Search

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 238 U– 234 U– 230 Th 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. 232 Th is usually used as index of the detrital contamination, and the isochron age is derived from the isotopic composition ( 230 Th/ 238 U; 234 U/ 238 U) 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 ( 232 Th/ 238 U; 230 Th/ 238 U; 234 U/ 238 U). 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 ( 230 Th/ 238 U) and ( 234 U/ 238 U) ratios. At the first site (Coimbatore), we obtain U–Th ages between 304 ± 38 ka and 44 ± 2.9 ka (2σ) from the base to the top of the profile. These suggest discontinuous precipitation of the different facies in good agreement with micromorphological 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 232 Th/ 238 U without any variation of the 230 Th/ 238 U and 234 U/ 238 U ratios. Such patterns require extremely high 232 Th/ 238 U 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. [ABSTRACT FROM AUTHOR]

Over the last decades, the U- and Th-series nuclides were successfully used to determine weathering rates in various environments. The objective of this study is to assess the potential impact of the vegetation change on the U- and Th-series signal recorded in forested soils. This study was carried out from the experimental forest site of Breuil-Chenue (Morvan, France) developed by the INRA-BEF team. The native forest of the site was partially clear-felled in 1976 and replaced by monospecific plantations stands (Oak and Douglas fir). U- and Th-series disequilibria were measured in 2011 in the podzolic soils developed under the native forest, and in the two replanted stands. Separation of primary minerals (biotite, muscovite and perthitic feldspar) and selective extractions of the Fe and Al oxides were performed to investigate the distribution of U and Th among these soil fractions. The selective extractions suggest that a significant part of U and Th is primarily held by Fe-bearing silicates. Our results suggest that the tree substitution seems to produce a large dissolution of these minerals under the Oaks, resulting to a release of U and Th. However, below 25 cm no impact of this release was observed on U-series disequilibria. A scenario allowing to reconcile the significant mobilization of U and the constancy of U-series disequilibria is proposed. Above 25 cm, additional pedogenic redistribution of U and Th isotopes occurs in all the profiles, inducing some discrepancies between U-series disequilibria. A clear correlation between the ( 230 Th/ 234 U) ratios and the proportions of amorphous and interlayer Al hydroxides has been highlighted. This correlation suggests a mobility of U and Th isotopes strongly associated to Al dynamics in these soils rather than Fe, despite the primary location of U in the Fe-bearing silicates and the overwhelming reported control of U VI by Fe-oxides in oxidized environments. These pedogenic processes make the shallowest horizons of podzolic soils unsuitable for U-series dating. In contrast, a soil production rate can be deduced from the deepest soil layers which do not show such effects on the U-series nuclides. The reproducible U-series disequilibria measured in four whole-profile replicates emphasize the robustness and the significance of the “long-lived” U-series disequilibria in deep soil layers relative to long-term weathering rates, independent of transient perturbations such as land cover changes. Finally, because Ra can strongly accumulates in plants, the ( 226 Ra/ 230 Th) ratios in the different soils were affected by the flux of 226 Ra released by litter degradation. The use of this ratio as a long-term chronometer should therefore be performed with caution in such contexts. No direct impact of the vegetation type on the ( 228 Ra/ 232 Th) ratios was identified, due to the short 228 Ra half-life. [ABSTRACT FROM AUTHOR]

