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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

International audience; Longgupo cave, located in Chongqing Municipality, China, was discovered in 1984. Sixteen Gigantopithecus teeth and two hominid fossils accompanied with abundant mammalian fossils and stone artifacts were unearthed from the site. Previous dating result is questioned because of the complexity of deposition history. In this study, seventeen mammalian fossil teeth collected from different layers of unit C II and C III (C III') during the 2003-2006 Sino-Franco joint excavation were analyzed by combined ESR and U-series methods, and calculated with a US-ESR model. In situ dose rates were remeasured in detail in 2012, in order to refine the external dose rate determination. Uranium-series analyses indicate that no obvious uranium leaching has occurred, and all the teeth underwent a very recent uranium uptake history, except one from layer C III 3 of the south wall. The US-ESR results show that the ages of ten teeth from unit C III' of the north wall are consistent in general, about 2.35 Ma. The ages of seven samples from south wall are not in accordance with the stratigraphic order: three teeth are much younger than the other four, probably attributed to the relative higher uranium concentration and U/Th ratio in the dental tissues and significant higher in situ gamma dose rate. Nevertheless, the three teeth from C III of the south wall give an average age of - 2.48 Ma. The fossil ages obtained in this work combined with paleontological evidence and new paleomagnetic results place Longgupo at the very beginning of the Early Pleistocene. Compared with other early hominid settlements, Longgupo is one of the earliest evidences of hominid settlement in China and East Asia to date.

Deep erosional gullies dissect landscapes around the world. Existing erosion models focus on predicting where gullies might begin to erode, but identifying where existing gullies were initiated and under what conditions is difficult, especially when historical records are unavailable. Here we outline a new approach for fingerprinting alluvium and tracing it back to its source by combining bulk sediment optically stimulated luminescence (bulk OSL) and meteoric 10Be (10Bem) measurements made on gully-derived alluvium samples. In doing so, we identify where gully erosion was initiated and infer the conditions under which such erosion occurred. As both 10Bem and bulk OSL data have distinctive depth profiles in different uneroded and depositional settings, we are able to identify the likely incision depths in potential alluvium source areas. We demonstrate our technique at Birchams Creek in the southeastern Australian Tablelands—a well-studied and recent example of gully incision that exemplifies a regional landscape transition from unchanneled swampy meadow wetlands to gully incision and subsequent wetland burial by post-European settlement alluvium. We find that such historic alluvium was derived from a shallow erosion of valley fill upstream of former swampy meadows and was deposited down the center of the valley. Incision likely followed catchment deforestation and the introduction of livestock, which overgrazed and congregated in valley bottoms in the early 20th century during a period of drought. As a result, severe gully erosion was likely initiated in localized, compacted, and oversteepened reaches of the valley bottom.

Clay minerals hold some key information on time and rate of weathering processes that could be deciphered by U-series disequilibria. However a removal of organic matter is generally required prior to clay mineral chemical analyses, especially in soils. The impact of two protocols of organic matter removal on the U-series disequilibria in three different clay minerals was therefore investigated. Natural or commercial kaolinite, illite and montmorillonite were treated with hydrogen peroxide and by calcination. Both investigated treatments impact the U-series signature of clay minerals. Yet the extent of this impact strongly depends on the clay-type: while kaolinite remains almost insensitive to the treatments, illite shows some equivocal shifts since the activity ratios measured after calcination could describe more accurately the pure clay signal than initial activity ratios, and montmorillonite displays the larger reactivity. Overall, the H2O2 treatment seems to induce more disturbance than the calcination process, mostly on U isotopes in illite and montmorillonite. Due to the oxidizing power of H2O2, the redox state of U held in the clays, i.e.: U(IV) or U(VI), strongly influences the extent of U leaching. In addition, a preferential leaching of U-234 relative to U-238 was observed in montmorillonite sample only, resulting to a U isotopic ratio fractionation. Conversely, during calcination, the most pronounced effect arises for Th isotopes. The congruent feature of Th loss, i.e.: Th loss approximate to mass loss, affecting the three clay minerals suggests a loss of Th through volatilization during this treatment. U or Ra are also affected, but to a lesser extent, by this volatilization. Overall, calcination causes a two- or three-fold reduced change of the (Th-230/U-234) and (U-234/U-238) activity ratios relative to H2O2 treatment. The (Ra-226/Th-230) ratio was clearly the less sensitive to any treatment. It was only slightly affected in kaolinite after calcination, and the observed change suggests an improvement of the measured value due to a removal of a Th-bearing external matter. Albeit not perfect, calcination should therefore be preferred to H2O2 treatment for organic matter removal before U-series analyses.

