Abundance Of Uranium Research Articles

Soil contamination by trace elements and radioelements and related environmental risks in agricultural soils of the M'Dhilla Basin (southwestern Tunisia).

Agricultural soil contaminated by phosphogypsum pile stocked in plan air remains a major problem in M'Dhilla city southwestern of Tunisia. The present effort aimed to enhance the knowledge of trace elements and radioactivity abundance and to assess the corresponding environment. X-ray fluorescence spectroscopy was used to evaluate the trace elements, radioactive elements, and major element concentrations. Our finding revealed that the mean values of U and Th in all the soil profiles ranged from 0.283 to 2.875mg.kg-1 and from 0.797 to 1.491mg.kg-1 respectively. The statistical analyses indicated that uranium abundance has non-significant correlation with the most of trace metals; it shows a moderate correlated with Sr and inverse correlation with P2O5. Contamination factors indicate that the studied soil ranged from uncontaminated to significantly contaminated. Thus, the pollution load index values classify the sites from background values to high pollution sites. The findings of this study will help improve the environmental conditions of M'Dhilla city by addressing contamination issues through targeted interventions. The study's findings highlight the importance of a periodic environmental monitoring such as soil remediation in the study area. This research fills a critical gap in the current understanding of contamination management in this region.

Identification and Quantification of Multiphase U(VI) Speciation on Gibbsite with pH Using TRLFS and PARAFAC of Excitation Emission Matrices.

The significant abundance of uranium in radioactive waste inventories worldwide necessitates a thorough understanding of its behavior. In this work, the speciation of uranyl(VI), (UO22+) in a gibbsite system under ambient conditions has been determined as a function of pH by deconvolution and analysis of luminescence spectroscopic data. Uniquely, a combined experimental and statistical approach utilizing time-resolved luminescence spectroscopy and parallel factor analysis (PARAFAC) of excitation emission matrices has been successfully utilized to identify four separate luminescent U(VI) species in the uranyl-gibbsite system for the first time. The speciation of all luminescent U(VI) species in an environmentally relevant system over a pH range of 6-11 is discerned through the analysis of emission fingerprints at low temperature (20 K). Comparison of the deconvoluted luminescence spectra with mineral standards and geochemical models of the system allows the assignment of the luminescent chemical species as metaschoepite, Na-compreignacite, surface adsorbed ≡AlO2-UO2(OH) and ≡AlO2-UO2(CO3)24- complexes, with assignments supported by fitting of extended X-ray absorption fine structure data. The combined spectroscopic techniques in this study show that assignment and quantification of uranyl(VI) species in a sorption system over a large pH range can be accurately achieved using PARAFAC to deconvolute a three way emission spectroscopic data set.

High-Performance Polyamidoxime Porous Membrane Prepared by the In Situ Modification/Nonsolvent-Induced Phase Separation Strategy for Uranium Extraction from Seawater.

The abundance of uranium (U(VI)) reserves in seawater makes it crucial to develop economically efficient methods for uranium extraction from seawater. In this work, an enhanced polyamidoxime porous membrane (PAOM) was fabricated by pre-in situ amidoxime modification combined with nonsolvent-induced phase separation (NIPS). The strategy of in situ modification of the polyacrylonitrile (PAN) solution served to enhance the homogeneity of the reaction and avoid the destruction of the membrane matrix and pore structure. Compared with the control sample (AOPM), PAOM possessed better mechanical strength and hydrophilicity. The introduction of polyvinylpyrrolidone (PVP) formed a porous structure in PAOM, improving spatial accessibility and facilitating the diffusion transport and capture of UO22+ inside the membrane. The more uniform and abundant distribution of amidoxime groups in PAOM gave it ultrahigh adsorption capacity and selectivity. The equilibrium adsorption capacity and Kd value of PAOM were 1.72 and 5.51 times higher than those of AOPM. Meanwhile, PAOM also demonstrated good recyclability, with only a 6.15% decrease in adsorption capacity after seven cycles. Additionally, PAOM exhibited excellent dynamic adsorption performance, and after 14 days of continuous filtration and adsorption, PAOM could extract 2.03 mg·g-1 U(VI) from natural seawater.