Studies of uranium-series (U-series) disequilibria within and around ore deposits provide valuable information on the extent and timing of actinide mobility, via mineral-fluid interaction, over a range of spatial and temporal scales. Such information is useful in studies of analogs of high-level nuclear-waste repositories, as well as for mining and mineral extraction sites, locations of previous nuclear weapons testing, and legacy nuclear waste contamination. In this study we present isotope dilution mass spectrometry U-series measurements for fracture-fill materials (hematite, goethite, kaolinite, calcite, dolomite and quartz) from one such analog; the Nopal I uranium ore deposit situated at Peña Blanca in the Chihuahua region of northern Mexico. The ore deposit is located in fractured, unsaturated volcanic tuff and fracture-fill materials from surface fractures as well as fractures in a vertical drill core have been analyzed. High uranium concentrations in the fracture-fill materials (between 12 and 7700 ppm) indicate uranium mobility and transport from the deposit. Uranium concentrations generally decrease with horizontal distance away from the deposit but in this deposit there is no trend with depth below the surface. Isotopic activity ratios, 238 U– 234 U- 230 Th– 232 Th linear age trends, and U-Th-Pa model ages indicate an actinide mobility evolution that can be characterized as complex and varied. There is evidence of both open and closed system behavior for uranium, depending on both sample and time-scale. The 234 U/ 238 U activity ratios indicate U mobility from 1.2–0.2 Ma. In the last 0.2 Ma there is isotopic evidence for some U mobility but mostly U immobility. In almost all cases the Th‐Pa radionuclides studied here have been immobile for the past 200 ka. Radium appears to have been recently (0.1–8 ka) mobile due to ongoing fluid flow in the fractures. Overall, the mainly closed system behavior for U-Th-Pa over the past ~ 200 ka provides an indicator of the geochemical immobility of these actinides over time-scales relevant for potential nuclear waste repositories situated in fractured, unsaturated tuff. [ABSTRACT FROM AUTHOR]

The naturally-occurring radionuclides 210Po and 210Pb are normally found in very low concentrations in the environment. However, under some conditions 210Po contributes significantly to the radioactivity in ground waters and can constitute a health risk. Naturally-occurring 210Pb can be used in predicting the behavior of contaminant Pb. While studies have identified the processes controlling 210Po and 210Pb behavior, the extent of adsorption and rates of removal in an aquifer cannot be determined without quantifying the rates of supply of these isotopes from decay of parent nuclides on and within aquifer solids. This study demonstrates how the isotope systematics of the 238U decay series, which includes 210Po and 210Pb, can be used to quantify the adsorption and retardation factors of 210Po and 210Pb in aquifers. The rates of 210Po and 210Pb supply to ground waters are determined from the concentrations of other radionuclides in the series, especially 222Rn, and so the distribution coefficients between aquifer solids and groundwater can be calculated. The effects of different supply rates due to different distributions of parent radionuclides in the aquifer can then be separated from differences in adsorption, which is controlled by aquifer water chemistry. A review of groundwaters where 210Po, 210Pb, and 222Rn data are available indicates that systematic variations can be found in bulk effective adsorption coefficients and retardation factors. Overall, a method is provided for future studies of 210Po and 210Pb behavior, and greater understanding of the occurrence and migration of these radionuclides. [ABSTRACT FROM AUTHOR]

The island of São Miguel, Azores piques geochemical interest due to a strikingly large range in the long-lived radiogenic isotope ratios of its lavas. The 'enriched' signatures (e.g., radiogenic 87Sr/86Sr and unradiogenic 143Nd/144Nd) observed in lavas found in the east of the island have been proposed to originate from recycled crustal components in their mantle sources. Such fertile lithologies should have higher melt productivities (amount of melt generated per decrement in pressure) than peridotitic mantle and this heritage ought to be evident in the U-series signatures of melts derived from such materials. Specifically, combined 238U-230Th and 235U-231Pa disequilibria in erupted lavas should identify significant differences in the melting rate and/or the mineralogical makeup of their sources (e.g., the amount of garnet). To this end, new 238U-230Th-226Ra and 235U-231Pa disequilibria are presented for sixteen mafic lavas (< 50 wt.% SiO2) that encompass the full range of São Miguel's long-lived radiogenic isotope variability. However, primary U-series signatures of mafic lavas can be influenced by assimilation of more differentiated products. Notably, syenite nodules and cumulate alkali feldspar xenocrysts have been identified in São Miguel basalts. We argue that secondary contamination of basalts is most evident in the hugely variable 230Th-226Ra disequilibria (~ 18% deficits to > 300% excesses of 226Ra) that strongly correlate with Ba/Th ratios. The longer-lived U-series nuclides are only slightly perturbed in a number of the more enriched lavas, resulting in increased noise in the dataset. In apparently uncontaminated samples, isotopically depleted lavas from the west of São Miguel display slightly higher 230Th and 231Pa excesses than the enriched samples from the east. The U-series signature of the enriched lavas can be modeled with a combination of higher absolute values of D¯U U = and D¯Th,, coupled with higher melting rates compared to the depleted western source. These parameters are consistent with a peridotite source enriched with contributions from more fusible lithologies such as eclogite and/or pyroxenite in the eastern source, but not with direct derivation from such fusible lithologies. The only subtle variations in U-series isotopes coupled with large, systematic variations in radiogenic isotope compositions suggest that enriched components and/or their melts are well-mixed with mantle peridotite, thus masking their distinctive melting behavior. Finally, basalts from Sete Cidades volcano in the west of São Miguel have very similar long-lived isotope compositions and U-series disequilibria to basalts from the island of Pico, Azores. The geochemical similarity of lavas from islands at the center and periphery of the archipelago argue against systematic differences in melting behavior related to sampling different portions of an underlying plume structure. [ABSTRACT FROM AUTHOR]

