K. Contributions from the quantum dynamics and thermal motion are sorted out by comparing differences between simulations with quantum and classical nuclei. The essential role of the quantum description of the protons is established.Comment: submitted to the Journal of Chemical Physic

Few-body reference data for multicomponent formalisms: Light nuclei molecules

We present full quantum statistical energetics of some electron-light nuclei systems. This is accomplished with the path integral Monte Carlo method. The effects on energetics arising from the change in the nuclear mass are studied. The obtained results may serve as reference data for the multicomponent density functional theory calculations of light nuclei system. In addition, the results reported here will enable better fitting of todays electron-nuclear energy functionals, for which the description of light nuclei is most challenging, in particular

A Numerical Study on Temperature Distribution of Line Heated Anisotropic Carbon Fiber Composites

Earlier we have described the various uses of infrared line scanner based thermal nondestructive testing equipment [1]. Time constants of measurements made with these kind of equipment are very suitable for testing carbon fiber composites. Scanning a line heat source over a sample surface causes a nonuniform temperature distribution in the sample. In addition to the heat flow normal to the surface, lateral heat flow exists in the surface plane. In the case of carbon fiber composites with a specific oriented structure, the surface temperature distributions depend on the direction where the line source moves. Generally, this is true of any sample having anisotropic thermal conductivity. In oriented carbon fiber composites the bulk thermal conductivity can be considered anisotropic, because the heat transfer in the composite is different in the direction of the fibers compared to perpendicular directions [2,3]. Varis et al. have discussed these phenomenon briefly with the testing of carbon fiber tubes using numerical methods [4]. Here, we represent a more detailed numerical analysis of the effects of line heating on a sample having anisotropic thermal conductivity

Tracking the Limnoecological History of Lake Hiidenvesi (Southern Finland) Using the Paleolimnological Approach

Correction: Water, Air and Soil Pollution, 230 :9 (2019), art. 228 DOI: 10.1007/s11270-019-4262-2We examined a sediment record from Lake Hiidenvesi in southern Finland using paleolimnological methods to trace its limnoecological history. In our record, beginning from the 1940s, chironomid (Diptera) assemblages shifted from typical boreal taxa towards mesotrophic community assemblages at similar to 1960-1980 CE being finally replaced by eutrophic taxa from the 1990s onward. The diatom (Bacillariophyceae) assemblages reflected relatively nutrient rich conditions throughout the record showing a further increase in eutrophic taxa from the 1970s onward. A chironomid-based reconstruction of late-winter hypolimnetic dissolved oxygen (DO) conditions suggested anoxic conditions already in the 1950s, probably reflecting increased inlake production due to allochthonous nutrient inputs and related increase in biological oxygen consumption. However, the reconstruction also indicated large variability in long-term oxygen conditions that appear typical for the basin. With regard to nutrient status, chironomid- and diatom-based reconstructions of total phosphorus (TP) showed a similar trend throughout the record, although, chironomids indicated a more straightforward eutrophication process in the benthic habitat and seemed to reflect the intensified human activities in the catchment more strongly than diatoms. The DO and TP reconstructions were mostly similar in trends compared to the measured data available since the 1970s/1980s. However, the increase in TP during the most recent years in both reconstructions was not visible in the monitored data. The results of our multiproxy study emphasize the significance of including both epilimnetic and hypolimnetic systems in water quality assessments and provide important long-term limnoecological information that will be useful in the future when setting targets for restoration.Peer reviewe

Recent changes in chironomid communities and hypolimnetic oxygen conditions relate to organic carbon in subarctic ecotonal lakes

A key question in aquatic elemental cycling is related to the influence of bottom water oxygen conditions in regulating the burial and release of carbon under climate warming. In this study, we used head capsules of Chironomidae larvae to assess community and diversity change between the past (estimated as Pre-Industrial Period) and present and to reconstruct changes in hypolimnetic oxygen conditions from 30 subarctic ecotonal lakes (northeastern Lapland) using the top-bottom paleolimnological approach applying surface sediment (topmost 0-2 cm) and reference (4-5 cm) samples. Subsequently, we tested the findings against dissolved organic carbon (DOC) concentration of the sites. We found that the benthic communities were statistically dissimilar between the past and the present with largest changes occurring in the more transparent oligo-mesohumic lakes. However, murky polyhumic lakes displayed uniformly a decrease in diversity. The chironomid-inferred oxygen values showed a general decrease toward the present with largest shifts in low-DOC lakes, whereas no significant changes were found in the hypolimnetic oxygen conditions of high-DOC lakes, which were often located in wet-land areas. These finding suggest that lakes associated with constant organic carbon inputs are more resilient toward climate-induced reductions in hypolimnetic oxygen. (c) 2018 Elsevier B.V. All rights reserved.Peer reviewe

