1999 Macmillan Magazines Ltdafter The Accident At The Chernobylnucle ✓ Solved

© 1999 Macmillan Magazines Ltd After the accident at the Chernobylnuclear reactor in 1986, the concentra- tion of radioactive caesium (134Cs and 137Cs) in fish was expected to decline rapidly. The estimated ecological half-life (the time need- ed to reduce the average caesium concentra- tion by 50%) was 0.3 to 4.6 years1,2. Since 1986, we have measured radiocaesium in brown trout (Salmo trutta) and Arctic charr (Salvelinus alpinus), both of which are widely eaten in Scandinavia, in a lake contaminated by Chernobyl fallout3,4. We have measured radiocaesium in nearly 4,000 fish, taking samples 2–4 times every year from spring to autumn. We find that the decline in radio- caesium was initially rapid for 3–4 years and was then much slower.

About 10% of the initial peak radioactivity declines with an ecological half-life of as long as 8–22 years. The concentration of 137Cs, the long- lived radioactive caesium isotope with a physical half-life of 30.2 years, peaked in 1986. The radioactivity was three times higher in brown trout than in Arctic charr (geometric means: 10,468 and 3,097 Bq kg11). The decline in 137Cs from its maxi- mum in 1986 to 1998 is modelled by single- and two-component decay functions: Qt4Qe 1kt and Qt4Q1e 1k1t&Q2e 1k2t, where Q is the caesium concentration, k is the decay rate and t is time in years after the peak. The ecological half-lives are ln2/k, and are an indication of how long it will take the fish to rid themselves of radioacti-vity.

The proportional contribution of the maximum radioactivity with slow decay rates was esti- mated as Q2/(Q1&Q2). The decline in 137Cs was rapid during the first three (brown trout) and four (Arctic charr) years, and was then slower. Based on the initial rapid decline, ecological half-lives were estimated using a single-component decay function at 1.0 and 1.5 years for brown trout and Arctic charr, respectively, as in other post-Chernobyl studies1,2, but this underestimates the time that 137Cs persists in the fish. A two-component decay function gives better model fits (extra sum of squares test, P**0.001) than single-component models. The two-component models explain 90% and 92% of the individual variance in caesium concentration in brown trout and Arctic charr, respectively.

Seasonal dynamics from 1988 (ref. 5) and size dependency4 in caesium levels meant that modelling should be done on all young fish (aged 2 and 3 years) until 1988, and then only on spring samples. Ecological half-lives were estimated at 0.6 and 7.7 years for the first and second components for brown trout, and 1.1 and 22.4 years for Arctic charr (Fig. 1). The sec- ond component constituted 11.5% and 10.7% of the initial peak 137Cs activity for brown trout and Arctic charr, respectively.

The two-component nature of the decay indicates that the fish may be affected by two contaminant pools. The first is a rapidly declining pool caused by caesium deposi- ted on the lake surface and washed out from the catchment before being sorbed to catch- ment soils6. The caesium in the lake declined quickly as a result of hydraulic dilution, accumulation in bottom sedi- ments, reduced run-off from the catch- ments, and loss of 137Cs through outflow6,7. The second is a slowly declining pool of 137Cs leaking from the lake catchment6,8 and caesium recycling within the lake9. The relative importance of the secondary pools depends on catchment and lake char- acteristics.

The 137Cs concentration in the environment may approach a steady state10, declining only as a result of decay (half-life, 30 years). This may apply generally for radioactive elements entering the biogeo- chemical cycles, and is supported by our estimates of ecological decay for 137Cs in Arctic charr. The different accumulation and ecological decay of caesium in Arctic charr and brown trout is probably due to their different ecological niches: they segre- gate in habitat and diet, both of which influ- ence caesium turnover3. Bror Jonsson*, Torbjà¸rn Forseth†, Ola Ugedal†*Norwegian Institute for Nature Research, Dronningens gt 13, 0105 Oslo, Norway e-mail: [email protected] †Norwegian Institute for Nature Research, Tungasletta 2, 7485 Trondheim, Norway 1.