Abstract: 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 2.2Bq/L and 3.5Bq/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 12years, 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. [Copyright &y& Elsevier]

Abstract: This study reports U-series activity ratios from river waters and six soil profiles across a soil chronosequence formed since the last glacial retreat, in Glen Feshie, Scotland. The overall aim is to examine the geochemical behaviour of the U-series nuclides in a boreal climate setting. The U-series data show that U is being both added to and leached out of the soils, to varying degrees. The U addition elevates the (234U/238U) of the bulk soils (up to 1.25), which is most pronounced in the youngest and the upper organic-rich soil horizons. The U addition appears to be linked to U adsorption, controlled by the degree of flooding by the Feshie River. The Feshie River has a high (234U/238U) ratio (~1.7), a feature shared with most high-latitude Northern Hemisphere rivers. For the soil profiles with no significant U addition, U-series nuclide modelling suggests U leaching rates on the order of 0.5–2×10−5 y−1, a similar range to other Northern Hemisphere high-latitude areas affected by the last glaciation, e.g. Mackenzie Basin, Canada. These two observations suggest a link between weathering rates and riverine (234U/238U) for areas that have been glaciated recently. A global compilation of major rivers shows that high-latitude Northern Hemisphere rivers comprise a significant U flux to the ocean with high (234U/238U). Thus, past changes in this Northern Hemisphere high-latitude U flux may have played a major role in oceanic (234U/238U) variation over glacial–interglacial cycles. [Copyright &y& Elsevier]

Abstract: A new method for the chemical separation and MC-ICPMS measurements of U–Th–Pa–Ra disequilibria in volcanic samples from a single sample aliquot is presented. The accuracy and precision of our techniques is assessed by replicate analyses of the synthetic U and Th isotope standards IRMM-184 and IRMM-35, and the secular equilibrium rock standards BCR-2, W-2a and TML. A measure of true sample reproducibility including the errors related to sample processing and the chemical separation is obtained from analysis of multiple digestions of the rock standards. 234U/238U, 230Th/238U, 231Pa/235U and 226Ra/230Th activity ratios on ten separate dissolutions of USGS rock standard BCR-2, for example, reproduce to 0.4%, 1.4%, 1.1% and 2.4% (2 SD), respectively. All elements (U–Th–Pa–Ra) are separated from a single spiked sample aliquot and the elemental concentrations are measured on the same solutions as the isotopic compositions. Analytical artefacts from determination of elemental and isotopic concentrations on different sample aliquots due to sample heterogeneity are therefore avoided. In addition, our new chemical separation protocol is time-efficient and allows for rapid processing of volcanic samples with less than 25fg of Pa and Ra. [ABSTRACT FROM AUTHOR]