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.

The application of uranium-thorium dating methods to authigenic carbonates provides unique constraints on the temporal evolution of methane seeps at ocean margins. In this study, we report U-Th isotope measurements for carbonate breccias collected from within a hydrate-bearing pockmark located at the Niger Delta margin. These concretions were extracted from a carbonate-rich layer in the upper two meters of a sediment core (N2-KS-44; ~ 1200 m water depth), well above the present-day sulphate-methane transition zone (about 3 m depth) and the presence of gas hydrates in the sediment. The stratigraphy of core N2-KS-44 was established by tuning its downcore Al/Ti profile to a well-dated nearby reference core, and carbonate 230 Th/U ages were calculated using isochron methods. Our results indicate that a major event of aragonite precipitation occurred between about 13 and 2.5 ka at the studied location. Comparison of sediment accumulation rates at both core N2-KS-44 and the nearby reference site suggests that the initial stage of carbonate precipitation, between 13 and 10 ka, was associated with sediment winnowing, probably related to intense fluid seepage. In contrast, our data indicate that sedimentation rates rapidly increased within the pockmark after 7 ka. In agreement with the presence of carbonate breccias exhibiting U-Th ages older than their corresponding stratigraphic age, this observation would suggest that sediment reworking took place after that time, possibly caused by erosion of the surrounding sediment within the pockmark. We hypothesize that the period of carbonate formation between 13 and 2.5 ka was related to an upward migration of gas-hydrate reservoirs to the near seafloor environment. After this pulse of enhanced fluid flow, the diminution of methane fluxes at the base of the local gas-hydrate occurrence zone would have led to hydrate dissolution in sub-surface sediments and pockmark formation, thereby explaining the progressive increase in sedimentation rates, the absence of recent carbonate concretions and the deepening of the sulphate-methane transition zone at site N2-KS-44 inferred from pore water data. Overall, these results provide further constraints about the relationship between gas hydrate dynamics and the evolution of pockmarks at ocean margins through time.

Une première chronologie des occupations moustériennes de la grotte des Teixoneres (province de Barcelone, Espagne) a été établie grâce à la datation uranium-thorium de deux planchers stalagmitiques encadrant les niveaux moustériens. Des âges compris entre 13 et 17 ka, après correction du 230Th détritique, ont été déterminés sur le plancher supérieur, indiquant que le dépôt de ces spéléothèmes pourrait alors correspondre à l'oscillation climatique Bølling-Allerød. Le plancher inférieur a pour sa part été attribué au stade isotopique 5.3. A first chronology of the Mousterian levels of the Teixoneres cave (province of Barcelona, Spain) was established using U-series dating method on two stalagmitic flowstones located respectively above and under the mousterian levels. For the upper one, ages range from 13 to 17 ka, after correction of the detritic 230Th, indicating that the deposit of these speleothems could then correspond to the Bølling-Allerød oscillation. The lower flowstone was deposited during the oxygen isotopic stage 5.3.