Uranium Abundances and Ages of r-process Enhanced Stars with Novel U ii Lines* *Some of the data presented herein were obtained at the W. M. Keck Observatory, which is operated as a scientific partnership among the California Institute of Technology, the University of California, and the National Aeronautics and Space Administration. The Observatory was made possible by the generous financial

The ages of the oldest stars shed light on the birth, chemical enrichment, and chemical evolution of the universe. Nucleocosmochronometry provides an avenue to determining the ages of these stars independent from stellar-evolution models. The uranium abundance, which can be determined for metal-poor r-process enhanced (RPE) stars, has been known to constitute one of the most robust chronometers known. So far, U abundance determination has used a single U ii line at λ3859 Å. Consequently, U abundance has been reliably determined for only five RPE stars. Here, we present the first homogeneous U abundance analysis of four RPE stars using two novel U ii lines at λ4050 Å and λ4090 Å, in addition to the canonical λ3859 Å line. We find that the U ii lines at λ4050 Å and λ4090 Å are reliable and render U abundances in agreement with the λ3859 U abundance, for all of the stars. We, thus, determine revised U abundances for RPE stars, 2MASS J09544277+5246414, RAVE J203843.2–002333, HE 1523–0901, and CS 31082–001, using multiple U ii lines. We also provide nucleocosmochronometric ages of these stars based on the newly derived U, Th, and Eu abundances. The results of this study open up a new avenue to reliably and homogeneously determine U abundance for a significantly larger number of RPE stars. This will, in turn, enable robust constraints on the nucleocosmochronometric ages of RPE stars, which can be applied to understand the chemical enrichment and evolution in the early universe, especially of r-process elements.

Radiogeochemistry, mineralogy, lithology, radiogenic heat production, and health implication using airborne radiometric data of Ilesha and its surroundings.

The current study analyzed and interpreted airborne radiometric data from Ilesha's basement complex rock and its surroundings. At the surface, the concentrations of the most frequent primordial radionuclides notably K, elemental concentration of uranium eU, and elemental concentration of thorium eTh were measured. The weighted mean elemental and activity concentrations were 0.85%, 2.75ppm, 10.22ppm, and 267.54Bqkg-1, 34.41Bqkg-1, 41.51Bqkg-1 for 40K, 238U, and 232Th, respectively. The low concentration of 40K was certainly due to the effects of weathering, kaolinization of granites, and pedogenesis activities. The abundance of uranium was ascribed to the availability of uranium minerals such as allanite, apatite, and sphene with accessories minerals, while that of thorium was due to minerals such as cheralite, thorite, uranothorite, thorianite, and uranothorianite with accessories minerals. The RPHR weighted mean 1.48 µWm-3 compared to the earth's crust mean between 0.8 and1.2 µWm-3 was higher due to significant presence of gneiss rocks in all the studied profiles. Radiological hazard, in particular, dose rates, external hazard index, internal hazard index, radium equivalent, annual gonadal dose, effective dose dispensed to various organs of the body were computed to determine the deleterious effects of rocks in the area. The weighted means of annual gonadal dose of 363.98 µSv y-1 and outdoor 0.91 × 10×3 and indoor 1.65 × 10-3 excessive life cancer risks were more than the global average 300 µSv y-1, 0.29 × 10-3 and 1.16 × 10-3. As a result, proper surveillance is required in the area in order to prevent epidemics occurrence in future.

Frontier of Understanding Earth's Dynamics

Frontier of Understanding Earth's Dynamics

Geochemical Studies of Detrital Zircon Grains from the River Banks and Beach Placers of Coastal Odisha, India

Zircon grains are reasonably ubiquitous in river banks and beach placers as detrital minerals, including along the ~1700 km long Indian east coast, from Odisha state to the state of Kerala. Zircons from beach placers and river banks located along the eastern part of Odisha, India, were studied using LA-ICP-MS in order to delineate their geochemical characteristics. Hf (mean = 11270 ppm) and Y (mean = 1064 ppm) were the two most abundant trace elements found within zircon grains as compared to other trace elements. The abundance of uranium was observed to be 2–4 times larger than that of thorium. Zircon overgrowths formed in equilibrium with a partial melt and were similar to magmatic zircon in terms of the high Y, Hf and P content, steep heavy-enriched REE pattern, positive Ce anomaly and negative Eu anomaly. The average low Th/U ratio of the studied zircon grains distinguished them from the magmatic ones. The REE present in zircon grains was restricted to high-grade metamorphic events. The result of the present study would be useful for delineating the source region and the efficacy of resource potential and indigenous export.

