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International audience; Understanding the uranium (U) cycle – reservoirs and processes – at the watershed scale is key to manage contaminated areas, to elucidate ore formation processes as well as to implement paleoenvironmental research. Here, we investigated the different steps of the U cycle, from sources to sinks, and the relative roles of redox processes and organic matter in the control of U mobility in the naturally U-rich small mountainous watershed of Lake Nègre (France). We interpret the U repartition in U reservoirs through chemical, isotopic (δ238U and (234U/238U)) and speciation analyses, in the light of anterior studies of the site. We show that U(VI) originates from the leaching of U-rich rock fractures and is transported in dissolved forms. Wetlands and meadow soils then act as intermediary sinks where U(VI) is complexed by organic matter (up to > 5000 µg/g) and subsequently partly reduced to U(IV). Dissolved U is also supplied to the lake, in addition to particulate and colloidal U resulting from soil physical erosion. After entering the lake, most U(VI)-bearing organic particles settle in the sediments and U(VI) is reduced to U(IV), resulting in high sedimentary U concentrations (up to > 1000 µg/g), while a fraction of U is potentially desorbed from particles. Remaining dissolved U is exported from the watershed through the lake outlet stream. In this high-mountain lake catchment, the U cycle is mainly controlled by organic matter complexation and particulate transport, though U reduction in the lake sediments may help to its long-term immobilization.

Anthropogenic contamination, such as agricultural or urban activities, has led in less than a hundred years to the degradation of the quality of water resources. To assess the vulnerability of groundwater regarding organic and inorganic pollution (i.e. nitrates, pesticides, and pharmaceutical compounds) isotopic tracers may be used. However, in areas where multiple pollution sources occur, isotopic tracers may not be sufficiently discriminant, making source identification difficult. To overcome this problem, a combination of different tracers, including pharmaceutical compounds, along with the multi-isotopic approach is considered. Here, we present the preliminary results of the interpretation of a multi-tracer approach that show a mixed origin of NO3- contamination from agriculture and urban origin as well as the interaction between surface water (Waste-Water Treatment Plant effluent) and groundwater. The multi-tracer approach was applied in the Vistrenque basin area (Gard, France) to characterize NO3- sources. A combination of natural and anthropogenic tracers is implemented, including tracers of water origin (major ions, trace elements (Br, Li, and Sr)), nitrogen and oxygen stable isotopes as a tracer of NO3- (δ15N/δ18O-NO3-), boron stable isotopes (δ11B), water stable isotopes (δ18O/δ2H-H2O) and strontium isotopes (87Sr/86Sr). In addition, a suite of 80 organic molecules including pesticides and pharmaceutical compounds are trialed as tracers of the origin of the contamination. Analyses were carried out on groundwater samples, surface water, and soil samples to characterize the geochemical and isotopic signature of end-members. The dual-isotope approach δ15N/δ18O-NO3- for nitrate sources highlights differences between nitrate influenced by nitrification of NH4+ in fertilizer & precipitation origin on one hand, and, nitrates nitrate influenced by manure and septic waste origin on the other hand. However, little contrast between agricultural contamination (i.e. manure) and urban contamination (i.e. sewage) was seen with this method. In parallel, a mixing trend between groundwater and WWTP effluent is evidenced by the combination between δ15N-NO3- and δ11B. Moreover, this trend is confirmed with the occurrence of pharmaceutical compounds and a positive anomaly in Gadolinium (i.e. contrast-agent) in groundwater. As pharmaceuticals are found in several water supply boreholes the potential of pharmaceutic to discriminate the origin of organic nitrate will be investigated.

The boron isotopic ratio (δ11B) of marine biogenic carbonates is a proxy for ocean pH through the geological times, provided that δ11B measurements reach precisions better than 0.8‰ (2 SD), corresponding to 0.1 pH-unit (magnitude of the Anthropocene ocean acidification). This level of precision is however challenging to achieve for samples with a small size and low B content used in paleoceanographic reconstructions. We developed a protocol combining (i) the fast-handling and very low blank (9 ± 7 pg, 1 SD, n = 7) microsublimation technique to purify boron from the carbonate matrix and (ii) the direct injection of the samples into the MC-ICP-MS plasma with the new μ-dDIHEN device, allowing for small sample volumes and low uptake rates. We further used Jet sampler and X skimmer cones and implemented two 1013 Ω amplifiers to respectively increase the sensitivity and enhance the signal/noise ratio, enabling the precise measurement of small B signals. Our protocol, that consumes only 240 μL for one triplicate sample-standard bracketing (SSB) measurement, was validated through repeated B extractions and measurements of the carbonate reference material MVS-1 at different B concentrations. It yielded 15.93 ± 0.24‰ (2 SD, n = 10) with only 1.2 ng consumed for one triplicate SSB measurement, with individual external repeatabilities (n = 5, 2 SD) of 0.19 ± 0.14‰. We further measured δ11B of modern biogenic carbonates (arthropods, brachiopods, molluscs, red algae and reef carbonate mixes) down to 0.5–0.7 ng of B and successfully compared these δ11B values with those obtained after B extraction of the same samples through ion exchange chromatography.

