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Isotopic signature of plutonium accumulated in cryoconite on glaciers worldwide

Science Direct, Volume 951, 15 November 2024,

Edyta Łokas ^a, Giovanni Baccolo ^b, Anna Cwanek ^a, Jakub Buda ^c, Katarzyna Kołtonik ^a, Nozomu Takeuchi ^d, Przemysław Wachniew ^e, Caroline Clason ^f, Krzysztof Zawierucha ^c, Dylan Bodhi Beard ^g, Roberto Ambrosini ^h, Francesca Pittino ⁱ, Andrea Franzetti ⁱ, Philip N. Owens ^j, Massimiliano Nastasi ^kl, Monica Sisti ⁱ, Biagio Di Mauro ^m

Highlights

• •Cryoconite samples show larger deposition of ^239+240Pu, but not of ²³⁸Pu, in the Northern than in the Southern Hemisphere
• •Isotopic signatures of Pu in cryoconite show that besides the global fallout the regional contributions may be significant
• •First evidence of ²³⁸Pu contamination from the crash of Interplanetary Station “Mars’96”

Abstract

Glaciers are recognized as repositories for atmospheric pollutants, however, due to climate change and enhanced melting rates, they are rapidly transitioning from being repositories to secondary sources of such apollutants. Artificial radionuclides are one of the pollutants found on glaciers that efficiently accumulate onto glacier surfaces within cryoconite deposits; a dark, often biogenic sediment. This work provides information about the accumulation, distribution and sources of plutonium (Pu) isotopes in cryoconite samples from glaciers worldwide.

 Plutonium is an artificial radionuclide spread into the environment in the last decades as a consequence of nuclear test explosions, accidents and nuclear fuel re-processing. Samples collected from 49 glaciers across nine regions of Earth are considered. Activity concentrations of plutonium in cryoconite are orders of magnitude higher than in other environmental matrices typically used for environmental monitoring (e.g. lichens, mosses, soils and sediments), particularly in the Northern Hemisphere. 

 Isotopic ratios indicate that plutonium contamination of cryoconite is dominated by the global signal of stratospheric fallout related to atmospheric nuclear tests. However, specific glaciers in Svalbard reveal a signature compatible with a contribution from the re-entry of the SNAP-9A satellite in 1964, which was equipped with a ²³⁸Pu radioisotope thermoelectric generator. Similarly, an excess of ²³⁸Pu is observed in cryoconite from the Exploradores Glacier (Chile). This could be associated with the November 1996 crash of the automatic Interplanetary Station “Mars ’96” which was carrying a ²³⁸Pu thermoelectric generator. This is the first time ever that an isotopic evidence for this event is reported. These findings highlight the role that cryoconite can play in reconstructing the radioactive contamination history of different glaciated regions of the Earth.

Introduction

Atmospherically derived radioactivity is the component of environmental radioactivity that is deposited on the Earth’s surface through wet and dry deposition from the atmosphere. The deposited radionuclides are also named fallout radionuclides (FRNs). Some FRNs have a natural origin, such as cosmogenic ⁷Be and ¹⁴C, or are decay products of primordial isotopes. This is the case for ²¹⁰Pb, which derives from ²³⁸U.

However, most FRNs are artificial and occur globally as a result of atmospheric nuclear tests and unintentional nuclear accidents (UNSCEAR, 1982, UNSCEAR, 2000). A key requirement when dealing with environmental radioactivity is the assessment of contamination levels, including the reconstruction of contamination histories, the identification of transport pathways, and of the fate of the radioactivity released into the diverse environmental compartments (Engelbrecht and Schwaiger, 2008).

Glaciers are especially important for studying atmospheric fallout history (Jaworowski et al., 1978). First, glaciers consist of deposits of atmospheric precipitation and intrinsically accumulate fallout species, including FRNs. Under specific conditions (i.e. no melting, low horizontal ice flow), by studying the stratigraphy of ice and snow layers, it becomes possible to reconstruct the depositional history of FRNs (Gabrieli et al., 2011; Olivier et al., 2004). In addition to glacier ice, attention has recently turned to another environmental matrix typical of glaciated landscapes which accumulates radioactivity; cryoconite that is a type of sediment found on the surface of glaciers worldwide (Cook et al., 2016). …………………………………..

