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We report mineralogical, elemental (major and trace elements) and Sr-Nd isotopic data for suspended particulate matter (SPM) samples from the Amazon, Orinoco and Maroni Rivers collected on a monthly basis over a one-year long hydrological cycle. The aim of this study was i) to characterize the mineralogical and geochemical composition of major South American tropical rivers and ii) to evaluate the effect of seasonal hydroclimate variations and changes in sediment discharge on the composition of these SPM. In addition to displaying particular mineralogical and geochemical signatures (e.g. Al/Si ratios, weathering indices), the Amazon, Orinoco and Maroni SPM are characterized by marked differences in Sr-87/Sr-86 (0.7213 +/- 0.0030, 0.7288 +/- 0.0018 and 0.7335 +/- 0.0019, respectively), and eNd values (-10.6 +/- 0.6, -14.1 +/- 0.3 and - 23.7 +/- 1.2), which reflect differences in source rock lithology. While we find no effect of the hydrological cycle on the geochemistry of Orinoco SPM, particulate eNd and Cr/Th signatures fluctuate with the hydrological cycle in the Maroni basin, indicating that they are controlled by variation in rainfall distribution linked to the latitudinal migration of the Intertropical Convergence Zone (ITCZ). In contrast to Maroni and Orinoco SPM, the Amazon SPM are characterized by significant Sr isotope annual variability correlated with suspended sediments discharge and a small but significant Nd isotopic variability over the year. This latter variation is related to seasonal changes in the rainfall distribution patterns across the Amazon basin, associated with latitudinal migrations of the ITCZ. This suggests that the geochemical composition of the SPM exported over the year from the Amazon Basin faithfully responds to hydroclimate changes related associated with the migration of the rain belt over regions of contrasted geochemical signatures. These findings have implications for the application of Sr and Nd isotopes as provenance proxies in sedimentary archives and paleoclimatic studies. The Sr isotopic composition of exported SPM appears to be mostly controlled by weathering processes and/or mineralogical sorting, rather than being indicative of sediment provenance. In contrast, the relationship documented between Nd isotopes and the hydrological variability indicates that their application to archives of past river sediment discharges can provide unique insights on paleo-hydroclimate changes over tropical South America.

International audience; Chemical weathering of silicate rocks on continents acts as a major sink for atmospheric carbon dioxide and has played an important role in the evolution of the Earth's climate. However, the magnitude and the nature of the links between weathering and climate are still under debate. In particular, the timescale over which chemical weathering may respond to climate change is yet to be constrained at the continental scale. Here we reconstruct the relationships between rainfall and chemical weathering in northeast Africa for the last 32,000 years. Using lithium isotopes and other geochemical proxies in the clay-size fraction of a marine sediment core from the Eastern Mediterranean Sea, we show that chemical weathering in the Nile Basin fluctuated in parallel with the monsoon-related climatic evolution of northeast Africa. We also evidence strongly reduced mineral alteration during centennial-scale regional drought episodes. Our findings indicate that silicate weathering may respond as quickly as physical erosion to abrupt hydroclimate reorganization on continents. Consequently, we anticipate that the forthcoming hydrological disturbances predicted for northeast Africa may have a major impact on chemical weathering patterns and soil resources in this region. Erosion processes on continents include mechanical erosion and chemical weathering, both of which shape the Earth's surface and contribute to substantial drawdown of atmospheric carbon via export of organic-rich clay fractions and alteration of silicate minerals. However, while the links between climate and physical erosion rates have been well documented from river chemistry data 1 and sedimentary records 2 , the response of continental chemical weathering to climate change is not well understood and requires further investigation. At the large scale, the relative importance of physical erosion, temperature, rainfall, vegetation and lithology on chemical weathering over both long (> 10 6 yr) and short (≪ 10 4 yr) periods of time is still under debate. Most studies that have established links between climate and silicate weathering were based on the analysis of dissolved phases in modern river basins 3 or on the reconstruction of past ocean chemistry during the Cenozoic 4 from marine carbonates or deep-sea ferromanganese deposits. To date, only a few studies have investigated past variations in silicate weathering over short timescales and these have yielded contradictory results 5–9. However, this information is important for predicting the evolution of the short-term carbon cycle and its impact on the Earth's vital resources. Sediments formed in weathering profiles, exported by large rivers and accumulated on margins, can provide invaluable information on the short-term evolution of weathering at the sub-continental scale. To provide constraints on the links between hydro-climate and weathering, we have reconstructed the Late Quaternary evolution of rock chemical weathering in the Nile basin, using a marine sediment record recovered from the Nile deep-sea fan, off the coast of Egypt (Fig. 1). Tropical Africa is known to have experienced major hydrological changes during the Quaternary period, which dramatically affected fluvial discharge and particle delivery to the surrounding ocean margins 10–12. The onset of past humid periods was related to increased summer insolation in the Northern Hemisphere associated with northward migration of the Inter Tropical Convergence

International audience; This paper provides new data on the evolution of the Caspian Sea and Black Sea from the Last Glacial Maximum until ca. 12 cal kyr BP. We present new analyses (clay mineralogy, grain-size, Nd isotopes and pollen) applied to sediments from the river terraces in the lower Volga, from the middle Caspian Sea and from the western part of the Black Sea. The results show that during the last deglaciation, the Ponto-Caspian basin collected meltwater and fine-grained sediment from the southern margin of the Scandinavian Ice Sheet (SIS) via the Dniepr and Volga Rivers. It induced the deposition of characteristic red-brownish/chocolate-coloured illite-rich sediments (Red Layers in the Black Sea and Chocolate Clays in the Caspian Sea) that originated from the Baltic Shield area according to Nd data. This general evolution, common to both seas was nevertheless differentiated over time due to the specificities of their catchment areas and due to the movement of the southern margin of the SIS. Our results indicate that in the eastern part of the East European Plain, the meltwater from the SIS margin supplied the Caspian Sea during the deglaciation until ∼13.8 cal kyr BP, and possibly from the LGM. That led to the Early Khvalynian transgressive stage(s) and Chocolate Clays deposition in the now-emerged northern flat part of the Caspian Sea (river terraces in the modern lower Volga) and in its middle basin. In the western part of the East European Plain, our results confirm the release of meltwater from the SIS margin into the Black Sea that occurred between 17.2 and 15.7 cal kyr BP, as previously proposed. Indeed, recent findings concerning the evolution of the southern margin of the SIS and the Black Sea, show that during the last deglaciation, occurred a westward release of meltwater into the North Atlantic (between ca. 20 and 16.7 cal kyr BP), and a southward one into the Black Sea (between 17.2 and 15.7 cal kyr BP). After the Red Layers/Chocolate Clays deposition in both seas and until 12 cal kyr BP, smectite became the dominant clay mineral. The East European Plain is clearly identified as the source for smectite in the Caspian Sea sediments. In the Black Sea, smectite originated either from the East European Plain or from the Danube River catchment. Previous studies consider smectite as being only of Anatolian origin. However, our results highlight both, the European source for smectite and the impact of this source on the depositional environment of the Black Sea during considered period.