The Toba volcano super-eruption on the island of Sumatra occurred about 74,000 years ago[1], close to the transition between interglacial Marine Isotope Stage (MIS) 5 and glacial MIS 4. This eruption, called Youngest Toba Tuff (YTT), is currently described as the largest cataclysmic eruption of the Quaternary. However, the impact of this super-eruption on climate is widely debated and its effects on the ocean remains poorly understood.The aim of this work is to estimate its impact on oceanic pH at a site near the eruption center. To do so, we measured δ11B values (pH proxy) on monospecific samples of planktonic foraminifera Globigerinoides ruber and Pulleniatina obliquiloculata from sediment core BAR94-25 (Andaman Sea) using a recently developed method at the Institut de Physique du Globe de Paris (IPGP)[2]. G. ruber is a species that thrives preferentially in surface waters, while P. obliquiloculata lives at the thermocline. Therefore, δ11B measurements on their shells can reconstruct pH variations in surface and thermocline waters, respectively.We selected the interval from 258 to 355 cm, corresponding to an age between 57 and 82 ka. This interval contains two clearly visible tephra layers corresponding to the YTT, at the transition from MIS 5 to MIS 4, and to a post-YTT explosive activity during MIS 4. These layers are correlated with a significant decrease in carbonate content (CaCO3). Our results indicate a complex pH response during the two concerned volcanic episodes. Thermocline seawater doesn’t show significant pH decrease during the volcanic episodes compared to the overall signal recorded throughout the studied interval. Conversely, surface seawater shows a much more important pH decrease during part of the volcanic episodes than during the all studied interval. Such decrease in pH during the transition to a glacial state is particularly surprising because an increase in pH, due to the global reduction in atmospheric CO2, is rather expected, as shown by previous foraminifera δ11B records[3].The coupling of CaCO3 and pH decrease during tephra levels suggests acidification in the Andaman Sea as a consequence of the Toba volcanic eruptive activity. The seawater surface seems much more sensitive to pH changes than the thermocline zone. However, the reduction of carbonate in the two tephra layers may also be due to dilution from ash falling into the sediment. Other analyses, such as measuring the variation of calcification intensity in planktonic foraminifera, are therefore necessary to better interpret these paleo-pH data.[1] Storey et al., 2012, PNAS, 109 (46), 18684-18688[2] Buisson et al., 2021, JAAS, 36, 2116-2131[3] Foster et al., 2008, EPSL, 271, 254-266

Geochimica et cosmochimica acta 338, 322 - 346 (2022). doi:10.1016/j.gca.2022.10.018, Understanding the controls on uranium (U) mobility in the environment is key to improve the management of sites contaminated by U mining activities. Previous research has shown that natural or engineered wetlands are particularly able to scavenge high amounts of U(VI) and U(IV) under noncrystalline forms. However, questions remain on the respective roles of sorption and reduction processes in the removal of U from running waters in wetlands, as well as on the long-term stability of U storage. Here, we performed a series of geochemical, isotopic (δ238U, (234U/238U)), microscopic (SEM-EDXS, EPMA) and spectroscopic (µ-XRF, µ-XAS, XANES and EXAFS at the U L3 and M4-edges and Fe K-edge) investigations to determine the modes of U accumulation and assess U mobility in a natural exceptionally U-enriched (up to 5000 µg/g) wetland on the shore of Lake Nègre (Mediterranean Alps, France). Uranium (VI) was largely dominant in the two studied soil cores, except a few samples containing as much as ∼50 % U(IV). At the particle scale, U is associated to a variety of organic constituents of the soil matrix with a homogenous oxidation state. Bulk EXAFS spectroscopy at the U L3-edge shows that U is mostly mononuclear, with dominant monodentate binding to organic moieties (C neighbors at ∼3.45 Å). An additional minor fraction of U under polymeric forms is inferred from wavelet (CCWT) analysis of the EXAFS data. These observations are reinforced by 1 M bicarbonate extractions that result in the dissolution of 82–96 % of total U, including putative polymeric species. At the wetland scale, similar or slightly fractionated isotopic ratios (δ238U) between the wetland-feeding creek waters and the wetland soils are observed, supporting the idea that U(VI) sorption on organic matter is the primary U scavenging mechanism. Furthermore, it indicates that partial U(VI) reduction to U(IV) occurs as a second step, after sorption. Analysis of U decay chain disequilibria in the cores as a function of depth suggests that U accumulation in this wetland has lasted for several thousand years. We propose that the wetland acts as an active reactor where U has been massively accumulating for ∼14500 years, especially as U(VI) forms associated to organic matter, and is further partly exported to the lake through soil erosion., Published by Elsevier, New York, NY [u.a.]