High-precision Nuclear Chronometer for the Cosmos

Nuclear chronometers, which predict the ages of the oldest stars by comparing the present and initial abundances of long-lived radioactive nuclides, provide an independent dating technique for the cosmos. The idea of synchronizing Th/X, U/X, and Th/U chronometers can impose stringent constraints on the astrophysical conditions in the r-process simulations, giving rise to the Th-U-X chronometer. It is found that the astrophysical uncertainties of nuclear cosmochronology are significantly reduced from more than ±2 billion years to about 0.3 billion years. The Th-U-X chronometer is then applied to estimate the ages of the six metal-poor stars with observed uranium abundances, and the predicted ages are compatible with the cosmic age of 13.8 billion years predicted from the cosmic microwave background radiation but contradict the cosmic age of 11.4 billion years from the gravitational lens measurement.

Geochemical, mineralogical and textural nature of beach placers, north-east Sri Lanka: Implications for provenance and potential resource

The Pulmoddai placer sands in Sri Lanka (9°3′23.65″–8°51′38.83″N and 80°55′22.91″–81°3′32.65″E) is considered to be one of the major rare earth element (REE) prospects world-wide. This deposit has a global significance in terms of strategic economic resources and can provide valuable insight for resource estimation and for its economic use. A detailed study of the geochemical, mineralogical, and textural properties of the heavy minerals of this region is undertaken applying the scanning electron microscopy–electron dispersive X-ray spectroscopy (SEM–EDS), inductively coupled plasma–optical emission spectroscopy (ICP–OES), X-Ray diffraction (XRD), and X-Ray fluorescence (XRF) techniques. The results from these analyses are modeled to delineate the source(s) which contribute to the formation of the mineral deposits and their subsequent enrichment. The beach sediment from this region, exhibited elevated concentrations of major elements like silicon (Si) and manganese (Mn) and high abundance of minor and trace elements like yttrium (Y), strontium (Sr), lanthanum (La), barium (Ba), which indicates that the provenance is similar to calcium-silicate rocks. The highest (total) concentration of Y, La, scandium (Sc) and cerium (Ce) are mainly associated to fine grained sediment, signifying that their occurrences are governed by the grain size distribution and ambient hydrodynamic conditions primarily from lagoonal input. Monazite, zircon, ilmenite, rutile, and xenotime are more abundant than garnet in the study area, as is evident from the higher abundance of thorium (Th) and uranium (U) and the enrichment of zirconium (Zr), Ce, vanadium (V), chromium (Cr), and Y. Textural analysis of sediment shows sediment particles are poorly sorted and have more angular grain boundaries, indicating their lower textural maturity. This suggests that the sediment is fluvially derived from a close source rock of charnockite specific to the hinterland lithology. The sediment transported by perennial rivers and the hydrodynamic conditions of the Kokkilai Lagoon estuary, are the two primary sources of heavy mineral enrichment in the study area, which is quite unique in terms of the placer formation. Subsequently, these sediment particles formed the beach placer deposits enriched in REE–Th rich heavy minerals, which are of immense strategic importance.

Abundances of Uranium and Thorium Elements in Earth Estimated by Geoneutrino Spectroscopy

AbstractThe decay of the primordial isotopes 238U, 235U, 232Th, and 40K has contributed to the terrestrial heat budget throughout the Earth's history. Hence, the individual abundance of those isotopes are key parameters in reconstructing contemporary Earth models. The geoneutrinos produced by the radioactive decays of uranium and thorium have been observed with the Kamioka Liquid‐Scintillator Antineutrino Detector (KamLAND). Those measurements have been improved with more than 18‐year observation time, and improvement in detector background levels mainly with an 8‐year nearly reactor‐free period, which now permit spectroscopy with geoneutrinos. Our results yield the first constraint on both uranium and thorium heat contributions. The KamLAND result is consistent with geochemical estimations based on elemental abundances of chondritic meteorites and mantle peridotites. The High‐Q model is disfavored at 99.76% C.L. and a fully radiogenic model is excluded at 5.2σ assuming a homogeneous heat producing element distribution in the mantle.