Travertines overlying and impregnating Allier terraces have been dated and confirm previous research that they register past spring activity for the last 250 kyr New U-senes ages demonstrate that surface travertines were not only mainly deposited under Interglacial but also under Interstadial conditions Detailed reconstruction of the Holocene fluvial incision suggests that the position of surface travertine precipitation at terrace scarps bear a relationship with floodplain incision (terrace formation) Unfortunately the temporal resolution of current U-senes dates is too coarse to allow a detailed reconstruction for older cycles. Travertine dating with 14C has proved to be unreliable as a result of the contamination with fossil carbonates, Les travertins qui recouvrent et imprègnent les terrasses fluviatiles de l'Allier ont été dates par U/Th. Ces résultats confirment les travaux antérieurs et montrent que ces dépôts ont enregistré l'activité des sources hydrothermales depuis environ 250 ka BP. Les nouvelles datations U/Th montrent que les travertins ne se sont pas formés uniquement pendant les interglaciaires mais aussi au cours des épisodes interstadiaires. Une reconstruction détaillée de l'incision de la vallée de l'Allier au cours de l'Holocène suggère que la précipitation de dépôts travertineux sur le talus de la terrasse de 10 m est en rapport avec l'incision de la vallée actuelle de l'Allier. Malheureusement, la precision des âges détermines par la méthode U/Th est trop faible pour permettre une reconstitution détaillée pour les cycles plus anciens. La datation des travertins par le I4C a montré que cette technique n'était pas fiable en raison de leur contamination par des carbonates fossiles., Veldkamp Antonie, Kroonenberg Salomon, Heijnis Henk, Van Den Berg Van Saparoea Rob. The suitability of dated travertines as a record of fluvial incision: Allier (France) floodplain dynamics during the Late quaternary [ Validité des travertins datés pour la reconstitution de l'histoire de l'incision fluviatile. La dynamique de la nappe alluviale de l'Allier pendant le Quaternaire récent.]. In: Quaternaire, vol. 15, n°1-2, 2004. Fluvial Archives Group. Clermond-Ferrant 2002. pp. 159-165.

Fissure ridge travertines grown from geothermal springs of Denizli Basin, southwestern Turkey, are investigated through stratigraphic, structural, geochemical, and geochronological methods, with the aim of understanding the growth of these elongate mound-shaped structures. Two main types of travertine deposits are recognized: (1) bedded travertines, which grew as fl owstone on sloping surfaces and form the bulk of fi ssure ridges, and (2) banded travertines, which grew as veins within the bedded travertine chiefly along its central feeding conduit. Stratigraphic and structural observations shed light on the bedded-banded travertine relationships, where the banded features grew through successive accretion phases, crosscutting the bedded travertine or forming sill-like structures. The bedded and banded travertines alternated their growth, as demonstrated by complicated crosscutting relationships and by the upward suture, in places, of banded travertine by bedded travertine that was, in turn, crosscut by younger banded travertine. The bedded travertine is often tilted away from the central axis of the fi ssure ridge, thus leaving more room for the central banded travertine to form. U-series ages confi rm the bedded-banded travertine temporal relationships and show that the growth of the studied fi ssure ridges lasted up to several tens of thousands of years during Quaternary time. The banded travertine was deposited mainly during cold events, possibly in coincidence with seismic events that might have triggered the outfl ow of deep geothermal fl uids. C and O stable isotope and rare earth element data indicate a shallow feeding circuit for the studied structures with a fl uid component deriving from a deeper geothermal circuit. A crack-and-seal mechanism of fi ssure ridge growth is proposed, modulated by the interplay of local and regional infl uencing factors and mechanisms such as geothermal fl uid discharge, paleoclimate, tectonics, and the progressive tilting of bedded travertine limbs over a soft substratum creating the necessary space for the central veins to grow. © 2012 Geological Society of America.

Ice sheets and deep ice cores have yielded a wealth of paleoclimate information based on continuous dating methods while independent radiometric ages of ice have remained elusive. Here we demonstrate the application of ( 234 U/ 238 U) measurements to dating the EPICA Dome C ice core based on the accumulation of 234 U in the ice matrix from recoil during 238 U decay out of dust bound within the ice. Measured ( 234 U/ 238 U) activity ratios within the ice generally increase with depth while the surface areas of the dust grains are relatively constant. Using a newly designed device for measuring surface area for small samples, we were able to estimate reliably the recoil efficiency of nuclides from dust to ice. The resulting calculated radiometric ages range between 80 ka and 870 ka. Measured samples in the upper 3100 m fall on the previously published age-depth profile. Samples in the 3200–3255 m section show a marked change from 723–870 ka to 85 ka indicating homogenization of the deep ice prior to resetting of the ( 234 U/ 238 U) age in the basal layers. The mechanism for homogenization is likely enhanced lateral ice flow due to high basal melting and geothermal heat flux.

