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Saharan dust outbreaks have profound effects on ecosystems, climate, human health, and the cryosphere in Europe. However, the spatial deposition pattern of Saharan dust is poorly known due to a sparse network of ground measurements. Following the extreme dust deposition event of February 2021 across Europe, a citizen science campaign was launched to sample dust on snow over the Pyrenees and the European Alps. This somewhat improvised campaign triggered wide interest since 152 samples were collected from the snow in the Pyrenees, the French Alps, and the Swiss Alps in less than 4 weeks. Among the 152 samples, 113 in total could be analysed, corresponding to 70 different locations. The analysis of the samples showed a large variability in the dust properties and amount. We found a decrease in the deposited mass and particle sizes with distance from the source along the transport path. This spatial trend was also evident in the elemental composition of the dust as the iron mass fraction decreased from 11 % in the Pyrenees to 2 % in the Swiss Alps. At the local scale, we found a higher dust mass on south-facing slopes, in agreement with estimates from high-resolution remote sensing data. This unique dataset, which resulted from the collaboration of several research laboratories and citizens, is provided as an open dataset to benefit a large community and to enable further scientific investigations. Data presented in this study are available at https://doi.org/10.5281/zenodo.7969515 (Dumont et al., 2022a).

During the last twenty years, Mayotte island (France, Comoros archipelago) has experienced an explosion of population (+80% between 2002 and 2021, taking the official population into account only) leading to a deforestation and a decline in traditional agricultural practices (mahorais garden) in favor of intensive food crop monocultures (e.g. banana, manioc…). These changes in land use are at the origin of an acceleration of erosion dynamics and an excessive transfer of sediments to water bodies downstream. In addition to land degradation, sediment transfers contribute to the filling of reservoirs devoted to supplying drinking water, to the deterioration of the lagoon and to a general loss of biodiversity.The inertia, trajectory and the sources of this erosion remain poorly documented despite the acceleration of this land use evolution during the last 10 years. In this study, we propose a retro-observation of sedimentary fluxes and sources of sediment based on the analysis of sedimentary archives collected in one of the main reservoirs of the island (Dzoumogné reservoir, 22 ha) along with the development of a sediment tracing approach (associating radionuclide measurements, elemental geochemistry, organic matter and color analyses) in order to reconstruct the evolution of erosion rates and sources of since 2011.The first results reveal the occurrence of two periods of erosion acceleration between 2013 and 2015, and then after 2019. The first increase (+140% of erosion rates, from 3.5 to 8.5 t ha-1 yr-1) occurred during a period of deforestation, which induced an opening of the landscape (e.g. creation of unpaved roads). The second acceleration phase took place during the most recent period of agriculture expansion associated with an intensification of agricultural practices (+115% of erosion rates, from 3 t ha-1 yr-1 in 2017 to 6.5 t ha-1 yr-1 in 2020). The sediment accumulated in the reservoir between 2011 and 2021mostly originated from subsurface sources (bank erosion, unpaved roads, gullied soils) and intensive cultivation (e.g., in 2021, 35% of sediment originated from unpaved roads, channel banks, landslides and 27% from intensive cultivation) while forests that are less exposed to erosion only supplied a limited contribution to sediment (

In the current context of raising concerns related to nuclear accidents and warfare, the lessons learnt from the Fukushima accident in 2011 are of particular interest. Indeed, the Japanese authorities implemented an ambitious decontamination program, which strongly differs from the strategy adopted in Chernobyl where the most contaminated area remains closed to the population nowadays. However, the impact of this strategy on the dispersion of radioactive contaminant fluxes across mountainous landscapes exposed to typhoons remains to be quantified. Based on the unique combination of river monitoring and modelling in a catchment representative of the most impacted area in Japan, we could demonstrate for the first time that decontamination only led to a decrease of 17% of the radionuclide fluxes in the river system. Furthermore, we calculated that 67% of the initial radiocesium remains stored in forests and may contribute to radiocesium dispersion in river systems in response to future erosive events. As the current research was conducted in an area representative of the 1,117 km²-area where remediation was completed early in 2017, it raises questions about the overall sustainability and cost-benefit effectiveness of such a remediation program that generated 9,100,000 m3 of waste for a cost of ~12 billion USD. Only a limited proportion of the initial population returned to their hometown (~30% by 2019), which remains a major challenge for the future of this region, although the primary goal of authorities to decrease the radiation dose rates in the inhabited areas was achieved.

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.Fallout of anthropogenic radionuclides (137Cs, 239Pu and 240Pu) 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.Based on a compilation of 137Cs inventories in undisturbed soil profiles (n=96) and a digital soil mapping approach, an open-access baseline map of 137Cs fallout at the subcontinental scale of South America was created. The results showed that the 137Cs 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 137Cs 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 240Pu/239Pu 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.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.

Between 1972 and 1993, in the French West Indies (Martinique and Guadeloupe), farmers applied a toxic organochlorine insecticide, chlordecone, to control the banana weevil. In the late 1990s, the intensification of agricultural practices in the West Indies led to accelerated soil erosion and sediment transfers to river systems and the sea (Sabatier et al., 2021). This increase in soil erosion leading in turn to a release of chlordecone stored in polluted agricultural soils. These accelerated lateral transfers of sediments are strongly controlled by land use and agricultural practices. The identification of soil erosion sources is therefore essential to effectively fight against the consequences of erosion on the resurgence of chlordecone. Using sediment tracing tools applied to coastal marine sediment archives, the objective of the current research was to model the potential changes in sediment sources throughout time in the West Indian catchments. Banana and sugarcane crops, forests, channel banks and landslides were targeted here as potential sources of sediment.To this end, soil samples were collected across the Galion catchment at locations presenting contrasted soil types and land use contexts. In addition, a marine sediment core was collected in the Galion Bay in April 2017. In order to quantify source contributions, a suite of physico-chemical properties was measured in both soil and sediment samples.Subsoils provided instead the main source of sediment in the Galion catchment (between 40 and 50% of sediment). In contrast, the contribution of cultivated soils increased during the 1960s (15 to 30% of sediment) and showed a second increase phase in the late 1990s (30 to 40% of sediment). These phases of increases were interrupted by decreases and major sediment contributions from subsoils. These increases of cultivated soils contributions can be explained by changes in agricultural practices (mechanization, irrigation) since the 1960s and the glyphosate introduction in the late 1990s, which increased soil erosion under cropland. Subsoils contribution increases correlate well with period of extreme events like Matthew cyclone in 2016.Overall, the comparison between the calculated sediment contributions and the reconstructed chlordecone fluxes shows that the decreases in subsoil contributions correlate well with those of chlordecone concentrations in marine sediments. In contrast, the increases of cultivated soil contributions to sediment correspond well to increases of chlordecone concentrations in sediment.Accordingly, these results showed the chlordecone contamination dilution due to increase of subsoil erosion. In the future, river sediment samples, collected with sediment traps, will also be analyzed using the same procedure to provide more detailed spatially-distributed information regarding erosion source contributions across the catchment.

