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1.
ACS Omega ; 8(37): 33651-33657, 2023 Sep 19.
Artigo em Inglês | MEDLINE | ID: mdl-37744834

RESUMO

The fabrication of UO2 from U3O8 is an essential reaction in the nuclear fuel cycle. The oxygen isotope fractionation associated with this reaction has significant implications in the general field of nuclear forensics. Hence, the oxygen isotope fractionation during the reduction of U3O8 to UO2 was determined in the temperature range of 500-700 °C and for a duration of 2 to 6 h under a high-purity H2 atmosphere. Three U3O8 samples, possessing a different oxygen isotopic composition, were used to investigate key parameters involved with the fractionation during the reduction process. All UO2 products did not maintain the original isotope composition of the starting U3O8 under all conditions. The results show that the system UO2-H2O attains isotope equilibrium at 600 °C, provided the reduction process lasts at least 4 h or more. At 600 °C, UO2 was isotopically depleted by 2.89 ± 0.82‰ compared to the U3O8 from which it was produced. We find that the H2O formed during the reduction plays a major role in determining the final δ18O of UO2 prepared from U3O8. The isotope equilibrium of the system UO2-H2O at 600 °C was calculated, indicating that δ18O of the H2O was enriched by about 11‰ relative to the UO2 due to the uranium mass effect. These findings could potentially have important implications for nuclear forensics, as they provide a new method for determining the history of UO2 samples and tracing back their production process.

2.
Geobiology ; 21(1): 119-132, 2023 01.
Artigo em Inglês | MEDLINE | ID: mdl-36163640

RESUMO

Stable isotopes in mollusc shells, together with variable growth rates and other geochemical properties, can register different environmental clues, including seawater temperature, salinity and primary productivity. However, the strict biological control over the construction of biominerals exerted by many calcifying organisms can constrain the use of these organisms for paleoenvironmental reconstructions. Biologically controlled calcification is responsible for the so called vital effects that cause a departure from isotopic equilibrium during shell formation, resulting in lower shell oxygen and carbon compared to the equilibrium value. We investigated shell oxygen and carbon isotopic composition of the bivalve Chamelea gallina in six sites along with a latitudinal gradient on the Adriatic Sea (NE Mediterranean Sea). Seawater δ18 O and δ13 CDIC varied from North to South, reflecting variations in seawater temperature, salinity, and chlorophyll concentration among sites. Shell δ18 O and δ13 C differed among sites and exhibited a wide range of values along with the ~400 km latitudinal gradient, away from isotopic equilibrium for both isotopes. These results hampered the utilization of this bivalve as a proxy for environmental reconstructions, in spite of C. gallina showing promise as a warm temperature proxy. Rigorous calibration studies with a precise insight of environment and shell growth are crucial prior to considering this bivalve as a reliable paleoclimatic archive.


Assuntos
Bivalves , Oxigênio , Animais , Bivalves/química , Carbono , Isótopos de Carbono , Isótopos de Oxigênio , Água do Mar/química
3.
ACS Omega ; 7(50): 47040-47047, 2022 Dec 20.
Artigo em Inglês | MEDLINE | ID: mdl-36570191

RESUMO

As a major component in the nuclear fuel cycle, octoxide uranium is subjected to intensive nuclear forensics research. Scientific efforts have been mainly dedicated to determine signatures, allowing for clear and distinct attribution. The oxygen isotopic composition of octoxide uranium, acquired during the fabrication process of the nuclear fuel, might serve as a signature. Hence, understanding the factors governing the final oxygen isotopic composition and the chemical systems in which U3O8 was produced may develop a new fingerprint concerning the history of the material and/or the process to which it was subjected. This research determines the fractionation of oxygen isotopes at different temperatures relevant to the nuclear fuel cycle in the system of U3O8 and atmospheric O2. We avoid the retrograde isotope effect at the cooling stage at the end of the fabrication process of U3O8. The system attains the isotope equilibrium at temperatures higher than 300 °C. The average δ18O values of U3O8 in equilibrium with atmospheric oxygen have been found to span over a wide range, from -9.90‰ at 300 °C up to 18.40‰ at 800 °C. The temperature dependency of the equilibrium fractionation (1000 ln αU3O8-atm. O2 ) exhibits two distinct regions, around -33‰ between 300 °C and -500 °C and -5‰ between 700 °C and -800 °C. The sharp change coincides with the transition from a pseudo-hexagonal structure to a hexagonal structure. A depletion trend in δ18O is associated with the orthorhombic structure and may result from the uranium mass effect, which might also play a role in the depletion of 5‰ versus atmospheric oxygen at high temperatures.