International audience; The discovery of chemically and isotopically enriched mid-ocean ridge basalts (E-MORB) has offered substantial insight into the origin, time scales, and length scales of mantle heterogeneity. However, the exact processes involved in producing this E-MORB enrichment are vigorously debated. Additionally, because the ages of E-MORB are not well constrained, the petrogenetic, temporal, and geological relationships between E-MORB and normal (N)-MORB are not known. To investigate these relationships and to explore how melting and melt transport processes contribute to or modify enriched mantle source compositions and generate E-MORB melts beneath mid-ocean ridges, we measured major and trace elements, and Sr, Nd, Hf, Pb, and U-Th-Ra isotopes for a suite of lavas that were collected off-axis, including several E-MORB, at 9-10 degrees N along the East Pacific Rise (EPR). These data show coherent mixing trends among long-lived radiogenic isotopes, U-series nuclides, and incompatible trace elements, implying that mixing of melts from different sources occurs at different depths. Our results are consistent with previous studies that show that melting occurs in a two-porosity melting regime, with high-porosity channels forming deeply in the presence of garnet and transporting enriched melts with large (230)Th excesses to the crust, whereas low-porosity channels transport melts more slowly, allowing them to equilibrate at shallow depths and develop large (226)Ra excesses at the expense of diminished (230)Th excesses. Forward modeling of the trace element data also is consistent with mixing of melts in a two-porosity melting regime. U-series age constraints suggest that E-MORB neither erupt at systematically different times from N-MORB, nor necessarily through different pathways. Previous studies of E-MORB at 9-10 degrees N have suggested that E-MORB compositions could be explained by off-axis eruption. However, when considered in light of previously published magnetic paleointensity and U-series age constraints, recent geological studies, and the most widely accepted contemporary understanding of volcanic construction at 9-10 degrees N EPR, the asymmetric, off-axis distribution of E-MORB at 9-10 degrees N EPR is consistent with, and more simply explained by, a model in which E-MORB erupted within the axial summit trough (AST) and flowed down the ridge flanks (similar to 0-3 km). These E-MORB subsequently spread away from the AST, and, finally, were preserved on the seafloor through asymmetric construction of the extrusive layer. Taken together, the range of ages of E-MORB at 9-10 degrees N EPR and the geochemical and isotopic mixing trends suggest that enriched melts are continuously supplied to the ridge axis, but because of their small proportions relative to the volumetrically and volcanically dominant N-MORB, E-MORB preservation and exposure is comparatively scarce.

U-series disequilibria provide important constraints on the processes and time scales of melt production, differentiation, and transport in subduction settings. Such constraints, which are essential for understanding the chemical evolution of the continental crust, are conventionally based on the assumption that the U-series disequilibria measured in mafic lavas are produced during mantle metasomatism and melting, and that intracrustal differentiation and assimilation have limited impacts. Here we show that mantle-derived U-series disequilibria in mafic lavas erupted at Volcan Llaima, Chile are significantly diminished by assimilation of plutonic rocks forming Llaima's subvolcanic basement. This contamination process is extremely subtle in terms of “classical” indicators of crustal assimilation like Sr, Nd or Pb isotopes because it is a manifestation of assimilative recycling of the plutonic roots of the arc. This process results in variations in U-series disequilibria and incompatible trace element ratios that are significant compared to regional and global variability in arc magmas. Furthermore, it yields linear correlations between U-series excesses and incompatible trace element ratios that are generally interpreted as slab-fluid indicators and chronometers, or tracers of sediment recycling in subduction zone. Cannibalization of ancestral magmas by ascending melts warrants careful evaluation when considering the components and chemical fluxes in subduction zones. Linear arrays defined by activity ratios of U-series nuclides with different half-lives may be the most reliable indicators of assimilative recycling of ancestral intrusive magmas.