Be, V, and Cu in the halo star CS 31082-001 from near-UV spectroscopy

ABSTRACT The ‘First Stars’ programme revealed the metal-poor halo star CS 31082-001 to be r-process and actinide rich, including the first measurement of a uranium abundance for an old star. To better characterize and understand such rare objects, we present the first abundance estimates of three elements (Be, V, Cu) for CS 31082-001 from analysis of its near-ultraviolet spectrum. Beryllium is rarely measured in giant stars, and we confirm that its abundance in this star is low due to the rather cool effective temperature that causes destruction of both Be and Li in its atmosphere. Vanadium and copper are iron-peak elements that are starting to be used as chemical-tagging indicators to investigate the origin of stellar populations. We find V and Cu abundances for CS 31082-001 that are comparable to other metal-poor stars, and present new chemical evolution models to investigate our results. In the case of V, extra nucleosynthesis due to interaction of neutrinos with matter is included in the models to be able to reproduce the measured abundance. Given the availability of high-quality spectroscopy of CS 31082-001, we also explore other atomic lines in the near-ultraviolet as a template for future studies of metal-poor stars with the planned CUBES instrument in development for the Very Large Telescope.

Characterizing major and trace element compositions in fallout melt glass from a near-surface nuclear test

The chemical and isotopic compositions of fallout melt glasses from nuclear tests contain a range of information constraining the physical conditions within the fireball and the mechanisms of fallout formation but historic studies tended to exclude the behavior of stable major and trace elements. Here, we present a large study specifically focused on major and trace element relationships within a population of macroscale fallout samples from a single event. We interpret these data to better constrain how fallout melt glass formation in near surface environments is influenced by that environment and demonstrate how major and trace element abundances can provide useful insights into chemical processes within the fireball.Data confirm that the uranium in the fallout glass population derives from two isotopically distinct endmembers: isotopically enriched uranium (presumably from the weapon), and natural composition uranium that may be a combination of anthropogenic and environmental materials from within the blast zone. The similarity between major and trace element concentrations in fallout and corresponding local soils from the event site confirm the local soils as the most probable source of entrained material into the fireball and the source of carrier material into which the bomb vapor was incorporated. The lack of correlation between major and trace element abundances with size indicates that volatility driven processes, such as condensation from the fireball, do not control the composition of macroscale fallout melt glass.Although the fallout has major and trace element chemical characteristics broadly similar to those of the local, associated soils, some systematic differences are observed between the two populations. Fallout melt glass is depleted in volatile elements such as K, Na, Tl and Pb, consistent with heating to temperatures above ∼1000 °C for 3-10 s. This is supported by the results of laser heating experiments performed on rhyolitic soil at temperatures (1600-2200 °C) and timescales (1-120 s) that are broadly relevant to fallout formation conditions. Relative enrichments of metals such as Cu and Co do not correlate with the abundance of uranium, suggesting that fallout also records input of near field anthropogenic materials. Our observations suggest that major chemical features can be related to processing in the fireball and used to inform the thermal-chemical evolution of the system. Ultimately, these data are consistent with a fallout formation mechanism that involves rapid melting of surface materials to form carrier material melts with minor incorporation of bomb vapor and a degree of volumetric volatile loss due to heating.

The role of planetary interior in the long-term evolution of atmospheric CO2on Earth-like exoplanets

