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A precisely dated speleothem isotope record from the Sieben Hengste cave system, Switzerland, reveals a Last Interglacial (LIG) climate optimum centred at 128.1 ± 1.2 ka. The oxygen isotope signal is consistent with a temperature-dominated record and shows a strong correlation with the modelled Greenland d18O signal. The comparison with speleothems formed during the Last Glacial Maximum (LGM) in the same cave indicates a 4e5‰ difference in d18O between peak LIG and peak LGM conditions, suggesting peak LIG temperatures being up to 3 C higher than the Holocene average. Pollen extracted from the stalagmite indicates a vegetation response delayed by up to 3 ka with maximum pollen concentrations recorded at 125 ± 2.2 ka. Our results support a regional vegetation dominated by thermophile tree species including Corylus (hazel), Quercus robur-type (oak), Ulmus (elm), Tilia (lime tree) and Hedera (ivy), consistent with slightly warmer conditions than today. Despite the relatively low pollen concentration, the speleothem-based pollen spectra are consistent with regional pollen assemblage zones from paleolake sediments and provide the first direct paleovegetation evidence of thermophilization during the LIG at high altitudes in the Alps. ISSN:0277-3791

Based on previous radiocarbon and U-series (Diffusion/Adsorption) dating of bone samples, the Middle Palaeolithic has been thought to persist at Gruta da Oliveira until ∼37 thousand years (ka) ago. New U-series ages for stratigraphically constraining speleothems, coupled with new luminescence ages for sediment infill, show that the site’s ∼6 m-thick archaeological stratigraphy dates entirely within a

EASPEBIOmEAGROSUP; Pedogenic carbonates (calcretes) have often been proposed as possible markers of the pedogenetic processes in soils under semi-arid climate. However, precise chronological constraints on, their formation are required in order to investigate the climatic and paleo-environmental conditions that prevailed during and after their formation, and to improve our understanding of the, physical and chemical conditions that promoted their development and preservation. Moreover, these authigenic calcium carbonate precipitates provide us with an interesting test of the U-Th radioactive disequilibrium dating method, the reliability of which has been demonstrated extensively in aragonitic corals and calcitic continental speleothems, but which remains much more questionable in highly porous, chemically and mineralogically complex media such as soil profiles. We report here U-238-U-234-Th-230 radioactive disequilibrium analyses measured by Thermo-Ionization Mass Spectrometry in calcretes from two sites in southwest India. Since calcretes are impure carbonates mixed with various amounts of parent rock and weathered minerals, all ages must be computed using the isochron technique, involving the extraction of several coeval subsamples from the same soil horizon and U-Th analyses after total dissolution (TSD) method, in order to correct for the detrital component. Th-232 is usually used as index of the detrital contamination, and the isochron age is derived from the isotopic composition (Th-230/U-238; U-234/U-238) of the pure authigenic carbonate end-member of the mixing line. Our results show that each set of samples taken from decimetric blocks is characterized by a well-defined isochron line in a 3-D Osmond diagram (Th-232/U-238; Th-230/U-238; U-234/U-238). The regression lines obtained for most of the samples show probabilities of fit satisfying the mathematical assumptions of the isochron model (in most cases 0.05), and mean square weighted deviations (MSWD) values around 1. The compositions of the end-members are highly variable between the different isochrons, both for the carbonate phase, indicating various ages of cristallisation, and for the detrital phase, indicating various origins and compositions. Notably, our data demonstrate that the detrital component is in all cases out of secular equilibrium, by contrast with the standard paradigm of "dirty carbonate" dating, which is based on the assumption of average continental crust Th/U elemental ratio, and secular equilibrium values of the ((Th-230/U-238) and (U-234/U-238) ratios. At the first site (Coimbatore), we obtain U-Th ages between 304 +/- 38 ka and 44 +/- 2.9 ka (2 sigma) from the base to the top of the profile. These suggest discontinuous precipitation of the different fades in good agreement with micro morphological observations. At the second study site (Gundlupet), the carbonate nodules sampled from two different profiles, yield consistent ages of 200 ka, which suggests that precipitation of nodular carbonate occurred across the entire pediment toposequence during a single event. Surprisingly, in mere contradiction with the standard model of impure carbonate dating, two puzzling samples display "vertical isochrons", i.e., variable Th-232/U-238 without any variation of the Th-230/U-238 and U-234/U-238 ratios. Such patterns require extremely high Th-232/U-238 ratios, in an as yet unidentified detrital phase. Taken at face value, these results suggest that no clear correlation can be inferred between the precipitation of these pedogenic carbonates and major orbitally-driven climate variations recorded in other archives during the last glacial/interglacial cycles.