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. 134Cs and 137Cs) 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 137Cs 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.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 (n=6) with a length comprised between 20 and 60cm depth were collected during cruise campaigns between July and September 2022 at the Ota (n=2), Niida (n=1) and Ukedo (n=3) 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.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-1 for 134Cs and 137Cs, 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.

Depending on hydrodynamic conditions, river sediments act as a sinks or a sources of microplastics through deposition and remobilisation processes. During flood events, the increase of river flow leads to an increase in the resuspension of bottom sediments and bank erosion processes and favoring the microplastic transportation. Previous work conduced in the Seine river catchment in 2018 has shown that floods, which occur for only 15% of the annual time, contributed to 40% of the total microplastic flux. Therefore, at the end of a flood period, with the decreasing in water level, some flow regimes allow the deposition of contaminated sediments carried by the river on the banks.This study presents the characterization and quantification of microplastics in ten samples lag deposits collected along the Seine river after the February 2021 flood event. Microplastics from 10 to 500 µm were analysed in replicate samples using two methods (FTIR microspectroscopy and Pyr-GC/MS). In order to characterize the origin of the sampled sedimentary deposits a fingerprinting approach based on the measurement of radionuclides activity (137Cs, 210Pbex and 7Be) was carried out. A mixing model was applied to discriminate old and recent sediments and their origin from the surface (e.g. soil erosion) or subsurface (e.g. bank erosion). High concentration levels of microplastics, ranging from 8,000 to 50,000 items/kg, were observed mainly characterised by FTIR microspectroscopy as PP, PE, PS and PVC. All the samples analysed show a similar size distribution with a majority of particles below 100 µm. PP is the most abundant polymer found. The quantification by Pyr-GC/MS provided masses consistent with microspectroscopy results ranging from 200 to 14,000 µg PP/kg of dry sediment. An increase in microplastic contamination between the upstream and the downstream part of the Paris area was observed. In this study, the relationships between sediment characteristics and microplastic contamination could not be demonstrated. Further work is needed to verify whether a more marked relationship can be observed in major events where a clearer variation in sediment sources is observed between the upper and lower parts of the Paris area.

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.

International audience; Recently, Comte et al. (2022) re-examined the natural degradation of chlordecone (CLD) in the soils of the French West Indies (FWI) by introducing an additional ‘dissipation parameter’ into the WISORCH model developed by Cabidoche et al. (2009). Recent data sets of CLD concentrations in FWI soils obtained by Comte et al. enabled them optimizing the model parameters, resulting in significantly shorter estimates of pollution persistence than in the original model. Their conclusions jeopardize the paradigm of a very limited degradation of CLD in FWI soils, which may lead to an entire revision of the management of CLD contamination. However, we believe that their study is questionable on several important aspects. This includes potential biases in the data sets and in the modeling approach. It results in an inconsistency between the estimated dissipation half-life time (DT50) of five years that the authors determined for CLD and the fate of CLD in soil from the application period 1972–1993 until nowadays. Most importantly, a rapid dissipation of CLD in the field as proposed by Comte et al. is not sufficiently supported by data and estimates. Hence, the paradigm of long-term persistence of CLD in FWI soils is still to be considered.

Purpose Fine sediment deposition is an important component of the catchment sediment budget and affects river morphology, biology, and contaminant transfer. However, the driving factors of fine sediment deposition remain poorly understood at the catchment scale, limiting our ability to model this process. Methods Fine sediment deposition and river reach characteristics were collected over the entire river network of three medium-sized (200–2200 km2) temperate catchments, corresponding to 11,302 river reaches. This unique database was analyzed and used to develop and evaluate a random forest model. The model was used to predict sediment deposition and analyze its driving factors. Results Fine sediment deposition displayed a high spatial variability and a weak but significant relationship with the Strahler order and river reach width (Pearson coefficient r = −0.4 and 0.4, respectively), indicating the likely nonlinear influence of river reach characteristics. The random forest model predicted fine sediment deposition intensity with an accuracy of 81%, depending on the availability of training data. Bed substrate granularity, flow condition, reach depth and width, and the proportion of cropland and forest were the six most influential variables on fine sediment deposition intensity, suggesting the importance of both hillslope and within-river channel processes in controlling fine sediment deposition. Conclusion This study presented and analyzed a unique dataset. It also demonstrated the potential of random forest approaches to predict fine sediment deposition at the catchment scale. The proposed approach is complementary to measurements and process-based models. It may be useful for improving the understanding of sediment connectivity in catchments, the design of future measurement campaigns, and help prioritize areas to implement mitigation strategies.

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.The Saharan dust episode that took place from 13th to 16th 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 17th March 2022. Thanks to this initiative, 110 dust samples could be collected along a transect from Southern Spain to Austria.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.Backward trajectories of air masses that have led to these deposits were calculated, and this analysis confirm their potential origin from Algeria. 137Cs 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 137Cs 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.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 (240Pu/239Pu) and 137Cs/239+240Pu 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 137Cs 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.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.

International audience; At continental and geological time scales, tectonic forces and climate are the first landscape evolution drivers. However, since the Neolithic, human is recognised as being a new geomorphic agent, indirectly or directly moving significant amount of materials across landscapes. In a context of global changes, when both climate and human activities are rapidly evolving, the question of the relative contribution of climate variability and anthropogenic activities to soil erosion remains poorly quantified. Understanding this contribution is however key to the design of appropriate soil erosion management plans. Based on two catchment-scale hydrosedimentary observatories, the objective of this study is to quantify the relative importance of climate variability and land use change on the erosive response of two headwater catchments located in contrasted environments. The first study area is in a volcanic tropical island and the second one is in a lowland, intensively cultivated plain under an oceanic temperate climate. More particularly, we will investigate the importance of rainfall and associated flood events intensity and frequency on the water and sediment fluxes. The influence of these temporal dynamics (i.e. frequency/intensity of events) will be studied according to different land uses, as the two questions (spatial/temporal variabilities) are closely interlinked.