The Gruta Nova da Columbeira is recurrently mentioned in the literature concerning the Middle-to-Upper Palaeolithic transition in Iberia as documenting the persistence beyond 30 000 calendar years ago of a Neanderthal-associated Mousterian. This claim is based on conventional radiocarbon dates obtained in the 1960’s and the 1970’s. In order to assess its validity, we undertook archival research to obtain unpublished details concerning the actual composition and chemistry of the dated samples, replicated the dating of samples of the same kind (carbonaceous sediments) and collected in the same deposits from the back of the cave whence came the 1970’s results, and obtained an U-series age estimate for a bone tool from the base of the Mousterian sequence excavated at the entrance of the cave in 1962. We then cross-checked all the stratigraphic and dating information thus assembled against the original field documents. Our results show that (a) the cave entrance sequence formed between MIS-5 and early MIS-3, (b) the deposits at the back of the cave probably formed in the Tardiglacial, and (c) the presence in these deposits of significant amounts of inherited charcoal derived from the entrance area explains the “Early Upper Palaeolithic” (EUP) age determinations obtained for the 1970’s samples. The association of such determinations with the Mousterian has been based on an unwarranted assumption of lateral stratigraphic continuity. While the entrance deposits correspond to an in situ Mousterian sequence, those from the back of the cave are primarily made of clay accumulated under temporary waterlogged conditions, with the few artefacts of Middle Palaeolithic affinities recovered therein being in secondary position. The evidence from Gruta Nova can no longer be used to counter the existence of a late Aurignacian in the region. In southern and western Iberia, the Neandertal-to-modern and Middle-to-Upper Palaeolithic transitions occurred no later than about 37 000 years ago. Auf der Grundlage konventioneller Radiokarbondaten aus den 1960er und 1970er Jahren werden Grabungsergebnisse aus der Gruta Nova da Columbeira (Bombarral) in der Fachliteratur auch heute noch verschiedentlich als Beleg für eine zeitliche Überlappung des späten Moustérien mit dem älterem Jungpaläolithikum genannt. Die unerwartet jungen 14C-Daten werden weiterhin als Beleg für eine Fortdauer der Neanderthaler bis in eine Zeit jünger als 30 000 Jahre vor heute herangezogen. Zur Überprüfung dieser Hypothese haben wir zahlreiche historische Dokumente zu den früheren Grabungen, wie auch der 14C-Datierungen, zusammengestellt und einer kritischen Sichtung unterzogen. Mit Hilfe der historischen Grabungsdokumente konnten zahlreichen Details der ursprünglichen Stratigraphie von Gruta Nova rekonstruiert werden. Auf dieser Grundlage wurden dann gezielt Nachuntersuchungen vorgenommen, um Probenmaterial zur erneuten 14C-Datierung aus gleicher stratigraphischer Position zu erhalten. Ferner wurde eine U/Th-Datierung an einem Knochenwerkzeug des Moustérien aus der Basis der Schicht 8 vorgenommen. Die Datierungsergebnisse zeigen, (1) dass es am Höhleneingang tatsächlich Fundschichten mit Artefakten gibt, die zwischen MIS-5 und MIS-3 datieren, aber (2), dass die ursprünglich anhand der 14C-Daten als ein „frühes Jungpaläolithikum“ interpretierten Schichten im rückwärtigen Teil der Höhle wahrscheinlich aus dem Spätglazial und durch Sedimente mit alten Holzkohlen aus dem Eingangsbereich kontaminiert wurden. Wie die Rekonstruktion der stratigraphischen Situation zeigt, gibt es in Gruta Nova — bei den heute nicht mehr akzeptablen 14C-Daten — keine ernstzunehmenden Indizien für die Existenz eines späten Moustérien. Damit liegt auch kein Beleg für einen späten Übergang - nach 37 000 Jahren vor heute - vom Mittel- zum Jungpaläolithikum im südlichen und westlichen Bereich der Iberischen Halbinsel vor.

