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Paleosols (PSs) contain valuable information about the climatic conditions under which they formed and constitute an outstanding archive of past weathering processes. Nevertheless, paleosol dating over most of the Quaternary remains challenging. Volcanic environments are unique sites for such purposes, as precise radiometric age determination can be achieved on volcanic units ‘bracketing’ PSs. Here, we present a combined geochemical and geochronological study of PSs spanning the last Myr in the Central Azores archipelago (Pico, Faial and São Jorge Islands; central North Atlantic). Precise K-Ar dating of lava flows on groundmass separates (unspiked Cassignol-Gillot technique) yield ages with a typical uncertainty of a few kyr, allowing us to tightly constrain PS ages and weathering rates near key paleoclimatic transitions. PS geochemistry further allowed us to reconstruct weathering conditions and estimate Mean Annual Precipitation and Temperature (MAP & MAT) by two proxies previously validated for other volcanic terranes (CIA-K and Clayeyness).Four periods of PSs formation are constrained at 870-845 ka, ~725 ka, 320-280 ka and 130-45 ka. Most PSs formed just after interglacial peaks, with a few exceptions. Our MAP reconstructions are variable (600-1,500 mm/yr), but generally lower than current annual precipitations (~1,000 mm/yr). MAT estimates (14-28°C) are higher than present-day annual temperatures (~17.5°C). MAP & MAT variations are in general agreement with global climatic curves; the highest values (28°C, 1,500 mm/yr) are reached at ~855 ka, coinciding with an interglacial peak. The younger PSs (130-45 ka) indicate more stable MAP & MAT in the ranges 650-1,000 mm/yr and 15-20°C, respectively and seem to show a temperature decrease after the MIS5e interglacial stage.Most paleosols were formed in a few kyr under high MAT (>17°C) and moderate to high MAP (>700 mm/yr), supporting a major influence of temperature on weathering kinetics. Parental material texture also had an important role, as several PSs formed upon pyroclastic deposits over most of their depth, whereas those developed on lava flows were generally restricted to the highly fragmented upper brecciated parts. Minimum vertical soil formation rates are in the range of ~0.3-4.5 cm/kyr, with a mean of ~1.7 cm/kyr and an outlier around ~10 cm/kyr. Those generally high values can be explained by the highly vesicular parental material, and by an exceptionally feldspar-rich (easily weathered) parental rock for the outlier.As current precipitation and temperatures are higher than the threshold values of ~700 mm/yr and ~17°C under which most PSs formed, enhanced soil formation is expected for the near future, especially in the context of global warming and particularly in volcanic contexts. This may have important and fast impacts on local human activities, but also regarding CO2 consumption by rock weathering and geological hazards.

International audience; A rare dataset of in-situ 10Be from high-resolution depth profiles, soils, rock outcrops and stream sediments is combined with geochemical analysis and modeling of regolith evolution to understand the variability of denudation rates in a mountain watershed (Strengbach critical zone observatory). High-resolution depth profiles are key to detect the presence of mobile regolith and to highlight how it affects the critical zone evolution. The modeling of regolith evolution and 10Be concentrations along depth profiles allow to estimate both the cosmic ray exposure age (19 kyr) and the mean denudation rate (22 mm.kyr-1) of the regolith without any steady-state assumption on 10Be concentrations. Comparison with maximum denudation rates inferred from topsoil samples collected from the surface of the depth profiles and calculated using the temporal steady-state assumption of 10Be concentrations highlight an overestimation of denudation by a factor of two. Maximum spatially-averaged denudation rates determined from stream sediment samples also likely overestimate denudation rates by a factor of two. These biases are significant for investigating the geomorphological evolution and we propose a method to correct denudation rates using the inherited 10Be concentrations and the cosmic ray exposure age deduced from the high-resolution depth profiles. A key result is also that a steady-state of 10Be concentrations and a steady-state of regolith thickness are two different equilibrium states that do not necessarily coincide. The comparison between locally corrected and spatially-averaged denudation rates indicates that the watershed geomorphology is not in a topographic steady-state but is modulated by regressive fluvial erosion. Nonetheless, our study demonstrates that even in a watershed where the steady-state assumption of 10Be concentrations is not verified, the spatial variations of in-situ 10Be concentrations in sediments still carry qualitatively relevant information on the geomorphological evolution of landscapes.

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.