Context.The long-term carbonate silicate cycle plays an important role in the evolution of Earth’s climate and, therefore, may also be an important mechanism in the evolution of the climates of Earth-like exoplanets. However, given the large diversity in the possible interiors for Earth-like exoplanets, the ensuing evolution of the atmospheric CO2pressure may be widely different.Aims.We assess the role of the thermal evolution of the planetary interior on the long-term carbon cycle of Earth-like exoplanets. In particular, we investigate the effects of radiogenic mantle heating, core size, and planetary mass on the atmospheric partial CO2pressure, and the ability of a long-term carbon cycle driven by plate tectonics to control the atmospheric CO2pressure.Methods.We developed a box-model which connects carbon cycling to parametrized mantle convection. Processes considered in the carbon cycle are temperature-dependent continental weathering, seafloor weathering, subduction, and degassing through ridge and arc volcanism. The carbon cycle was coupled to the thermal evolution via the plate speed, which was parametrized in terms of the global Rayleigh number.Results.We find decreasing atmospheric CO2pressure with time, up to an order of magnitude over the entire main sequence lifetime of a solar-type star. High abundances of radioactive isotopes allow for more efficient mantle degassing, resulting in higher CO2pressures. Within the spread of abundances found in solar-type stars, atmospheric CO2pressures at 4.5 Gyr were found to vary from 14 Pa to 134 Pa. We find a decreasing Rayleigh number and plate speed toward planets with larger core mass fractionsfc, which leads to reduced degassing and lower atmospheric CO2pressure. In particular forfc≳ 0.8, a rapid decrease of these quantities is found. Variations in planet mass have more moderate effects. However, more massive planets may favor the development of more CO2rich atmospheres due to hotter interiors.Conclusions.The dependence of plate tectonics on mantle cooling has a significant effect on the long-term evolution of the atmospheric CO2pressure. Carbon cycling mediated by plate tectonics is efficient in regulating planetary climates for a wide range of mantle radioactive isotope abundances, planet masses and core sizes. More efficient carbon cycling on planets with a high mantle abundance of thorium or uranium highlights the importance of mapping the abundances of these elements in host stars of potentially habitable exoplanets. Inefficient carbon recycling on planets with a large core mass fraction (≳0.8) emphasizes the importance of precise mass-radius measurements of Earth-sized exoplanets.

Neutronics analysis of the stellarator-type fusion-fission hybrid reactor based on the CAD optimization method

The stellarator plasmas devices have some advantages in steady state operation with less magneto hydrodynamic activities, and the development of stellarator-type FFHR (Fusion-Fission Hybird Reactor) can provide viable energy in the foreseeable future with less requirements to the plasmas technology compared with the pure fusion system. However, the neutronics design of the stellarator-type FFHR has some challenges and difficulties, since the construction of helical structures with large number of high order spline surfaces are the extremely complicated process, and the physical parameters of the complex model have to be changed frequently in the primary design stage, which make the corresponding manual input simulation work into the time consuming and error prone tasks. In this context, the CAD optimization method has been used in converting the dense mesh oriented CAD structure to the low poly style, and the CAD-PSFO code has been employed for the modeling transformation process in Monte Carlo calculation. The thorium resource has been selected as the breeding material for the stellarator-type FFHR, and the polonium and natural abundance of uranium have been used as the seed fuel for the initial loading in order to get the high energy multiplication factor. The OMCB (OpenMC basedBurnupcode) has been introduced to study the burnup features, and stellarator-type FFHR has been designed as the multi-purposes facility that can keep tritium self-sufficiency and get the high breeding ratio with steady energy output.

INCREASE IN CARBONATE CONTRIBUTION FROM FRAMEWORK-BUILDING METAZOANS THROUGH EARLY CAMBRIAN REEFS OF THE WESTERN BASIN AND RANGE, USA

During the early Cambrian, organisms with robust skeletons began to integrate with microbialite reef structures. Specifically, archaeocyathan sponges were among the first metazoan reef-builders. Here we investigate the transition from microbial-dominated reef environments to metazoan-based reefs from strata in the western Basin and Range of California and Nevada. This study integrates point count data from petrographic thin sections with stable carbon isotopic and elemental composition of carbonates. From the earliest reef bearing formations to the latest, metazoan framework contribution increases from zero to 29.7%. This increase is linked to the addition of new framework-building organisms, namely coralomorphs, as well as an increase in archaeocyath body size. Correspondingly, Shannon's diversity increases from 0.652 to 1.492. However, skeletal contributions from additional organisms within the reefs (e.g., trilobites, echinoderms) appear unchanged and their diversity is not correlated with framework-builder diversity. A positive carbon isotopic excursion within the Lower Poleta Formation correlates with decreases in the abundances of uranium and molybdenum that suggest a global change in organic carbon burial as opposed to localized or diagenetic factors. This allows for chemostratigraphic correlation to published carbon isotopic data and provides a proposed regional age constraint of roughly 517 million years. Overall, early Cambrian reefs at this location exhibit a pattern of increasing metazoan contribution during the transition from microbial- to metazoan-based reef support, however, diversity remained low until additional organisms evolved to inhabit these ecosystems.