4.
ACS Omega ; 7(9): 7973-7980, 2022 Mar 08.
Artigo em Inglês | MEDLINE | ID: mdl-35284730

RESUMO

Triuranium octoxide (U3O8) is one of the main compounds in the nuclear fuel cycle. As such, identifying its processing parameters that control the oxygen isotopic composition could be developed as a new signature for nuclear forensic investigation. This study investigated the effect of different synthesis conditions such as calcination time, temperature, and cooling rates on the final δ18O values of U3O8, produced from uranium metal, uranyl nitrate hydrate, and uranium trioxide as starting materials. The results showed that δ18O of U3O8 is independent of the above-listed starting materials. δ18O values of 10 synthetic U3O8 were similar (9.35 ± 0.46‰) and did not change as a function of calcination time or calcination temperature. We showed that the cooling rate of U3O8 at the end of the synthesis process determines the final oxygen isotope composition, yielding a significant isotope effect on the order of 30‰. Experiments with two isotopically spiked 10 M HNO3, with a difference of δ18O ∼75‰, show that no memory of the starting solution oxygen isotope signature is expressed in the final U3O8 product. We suggest that the interaction with atmospheric oxygen is the main process parameter that controls the δ18O value in U3O8. The uranium mass effect, the tendency of uranium ions to preferentially incorporate 16O, is expressed during the solid-gas oxygen exchange, which occurs throughout cooling of the system.

5.
Glob Chang Biol ; 28(10): 3349-3364, 2022 05.
Artigo em Inglês | MEDLINE | ID: mdl-35218086

RESUMO

Coral reefs are in global decline due to climate change and anthropogenic influences (Hughes et al., Conservation Biology, 27: 261-269, 2013). Near coastal cities or other densely populated areas, coral reefs face a range of additional challenges. While considerable progress has been made in understanding coral responses to acute individual stressors (Dominoni et al., Nature Ecology & Evolution, 4: 502-511, 2020), the impacts of chronic exposure to varying combinations of sensory pollutants are largely unknown. To investigate the impacts of urban proximity on corals, we conducted a year-long in-natura study-incorporating sampling at diel, monthly, and seasonal time points-in which we compared corals from an urban area to corals from a proximal non-urban area. Here we reveal that despite appearing relatively healthy, natural biorhythms and environmental sensory systems were extensively disturbed in corals from the urban environment. Transcriptomic data indicated poor symbiont performance, disturbance to gametogenic cycles, and loss or shifted seasonality of vital biological processes. Altered seasonality patterns were also observed in the microbiomes of the urban coral population, signifying the impact of urbanization on the holobiont, rather than the coral host alone. These results should raise alarm regarding the largely unknown long-term impacts of sensory pollution on the resilience and survival of coral reefs close to coastal communities.


Assuntos
Antozoários , Microbiota , Animais , Antozoários/fisiologia , Recifes de Corais , Periodicidade , Urbanização
6.
Science ; 365(6452): 469-473, 2019 08 02.
Artigo em Inglês | MEDLINE | ID: mdl-31371609

RESUMO

The oxygen isotope composition (δ18O) of marine sedimentary rocks has increased by 10 to 15 per mil since Archean time. Interpretation of this trend is hindered by the dual control of temperature and fluid δ18O on the rocks' isotopic composition. A new δ18O record in marine iron oxides covering the past ~2000 million years shows a similar secular rise. Iron oxide precipitation experiments reveal a weakly temperature-dependent iron oxide-water oxygen isotope fractionation, suggesting that increasing seawater δ18O over time was the primary cause of the long-term rise in δ18O values of marine precipitates. The 18O enrichment may have been driven by an increase in terrestrial sediment cover, a change in the proportion of high- and low-temperature crustal alteration, or a combination of these and other factors.

7.
PeerJ ; 5: e3590, 2017.
Artigo em Inglês | MEDLINE | ID: mdl-28740755

RESUMO

Reef building corals precipitate calcium carbonate as an exo-skeleton and provide substratum for prosperous marine life. Biomineralization of the coral's skeleton is a developmental process that occurs concurrently with other proliferation processes that control the animal extension and growth. The development of the animal body is regulated by large gene regulatory networks, which control the expression of gene sets that progressively generate developmental patterns in the animal body. In this study we have explored the gene expression profile and signaling pathways followed by the calcification process of a basal metazoan, the Red Sea scleractinian (stony) coral, Stylophora pistillata. When treated by seawater with high calcium concentrations (addition of 100 gm/L, added as CaCl2.2H2O), the coral increases its calcification rates and associated genes were up-regulated as a result, which were then identified. Gene expression was compared between corals treated with elevated and normal calcium concentrations. Calcification rate measurements and gene expression analysis by microarray RNA transcriptional profiling at two time-points (midday and night-time) revealed several genes common within mammalian gene regulatory networks. This study indicates that core genes of the Wnt and TGF-ß/BMP signaling pathways may also play roles in development, growth, and biomineralization in early-diverging organisms such as corals.

