For decades the artificial radionuclide 137Cs has been used as an independent time marker to ascertain the 210Pb chronology to date recent sediments from the Anthropocene period (

Past nuclear weapons testing and nuclear power plant accidents resulted in the ubiquitous deposition of radionuclides in the environment. While the risks associated with radionuclide contamination are apparent, these fallout radionuclides (FRNs) provide the privileged markers (“golden spikes”) of the Anthropocene stratigraphic layers. The onset of their emissions in the 1950s coincided with the “Great Acceleration”, which is characterized by large-scale shifts in the biophysical and socio-economic aspects of the Earth System, including an increase in soil degradation, triggered mainly by land-use change. Among the host of FRNs deposited globally, 137Cs has been the most commonly used and 239+240Pu is a new emerging tracer and chronological marker to assess soil erosion and/or chronology of sediment deposition.In this meta-analysis, we compiled existing 137Cs and 239+240Pu data analyzed from undisturbed soils in the literature to get an overview of the spatial distribution and constraints of fallout 137Cs and 239+240Pu in Equatorial and Southern Hemisphere soils, as well as the possible sources of these FRNs through their isotopic ratios. A database composed of 1087 reference cores was built from the literature published on Equatorial and Southern hemisphere soils.Aside from the cores collected from the north equatorial regions, high 137Cs inventories were also found in reference soils collected at the 40-50° S latitudinal band, which were mostly from South America. On the other hand, high 239+240Pu inventories were found at the 20-30° S latitudinal band, but this was influenced by the unusually high inventories measured from the French Polynesia, where many nuclear weapons testing occurred. The 240/239Pu atomic ratios indicated that sources other than the global fallout (240/239Pu = 0.18) contributed to the reference inventories in the Southern Hemisphere. As some areas lacked measurements, specific points where additional data could be obtained were identified through a GIS-based approach to represent the entire land surface areas of interest adequately. Together with new measurements, the compiled reference soil data will be used to construct a detailed baseline map of 137Cs and 239+240Pu fallout mainly for regional soil erosion assessments.

In mining and smelting sites, where diverse potentially toxic element-rich wastes (e.g., waste rocks and slags) are stored over a mineralized background, difficulties arise to decipher the influence of multiple sources in the contaminated compartments of the critical zone. This paper aimed at testing the coupling of Pb and Sb isotopes to distinguish the sources of an inherited anthropogenic contamination in a mining and smelting environment. To do so, a selection of contamination sources (ore-bearing rocks, n = 4; slags, n = 2; stack residues - matte, n = 2), mine dewatering gallery sediments (n = 3), river sediments (n = 3) and soil horizons (n = 31) were sampled at the Peisey-Nancroix mining and smelting site (PbAg, French Alps), which has been abandoned for >150 years. The contamination sources, the soils and some sediments display similar Pb-206/Pb-207 ratios (1.173 +/- 0.003), but distinct Sb isotopic signature (delta Sb-123). Slags and stack residues have similar high delta Sb-123 signature (+0.62 to +0.78 parts per thousand), whereas the ore-bearing rocks display lower delta Sb-123 signatures (-0.28 to +0.10 parts per thousand). Such a result indicates Sb isotopes fractionation during smelting, suggesting Sb may be used to distinguish the contribution of metallurgic wastes from that of ore-bearing rocks to the environmental contamination. However, our results indicate that Sb isotopes alone cannot be used to discuss the degree of anthropogenic contamination, and must be coupled to another isotopic system. Coupling Pb and Sb isotopic systems, makes it possible to determine the degree of anthropogenic contamination (Pb isotopes) and the contribution of local ore vs slags (Sb isotopes) to it. This multi-isotope approach provides an added advantage to identify mining and metallurgical sources of contamination more accurately than using a single isotopic system.

Over the last millennia, mining and smelting activities have produced large amounts of mine and metallurgical wastes that remain enriched in potentially toxic trace elements (PTE). A spatial distribution of Pb content was coupled to mineralogical observations and single extraction tests to characterise the Pb contamination legacy and Pb dispersion trajectories in an ancient mining and smelting site that has been abandoned for approximatively 200 years. In the Peisey-Nancroix Pb–Ag mine (Savoy, France), extreme anthropogenic Pb contamination is located close to the slag heaps and along ore and slag transport paths. The contamination gradient is restricted to a few hundred metres downhill, down to background Pb values. The Pb-bearing phases change along the contamination gradient. The most contaminated soils contain significant amounts of galena and slags that are more or less weathered into pyromorphite and cerussite. Pb-bearing Mn (hydr-)oxides are the most stable and ubiquitous forms of Pb, which proportions increase downgradient. Despite the presence of some stable Pb-bearing phases (pyromorphite, Pb-bearing Mn (hydr-)oxides), extraction tests indicate that a small proportion of Pb may still be mobile over time.

