240Pu/239Pu signatures allow refining the chronology of radionuclide fallout in South America.

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 240Pu/239Pu 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 240Pu/239Pu atom ratios to refine the age models of environmental archives in this region of the world.

Evidence of Chlordecone Resurrection by Glyphosate in French West Indies.

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.

When archives are missing, deciphering the effects of public policies and climate variability on the Brazilian semi-arid region using sediment core studies.

The northeastern region of Brazil is the most densely populated and biodiverse semi-arid regions of the planet. Effects of the natural climate variability and colonization on the landscape have been described since the beginning of the 16th century but little is known about their effects on natural resources. Climate projections predict temperatures above 40 °C and an increase in the number and duration of droughts at the end of the 21st century with strong societal impacts. Here, we analyze the influence of public policies, human activities and natural climate variability on the environment over the last 60 years. Our study is based on sedimentological and environmental reconstructions from two sediment cores collected in two dam lakes on the river Acaraú in the State of Ceará. Multiproxy analyses of both cores (inorganic geochemistry, pollen, charcoal, remote sensing) at an annual resolution showed that 1) at interannual scale composition and distribution of the dry forest (known as Caatinga) were not affected by the alternance of drought and high moisture episodes; 2) at decadal scale human activities such as agriculture were reflected by changes in vegetation cover and fishery by progressive changes in lake trophic status; 3) public policies were able to promote changes in the landscape e.g., land colonization with the regression of the dry forest and irrigation plan able to amplify the deforestation and change the floristic composition. Thanks to paleo-science approach, our environmental diagnosis should help future decision-making and provide guidelines for preservation of resources and wellbeing of the inhabitants.

Backwash sediment record of the 2009 South Pacific Tsunami and 1960 Great Chilean Earthquake Tsunami.

Following recent tsunamis, most studies have focused on the onshore deposits, while the offshore backwash deposits, crucial for a better understanding of the hydrodynamic processes during such events and offering an opportunity for sedimentary archives of past tsunamis, have mostly been omitted. Here, we present a unique sedimentary record of the backwash from two historical tsunamis sampled in a sheltered bay in American Samoa, namely the 2009 South Pacific Tsunami and the 1960 Great Chilean Earthquake Tsunami. Although not always concomitant with a marked grain size change, backwash deposits are identified by terrestrial geochemical and mineralogical signatures, associated with basal soft sediment micro-deformations. These micro-deformations, including asymmetric flame structures, are described for the first time in historic shallow marine backwash deposits and lead us to propose an improved depositional mechanism for tsunami backflow based on hyperpycnal currents. Moreover, this study brings a potential new criterion to the proxy toolkit for identifying tsunami backwash deposits, namely the basal soft sediment micro-deformations. We suggest that further studies focus on these micro-deformations in order to test the representability of this criterion for tsunami backwash deposits. Sheltered shallow marine environments in areas repeatedly impacted by tsunamis have a higher potential for the reconstruction of paleo-tsunami catalogs and should be preferentially investigated for coastal risk assessment.