International audience; The present study reports data on CO2-rich geothermal waters and their associated carbonate deposits from the SW part of the Permian Lodève Basin (South of France). Both waters and carbonates are enriched in 226Ra, with activities up to 2.2 Bq/L and 3.5 Bq/g respectively. A series of carbonate samples precipitated around the studied wells since 1990 were dated through the 226Ra-210Pb and 228Ra-228Th methods, and the results are discussed in a new Concordia diagram. While both methods give comparable dates for many samples, Ra-Pb ages are sometimes much older than Ra-Th ages. We propose that these discrepancies result from the adsorption of 222Rn daughter nuclides in such a Rn-rich environment. The (228Ra/226Ra)0 activity ratios calculated at the time of deposition are remarkably constant for about 12 years, suggesting the presence of a steady-state aquifer. The low (228Ra/226Ra)0 ratios (0.348 ± 0.008) are best explained if most of the Ra derives from the Cambrian carbonate series (mainly dolostone) underlying the Permian Basin, which have low Th/U and similar (228Ra/226Ra) ratios. This study suggests that Ra and other alkaline-earth elements do not necessarily originate from the rocks surrounding the deep geothermal reservoir, but rather may be introduced into the CO2-rich water during its transfer towards the surface and interaction with carbonate rocks.

Long-term leaching experiments were done using aquifer rocks of the Aquitaine Basin, ranging in age from Santonian to upper Eocene. Leachates were sampled after one hour, one day, one week, and one month, and were analyzed for U concentrations and activity ratios. Overall, U contents in the leachate decreased with increasing time. U activity ratios were characteristic of each aquifer rock. Only slight variations, when present, were found between the one-week and the one-month leachates. In most cases, the short-lived leachates (one hour, one day) displayed U activity ratios closer to the equilibrium value.

On continental margins, oxidation of methane-rich fluids from the sediment often leads to formation of authigenic carbonate pavements on the seafloor. The biogeochemical processes involved during this carbonate precipitation are increasingly understood, but little is known about the duration or mode of carbonate crust formation. Here, we report uranium and thorium concentrations and isotope compositions for a set of 14 samples drilled across an authigenic carbonate pavement, which provide the first stratigraphy for a cold-seep carbonate crust. The 5.5-cm thick crust (NL7-CC2) was collected by submersible on the Nile deep-sea fan in an area of active fluid venting. U-Th analyses must be corrected for initial Th and measurement of co-existing sediments indicates the presence of both scavenged and detrital initial 230Th, which must be considered during this correction. The calculated 230Th/U age-depth profile for NL7-CC2 provides evidence for continuous downward carbonate precipitation at the studied location over the last ~ 5000 years. Three distinct phases can be distinguished from top to bottom with average growth rates of ~ 0.4, 5 and 0.8 cm/kyr, respectively, corresponding to carbonate precipitation rates ranging from ~ 7 to 92 μmol m2 h- 1 (rates consistent with previous estimates). High-resolution δ13C profiles [Gontharet, S., Pierre, C., Blanc-Valleron, M.-M., Rouchy, J.M., Fouquet, Y., Bayon, G., Foucher, J.P., Woodside, J., Mascle, J., The Nautinil Scientific Party, 2007. Nature and origin of diagenetic carbonate crusts and concretions from mud volcanoes and pockmarks of the Nile deep-sea fan (eastern Mediterranean Sea). Deep Sea Res. II 54, 1292-1311] and major elements across NL7-CC2 show that the variations in carbonate precipitation rates were also accompanied by changes in carbonate mineralogy and fluid composition. We suggest that these changes primarily reflect modification of the diagenetic environment, i.e. a progressive depletion of dissolved sulphate through anaerobic oxidation of methane, caused by the initial carbonate crust formation and the resulting reduction in bioirrigation. Overall, U-Th dating of cold seep carbonates offers a promising tool to bring new insights into biogeochemical processes at cold seeps and to assess the timing and duration of fluid venting on continental margins. © 2008 Elsevier B.V. All rights reserved.

International audience; U-series studies in oceanic basalts are critical for understanding melting and melt extraction in the Earth's mantle. The combined results of a detailed geochemical study of melting and melt extraction at Theistareykir, northern Iceland, provide a strong case for melt extraction via channeled melt flow at an active spreading ridge. It has often been argued, however, that widely used melting and melt extraction models, which simulate channeled melt extraction (i.e. fractional and/or dynamic melting), can only partially explain the global U-series systematics in oceanic basalts. As a consequence, more complicated models have been invoked, which suggest different styles of melt extraction at different depths/pressures in the mantle, so-called “two-porosity models”. Alternatively, diffusion-controlled mechanisms have been proposed. Here we show that U–Th–Pa–Ra systematics in oceanic basalts can indeed be explained by models where melt transport occurs without chemical equilibrium between melt and solid when variations in all three critical melting parameters (residual porosity, upwelling rate of the solid mantle and melt velocity) are taken into account. Melting at ridges requires systematic variation of at least two critical melting parameters, most likely upwelling and melt extraction rate. Melts generated with increasing lateral distance to the ridge axis are generated with slower upwelling rates and are extracted with lower velocities than melts created closer to the ridge axis. Melting at ocean islands, on the other hand, can successfully be explained by variations in upwelling rate only. Global U-series systematics in OIB originate from superimposed global variations in upwelling velocity due to different buoyancy fluxes and from local variation in upwelling velocity as a function of radial distance to the plume center. The model proposed here is consistent with other geochemical data for oceanic basalts and strongly supports melt extraction via high-porosity channels as a general means of melt extraction from the Earth's upper mantle.