Metal-rich fumes emitted during ore smelting contribute to widespread anthropogenic contamination. Environmental archives (such as lake sediments) record fallouts deposited on lake and terrestrial surfaces during ancient mining and smelting activities. However, very few is known about the potential buffering effect of soils upon which metal falls out, prior to be released through runoff and or/erosion, hence leading to pervasive contamination fluxes long after the ceasing of metallurgical activities. Here we aim at assessing this long-term remobilisation in a mountainous catchment area. Lake sediments and soils were collected 7 km upward a 200-year-old historic mine. The Pb\\Ag mine of Peisey-Nancroix was operated between the 17th and the 19th centuries with a documented smelting period of 80 years. In lake sediments, the total Pb content varies from 29 mg.kg-1 prior smelting to 148 mg.kg-1 during ore smelting. Pb isotopes in lake sediments and soils provide evidence of anthropogenic Pb from the local ore (206Pb/207Pb = 1.173; 208Pb/206Pb = 2.094) during and after smelting, suggesting anthropogenic Pb remobilisation for 200 years. The accumulation rates of anthropogenic Pb calculated in lake sediments after the smelting period confirm such a remobilisation. Despite a decrease in this accumulation rate through time, soils still contain significant stocks of anthropogenic Pb (54-89 % of PbANTH). The distribution of present-day anthropogenic Pb in the catchment area depends mainly on topographic characteristics. Coupling lake sediments and soils investigations is thus necessary to constrain the long-term persistence and remobilisation of a diffuse contamination related to mining activities.

The Pb-Ag mine of Peisey-Nancroix was operated between 1734 and 1824. The associated smelters emitted Pb-rich fumes that were reported as threatening for local people all along their period of activity. Lake La Plagne is located at 2 100 m a.s.l., 7 km uphill the former mine. Considering that smelters fumes were transported uphill by prevailing winds, studying the metal contamination within Lake La Plagne sediments offers the rare opportunity to reconstruct the local atmospheric contamination, as well as the remnant catchment area contamination in a context where historical conditions of exploitation are well-constrained. Sediments deposited before mining and smelting only contains 30 mg/kg of Pb, whereas the sediments deposited during smelting contains up to 148 mg/kg of Pb. Recent sediments deposited after mining activity period also present an enrichment in Pb (up to 58 mg/kg). Mineralogical observations (SEM-FEG) suggest that within contamination peaks, Pb is essentially associated with infra µm-scale Mn-Fe (hyrdr-)oxides. Pb isotopes were measured on selected samples collected along the lake sediment core prior mining, during mining and smelting, and after mining. The Pb isotopic ratios of all lake sediments (n=12) indicate mixing between the isotopic ratios of the ore (n=39; galena : 208Pb/206Pb =2.092 ± 0.004 and 206Pb/207Pb= 1.173 ± 0.002), and those of the deepest lake sediments (n=2; 208Pb/206Pb =2.041 ± 0.002 and 206Pb/207Pb= 1.209 ± 0.0004) that are representative of the geochemical background. The sediments deposited long after mining still present a significant influence of local ore-derived Pb, suggesting remobilisation from the watershed of Pb inherited from the smelting period.

Mining produces mine wastes (waste dumps, tailings) and metallurgical wastes (slags and fumes) enriched in trace metals (TMs, e.g. Pb, Cu, As, Zn, Cd and Sb) that are mobilized by leaching and particle translocation in the critical zone long after mine closure. A multi-compartments study, integrating ore, slags, soils, stream sediments, lake sediments, and vegetation, has been developed to better understand the transfer mechanisms and remobilization processes of TMs over time (>50 years) in the critical zone. An interdisciplinary approach was used, coupling geochemical analyses with mineralogical observations and ecotoxicology (i) to discuss the processes involved in TMs persistence and remobilization in each environmental compartment contaminated by mining activity, and (ii) to assess the effects of TMs contamination on biota and on the ecosystem functioning.