Uranium (U) presents a unique challenge for ecological risk assessments (ERA) because it induces both chemical and radiological toxicity, and the relative importance of these two toxicities differs among the various U source terms (i.e., natural, enriched, depleted). We present a method for the conversion between chemical concentrations (μg L -1) and radiological dose rates (μGy h -1) for a defined set of reference organisms, and apply this conversion method to previously derived chemical and radiological benchmarks to determine the extent to which these benchmarks ensure radiological and chemical protection, respectively, for U in freshwater ecosystems. Results show that the percentage of species radiologically protected by the chemical benchmark decreases with increasing degrees of U enrichment and with increasing periods of radioactive decay. In contrast, the freshwater ecosystem is almost never chemically protected by the radiological benchmark, regardless of the source term or decay period considered, confirming that the risks to the environment from uranium's chemical toxicity generally outweigh those of its radiological toxicity. These results are relevant to developing water quality criteria that protect freshwater ecosystems from the various risks associated with the nuclear applications of U exploitation, and highlight the need for (1) further research on the speciation, bioavailability, and toxicity of U-series radionuclides under different environmental conditions, and (2) the adoption of both chemical and radiological benchmarks for coherent ERAs to be conducted in U-contaminated freshwater ecosystems. © 2009 American Chemical Society.

International audience; Travertine and calcareous tufa deposits, are important archives for Quaternary continental climate and archaeology. Here we present a comprehensive study on Holocene travertine deposition in the Thuringia basin based on a detailed survey of U-series measurements and dating that demonstrates rapid accumulation of carbonate during the early Holocene until 8000 years (up to 8 mm yr-1) followed by significant change in tra-vertine accumulation that ended around 5800 years BP at the investigated site. We further demonstrate that systematic changes of the U-isotopic composition through time occur in agreement with major changes in travertine accumulation and texture, which are likely to reflect changes of spring water U-isotopic composition rather than U-series system opening or contamination with non-carbonate particles.; Les travertins et les tufs calcaires constituent des archives importantes pour les études sur le climat continental quaternaire et l’Archéologie. Nous présentons l’étude de la formation d’un travertin holocène du bassin de Thuringe, basée sur un exposé détaillé des données uranium-thorium et des âges U-Th obtenus, qui démontrent une accumulation rapide du carbonate pendant le début de l’Holocène jusque vers 8000 ans (jusqu’à 8 mm par an) suivie par un changement significatif dans le rythme d’accumulation du travertin qui s’achève vers 5800 ans BP sur le site étudié. Nous démontrons ensuite que les changements systématiques dans la composition isotopique de l’uranium au cours du temps sont en accord avec les variations majeures de texture et d’accumulation du travertin et reflètent les changements de composition isotopique de l’uranium des eaux de source plutôt qu’une ouverture du système des séries de l’uranium voire une contamination par des particules non carbonatées.

International audience; The archaeological site of Payre (France) yielded Middle Palaeolithic layers with Neanderthal remains, which are partly bracketed by two stalagmitic flowstones. To obtain a precise chronological framework for the human occupation and the faunal remains of this site, several dating methods were used: uranium-series (U-series by alpha spectrometry and TIMS) and electron spin resonance (ESR) on stalagmitic flowstones, combined ESR/U-series on teeth and bones, and thermoluminescence (TL) on burnt flints. Most of the ages obtained for levels H to C range from 300 to 140 ka. The age-spread obtained for the anthropic (burnt flints) and faunal remains (bones and teeth) suggests that the dated levels represent several human occupation periods, near the transition from MIS8 to MIS7 (levels G–F) and from MIS6 to MIS5 (levels D–E).