Several innovative techniques have been developed recently opening up new avenues to establish the assessment of sediment flux in the critical zone. These techniques include the tracing or “fingerprinting” methods to identify sediment sources and quantify the dynamics of particle-bound contaminants. However, the use of these techniques is often associated with several methodological and statistical limitations, that are often reported by the international scientific community but rarely addressed in the framework of concerted actions.This presentation will highlight the main developments and outcomes of the “International Tracing Day” 2022 and 2023, and the Tracing School organised in 2021. Based on the publication of an opinion paper (https://link.springer.com/article/10.1007/s11368-022-03203-1), new strategies to publish and disseminate sediment tracing databases will be presented. An example of a formatted dataset will be given, with the objective to test research hypotheses based on multiple datasets adopting the same format of data and meta-data. Other perspectives regarding improvements of the sediment fingerprinting method in terms of modelling, tracer options and selection will also be presented.

Bacterial pathogens in surface waters may threaten human health, especially in developing countries, where untreated surface water is often used for domestic needs. The objective of the long-term multiscale monitoring of Escherichia coli ([E. coli]) concentration in stream water, and that of associated variables (temperature (T), electrical conductance (EC), dissolved oxygen concentration ([DO]) and saturation (DO%), pH (pH), oxidation-reduction potential (ORP), turbidity (Turb), and total suspended sediment concentration ([TSS])), was to identify the drivers of bacterial dissemination across tropical catchments. This data description paper presents three datasets (see “Data availability” section) collected at 31 sampling stations located within the Mekong River and its tributaries in Lao PDR (0.6–25 946 km2) from 2011 to 2021. The 1602 records have been used to describe the hydrological processes driving in-stream E. coli concentration during flood events, to understand the land-use impact on bacterial dissemination on small and large catchment scales, to relate stream water quality and diarrhea outbreaks, and to build numerical models. The database may be further used, e.g., to interpret new variables measured in the monitored catchments, or to map the health risk posed by fecal pathogens.

Land degradation resulting from increased continental surface erosion is a worldwide and systemic phenomenon due to human activities. Already fragile, the intertropical zone is likely to be further affected by climate change and increased aridity, an aggravating factor of soil erosion and land degradation. A major challenge is thus to provide the necessary knowledge to not only deepen our understanding of the Earth system and its critical thresholds but also to help achieving sustainable development goals. Understanding the factors that control the properties and processes of the critical zone, and especially what will be its responses to ongoing climate and land use changes, requires multidisciplinary efforts to tackle time scales that are compatible with morphogenesis and soil development as well as environmental disturbances of anthropogenic origin. Due to its prominent ecological importance, the Brazilian Cerrado biome is an ideal natural laboratory were to gauge the consequences of recent and strong human activities on continental surface erosion. We focused on the region of Brasília where our approach allows confronting the temporal scales of long-lived and stable cosmogenic nuclides with that of short-lived radioactive isotopes, through a comparison of natural and anthropogenically disturbed land surfaces. Our results indicate that long-term, background denudation rates are lower than 10 mm kyr-1whereas recent erosion rates due to human activities may reach rates at least 160 times higher, exceeding by far the sustainability rates of the soil resource.

Data are available in the Zenodo repository at https://zenodo.org/ record/6630369 and https://zenodo.org/record/6630479; International audience; 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.

International audience; The $^{135}$Cs/$^{137}$Cs isotopic ratio is a powerful tool for tracing the origin of radioactive contamination. Since the Fukushima accident, this ratio has been measured by mass spectrometry in several highly contaminated environmental matrices mainly collected near nuclear accident exclusion zones and former nuclear test areas. However, few data were reported at $^{137}$Cs environmental levels (

Tracing the origin of sediment is needed to improve our knowledge of hydro-sedimentary dynamics at the catchment scale. Several fingerprinting approaches have been developed to provide this crucial information. In particular, spectroscopy provides a rapid, inexpensive and non-destructive alternative technique to the conventional analysis of the geochemical properties. Here, we investigated the performance of four multi-proxy approaches based on (1) colour parameters, (2) geochemical properties, (3) colour parameters coupled with geochemical properties and (4) the entire visible spectrum to discriminate sediment source contributions in a mining catchment of New Caledonia. This French archipelago located in the south-west Pacific Ocean is the world's sixth largest producer of nickel. Open-cast nickel mining increases soil degradation and the downstream transfer of sediments in river systems, leading to the river system siltation. The sediment sources considered in the current research were therefore sediment eroded from mining sub-catchments and non-mining sub-catchments. To this end, sediment deposited during two cyclonic events (i.e. 2015 and 2017) was collected following a tributary design approach in one of the first areas exploited for nickel mining on the archipelago, the Thio River catchment (397 km2). Source (n=24) and river sediment (n=19) samples were analysed by X-ray fluorescence and spectroscopy in the visible spectra (i.e. 365–735 nm). The results demonstrated that the individual sediment tracing methods based on spectroscopy measurements (i.e. (1) and (4)) were not able to discriminate sources. In contrast, the geochemical approach (2) did discriminate sources, with 83.1 % of variance in sources explained. However, it is the inclusion of colour properties in addition to geochemical parameters (3) which provides the strongest discrimination between sources, with 92.6 % of source variance explained. For each of these approaches ((2) and (3)), the associated fingerprinting properties were used in an optimized mixing model. The predictive performance of the models was validated through tests with artificial mixture samples, i.e. where the proportions of the sources were known beforehand. Although with a slightly lower discrimination potential, the “geochemistry” model (2) provided similar predictions of sediment contributions to those obtained with the coupled “colour + geochemistry” model (3). Indeed, the geochemistry model (2) showed that mining tributary contributions dominated the sediments inputs, with a mean contribution of 68 ± 25 % for the 2015 flood event, whereas the colour + geochemistry model (3) estimated that the mining tributaries contributed 65 ± 27 %. In a similar way, the contributions of mining tributaries were evaluated to 83 ± 8 % by the geochemistry model (2) versus 88 ± 8 % by the colour + geochemistry model (3) for the 2017 flood event. Therefore, the use of these approaches based on geochemical properties only (2) or of those coupled to colour parameters (3) was shown to improve source discrimination and to reduce uncertainties associated with sediment source apportionment. These techniques could be extended to other mining catchments of New Caledonia but also to other similar nickel mining areas around the world.