Plutonium (Pu) is a toxic, radioactive and predominately anthropogenic element produced through neutron irradiation of uranium in nuclear reactors and during nuclear weapon detonations (Zhong et al., 2019). The most significant releases of plutonium in the Northern Hemisphere were associated with global fallout (GF) resulting from atmospheric nuclear weapon tests carried out between 1945 and 1980, with a peak in the 1960s (UNSCEAR, 1982, UNSCEAR, 2000). 

Other important sources are related to catastrophic events such as the 1978 crash of the Cosmos-954 satellite, which had a nuclear reactor on board (Krey et al., 1979; Tracy et al., 1984), as well as the Chernobyl nuclear power plant disaster in 1986 (UNSCEAR, 2010) and the Fukushima Daiichi accident in 2011 (Povinec et al., 2013a; Povinec et al., 2013b).  Moreover, from 1964 to 1980, China conducted atmospheric nuclear testing at the Lop Nor test site in north-western Chi

The Northern Hemisphere has received two-thirds of global plutonium deposition (Clark et al., 2019). Fig. 1 illustrates the most significant atmospheric nuclear testing and accident sites in the Northern and Southern Hemispheres, including those near the Equator. 

The tests conducted in the Northern Hemisphere have received significant interest but much less is known regarding the deposition that took place in the Southern Hemisphere. The United Kingdom (UK) was at the forefront of the atmospheric nuclear testing program in the Southern Hemisphere between 1952 and 1957 in Australian territory (Johansen et al., 2019), while France conducted extensive open-air nuclear testing in French Polynesia in the South Pacific Ocean from 1966 to 1974 (Bouisset et al., 2021). The UK tests resulted in a substantial amount of regional fallout (i.e., tropospheric fallout), compared to the higher-yield French tests, which contributed to the stratospheric fallout.

In 1964, the Transit 5BN3 satellite carrying a SNAP 9A radioisotope thermoelectric generator, launched by the United States of America (USA), failed to achieve orbit. The satellite burned up when descending into the upper atmosphere over Madagascar. The ²³⁸Pu load (1 kg) was dispersed worldwide and was detected globally in the environment, even in remote areas. Most of the fallout of ²³⁸Pu from this satellite occurred in the Southern Hemisphere (Hardy et al., 1972, Hardy et al., 1973). 

Another important event, although not well-documented, was reported by the International Atomic Energy Agency (Radiation and Safety, 2001) in their inventory of accidents and losses at sea involving radioactive material. According to the report, it involved the atmospheric re-entry of the automatic Interplanetary Station “Mars ’96”, which was launched on November 16th, 1996. The station fell off the coast of Chile near the border with Bolivia and has not been located to date.

Plutonium isotope deposition after weapons testing can be local, regional and global, depending on detonation height, yield and meteorological conditions ………………….

This study, for the first time, presents a comprehensive global analysis of the variation in activity concentrations of ²³⁸Pu and ^239+240Pu, along with activity (²³⁸Pu/^239+240Pu) and atomic (²⁴⁰Pu/²³⁹Pu) ratios, observed in cryoconite on glaciers from both hemispheres. 

…………………………….Conclusions

This study provides new insights into the provenance of Pu isotopes (²³⁸Pu, ²³⁹Pu, ²⁴⁰Pu) in glaciers based on cryoconite samples collected from nine glaciated regions of six continents. The ^239+240Pu activity concentrations are significantly higher in the Northern Hemisphere than in the Southern Hemisphere, which reflects the uneven deposition of global fallout between hemispheres. Within the Northern Hemisphere the highest concentrations occur in Scandinavia and the European Alps…………………………………………….. more https://www.sciencedirect.com/science/article/abs/pii/S0048969724055062

October 27, 2024 - Posted by Christina Macpherson | - plutonium