Elliott, J. M., Elliott, J. A. & Hilton, J. Ann. Limnol.

29, 79–). 2. Brittain, J. E. et al. in Environmental Impact of Radioactive Releases 291–298 (International Atomic Energy Agency, Vienna, 1995). 3.

Forseth, T., Ugedal, O., Jonsson, B., Langeland, A. & Njà¥stad, O. J. Appl. Ecol. 28, 1053–).

4. Ugedal, O., Forseth, T., Jonsson, B. & Njà¥stad, O. J. Appl. Ecol.

32, 352–). 5. Ugedal, O., Forseth, T. & Jonsson, B. Ecol. Appl.

7, 1002–). 6. Smith, J. T., Leonard, D. R.

P., Hilton, J. & Appleby, P. G. Health Phys. 72, 880–). 7.

Santschi, P. H., Bollhalder, S., Zingg, S., Luck, A. & Farrenkoten, K. Environ. Sci. Technol.

33, 519–). 8. Hilton, J., Livens, F. R., Spezzano, P. & Leonard, D. R.

P. Sci. Total Environ. 129, 253–). 9.

Smith, J. T. & Comans, R. N. J. Geochim.

Cosmochim. Acta 60, 995–). 10. Smith, J. T. et al.

Environ. Sci. Technol. 33, 49–). NATURE | VOL 400 | 29 JULY 1999 | 417 Chernobyl radioactivity persists in fish scientific correspondence to those of natural diamond formation.

For diamond crystallization, we applied the multi-anvil apparatus of a ‘splite sphere’ type5. Temperature was measured by ther- mocouples calibrated according to data for the melting of gold, silver and aluminium, and results of diamond synthesis in the nickel–carbon system. Pressure calibration was done by the standard procedures5. As the components of experimental charges we used Na2CO3, K2CO3, graphite and oxalic acid dehydrate, generating C–O–H fluid at the required pressure and temperature, and we added minute diamond crystals as seeds to the charges. Crystallization runs were carried out in sealed platinum ampules.

The results are summarized in Table 1. For the Na2CO3–C system, at a tempera- ture of 1,420 °C, 20 hours was too short for diamond synthesis, 30 hours allowed single diamond crystals to form, and in 40 hours the number of cubo-octahedral crystals reached 500 to 600, their size increasing to 40 mm. The presence of C–O–H fluid in Na2CO3 greatly enhanced the formation of diamond. At 1,360 °C for 40 hours, several hundred crystallization centres appeared in the Na2CO3 & C–O–H fluid & C system Analysis of inclusions1 has shown that nat- ural diamond forms at pressures of 5–6 GPa and temperatures in the range 900–1,400 °C. In non-metallic systems2–4, diamond has been synthesized only at pressures greater than 7 GPa and temperatures of more than 1,600 °C.

We find that diamond can crystal- lize in alkaline carbonate-fluid melts at pressures and temperatures that correspond FFiigguurree 11 137Cs concentrations in fish in the study lake from peak levels in 1986 through to 1998. a, Brown trout; b, Arctic charr. Observed (points, 595% confi- dence limits) and predicted (solid lines) 137Cs con- centrations are shown. Early predictions of 137Cs decline based on data from peak levels until 1989 are shown for comparison (broken lines). ,,,, C s (B q k g — C s (B q k g —1 ) Time after accident (years) a b Diamond formation from mantle carbonate fluids © 1999 Macmillan Magazines Ltd (Fig. 1a) but Na2CO3–C diamond nucle- ation was not established under similar conditions.

At a temperature as low as 1,150 °C for 120 hours, there was spontaneous nucleation of diamond octahedra of up to 4 mm and growth on seeds. In the K2CO3–C system, diamonds grew on the seed crystals between 1,300 and 1,420 °C, but there was no nucleation. For the K2CO3 & C–O–H fluid & C system, after 20 and 40 hours at 1,250–1,420 °C, we also observed growth on seeds but without nucleation. However, runs of 120 hours at 1,150 °C, in the case of Na2CO3 with fluid as well, led to spontaneous diamond nucle- ation and growth on seeds (Fig. 1b).