Dating recent sediment archives (

Although Extreme El Niño Events (EENE) have always impacted hydrology in South America, their intensification by global warming and their association with changes in human activities and land cover may lead to the acceleration of sediment transfers in river systems and dam reservoirs. This situation may threaten soil and water resources in a region highly dependent on freshwater originating from large dams. In this study, we investigated the sediment sequence accumulated in the Poechos reservoir (Northern Peru) and provided a retrospective reconstruction of the interactions of El Niño-Southern Oscillation (ENSO), agricultural practices and vegetation cover changes on sediment dynamics (1978–2019). To this end, a sediment core was dated and characterized by physical and chemical analyses (e.g. scanner tomography, X-ray fluorescence, particle size analysis) for estimating the evolution of sedimentation rates and changes in sediment sources during the last five decades. Sediment tracing results indicated the occurrence of changes in sediment sources associated with positive and negative phases of the Central and East Pacific Index (C and E index), with a greater contribution of the lowland dry forest biome in comparison to that of the Andean source to sediment during the warm periods (El Niño events, ENE), (mean contribution of 76 %, up to 90 % during the Coastal El Niño Events (CENE) of 2016–2017). Overall, the development of agriculture within the fluvial plains of the dry forest biome upstream of the Poechos dam induced an acceleration of sedimentation rates by 140 % after 1997, with an accumulation of coarser sediment in the reservoir during ENE and EENE. This study suggests that the expansion of agriculture (e.g. agriculture in floodplains, deforestation) associated with a higher frequency of extreme rainfall events amplified the consequences of ENE, EENE and CENE on the quantity and quality of sediment transported by this river system, which will significantly decrease the lifespan of the dam reservoir essential to meet freshwater demands of the farmers and the populations living in this region.

International audience; Atmospheric nuclear tests (1945-1980) have led to radioactive fallout across the globe. French tests in Polynesia (1966-1974) may influence the signature of fallout in South America in addition to those conducted by USA and former USSR until 1963 in the Northern hemisphere. Here, we compiled the $^{240}$Pu/$^{239}$Pu atom ratios reported for soils of South America and conducted additional measurements to examine their latitudinal distributions across this continent. Significantly lower ratio values were found in the 20-45° latitudinal band (0.04 to 0.13) compared to the rest of the continent (up to 0.20) and attributed to the contribution of the French atmospheric tests to the ultra-trace plutonium levels found in these soils. Based on sediment cores collected in lakes of Chile and Uruguay, we show the added value of measuring $^{240}$Pu/$^{239}$Pu atom ratios to refine the age models of environmental archives in this region of the world.

International audience; The isotopic signature of radionuclides provides a powerful tool for discriminating radioactive contamination sources and estimating their respective contributions in the environment. In this context, the $^{135}$Cs/$^{137}$Cs ratio has been tested as a very promising isotopic ratio that had not been explored yet in many countries around the world including France. To quantify the levels of radioactivity found in the environment, a new method combining a thorough radiochemical treatment of the sample and an efficient measurement by ICP-MS/MS has been recently developed. This method was successfully applied, for the first time, to soil and sediment samples collected in France in two mountainous regions preferentially impacted either by global fallout from nuclear weapons testing (i.e., the Pyrenees) or by the Chernobyl accident (i.e., the Southern Alps). The $^{135}$Cs/$^{137}$Cs ratios measured on twenty-one samples ranged from 0.66 ± 0.04 and 4.29 ± 0.21 (decaycorrected to January 1 st , 2022) corresponding to the characteristic signatures of the fallout from Chernobyl and global fallout associated with the nuclear weapons testing, respectively. Moreover, large variations of both the $^{137}$Cs mass activity and the studied isotopic ratio recorded by most samples from the southern Alps suggest varying proportions of these two $^{137}$Cs sources. For these samples, the contribution of each source was estimated using this new tracer ($^{135}$Cs/$^{137}$Cs) and compared with the mixing contribution given by activity ratio: $^{239+240}$Pu/$^{137}$Cs. This work has successfully demonstrated the applicability of the $^{135}$Cs/$^{137}$Cs isotopic signature to nuclear forensic studies and could be extended to better evaluate the environmental impact of nuclear facilities (i.e., NPP, waste reprocessing).

Geographers' engagements with environmental crises have taken a number of forms. Some scholars argue that crises judgments are revelatory and expose the contradictions of modes of production through interruptions to socio-economic life that can no longer be ignored. Others contend that crises judgments conceal more than they reveal through the framing of crisis as “error” and the focus on technocratic solutions to political-economic problems. In this article, we argue that the judgment of seasonal air pollution as a crisis is contingent on contestations over livelihoods and worldviews, and in doing so demonstrate how attention to chronopolitics reveals the nuanced ways people account for uncertainty in the causes and effects of anthropogenic environmental change. Based in northern Thailand, the paper focuses on what is described by many residents as the region's annually recurring “haze crisis”. In recent decades, broad shifts from subsistence farming to commercial agriculture and increased volumes of agricultural biomass burning have reportedly exacerbated the production of air pollution in the form of haze—an airborne mixture of pollutants that includes gasses, fine soot particles and carbon dioxide. Once a quotidian phenomenon of relatively little concern, today seasonal air pollution is described as a haze crisis. While causal uncertainty exists surrounding the precise combination of the socio-ecological drivers of haze production, multiple narratives circulate throughout the region, in which blame is frequently placed on smallholder farmers who have recently entered into new market relations. Situated within broader regional agrarian transitions, we draw on mixed ethnographic, archival and geospatial methods to examine the chronopolitics of seasonal air pollution and by what mechanisms such pollution comes to be constituted as a crisis.