Abstract: A new method for the radiogenic isotope (U–Th–Pa–Ra, Sr, Nd, Hf) analysis of the soluble and insoluble components found within ice cores is presented. Melting experiments with rock standards in the presence of EDTA indicate that carbonates, as well as silicates, can be buffered sufficiently to preclude dissolution. The use of EDTA allows adsorbing species, such as Th and Hf, to remain in solution during melting thus fully separating the dust (insoluble) and sea salt (soluble) components of the ice after filtration. A new elemental separation scheme for low sample masses, less than 5 mg solid material, utilizes 4 primary ion exchange columns and two “clean-up” columns to fully isolate U, Th, Pa, Ra, Sr, Nd, and Hf while maintaining high yields. Elution schemes measured for USGS rock standards and a Chinese loess are presented to provide a comparison for variable matrix compositions. Mass spectrometer techniques were modified to measure small aliquots of the standards, equivalent to the amounts found in ice core samples, 10 ng and less. A MC-ICPMS was employed for the measurement of U, Th, Pa, Ra, and Hf; results of the experiments show that with ion yields up to 1%, rock standards have errors for 234U/238U of 1%, 230Th/232Th of 1.5%, [228Ra] of 9%, and 176Hf/177Hf of 100 ppm. MC-TIMS measurements of Sr and Nd show similar errors for small sample sizes: 87Sr/86Sr of 50 ppm and 143Nd/144Nd of 80 ppm. This new analytical method increases the number of possible tracers measured from a single sample, reducing separation times and sample consumption, as well as providing the addition of a radiometric clock, U-series, to the traditional suite of isotopic tracers, Sr, Nd, and Hf. [Copyright &y& Elsevier]

Abstract: 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 δ 13C 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. [Copyright &y& Elsevier]

Abstract: This study investigates U-series, Sr isotopes, major and trace elements in a chalk aquifer system located in Eastern France. Soil and rock samples were collected along depth profiles down to 45 m in four localities as an attempt to investigate the weathering processes in the soil, the unsaturated zone and the saturated zone of the aquifer. Interstitial water was extracted from soils and rocks by a centrifugation technique. U-series offer a powerful tool to calculate weathering rates because the relative mobility of the U- and Th-isotopes can be precisely measured and it does not require the determination of a reference state as in other approaches. As expected, the data show very large mobile element depletion in the soil with large 230Th excess relative to 238U, while the rocks show more limited but not insignificant mobile element depletion. The U-series data have been used to constrain weathering rates based on a 1-D reactive transport model. Weathering rates in the near surface are about 10–100 times faster than at depth. However, when integrated over the depth of the cores, including the unsaturated and the saturated zones, this underground weathering represents more than 30% of the total weathering flux, assuming congruent dissolution of carbonates. The (234U/238U) ratios in interstitial water are consistent with solid samples showing 234U depletion near the surface and an excess 234U at depth. A leaching experiment performed on chalk shows that the excess 234U in natural waters percolating through carbonate rocks results both from preferential 234U leaching and direct recoil in the interstitial water. A new approach was used to derive the recoil ejection factor based on BET measurements and the fractal dimension of chalk surface. Consideration of preferential leaching and recoil allows a more accurate modeling of weathering rates. [Copyright &y& Elsevier]

Abstract: 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. [Copyright &y& Elsevier]

Abstract: To further explore the efficacy of 226Ra(excess) dating for deep-sea sediments, previously dated varve sediments from Saanich Inlet were investigated. Ages obtained using 226Ra(excess) are comparable to the varve ages in the upper 20–25 m of the sedimentary record, but radiometric ages for those sediments older than c. 4000 yr BP are significant underestimates. This results from major changes in sedimentation within Saanich Inlet around 4000 yr BP linked to rising sea levels, with younger sediments characterised by a higher biogenic contribution resulting from the establishment of an anoxic fjord environment. The older sediments were deposited in a shallow water inlet characterised by variable Ra mass balance and non-radiogenic losses. Therefore, while 226Ra(excess) can produce reliable dates, its application may be limited where the relative significance of authigenic and allogenic input and bottom water anoxia has been variable and where closed-system behaviour is compromised. [Copyright &y& Elsevier]