Numerical evaluation of gamma radiation monitoring

Abstract Airborne Gamma Ray Spectrometry (AGRS) with its important applications such as gathering radiation information of ground surface, geochemistry measuring of the abundance of Potassium, Thorium and Uranium in outer earth layer, environmental and nuclear site surveillance has a key role in the field of nuclear science and human life. The Broyden–Fletcher–Goldfarb–Shanno (BFGS), with its advanced numerical unconstrained nonlinear optimization in collaboration with Artificial Neural Networks (ANNs) provides a noteworthy opportunity for modern AGRS. In this study a new AGRS system empowered by ANN-BFGS has been proposed and evaluated on available empirical AGRS data. To that effect different architectures of adaptive ANN-BFGS were implemented for a sort of published experimental AGRS outputs. The selected approach among of various training methods, with its low iteration cost and non-diagonal scaling allocation is a new powerful algorithm for AGRS data due to its inherent stochastic properties. Experiments were performed by different architectures and trainings, the selected scheme achieved the smallest number of epochs, the minimum Mean Square Error (MSE) and the maximum performance in compare with different types of optimization strategies and algorithms. The proposed method is capable to be implemented on a cost effective and minimum electronic equipment to present its real-time process, which will let it to be used on board a light Unmanned Aerial Vehicle (UAV). The advanced adaptation properties and models of neural network, the training of stochastic process and its implementation on DSP outstands an affordable, reliable and low cost AGRS design. The main outcome of the study shows this method increases the quality of curvature information of AGRS data while cost of the algorithm is reduced in each iteration so the proposed ANN-BFGS is a trustworthy appropriate model for Gamma-ray data reconstruction and analysis based on advanced novel artificial intelligence systems.

Single Grain (U-Th)/He Ages from EET14074, an Acapulcoite Meteorite

EET14074 is a member of the acapulcoite-lodranite family of meteorites and was discovered in Antarctica in 2014 by the Korean Polar Research Institute (KOPRI). Herein, we obtained (U-Th)/He ages from 20 phosphate aggregates in EET14074 to constrain the sample’s thermal history. The ages range between 116.8 Ma ± 145.7 Ma (1σ) and 4211.5 Ma ± 1089.1 Ma, with an average of 2530 ± 260 Ma (n=20). Excluding outlier ages with large uncertainties, the 13 most concentrated ages yielded a theoretical minimum age of 3000 ± 150 Ma. This age corresponds to a fraction helium loss (f) of ~31% assuming crystallization age of ~4.55 Ga. The uranium abundances in single aliquots are in the range of 1.52 - 289.93 fmol, with an average of 58.00 fmol (n = 20), whereas the thorium abundance ranges between 2.51 and 2337.06 fmol with an average of 1149.29 fmol (n = 20). To explain the observed He loss, thermal diffusion modeling was performed with an assumption that the He loss occurred during a recent passage of the meteorite in Earth’s atmosphere. For the most likely t-T condition of compressional heating in Earth’s atmosphere of this meteorite (T = ~430 ˚C, t = ~10 sec), a fractional loss of 29.3% was calculated when the diffusion domain radius (r) of 92.8 µm was assumed. This estimation is nearly indistinguishable from the observed fractional loss of 31.3%. Additionally, diffusion modeling for another set of data with a different size (r = 40.4 µm) yielded a very similar fractional loss. Therefore, the observed (U-Th)/He age distribution is likely derived from compressional heating during the passage of EET14074 in Earth’s atmosphere.