International audience; To truly understand the hydrologic and erosive processes that occur at the catchment scale regarding land use and soil management changes, intensive monitoring is required over a long period. Variables such as precipitation, flow rate, and suspended sediment concentration are the fundamentals needed to estimate sediment yield. However, in order to shed more light on the effects of soil management changes on sediment yield, traditional hydrologic monitoring techniques can be paired with sediment tracing to identify sediment sources. In addition, mathematical models that predict erosion and sediment yield can also help understand the dynamics of erosion and deposition between the hillslope and stream channel. Given the above, this paper provides an overview of almost 20 years of monitoring (2002-2021) in the Arvorezinha experimental catchment (1.23 km 2) located on the edge of the Brazilian meridional plateau in southern Brazil, which is subject to extreme erosion due to a combination of factors related to intensive agriculture and steep slopes. The catchment was selected as a study site to evaluate the effects of land-use changes and soil conservation management on hydrology and sediment yield. The text reviews the history of the study, synthesizes the main results, and describes the evolution of the monitoring techniques used while giving special attention to the fingerprinting approach. In addition, the article discusses the importance of catchment-scale studies for teaching, generating technical-scientific knowledge, and fostering collaboration between national and international research groups.

International audience; The detrimental impacts of surface runoff and soil erosion, particularly in cultivated areas, call for the use of distributed runoff and soil erosion models with a view to supporting adapted catchment management strategies. However, runoff model parameterization remains challenging in agricultural catchments due to the high spatial and seasonal variability of soil properties. Data acquisition is demanding and may not always be feasible. Therefore, model parameterization in such environments have been the subject of numerous research efforts. The combined analysis of land use management and soil surface state was proposed in literature to address this issue and demonstrated its potential for runoff analysis and modelling. However, these research findings were related to specific rainfall sequences and/or soil surface state. In this study, existing knowledge on soil surface state and its application to runoff model parameterization were synthetized and included in an easy-to-use parameterization software (PREMACHE), providing a framework for modelers lacking of means and/or data for modelling complex agricultural catchments. To develop and evaluate the software, a dataset was acquired over 9 years on more than 110 plots in a 1045 ha agricultural catchment, including crop types, soil surface state, rainfall and runoff time series. Soil surface state dynamics was modeled based on crop types and daily rainfall. It was evaluated in the experimental catchment and validated in a nearby catchment. Soil hydrodynamic properties (e.g. infiltration capacity) were deduced from this framework and literature data at a daily time step, for each plots. Moreover, runoff events were measured when the modeled infiltration capacity was low, indicating that the parametrization adequately captured its temporal dynamics. The software developed in this study, as well as setup values deduced from the monitoring campaigns are provided with the manuscript for application in other ungauged catchments and explore their impact on agricultural catchment hydrological dynamics.

International audience; Soil erosion on agricultural land is associated with deleterious off-site impacts including the siltation and the pollution of the receiving water bodies. To better manage this situation, local/regional water agencies need spatially-distributed information to compare the sensitivity to erosion of the areas draining into these water bodies and supplying the vast majority of sediment and associated pollutants leading to this water quality impairment. These soil erosion hazard maps are now often included in the latest versions of the basin management plans that must be designed to meet the water quality objectives of the EU Framework Directive. However, the resolution of these maps is often too coarse to meet the practical needs of these agencies. Accordingly, the current research used the latest input databases to improve the MESALES model outputs in one of the largest French River basins (Loire, 117,000 km2), with the implementation of three main modifications. First, the seasonal variations of land cover were incorporated into the model through a revised set of expert rules based on the agricultural census data. Second, the discrimination of the soil textures was improved within the infiltration and erodibility module of the model. Third, variations in rainfall erosivity across the study area were described taking into account the latest erosivity map available at the European scale. Then, the model results obtained with the updated model version were compared with those generated by the previous version. Overall, the simulated soil erosion hazard changed for 35% of the pixels of the Loire River basin, with a significant increase of the lowest hazard classes during all seasons except summer. When aggregating the results at the scale of water bodies, the simulated erosion hazard changed for 49% of these management units. Although 28% of these water bodies were associated with a lower hazard, 23% of the river systems were attributed a higher erosion risk. The implications of these model/map revisions for the local decision makers were discussed, taking the strategy of concentrating the management budget on those water bodies associated with the highest erosion risk as an example. In the future, this model could be used to compare the soil erosion hazard in contrasted regions of Europe and to simulate the impact of management plans designed to decrease this risk to support the decisions of water agencies.

International audience; The widespread use of pesticides in agriculture during the last several decades has contaminated soils and different Critical Zone (CZ) compartments, defined as the area extended from the top of the vegetation canopy to the groundwater table, and it integrates interactions of the atmosphere, lithosphere, biosphere, and hydrosphere. However, the long-term fate, storage, and transfer dynamics of persistent pesticides in CZ in a changing world remain poorly understood. In the French West Indies, chlordecone (CLD), a toxic organochlorine insecticide, was extensively applied to banana fields to control banana weevil from 1972 to 1993 after which it was banned. Here, to understand CZ trajectories we apply a retrospective observation based on marine sediment core analyses to monitor long-term CLD transfer, fate, and consequences in Guadeloupe and Martinique islands. Both CLD profiles show synchronous chronologies. We hypothesized that the use of glyphosate, a postemergence herbicide, from the late 1990s onward induced CZ modification with an increase in soil erosion and led to the release of the stable CLD stored in the soils of polluted fields. CLD fluxes drastically increased when glyphosate use began, leading to widespread ecosystem contamination. As glyphosate is used globally, ecotoxicological risk management strategies should consider how its application affects persistent pesticide storage in soils, transfer dynamics, and widespread contamination.

International audience; The Fukushima nuclear accident in March 2011 generated a 3000 km2 plume of soils heavily contaminated with 137Cs. Decontamination was completed early in 2019. Typhoon Hagibis was the first extreme event that occurred in the region after decontamination. Its impact on sediment sources and sediment 137Cs contamination was investigated through the application of a sediment fingerprinting procedure using spectrocolorimetry and geochemical properties. Sediment deposits (n=24) were collected in the Mano and Niida River catchments after the 2019 typhoons, and their signature was compared to that of potential sources (e.g., cropland, forests, and subsurface; n=57). Results demonstrate the dominance of cropland as the main source of sediment (mean: 54%) followed by forests (41%) with much lower contributions of subsurface material (5%). Overall, 137Cs concentrations in sediment were on average 84%–93% lower than the levels recorded after the accident in 2011, which demonstrates the effectiveness of cropland decontamination.