Cladoceran (Crustacea) Niches, Sex, and Sun Bathing—A Long-Term Record of Tundra Lake (Lapland) Functioning and Paleo-Optics

Under fundamental ecosystem changes in high latitude lakes, a functional paleolimnological approach may increase holistic understanding of lake responses and resilience to climate warming. A ~2000-year sediment record from Lake Loažžejávri in the tundra of northern Finnish Lapland was examined for fossil Cladocera assemblages to examine long-term environmental controls on aquatic communities. In addition, cladoceran functional attributes, including functional diversity (FD), UV absorbance (ABSUV) of Alona carapaces, and sexual reproduction (ephippia) in Bosmina and Chydoridae were analyzed. Cladoceran communities responded to a major change in benthic habitat quality, reflected as elevated (increasingly benthic) sediment organic matter δ13C signal since the 17th century. FD fluctuations showed association with climate oscillation, FD being generally higher during warm climate periods. These ecological changes were likely attributable to diversification of littoral-benthic consumer habitat space. ABSUV, irrespective of increases during the Little Ice Age (LIA) due to higher UV transparency of lake water, was lower under increasing autochthony (benthic production) suggesting establishment of physical UV refugia by the benthic vegetative substrata. Bosmina ephippia exhibited a decreasing trend associated with increasing benthic production, indicating favorable environmental regime, and, together with chydorid ephippia, transient increases during the climate cooling of the LIA driven by shorter open-water season

Late-Holocene variability in chironomid functional assemblages and carbon utilization in a tundra lake food web

High latitude freshwater systems are facing changes in catchment-mediated allochthonous input, as well as physical and chemical controls triggered by on-going climate change, which may alter their carbon processing and ecological characteristics. To explore changes in chironomid functional responses and carbon utilization in relation to longterm environmental change, we studied a sediment core covering ca. 2000 years from a tundra lake in northern Finland, which was analysed for sediment geochemistry, isotopic composition of chironomid remains and their functional assemblages. We aimed to relate changes in chironomid functional feeding assemblages and resource utilization, based on Bayesian stable isotope modelling, and determined that the long-term resource utilization was more controlled by sediment geochemistry (resource availability) and climatic variables, reflecting changes in habitat and lake ontogeny, rather than the functional feeding assemblage composition. Change horizons were observed for both sediment geochemistry and functional assemblage composition. However, different timing of these changes suggests different drivers affecting the dynamics of primary production and chironomid community functionality. We also compared the recent warming period to Medieval Climate Anomaly (MCA), observing divergent patterns, which suggests that MCA may not be a good analogue for changes induced by on-going climate warming.Peer reviewe

Air temperature and water level inferences from northeastern Lapland (69 degrees N) since the Little Ice Age

Independent Arctic records of temperature and precipitation from the same proxy archives are rare. Nevertheless, they are important for providing detailed information on long-term climate changes and temperature-precipitation relationships in the context of large-scale atmospheric dynamics. Here, we used chironomid and cladoceran fossil assemblages to reconstruct summer air-temperature and water-level changes, during the past 400 years, in a small lake located in Finnish Lapland. Temperatures remained persistently cold over the Little Ice Age (LIA), but increased in the 20th century. After a cooler phase in the 1970s, the climate rapidly warmed to the record-high temperatures of the most recent decades. The lake-level reconstruction suggested persistently wet conditions for the LIA, followed by a dry period between similar to 1910 and 1970 CE, when the lake apparently became almost dry. Since the 1980s, the lake level has returned to a similar position as during the IAA. The temperature development was consistent with earlier records, but a significant local feature was found in the lake-level reconstruction the LIA appears to have been continuously wet, without the generally depicted dry phase during the 18th and 19th centuries. Therefore, the results suggest local precipitation patterns and enforce the concept of spatially divergent LIA conditions.Peer reviewe