Information on the main sources supplying deleterious sediment loads to river systems is needed to improve our understanding of soil erosion processes. In particular, it is important to quantify the respective contributions of surface and subsurface sources to material degrading waterbodies. Radiocesium (137 Cs), emitted during thermonuclear bomb testing (~1950-1980) and nuclear accidents provides significant discrimination between surface material exposed to atmospheric fallout and subsurface material sheltered from it. A systematic worldwide review of research articles (n=123) that used 137 Cs to trace sediment sources showed that the United Kingdom (n=24), Australia (n=23) and the United States (n=20) had the highest number of publications utilizing 137 Cs in a sediment tracing framework. In contrast, few studies were published for catchments in Africa (n=9) or South America (n=2). In the northern hemisphere, positive relationships were evident between 137 Cs activities in surface and subsurface sources and the proportion of thermonuclear bomb fallout. However, given the low proportions of fallout received in regions between 0-20°N and 0-20°S, the potential application of 137 Cs tracing techniques may be limited in this area of the world as well as in agricultural regions with severe soil erosion (i.e. Chinese Loess Plateau and South Africa). In total, 94% of the studies researching surface and subsurface sources that analyzed 137 Cs as a potential tracing property included this parameter in end-member mixing models. The main challenges for the future of this technique are mainly related to the access to ultra-low background gamma spectrometry facilities and the development of surrogate tracers. Future research should ensure that basic catchment information and details on the sampling design are properly documented to ensure studies are reproducible. Moreover, researchers should provide raw measurement data to help improve our global understanding of 137 Cs dynamics in soil erosion research.

Owing to the rapid expansion of agriculture in South America in recent decades, soil erosion and fine 26 sediment supply to river networks, which lead to deleterious on-site and off-site environmental 27 impacts, are exacerbated in intensively cultivated catchments. Measuring soil inventories of bomb-28 derived fallout radiocesium ( 137 Cs) bound to fine particles is one of the few techniques available to reconstruct soil redistribution rates and evaluate the sustainability of farming practices over the recent 30 phase of agricultural intensification (1960s-2020). However, information about the spatial distribution 31 of 137 Cs fallout across the soils of South America remains scarce, and the published data has not been 32 synthesized at the scale of this subcontinent so far. The objective of the current research is therefore 33 to quantify and map the initial 137 Cs fallout at the scale of South America, based on the compilation of 34 published 137 Cs inventories, additional measurements conducted on undisturbed soil profiles and 35 digital soil mapping as this baseline information may be useful for a wide range of Earth Science 36 applications. A database of 137 Cs inventories at 96 reference sites (i.e. areas without soil erosion nor 37 accumulation) has been compiled for a variety of soil profiles (Argentina = 10, Brazil = 34, Chile = 46, 38 Uruguay = 5, French Guiana = 1) located between 5.3° North latitude and 53° South latitude. The spatial 39 distribution of 137 Cs fallout was shown to be highly latitude-dependent, with a maximum in the 30-50° 40 South latitude band. There were higher fallout levels than expected between 20 to 60° South latitude 41 compared to the previous estimations made by UNSCEAR. A partial least square regression approach 42 based on rainfall data and geographical information as covariates was used to create a baseline map 43 of 137 Cs fallout in soils of continental South America. This baseline map provides a powerful reference 44 dataset to anticipate the order of magnitude of 137 Cs inventories in undisturbed soil profiles collected 45 in Brazil and Southern Chile and for numerous other applications in Earth Sciences. The potential 46 application of the 137 Cs inventory technique in countries of South America in general, and in regions 47 investigate soil redistribution rates in most of South American countries where detectable levels of 52 137 Cs can be expected to be found in sites exposed to erosion.

From 1985 onwards, South America has undergone a major expansion of agriculture at the expense of native vegetation (e.g. native Pampa grassland). As an emblematic crop, the surface area cultivated with soybeans has increased by 1000% between 1990 and 2020 in Uruguay. The environmental consequences of this massive land use conversion on soil degradation remain poorly documented although the agriculture expansion is projected to continue to increase in the coming years in South America. In this study, sediment cores were collected in reservoirs located downstream of two contrasted agricultural catchments draining the Rio Negro River (Uruguay) for reconstructing the sediment dynamics and the sources of erosion associated with this expansion.Results demonstrated the occurrence of two periods of acceleration of sediment delivery since the 1980s. The first period of acceleration was recorded in the mid-1990s and was related to afforestation programs. The second and larger acceleration phase was recorded after 2000 during the soybean crop expansion. This period has been marked by a greater supply of sediment from the native grassland source highlighting the impact of agriculture expansion at the expense of native vegetation. Conservation measures should therefore be urgently taken to preserve biodiversity and soil functions in this region.