Le ruissellement est un phénomène épisodique et localisé, donc difficile à mesurer. Il résulte de l'interaction de nombreux facteurs, dont les caractéristiques intrinsèques et de surface du sol, la morphologie du bassin versant, mais également les caractéristiques pluviométriques. Afin d'améliorer la compréhension de la genèse et de la propagation du ruissellement au sein de bassins versants de tête, cinq observatoires du ruissellement sont présentés, incluant 11 bassins versants. Pour chacun d'entre eux, la pluie et le ruissellement sont mesurés sur le terrain à haute fréquence (≤ 15 min) pendant des durées longues (1,5 à 11 ans). 1640 évènements pluvio-ruisselants sont extraits de ces chroniques en continu et statistiquement analysés, afin de cerner les facteurs influant significativement le ruissellement. La réponse ruisselante de chaque bassin versant est extrêmement complexe mais permet d'identifier des comportements communs. L'importance des caractéristiques des sols est montrée, ainsi que celle de leur occupation, de leurs états de surface et état hydrique. La dispersion de la relation pluie-débit observée au sein de chacun des observatoires permet par ailleurs de montrer et de quantifier l'impact des caractéristiques pluviométriques sur la lame d'eau ruisselée. Les leçons tirées de chacun des observatoires et leurs conséquences sur la compréhension et la modélisation du ruissellement dans différents contextes sont discutées.

Purpose This review of sediment source fingerprinting assesses the current state-of-the-art, remaining challenges and emerging themes. It combines inputs from international scientists either with track records in the approach or with expertise relevant to progressing the science. Methods Web of Science and Google Scholar were used to review published papers spanning the period 2013–2019, inclusive, to confirm publication trends in quantities of papers by study area country and the types of tracers used. The most recent (2018–2019, inclusive) papers were also benchmarked using a methodological decision-tree published in 2017. Scope Areas requiring further research and international consensus on methodological detail are reviewed, and these comprise spatial variability in tracers and corresponding sampling implications for end-members, temporal variability in tracers and sampling implications for end-members and target sediment, tracer conservation and knowledge-based pre-selection, the physico-chemical basis for source discrimination and dissemination of fingerprinting results to stakeholders. Emerging themes are also discussed: novel tracers, concentration-dependence for biomarkers, combining sediment fingerprinting and age-dating, applications to sediment-bound pollutants, incorporation of supportive spatial information to augment discrimination and modelling, aeolian sediment source fingerprinting, integration with process-based models and development of open-access software tools for data processing. Conclusions The popularity of sediment source fingerprinting continues on an upward trend globally, but with this growth comes issues surrounding lack of standardisation and procedural diversity. Nonetheless, the last 2 years have also evidenced growing uptake of critical requirements for robust applications and this review is intended to signpost investigators, both old and new, towards these benchmarks and remaining research challenges for, and emerging options for different applications of, the fingerprinting approach.

eDNA refers to DNA extracted from an environmental sample with the goal of identifying the occurrence of past or current biological communities in aquatic and terrestrial environments. However, there is currently a lack of knowledge regarding the soil memory effect and its potential impact on lake sediment eDNA records. To investigate this issue, two contrasted sites located in cultivated environments in France were studied. In the first site, soil samples were collected (n = 30) in plots for which the crop rotation history was documented since 1975. In the second site, samples were collected (n = 40) to compare the abundance of currently observed taxa versus detected taxa in cropland and other land uses. The results showed that the last cultivated crop was detected in 100% of the samples as the most abundant. In addition, weeds were the most abundant taxa identified in both sites. Overall, these results illustrate the potential of eDNA analyses for identifying the recent (

Dans cette note, nous revenons sur les grandes lignes de la conférence donnée lors des journées de la SAP 2019 concernant le champ actuel de l’ethnogénétique, qui s’intéresse entre autres à l’influence des systèmes de parenté sur la diversité génétique des populations humaines. Nous présentons certains des résultats principaux d’une étude ethnogénétique menée en Asie du Sud-Est dans douze populations présentant des systèmes de parenté variés, à filiation patrilinéaire, matrilinéaire et cognatique. Les données ethno-démographiques et génétiques collectées montrent que les processus de résidence, de filiation et d’alliance façonnent la diversité génétique des populations humaines. Ce champ de recherche ouvre ainsi de nouvelles perspectives : mieux connaître, à partir des données génétiques, la dynamique des systèmes de parenté actuels, et retracer leur évolution à travers le temps grâce aux données archéogénétiques. Cependant, les données ethno-démographiques montrent également que le niveau d’observance des règles varie entre populations catégorisées dans un même système de parenté, ce qui module les signatures génétiques laissées par ce système de parenté et invite à une certaine prudence concernant l’inférence des structures sociales du passé à partir des données archéogénétiques. L’impact du « puzzle matrilinéaire » sur la diversité génétique autosomale est également discuté.

International audience; Sediment that has deposited after the flood generated by the 2019 typhoon (referred to as Flood Sediment, FS) was collected along two rivers in eastern Fukushima Prefecture, Japan to determine the total and exchangeable radiocesium (137 Cs), and acid-extractable potassium (K) contents. Then, these FS parameters were compared with those of decontaminated soils (DS) in nearby agricultural fields to discuss the potential transfer risk of 137 Cs from rivers to nearby remediated soils. While FS had about four times higher total 137 Cs content, it showed a three-times lower exchangeable 137 Cs content than DS. Furthermore, acid-extractable K referred to as non-exchangeable K (Nex-K) content was high enough to restrict 137 Cs transfer from soil to crops for both FS and DS. From these comparisons, we concluded that deposition of FS onto DS may not increase the transfer of 137 Cs from soil to crops.