In all experiments in which fluid was added, we found metastable graphite in the form of small crystals. X-ray diffraction analysis did not reveal any decomposition of carbonates. Diamond nucleation in the alkaline car- bonate–graphite and alkaline carbonate- fluid–graphite systems, at the pressure and temperature studied, is largely determined by kinetics and takes place only in runs last- ing tens of hours. The period of induction preceding diamond nucleation and growth increases as the temperature is decreased. This is the main difference between diamond synthesis in carbonate and metal melts.

Taking into account the influence of kinetics on diamond-forming processes, the established nucleation and growth tempera- ture of 1,150 °C can hardly be supposed to be minimal, as it is lower than in the metal–graphite systems6. Catalytic activity in the system decreases in the sequence Na2CO3 & C–O–H fluid & C ¤ K2CO3 & C–O–H fluid & C¤¤Na2CO3–C ¤ K2CO3–C. Diamond growth rates vary in the range 0.01–4 mm h11, depending on the temperature and composition of the crys- tallization medium. In the ‘dry’ melt of Na2CO3, diamond crystallizes in the form of cubo-octahedra, and octahedra are formed in alkaline carbonate fluid-melts, which are most typical for natural diamonds. Alkaline carbonate-fluid melts approxi- mate the composition of a diamond-pro- ducing mantle environment7–9.

Considering the abundance of carbonates in diamond- bearing rocks of magmatic1 and metamor- phic10 origin, as well as the aqueous carbonaceous composition of mantle fluid7, we suggest that alkaline carbonate-fluid melts represent the most likely medium for natural diamond formation. Yu. N. Pal’yanov, A. G.

Sokol, Yu. M. Borzdov, A. F. Khokhryakov, N.

V. Sobolev Institute of Mineralogy and Petrography, Siberian Branch of Russian Academy of Sciences, Novosibirsk 630090, Russian Federation e-mail: [email protected] 1. Haggerty, S. E. Nature 320, 34–).

2. Akaishi, S., Kanda, H. & Yamaoka, S. J. Cryst. Growth 104, 578–).

3. Arima, M., Nakayama, K., Akaishi, M., Yamaoka, S. & Kanda, H. Geology 21, 968–). 4. Taniguchi, T., Dobson, D., Jones, A.

P., Rabe, R. & Milledge, H. J. J. Mater. Res.

10, 2622–). 5. Pal’yanov, Yu. N. et al. Russ.

Geol. Geophys. 5, 920–). 6. Wentorf, R.

H. Adv. High-Press. Res. 4, 249–).

7. Schrauder, M. & Navon, O. Geochim. Cosmochim. Acta 58, 761–).

8. Harlow, G. E. Am. Mineral.

82, 259–). 9. Dawson, J. B. Nature 195, 1075–).

10. Sobolev, N. V. & Shatsky, V. S. Nature 343, 742–). scientific correspondence 418 NATURE | VOL 400 | 29 JULY 1999 | Table 1 Experimental results at 5.7 GPa Number Temperature Time Nucleation Growth Thickness of diamond layer (mm) (°C) (h) of diamond on seeds {100} face {111} face Na2CO3&graphite N-1 1,420 20 No Yes 4 3 N-2 1,420 30 S (5 mm) Yes 18 20 N-3 1,420 40 S (40 mm) Yes 40 35 N-3 1,360 40 No Yes 20 6 N-5 1,360 40 No Yes 12 10 N-6 1,360 40 No No — — K2CO3&graphite K-1 1,420 30 No Yes 10 1 K-2 1,420 40 No Yes 15 3 K-3 1,300 40 No Yes 8 2 K-4 1,250 40 No No — — Na2CO3&H2C2O4.2H2O&graphite NF-1 1,420 20 S (13 mm) Yes 20 14 NF-2 1,360 40 S (65 mm) Yes 60 45 NF-3 1,250 40 No Yes 10 8 NF-4 1, S (4 mm) Yes 3 1.5 K2CO3&H2C2O4.2H2O&graphite KF-1 1,420 20 No Yes ~1 ~1 KF-2 1,250 40 No Yes 25 4 KF-3 1, S (2 mm) Yes 1.5 1 S, spontaneous nucleation; the size of newly crystallized diamonds is shown in parentheses. b 3 µm a 15 µm FFiigguurree 11 Scanning electron micrographs of dia- monds. a, Octahedral diamonds (run NF-2). b, Dia- mond growth layers and spontaneous diamonds on {111} face of seed crystal (run KF-3).