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<p>The Fukushima-Daiichi Nuclear Power Plant (FDNPP) accident trigged by the Great East Japan Earthquake and subsequent tsunami in March 2011 released large quantities of radionuclides in terrestrial and marine environments of Fukushima Prefecture. Although radiocaesium (i.e. <sup>134</sup>Cs and <sup>137</sup>Cs) activity in these environments has decreased since the accident, the secondary inputs via the rivers draining and eroding the main terrestrial radioactive plume were shown to sustain high levels of <sup>137</sup>Cs in riverine and coastal sediments, which are likely deposited off the coast of the Prefecture. Accordingly, identifying the sources of sediment is required to elucidate the links between terrestrial and marine radiocaesium dynamics and to anticipate the fate of persistent radionuclides in the environment.</p><p>The objective of this study is to develop an original sediment source tracing technique to quantify the riverine sources of sediment and associated radionuclides accumulated in the Pacific Ocean. Target coastal sediment cores (<em>n=6</em>) with a length comprised between 20 and 60cm depth were collected during cruise campaigns between July and September 2022 at the Ota (<em>n=2</em>), Niida (<em>n=1</em>) and Ukedo (<em>n=3</em>) river mouths. Prior to gamma spectrometry measurements, sediment cores were opened and cut into 2 cm increments, oven-dried at 50°C for at least 48 hours, ground and passed through a 2-mm sieve.</p><p>Preliminary results regarding the spatial and depth distribution of radiocaesium in these samples show a strong heterogeneity, with highest radiocaesium levels (up to 134 ± 2 and 4882 ± 11 Bq kg<sup>-1</sup> for <sup>134</sup>Cs and <sup>137</sup>Cs, respectively) found in coastal sediment cores located at the Ukedo river mouth. On the opposite, no trace or low levels of Fukushima-derived radiocaesium were found in the Niida and in one sediment core of the Ota River mouths. Additional measurements will be conducted to determine the physico-chemical properties of this sediment, in order to select the optimal combination of tracers, which will then be introduced into un-mixing models. This increase knowledge will undoubtedly be useful for watershed and coastal management in the FDNPP post-accidental context.</p>

<p>Sedimentary sequences have received a growing interest as a support for conducting climatic and environmental reconstructions covering the 20th century period, which has been highly impacted by socio-environmental changes. South-America is one of the regions of the world the most impacted by these changes (e.g. agricultural expansion, extreme climatic events) which induce many deleterious consequences (e.g. increase of soil erosion, transfer of contaminants). However, quantitative information regarding soil erosion and sediment accumulation processes at the catchment scale is currently lacking to determine the magnitude of these phenomena and promote effective policies to mitigate their environmental and economic impacts.</p> <p>Fallout of anthropogenic radionuclides (<sup>137</sup>Cs, <sup>239</sup>Pu and <sup>240</sup>Pu) emitted by atmospheric nuclear weapon tests conducted between 1945 and 1980 provides an opportunity to overcome this lack of information. Indeed, artificial radionuclides bound to fine-grained sediment have been increasingly recognized as powerful tools to conduct environmental, climatic and soil redistribution rate reconstructions during the Anthropocene. Although spatial and temporal reconstructions of this fallout have been conducted worldwide, this information remains scarce in South America. In addition, scientific controversies emerged regarding the contribution of French atmospheric nuclear tests to the deposition of artificial radionuclides in this region of the world, requiring further investigation.</p> <p>Based on a compilation of <sup>137</sup>Cs inventories in undisturbed soil profiles (n=96) and a digital soil mapping approach, an open-access baseline map of <sup>137</sup>Cs fallout at the subcontinental scale of South America was created. The results showed that the <sup>137</sup>Cs inventory technique should be appropriate to reconstruct soil erosion in intensive agricultural landscapes of Chile, Argentina, Uruguay and southern Brazil and theoretically applicable in Paraguay, Bolivia and Peru. Compared to previous estimations, higher levels of <sup>137</sup>Cs fallout were observed between 20 and 60° South latitude. Additional measurements were therefore conducted in undisturbed soils and lake sediment cores collected at these latitudes by analyzing the <sup>240</sup>Pu/<sup>239</sup>Pu atomic ratios, which is a powerful tool to determine the sources and their respective contributions to the deposition of anthropogenic radionuclides. Significantly lower plutonium atom ratios were found and attributed to the higher contribution (up to 60% in Uruguay) of the fallout following French atmospheric nuclear tests between 1966 and 1974.</p> <p>This refined chronology and spatial distribution of bomb-derived fallout will undoubtedly be useful to avoid misinterpretations of sediment core dating and reconstruct soil redistribution rates during the Anthropocene in South America.</p>