8.
Sci Rep ; 6: 20191, 2016 Feb 05.
Artigo em Inglês | MEDLINE | ID: mdl-26847144

RESUMO

Reef-building scleractinian (stony) corals are among the most efficient bio-mineralizing organisms in nature. The calcification rate of scleractinian corals oscillates under ambient light conditions, with a cyclic, diurnal pattern. A fundamental question is whether this cyclic pattern is controlled by exogenous signals or by an endogenous 'biological-clock' mechanism, or both. To address this problem, we have studied calcification patterns of the Red Sea scleractinian coral Acropora eurystoma with frequent measurements of total alkalinity (AT) under different light conditions. Additionally, skeletal extension and ultra-structure of newly deposited calcium carbonate were elucidated with (86)Sr isotope labeling analysis, combined with NanoSIMS ion microprobe and scanning electron microscope imaging. Our results show that the calcification process persists with its cyclic pattern under constant light conditions while dissolution takes place within one day of constant dark conditions, indicating that an intrinsic, light-entrained mechanism may be involved in controlling the calcification process in photosymbiotic corals.


Assuntos
Antozoários/fisiologia , Calcificação Fisiológica/fisiologia , Animais , Relógios Biológicos , Carbonato de Cálcio/química , Oceano Índico , Luz , Microscopia Eletrônica de Varredura , Nanotecnologia , Espectrometria de Massa de Íon Secundário , Isótopos de Estrôncio/química
9.
Sci Rep ; 5: 8779, 2015 Mar 05.
Artigo em Inglês | MEDLINE | ID: mdl-25739995

RESUMO

Contemporary cnidarian-algae symbioses are challenged by increasing CO2 concentrations (ocean warming and acidification) affecting organisms' biological performance. We examined the natural variability of carbon and nitrogen isotopes in the symbiotic sea anemone Anemonia viridis to investigate dietary shifts (autotrophy/heterotrophy) along a natural pCO2 gradient at the island of Vulcano, Italy. δ(13)C values for both algal symbionts (Symbiodinium) and host tissue of A. viridis became significantly lighter with increasing seawater pCO2. Together with a decrease in the difference between δ(13)C values of both fractions at the higher pCO2 sites, these results indicate there is a greater net autotrophic input to the A. viridis carbon budget under high pCO2 conditions. δ(15)N values and C/N ratios did not change in Symbiodinium and host tissue along the pCO2 gradient. Additional physiological parameters revealed anemone protein and Symbiodinium chlorophyll a remained unaltered among sites. Symbiodinium density was similar among sites yet their mitotic index increased in anemones under elevated pCO2. Overall, our findings show that A. viridis is characterized by a higher autotrophic/heterotrophic ratio as pCO2 increases. The unique trophic flexibility of this species may give it a competitive advantage and enable its potential acclimation and ecological success in the future under increased ocean acidification.


Assuntos
Antozoários/fisiologia , Processos Autotróficos , Dióxido de Carbono , Animais , Isótopos de Carbono/análise , Clorofila/metabolismo , Índice Mitótico , Isótopos de Nitrogênio/análise , Água do Mar/análise , Água do Mar/química , Simbiose
10.
Proc Natl Acad Sci U S A ; 111(46): 16303-8, 2014 Nov 18.
Artigo em Inglês | MEDLINE | ID: mdl-25368148

RESUMO

Basin-scale calcification rates are highly important in assessments of the global oceanic carbon cycle. Traditionally, such estimates were based on rates of sedimentation measured with sediment traps or in deep sea cores. Here we estimated CaCO3 precipitation rates in the surface water of the Red Sea from total alkalinity depletion along their axial flow using the water flux in the straits of Bab el Mandeb. The relative contribution of coral reefs and open sea plankton were calculated by fitting a Rayleigh distillation model to the increase in the strontium to calcium ratio. We estimate the net amount of CaCO3 precipitated in the Red Sea to be 7.3 ± 0.4·10(10) kg·y(-1) of which 80 ± 5% is by pelagic calcareous plankton and 20 ± 5% is by the flourishing coastal coral reefs. This estimate for pelagic calcification rate is up to 40% higher than published sedimentary CaCO3 accumulation rates for the region. The calcification rate of the Gulf of Aden was estimated by the Rayleigh model to be ∼1/2 of the Red Sea, and in the northwestern Indian Ocean, it was smaller than our detection limit. The results of this study suggest that variations of major ions on a basin scale may potentially help in assessing long-term effects of ocean acidification on carbonate deposition by marine organisms.


Assuntos
Antozoários/metabolismo , Calcificação Fisiológica , Carbonato de Cálcio/análise , Recifes de Corais , Efeito Estufa , Plâncton/metabolismo , Água do Mar/química , Estruturas Animais/química , Estruturas Animais/metabolismo , Animais , Antozoários/anatomia & histologia , Antozoários/química , Antozoários/efeitos dos fármacos , Cálcio/metabolismo , Dióxido de Carbono/farmacologia , Precipitação Química , Estudos de Viabilidade , Foraminíferos/química , Foraminíferos/efeitos dos fármacos , Foraminíferos/metabolismo , Sedimentos Geológicos/análise , Sedimentos Geológicos/química , Concentração de Íons de Hidrogênio , Oceano Índico , Plâncton/química , Plâncton/efeitos dos fármacos , Salinidade , Solubilidade , Estrôncio/metabolismo
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