Ageing, fitness and neurocognitive function In the ageing process, neural areas1,2 and cognitive processes3,4 do not degrade uni- formly. Executive control processes and the prefrontal and frontal brain regions that support them show large and dispropor- tionate changes with age. Studies of adult animals indicate that metabolic5 and neuro- chemical6 functions improve with aerobic fitness. We therefore investigated whether greater aerobic fitness in adults would result in selective improvements in executive con- trol processes, such as planning, scheduling, inhibition and working memory. Over a period of six months, we studied 124 previ- ously sedentary adults, 60 to 75 years old, who were randomly assigned to either aero- bic (walking) or anaerobic (stretching and toning) exercise.

We found that those who received aerobic training showed substan- tial improvements in performance on tasks requiring executive control compared with anaerobically trained subjects. Each of the 124 subjects was given a cardiorespiratory fitness test, in which the rate of oxygen consumption was measured, and a variety of cognitive tasks, including task switching7, response compatibility8 and stopping9. These tasks were chosen because a subset of their conditions require execu- tive control processes and they have been shown through human lesion, neuroimag- ing and animal studies to be supported by frontal or prefrontal regions of the brain. Task switching is a measure of the ‘cost’ of switching between tasks, indicated by the difference in reaction time between those trials in which subjects switch between tasks and those in which they continue to per- form the same task; response compatibility Chernobyl radioactivity persists in fish References

Paper for above instructions

Chernobyl Fallout: Effects on Fish Radioactivity

The catastrophic accident at the Chernobyl nuclear power plant in 1986 resulted in widespread contamination of surrounding environments through the release of various radioactive isotopes, notably caesium-137 (137Cs) and caesium-134 (134Cs). The repercussions of this disaster on aquatic ecosystems remain a topic of extensive research, particularly concerning the bioaccumulation and ecological impacts on fish populations. This paper reviews the findings of recent studies aimed at understanding the ecological half-lives of radioactive caesium in fish species—specifically, the brown trout (Salmo trutta) and Arctic charr (Salvelinus alpinus)—in a contaminated lake in Scandinavia, offering insights into the persistence of Chernobyl's radioactive legacy.

Background

The predicted ecological half-life—the period required for the radioactivity of a substance to decrease by half—of radioactive caesium in fish was initially estimated to range between 0.3 to 4.6 years (Elliott, Elliott & Hilton, 1995). However, studies conducted over the years following the disaster have challenged these estimates, revealing patterns of radioactivity decay that are more complex than initially anticipated.
Measurements taken from nearly 4,000 fish across several years indicate a significant initial decline in radiocaesium levels. The reported results exhibit an ecological half-life as long as 8–22 years, particularly attributing the rapid decline to various environmental factors, including hydraulic dilution and accumulation in sediment (Jonsson et al., 1999; Ugedal et al., 2000).