<p>Air masses loaded with mineral dust and originating from the Sahara arrive frequently in Europe, which has multiple impacts on global and regional cycles. However, the occurrence of these processes may further accelerate in the future in response to climate change, and more knowledge is therefore required on the characteristics of the particles transported during these massive dust transport and deposition episodes. Furthermore, questions arise regarding the content of this dust in radionuclides, in relationship with the atmospheric nuclear bomb testing conducted around the world between the 1950s and the 1970s in general, and those tests conducted by France in the Sahara in the early 1960s in particular.</p><p>The Saharan dust episode that took place from 13<sup>th</sup> to 16<sup>th</sup> March 2022 led to the occurrence of dense dust deposition across multiple European countries, which raised concerns among the population regarding the potential radioactivity content of this dust. To address this question with a representative sample set, a participative science campaign to collect dust across Europe was launched on Twitter on 17<sup>th</sup> March 2022. Thanks to this initiative, 110 dust samples could be collected along a transect from Southern Spain to Austria.</p><p>This unique sample bank was regrouped at University Paris-Saclay, France, to conduct a set of physico-chemical analyses on a selection or on the totality of these dust samples including particle size, colourimetry, mineralogy and fallout radionuclides.</p><p>Backward trajectories of air masses that have led to these deposits were calculated, and this analysis confirm their potential origin from Algeria. <sup>137</sup>Cs was detected in all dust samples, with variable activity concentrations. A strong relationship was found between the particle size of the analysed particles and the <sup>137</sup>Cs activity concentrations, which is consistent with the literature on this topic. Particle size was found to decrease with increasing distances from the source. The colour and mineralogy analyses demonstrated that the dust collected in Austria showed different properties than those samples collected in Spain, France, Luxembourg and Germany, which likely indicates that this material did not fully consist of Saharan dust deposited during the March 2022 episode. Accordingly, the following interpretations did not take the properties of Austrian dust into account.</p><p>The mineralogical analyses confirmed the potential origin of the dust from the Maghreb region, including a vast area in Southern Morocco and Southern Algeria. In contrast, the analysis of plutonium isotopic ratios (<sup>240</sup>Pu/<sup>239</sup>Pu) and <sup>137</sup>Cs/<sup>239+240</sup>Pu activity ratios, which provide diagnosis tools to investigate the source of artificial radionuclides, in a selection of dust samples collected between Southern Spain and Luxembourg showed that the dust signature was consistent with that of the global fallout largely dominated by the nuclear tests conducted by the USA and the Soviet Union. The <sup>137</sup>Cs contained in the dust transported and deposited during this episode was therefore very likely not associated with the French nuclear tests conducted in the early 1960s in Sahara.</p><p>In the future, elemental geochemistry analyses will provide additional information on their source provenance. All results will also be published in open-access database and disseminated to the public.</p>

Abstract. Twelve years after the nuclear accident that occurred at the Fukushima Dai-ichi Nuclear Power Plant in March 2011, radiocesium contamination (with a large dominance of 137Cs, with a 30-years half-life) remains a major concern in various municipalities of Northeastern Japan. The Japanese authorities completed an unprecedented decontamination programme in residential and cultivated areas affected by the main radioactive plume (8953 km²). They implemented a complex remediation programme scheme relying on different decision rules depending on the waste type, its contamination level and its region of origin, after delineating different zones exposed to contrasted radiation rates. The central objective was not to expose local inhabitants to radioactive doses exceeding 1 mSv yr-1 in addition to the natural levels. At the onset of the full reopening of the Difficult-to-Return Zone in Spring 2023, the current review provides an update of a previous synthesis published in 2019 (Evrard et al., 2019). Although this ambitious remediation and reconstruction programme is almost completed, in the 12 municipalities of Fukushima Prefecture in which an evacuation order was imposed in at least one neighbourhood in 2011, from the 147,443 inhabitants who lived there before the accident, only 29.9 % of them had returned by 2020. Waste generated by decontamination and tsunami cleaning/demolition work is planned to have been fully transported to (interim) storage facilities by the end of 2023. The cost of the operations conducted between 2011–2020 for the so-called ‘nuclear recovery’ operations (including decontamination) was estimated by the Audit Board of Japan in 2023 to 6122.3 billion yen (~44 billion euro). Decontamination of cropland was shown to have impacted soil fertility, and potassium fertilization is recommended to limit the transfer of residual radiocesium to new crops. In forests that cover 71 % of the surface area of the Fukushima Prefecture and that were not targeted by remediation, radiocesium is now found in the upper mineral layer of the soil in a quasi-equilibrium state. Nevertheless, 137Cs concentrations in forest products (including wood for heating and construction, wild plants, wildlife game, mushrooms) often keep exceeding the threshold values authorized in Japan, which prohibits their exploitation in the area affected by the main plume. Radionuclides from forest were shown to be exported in dissolved and particle-bound forms to downstream river systems and floodplains, although multiple monitoring records showed the continuous decrease in radiocesium concentrations in both river water and sediment across the main plume between 2011–2021. Fish contamination is now generally found below the threshold limits although reputational damage remains a major concern for local fishing communities. The remobilisation of radiocesium from sediment accumulated in reservoirs of the region is also of potential concern as it may lead to secondary contamination of fish or irrigation waters supplied to decontaminated fields. Overall, this synthesis demonstrates the need to continue monitoring post-accidental radiocesium transfers in these environments and to keep sharing data in order to refine our predictive understanding of radiocesium mobility and consolidate the tools available to model contaminant transfers in ecosystems. In forests in particular, novel countermeasures and wood uses remain to be developed and tested. Furthermore, the hydrologic connectivity between ecosystems is of great influence on long term radiocesium transport. The consequences of extreme phenomena (e.g., typhoons, forest fires) that may become more frequent in the future as a result of global change in these contaminated environments should be further anticipated.