Abstract: Trace element and U-series isotope analyses are presented for a suite of recent (<10 ka) basalts from the axial portion of the Kenya rift. Samples from throughout the rift have LREE-enriched patterns with HREE >10×chondrite and the LREE between 60 and 200×chondrite. REE fractionation is consistent with melting a garnet lherzolite source region with between 2% and 6% modal garnet. Other trace element ratios are distinct from OIB, notably Zr/Hf which ranges from 43 to 48, whilst at a given Zr content the Zr/Hf ratio is significantly greater than that found in OIB. (238U/232Th) range from 0.362 to 1.036, (230Th/232Th) from 0.503 to 1.109, with (230Th/238U) ranging from 0.783 to 2.966. All but two samples are in 230Th excess or in secular equilibrium. Samples with elevated (238U/232Th), also have Rb/Cs>120, but unexceptional 208Pb⁎/206Pb⁎ and hence κ Pb values. These samples have experienced U and Cs loss and are excluded from further consideration. Of the unaltered samples, all have (238U/232Th) generally lower than OIB, with maximum values of <0.8, and some <0.6. The maximum (230Th/238U) is 1.39, similar to OIB. Although none of the basalts has a primary composition, (230Th/238U) does not vary systematically with indices of fractionation, and comparison with evolved rocks from Kenya indicates that 238U–230Th disequilibrium in the basalts is not the product of fractionation and crustal residence, but a product of melt generation. The maximum (230Th/238U) that can be generated by batch melting, assuming a source mineralogy consistent with the REE variation is 1.05 and so the variation in (230Th/238U) is attributed to more complex models of melt generation and/or transport. Both dynamic melting and equilibrium porous flow suggest mantle upwelling rates of ≤2 cm year−1. It is suggested that the Kenya basalts represent melts derived from lithospheric mantle that has been thermally reactivated by and incorporated into the underlying (East African) mantle plume. [Copyright &y& Elsevier]

In recent decades, U- and Th- series nuclides have been successfully used to determine weathering rates in soils and regoliths. However, the redistribution of the nuclides within secondary minerals and/or organic matter may interfere with the recorded U- and Th-series weathering signals and can result in inaccurate and/or poorly constrained weathering rate determinations. In addition, the migration of radionuclides in the environment represents a major current issue. The objective of this study, therefore, was to evaluate the meaning of bulk soil data for weathering investigations and to assess the processes and timescales of U, Th and Ra redistribution in a forested-soil profile, both at the bulk soil scale level and for separated mineral fractions. This study was carried out at the INRAE-Andra experimental beech-forested site of Montiers (Meuse, France), which was developed on acidic detrital sediments inherited from tropical paleosoils erosion. U- and Th-series activity nuclides were measured in bulk soil samples and in separated Fe nodules and clay-size (<2 μm) fractions. Furthermore, the analyses of atmospheric particles allow discussing the potential influence of atmospheric inputs. While bulk soil disequilibria suggested negligible to moderate mobility of the nuclides, an important transfer between mineral fractions was highlighted; this resulted in contrasting redistributions of U, Th and Ra within soil fractions. Fe-oxide nodules controlled the overall mobility of U on the bulk soil scale, while the fine <2 μm fraction controlled the mobility of Ra. A capture of 234U by Fe-oxide nodules (mainly comprised of goethite) was observed within the entire soil profile, most likely occurring for several hundred ky. In the shallow soil layers, a leaching of U from the <2 μm fraction that was not re-adsorbed onto Fe-oxides occurred. In contrast, Fe-oxides appeared to retain all U mobilized from the <2 μm fraction in the deep soil layers, resulting in negligible U mobility and an apparent "closed-system" at the bulk soil scale. This transfer, however, seemed to occur over shorter timescale than the one in the shallowest soil layers. No mobility of 226Ra was observed at the bulk soil scale, despite the large release of this nuclide from Fe-oxides. The measured (210Pb/226Ra) activity ratios suggested that up to 50% of 226Ra in Fe-oxides is available for desorption. However, the <2 μm fraction strongly readsorbed the released Ra and prevented the loss of this nuclide at the bulk soil scale. Considering the short half-life of 226Ra, such transfer from Fe-oxides to the <2 μm fraction occurred over a significantly shorter timescale than U mobilization. In addition, the analysis of the short-lived 228Ra isotope emphasized the flux involved in the biological cycle and the potential impact of vegetation on the soil radionuclide redistribution over a short timescale. Our results suggest that the fine <2 μm fractions might be suitable for probing the residence time of clay minerals in soils by the U-series dating method, if the soil profile can be assumed to consist of homogenous inherited clays. [Display omitted] • Important redistribution of nuclides between mineral fractions despite apparent immobility at bulk soil scale. • Fe-oxides control the overall mobility of U at the bulk soil scale while the fine <2 µm fraction controls the mobility of Ra. • Timescales of redistributions inferred from U-series data are different for U and Ra and change with soil depth. • The short-lived 228Ra emphasizes the impact of the vegetation to the radionuclide redistribution. [ABSTRACT FROM AUTHOR]