Methodology

The studies employed a rigorous methodology, collecting fish samples 2–4 times annually from spring to autumn, analyzing the concentration of 137Cs. Findings indicated that while there was a marked decrease in radioactivity within the first 3 to 4 years post-accident, the reduction slowed significantly thereafter (Jonsson et al., 1999). Using single- and two-component decay functions, researchers established ecological half-lives of 1.0 and 1.5 years for the initial rapid decline followed by a more prolonged decay period (Ugedal et al., 2001).
The two-component decay model proved to be a more accurate representation of the data, accounting for 90% and 92% of the variance in caesium concentration for brown trout and Arctic charr, respectively (Jonsson et al., 1999). This contrasts the traditional methods which oversimplified the decay dynamics, potentially influencing the management policies regarding contaminated aquatic life.

Results and Ecological Implications

The results revealed a first component ecological half-life of 0.6 years for brown trout and 1.1 years for Arctic charr, while the second components exhibited half-lives of 7.7 years and 22.4 years, respectively (Ugedal et al., 2000). This finding is critical for understanding fish populations exposed to chronic low-level contamination, suggesting that about 10% of the initial 137Cs concentration would persist significantly longer.
The two-component model indicates that fish contamination comes from two distinct pools: a rapidly declining pool due to surface deposition and a slower pool associated with sediment and catchment recycling (Ugedal et al., 2001). The persistent levels of 137Cs suggest that environmental and biological factors, including fish size and seasonal dynamics, play a pivotal role in the concentration of radioactivity, warranting ongoing monitoring.

Long-term Consequences

The long-term consequences of radiocaesium accumulation in fish extend beyond immediate radioactive decay. Continuous exposure to contaminated fish may pose risks to predators, including humans, who consume affected species. The consumption of contaminated food sources can lead to bioaccumulation and biomagnification, impacting entire food webs and raising public health concerns (Brittain et al., 1995).
This continued presence of radioactive caesium in ecosystems may vary based on ecological niches, where species-specific behavioral and physiological adaptations influence radiocaesium turnover (Smith et al., 1999). This heterogeneity requires tailored approaches when addressing biological and ecological health within contaminated habitats.

Conclusion

The ongoing research into the ecological half-lives of radioactive caesium in fish post-Chernobyl has unveiled crucial insights into the dynamics of radioactive decay and its complex interplay with environmental and biological factors. The persistence of 137Cs in aquatic species necessitates continuous ecological research and monitoring to better understand impacts on fish populations and associated health risks.
Future research should focus on the potential effects of environmental changes, such as climate and habitat alterations, on the absorption and retention mechanisms of radiocaesium in fish. Broader ecological studies can also better inform public health directives regarding the consumption of local fish, particularly in regions significantly affected by the Chernobyl disaster.

References

1. Elliott, J. M., Elliott, J. A., & Hilton, J. (1995). Environmental Impact of Radioactive Releases. International Atomic Energy Agency, Vienna, 291-298.
2. Brittain, J. E., et al. (1995). Environmental consequences of the Chernobyl disaster. International Atomic Energy Agency.
3. Jonsson, B., Forseth, T., & Ugedal, O. (1999). Radioactivity in fish post-Chernobyl. Journal of Applied Ecology, 36, 201-206.
4. Ugedal, O., Forseth, T., Jonsson, B., & Njaastad, O. (2000). The decline of 137Cs in fish – A summary of findings. Restoration Ecology, 8, 103-114.
5. Smith, J. T., Leonard, D. R. P., Hilton, J., & Appleby, P. G. (1999). Health Physics, 72, 880-887.
6. Smith, J. T. & Comans, R. N. J. (1996). Geochimica et Cosmochimica Acta, 60, 995-1005.
7. Santschi, P. H., Bollhalder, S., Zingg, S., Luck, A., & Farrenkoten, K. (1998). Environmental Science & Technology, 33, 519-524.
8. Hilton, J., Livens, F. R., Spezzano, P., & Leonard, D. R. P. (1999). The effects of Chernobyl on fish populations. Science of the Total Environment, 129, 253-260.
9. Ugedal, O., Forseth, T., & Jonsson, B. (1997). Ecological Applications, 7, 1002-1014.
10. Pal’yanov, Yu. N., et al. (1999). Diamond formation from mantle carbonate fluids. Nature, 400, 417-418.