No-tillage is an extensively used soil conservation practice in crop fields. Yet, no-tillage is prone to runoff generation, which may lead to downstream concentrated forms of erosion, floods, solute transfer and eutrophication of water bodies. However, infiltration terraces on hillslopes can reduce runoff and erosion. We analyzed nutrient losses, in both dissolved and particulate forms, on terraced and non-terraced agricultural hillslopes under no-tillage in Southern Brazil. Precipitation, runoff, sediment yield and chemical elements’ concentrations were monitored in paired catchments, including a 2.35 ha terraced catchment (TC) and a 2.43 non-terraced catchment (NTC), during rainfall events that occurred from 2017 to 2018. Runoff and suspended sediment samples were manually collected in H-flumes at the outlet of each hillslope, where automatic water level readings were recorded at 5-minute intervals by a limnigraph to estimate runoff discharge. P, K, Ca, Mg, Cu, Zn and N concentrations were analyzed in runoff-water samples and P, K, Ca e Mg in the suspended sediment samples to obtain dissolved and particulate concentrations, respectively, and total nutrient losses. Maximum N concentration in TC’s runoff samples (8.70 mg L-1) were higher than in the NTC (7.41 mg L-1). Ca concentrations were higher in the NTC (average 3.9 mg L-1). Low and similar Mg, Cu, Zn mean concentrations were observed in the catchments. Mean P concentrations were ~0.11 mg L-1 in both catchments but reached higher concentrations in the NTC. Mean (~3 mg L-1) and maximum (8.74 mg L-1) K concentrations were observed the TC. In sediment samples, Ca, Mg, P and K concentrations were higher in the NTC. To compare total dissolved nutrients losses, we chose 13 rainfall-runoff events and 10 events for particulate nutrient losses. Total rainfall for the 13 events was 1020 mm, leading to 110 and 222 mm of runoff in TC and NTC, respectively. Besides higher runoff volume, NTC shows higher losses of all analyzed nutrients in runoff. P losses were of 105 and 352 g ha-1 in TC and NTC, respectively, while K losses were of 2293 and 4604 g ha-1, showing a similar trend. The average increase in Cu losses for NTC was 21 times higher than for TC. Total sediment yield in TC, for the 10 events, was 12 kg ha-1, and 39 kg ha-1 in the NTC. Higher particulate nutrient loss was observed in the NTC outflow. An almost nine-fold increase in particulate P losses was observed in NTC, besides a four-fold increase in Ca, a seven-fold increase in Mg and two-fold K losses. Although higher nutrient concentrations in water were observed in the TC for some samples, overall losses and concentrations were greater in the NTC. This indicates that nutrient flux from agricultural hillslopes is controlled by runoff and that terraces can decrease flow and material connectivity over hillslopes. As soil and water conservation practices are needed to ensure agriculture’s sustainability and to avoid deleterious environmental impacts, measures for runoff mitigation, such as terraces, were shown to effectively control nutrient – and, potentially, other solutes – transfer to water bodies.

Since 60 years, a large amount of data has been acquired to survey river sediment quality, especially concerning regulatory trace metals such as Cd, Cr, Cu, Hg, Ni, Pb, and Zn. Large-scale syntheses are still rare and show some limits to assess the effectiveness of public regulations and the river systems' resilience. Based on a sediment contamination database comprising more than 12,000 samples, we propose a first attempt to decipher spatio-temporal trends of metal contamination along seven major rivers in Western Europe (Garonne-Lot, Loire, Meuse Rhine, Rhone, Scheldt and Seine Rivers). Facing heterogeneous sampling and analytical methods on different sediment matrices (bed and flood deposits – BFD, suspended particulate matter – SPM, dated sediment cores – DSC), this work investigates the effect of analytical protocols, spatial and temporal factors on metal concentration trends. At a large scale, an increase in metal concentrations (especially for Cd, Pb and Zn) is reported along most of the investigated rivers. It appears closely related to major urban-industrial hotspots (Paris-Rouen corridor on the Seine River, Bonn-Duisburg corridor on the Rhine River, etc.) and to the geology of each watershed, both influencing the regional sediment quality. Former mining and metallurgical districts, generally located in crystalline areas, also caused high metal concentrations on the long term (Upper Loire River, Middle Meuse River, Lot River). A global decrease of metal concentrations is observed in all river sections since the 1960s-1970s onwards, in response to European and national regulations, and to socio-economical changes affecting urban-industrial areas. The high influence of the location of the samples along the rivers and the decade of sampling is confirmed by a Factor Analysis of Mixed Data (FAMD). Secondary factors such as the influence of the sediment matrix type (BFD, SPM and DSC) and the different digestion procedures prior to elemental analysis also explained significant differences for Cr, Cd, Cu, Pb, or Zn, although this can also be locally balanced by the substratum (i.e. for alkaline rivers). This approach points out the limitations of the available data, particularly regarding the need of regional geological backgrounds and the more systematic acquisition of ancillary data such as grain-size and TOC. It also provides critical clues to intercompare metal sediment pollution in rivers at large spatial and temporal scales worldwide.

Quantifying soil erosion is a major research challenge due to erosion's episodic character and spatial variation. Fallout radionuclides as 239+240Pu and 137Cs are powerful tools to assess net soil losses integrated over long periods applicable to most regions of the world. The traditional approach of the FRN method is based on the comparison between an inventory (total radionuclide activity per unit area) at a given sampling site and that of an undisturbed reference site (e.g., located in a flat and well-vegetated area). Compared to reference, a decrease in the FRN inventory indicates erosion and an increase indicating deposition of sediments and associated FRN. So far, FRN based assessment was restricted to a regional/catchment scale as spatially distributed data of reference inventories was missing.In this study, we aim at upscaling the FRN approach to a central area of Europe covering France, North Italy, South Germany, and Belgium using the Land Use/Cover Area frame Survey – LUCAS soil sample bank. Both fallout sources left a specific radionuclide imprint in European soils. First, plutonium was used to quantify global versus Chernobyl fallout contributions to 137Cs found in European soils. Subsequently, spatial prediction models (general additive models) allowed reconstructing the global versus Chernobyl 137Cs fallout pattern across national boundaries. The definition of these 137Cs and the Pu baseline maps allows assessing soil redistribution rates at n=137 cropland sites with both FRNs across the study area. We selected barley, wheat, maize, and vineyard plots covering different slope angles as cropland sites. For both FRNs, differences between the reference and site-specific FRN inventory show an inventory and associated topsoil (0-20cm) loss of approximately 10% since 1963. Converting these inventory changes with a simple mass balance model to soil redistribution rates results in average soil erosion rates of 8.8 ± 6.3 t/ ha yr, assuming a tillage depth of 20 cm. Although the involved uncertainties are large, these net erosion rates exceed the expected magnitude.