International audience; The high proportion of crystallized apatite forming the bones and teeth should theoretically allow the use of fission track analysis to date vertebrate fossils when burying of the fossiliferous series did not subject them to temperatures exceeding 60 degrees C. However several major obstacles arise such as the complexity of fossils' internal structures, diagenetic modifications and substitutions of the hydroxyapatite by other minerals, and massive U uptake or loss during diagenesis. In this work, those various problems are addressed combining optical microscope observation of the fossils, a systematic fission track analysis of the best samples, alpha and gamma spectrometry and Secondary Ion Mass Spectrometry. Even if the problems caused by internal structures and mineral substitutions can be solved, U movements in and out of the fossils are generally too complex to allow fission track analysis dating of the fossils.

International audience; Andesitic products of the last magmatic eruption at La Soufrière in Guadeloupe (1530 AD) were analyzed for 238U–230Th–226Ra in order to constrain the timescales of magmatic processes preceding the eruption. Magma composition evolved during eruption from acid andesitic scoria falls and flows to andesitic lavas during the final phase of dome extrusion, with some evidence of mixing in banded pumices at the base of the first eruptive unit. Most of the least differentiated products show 238U and 226Ra excesses relative to 230Th, like many other arc lavas. The (226Ra/230Th) activity ratios of whole rocks then decrease with the degree of differentiation from 1.507 to 0.983, whereas the (230Th/238U) ratios increase from 0.942 to 1.003. The groundmasses of the least differentiated lava show a (226Ra/230Th) ratio higher than 1, but all the other samples are in secular equilibrium. These variations can be modeled by time-dependent differentiation from a common parental melt. Assuming a closed-or open-magma chamber behaviour (closed or open system), the duration of magma differentiation ranges from 30 to 70 kyr with crystallization rates ranging from 0.012 to 0.019 kyr− 1. 238U–230Th disequilibria have also been measured in minerals from one of the least differentiated products and provide crucial information on their age of crystallization, their origin and the crystal dynamics in the magma chamber. A crystallization age of 34.9 ± 8.3 kyr (2σ) is obtained using U–Th internal isochrons on pyroxenes, which is older than the differentiation time of their host lava given by time-dependant differentiation models, but younger than the differentiation time of the most evolved magma compositions. Older cumulate crystals were thus recycled in the erupted magmas, consistent with petrologic evidence for crystal-melt disequilibria. The calculated (226Ra/230Th) ratios of mineral assemblages (> 1 for most samples) suggest the presence of plagioclases younger than 8000 yr, in agreement with the fact that plagioclases do not plot on the U–Th isochron defined by pyroxenes. This study thus exemplifies the hybrid nature of arc andesitic magmas such as the 1530 AD Soufriere magma, which is generated by mixing of crystals and melts produced by fractional crystallization of a common parental magma. Our results also confirm that the magma chamber below large volcanoes such as La Soufrière of Guadeloupe is long-lived (up to tens of thousands of years). This timescale is much longer than the cooling times predicted by a closed-system thermal model, suggesting that the magma chamber was sustained by inputs of fresh and hot magma and behaves as an open system.

Twenty two Uranium-series datings carried on seven stalagmites collected in the cold, high mountain karstic system of Cueva del Cobre (Cantabrian Mountains, N Spain) allow to distinguish three phases of major growth frequency of speleothems in the cave for the last 7 50 ky. This difference in growth frequency is interpreted to be climatically induced: stalagmite growth was impeded during the coldest time intervals when the cave was probably covered by a mountain glacier and, on the contrary, their maximum growth took place during warmer humid episodes. This hypothesis is supported by the correlation of the speleothem growth frequencies with the oxygen isotopic stages of the late Quaternary

Twenty two Uranium-series datings carried on seven stalagmites collected in the cold, high mountain karstic system of Cueva del Cobre (Cantabrian Mountains, N Spain) allow to distinguish three phases of major growth frequency of speleothems in the cave for the last 7 50 ky. This difference in growth frequency is interpreted to be climatically induced: stalagmite growth was impeded during the coldest time intervals when the cave was probably covered by a mountain glacier and, on the contrary, their maximum growth took place during warmer humid episodes. This hypothesis is supported by the correlation of the speleothem growth frequencies with the oxygen isotopic stages of the late Quaternary