The R-Process Alliance: 2MASS J09544277+5246414, the Most Actinide-enhanced R-II Star Known

Abstract We report the discovery of a new actinide-boost star, 2MASS J09544277+5246414, originally identified as a very bright (V = 10.1), extremely metal-poor ([Fe/H] = −2.99) K giant in the LAMOST survey, and found to be highly r-process-enhanced (r-II; [Eu/Fe] = +1.28]), during the snapshot phase of the R-Process Alliance (RPA). Based on a high signal-to-noise ratio (S/N), high-resolution spectrum obtained with the Harlan J. Smith 2.7 m telescope, this star is the first confirmed actinide-boost star found by RPA efforts. With an enhancement of [Th/Eu] = +0.37, 2MASS J09544277+5246414 is also the most actinide-enhanced r-II star yet discovered, and only the sixth metal-poor star with a measured uranium abundance ([U/Fe] = +1.40). Using the Th/U chronometer, we estimate an age of 13.0 ± 4.7 Gyr for this star. The unambiguous actinide-boost signature of this extremely metal-poor star, combined with additional r-process-enhanced and actinide-boost stars identified by the RPA, will provide strong constraints on the nature and origin of the r-process at early times.

Formation of Superheavy Elements in Nature

It is shown that superheavy elements may also be formed in the main r process responsible for the formation of the heaviest elements observed in nature. Under conditions of a high neutron density, the nucleosynthesis region lies close to the neutron drip line, so that the r process may circumvent the region where nuclei undergo spontaneous fissions and therefore have short lifetimes. However, a high induced-fission rate, which increases with the charge number, may prevent the nucleosynthesis wave from overcoming the region of isotopes heavier than curium, and the beta-decay chain leading to an increase in the charge number of product elements inevitably results in the spontaneous fission of the majority of product nuclei. Calculations of nucleosynthesis that were performed with available nuclear data within the scenario of a neutron-star merger reveal that only Z < 106 superheavy elements are formed. Their abundance at the end of the r process is commensurate with the abundance of uranium, but their lifetime does not exceed several years, so that they fast undergo decay.

Coupled Mo-U abundances and isotopes in a small marine euxinic basin: Constraints on processes in euxinic basins

Sedimentary molybdenum (Mo) and uranium (U) abundances, as well as their isotope systematics, are used to reconstruct the evolution of the oxygenation state of the surface Earth from the geological record. Their utility in this endeavour must be underpinned by a thorough understanding of their behaviour in modern settings. In this study, Mo-U concentrations and their isotope compositions were measured in the water column, sinking particles, sediments and pore waters of the marine euxinic Lake Rogoznica (Adriatic Sea, Croatia) over a two year period, with the aim of shedding light on the specific processes that control Mo-U accumulation and isotope fractionations in anoxic sediment.Lake Rogoznica is a 15 m deep stratified sea-lake that is anoxic and euxinic at depth. The deep euxinic part of the lake generally shows Mo depletions consistent with near-quantitative Mo removal and uptake into sediments, with Mo isotope compositions close to the oceanic composition. The data also, however, show evidence for periodic additions of isotopically light Mo to the lake waters, possibly released from authigenic precipitates formed in the upper oxic layer and subsequently processed through the euxinic layer. The data also show evidence for a small isotopic offset (∼0.3‰ on 98Mo/95Mo) between particulate and dissolved Mo, even at highest sulfide concentrations, suggesting minor Mo isotope fractionation during uptake into euxinic sediments. Uranium concentrations decrease towards the bottom of the lake, where it also becomes isotopically lighter. The U systematics in the lake show clear evidence for a dominant U removal mechanism via diffusion into, and precipitation in, euxinic sediments, though the diffusion profile is mixed away under conditions of increased density stratification between an upper oxic and lower anoxic layer. The U diffusion-driven precipitation is best described with an effective 238U/235U fractionation of +0.6‰, in line with other studied euxinic basins.Combining the Mo and U systematics in Lake Rogoznica and other euxinic basins, it is apparent that the two different uptake mechanisms of U and Mo can lead to spatially and temporally variable Mo/U and Mo-U isotope systematics that depend on the rate of water renewal versus removal to sediment, the sulfide concentration, and the geometry of the basin. This study further emphasises the potential of combining multiple observations, from Mo-U enrichment and isotope systematics, for disentangling the various processes via which redox conditions control the chemistry of modern and ancient sediments.