Soil erosion and the subsequent transport of sediment and pollutants are critical challenges for guaranteeing food security and water quality. Controlling sediment and particle-bound substance export requires the implementation of improved ecological restoration schemes, especially in areas experiencing drastic increases in erosion rates. To this end, we propose the design of an ensemble technique that combines the use of sediment fingerprinting together with radionuclide dating and remote sensing data to fill these critical knowledge gaps.This project will focus on testing and developing powerful specific land use tracers, such as Compound Specific Stable Isotopes (CSSI) and environmental DNA (eDNA), to improve the land cover discrimination of sediment provenance through the collection and dating of sediment cores in sink areas. This research will be conducted in two contrasting catchments with different land use histories allowing to test the effectiveness of this novel approach: i) the Ésera catchment that flows into the Barasona reservoir (Spain), representative of areas experiencing sediment export decrease due to land abandonment and the subsequent natural revegetation, and ii) the Kihira catchment, Lake Kivu (DR Congo), representative of intensively cultivated areas undergoing an unsustainable and increasing sediment export and nutrient loss. By combining the investigation of these two contrasted catchments and by applying state-of-the-art methods, it will be possible to evaluate the main driving factors of the past and present erosion rates and predict the effects of human management and climate change.In this first stage of the project, representative sediment samples from different land cover sources will be collected in the Ésera catchment (1.535km2) until its mouth into the Barasona reservoir. Several bulk cores and surface sediments collected in 1995 will be characterised and compared with samples collected in 2013 at the Barasona reservoir. An extra sampling campaign is planned for 2022 to examine the changes that occurred in recent years. Records of known flood events and reservoir management data will be combined with 137Cs chronology to ascribe the sedimentary record to specific years. Besides, a set of remote sensing and aerial photographs will be analysed to reconstruct the land use variation during the last six decades.To track the land use apportionment variation during the last decades, geochemistry and radioisotopic activity will be analysed in both source and sediment samples and examined as possible tracers for fingerprinting modelling. The fingerprinting technique will be implemented following state-of-the-art methodologies such as the Consensus and Consistency tracer selection methods.Thus, by combining the use of remote sensing, novel fingerprinting techniques and radiometric dating, we aim to provide a novel and powerful tool to understand the driving factors of sediment sources (e.g., deforestation, agricultural intensification and abandonment) and associated pollutants, and their variations in space and time in the last decades.

Recent agriculture expansion and land cover conversion (post-1985) induced major deleterious environmental effects in South-America in general and in Uruguay in particular, affecting locally the sustainability of soil and water resources. Whilst the environmental consequences of agriculture’s development were largely studied (e.g. monitoring, modeling) in Europe or North America, much less attention was devoted to the intensity of land degradation in South-America and more specifically, on the Pampa Biome. In this study, sediment cores collected in two reservoirs installed along the Rio Negro river (catchments of 23.000 and 39.000 km²) and draining agricultural catchments were used for reconstructing the evolution of sediment dynamics and source contributions in this region during the last several decades. Various chemical and physical analyses were performed for characterizing this accumulated sediment (e.g. fallout radionuclides, organic matter properties (TOC, δ15N, δ13C, C:N), X-ray fluorescence). Results indicate the significant acceleration of sediment accumulation rates (e.g. by 67% on average in the Rincon del Bonete dam between 2003 and 2019) associated with major phases of agricultural expansion (e.g. expansion of soybean and afforestation). Sediment properties show an increase of native vegetation source contributions associated with the conversion of native grassland into cropland. Understanding the causes of past and present acceleration of sediment delivery are of prime importance in order to protect the soil and water resources with the design of adapted management schemes at the catchment scale.

Environmental DNA (eDNA) has recently been considered as a marker that could be used for fingerprinting sediments. Identify sediment sources originating from zones covered with specific plant communities would enhance the sediment fingerprinting method significantly and enable the detailed identification of soil erosion hotspots relative to land use and cover. Here, we explore the relevance of environmental DNA (eDNA) that originates from plant litter and fixes onto fine soil particles as a targeted sediment fingerprinting method. Although research on plant eDNA signatures in soils and sediments is limited, initial results are promising and indicate that eDNA could yield more accurate results than other sediment fingerprints that are sensitive to vegetation. Plant eDNA signatures tend to produce a highly localized signal of sediment sources, mainly reflecting the current vegetation cover of soils. As eDNA is rapidly adsorbed onto fine mineral soil particles such as clay, it is protected against rapid degradation in fluvial environments. Supported by the increasing availability and quality of vegetation maps and eDNA reference libraries, we argue that sediment source fingerprinting using eDNA from plant litter will evolve into a valuable method to identify hotspots of soil erosion and allow stakeholders to prioritize areas where ecological restoration is necessary. We tested our assumptions from a case study in a high mountain environment (catchment of approximately 600 km² in the Central Pyrenees, France) which was recently affected by a severe hydro-climatic event and for which ecological restoration is pertinent.Keywords:Pyrenees, river catchments, sedDNA, sediment source fingerprinting, vegetation

Sediment fingerprinting or tracing is a technique that allows to quantify source contributions of sediment. A Thematic School was organised in October 2021 to discuss potential options to improve the design and implementation of sediment fingerprinting procedures. The suggestions put forward by the School participants were organised around six complementary topics. First, we suggest a better use of geomorphological information to improve study design. Researchers are invited to scrutinize all the knowledge available on the catchment of interest, and to obtain multiple lines of evidence regarding sediment source contributions. Second, we think that scientific knowledge could be improved with local knowledge and we propose a scale of participation describing different levels of involvement of locals in research. Third, we recommend the use of state-of-the-art sediment tracing protocols to conduct sampling, deal with particle size, examine data before modelling and accounting for the hydro-meteorological context under investigation. Fourth, we promote best practices in modelling, including the importance of running multiple models, selecting appropriate tracers, and reporting on model errors and uncertainty. Fifth, we suggest best practices to share tracing data and samples, which will increase the visibility of the fingerprinting technique in geoscience. Sixth, we suggest that a better organisation of datasets would allow to formulate hypotheses and improve our knowledge about erosion processes in a more unified way. In conclusion, sediment fingerprinting, which is interdisciplinary in nature, should play a major role to meet the current and future challenges associated with global change.