International audience; The knowledge of the sources of base cations in stream water is a prerequisite to assess potential effects of changing environmental conditions such as changing rainfall, weathering or groundwater flows on cation export with stream water. This study use stable Mg isotopes to identify potential sources in the well-studied catchment of Krycklan located on gneissic bedrock covered by quaternary sediments in Sweden. Samples were collected from open filed rain, throughfall, stream, soil, rock and litterfall. The δ26Mg values of these samples was determined and the contributions of different sources to Mg fluxes in the stream were determined from the variation of the Mg isotope and Sr / Mg ratios.The results show an overall variation of 1.10‰ between all samples. In addition, Magnesium isotope ratios varied little in the streamwater and in soil solution, except during snowmelt periods during which a large portion of the annual runoff occurs. Magnesium in the streamwater is explained as a mixture of three pools (open field rain, soil solution and groundwater) with the latter two influenced by catchment processes. Outside the snowmelt period, Mg in streamwater mainly derived from the groundwater, assumed to be mineral weathering signature in this catchment, with a contribution ranging from 12 to 63% to Mg fluxes. Open field rain dominates Mg fluxes in streamwater during spring flood (0 to 78%) and may contribute significantly during larger summer and autumn rainfall events. Soil solution input to streamwater range from 16 to 59% of Mg fluxes in streamwater. Our results demonstrate that δ26Mg values together with Mg concentrations and Sr/Mg ratios can be used to constrain the Mg sources of stream water and quantify weathering release rates.

The non-classical crystallization pathway of carbonate minerals proceeds through the initial formation of an amorphous precursor phase and commonly takes place during biomineralization. Although the isotopic composition of marine calcifiers is often used to reconstruct paleo-environmental conditions, the impact of the crystallization pathway followed by these organisms on the isotope composition of their skeletons has rarely been quantified. This study presents the first examination of B partitioning and isotope fractionation during CaCO3 formation via an amorphous calcium carbonate phase, which provides new insights into the incorporation of B into marine calcite and aragonite that form via this route. Boron concentrations and isotope compositions of the fluids and the formed solids were characterized during the course of experiments that lasted two years and were performed at 25 °C under controlled pH conditions. The results suggest that during Mg-ACC formation, B distribution coefficients are 1.5–4 orders of magnitude higher than those reported earlier in the literature for calcite and aragonite. Additionally, B isotope fractionation during Mg-ACC formation is strongly affected by the continuous exchange of solutes between the nanoporous solid phase and the bulk fluid. The isotope composition of the transient amorphous phase is not inherited in the crystalline carbonate mineral phase that ultimately forms, because crystallization proceeds via a continuous dissolution/re-precipitation process and/or chemical/isotope exchange between the solid surface and the fluid. Interestingly, the isotope fractionation between the fluid and the final crystalline products is different from those achieved in earlier studies where mineral growth proceeded via the standard mechanism of ion-by-ion addition of solutes to advancing steps. The B isotope fractionations measured in this study are in good agreement with results of equilibrium first principle calculations for calcite and aragonite (Balan et al., 2018) except in the case of calcite formed at pH = 8.9, suggesting that changes in aqueous speciation (i.e. increase of NaB(OH)4° and CO32− concentrations) may be responsible for slight changes of the calcite BO4 environment and isotope composition. It is expected that this study will help to better decipher the mechanisms controlling B isotope fractionation during the non-classical growth of calcium carbonates involving the formation of transient amorphous precursors.

International audience; Uranium (U) isotopic signatures and concentration in sediments are widely used as paleo-redox proxies, as the behavior of U is often controlled by bottom water oxygenation. Here, we investigated the processes controlling U accumulation in the sediments of Lake Nègre (Mediterranean Alps, South-East France) over the past 9200 years. Exceptionally high natural U concentrations (350-1250 μg.g-1) allowed the measurement of U along with other elements by high-resolution X-Ray Fluorescence core-scanning. Weathering and erosion proxies (Ti content, Zr/Al and K/Ti ratios) indicate that sedimentary inputs were controlled by Holocene climatic variations. After a period of low erosion during the Holocene Climatic Optimum, a major regime shift was recorded at 4.2 kyr BP when terrigenous fluxes consistently increased until present with high sensitivity to centennial-scale climatic events. Sedimentary organic matter (OM) inputs were dominated by terrigenous OM from the catchment soils until 2.4 kyr BP, as attested by carbon to nitrogen (C/N) and bromine to organic carbon (Br/TOC) ratios. From 2.4 kyr BP to present, lake primary production and soils equally contributed to sedimentary OM. Uranium fluxes to the sediments were well correlated to terrigenous OM fluxes from 7 kyr BP to present, showing that U supply to the lake was controlled by U scavenging in the soils of the watershed followed by transport of U bound to detrital organic particles. Higher U/OM ratios before 7 kyr BP likely reflect the development of the upstream wetland. The fluctuations of U sedimentary inputs appear to be independent of bottom water oxygenation, as estimated from constant Fe/Mn ratios and δ238U isotopic signatures, and rather controlled by the production, erosion and sedimentation of terrigenous OM. This finding confirms that the use of U (and potentially other metals with high affinity to OM) concentrations alone should be used with caution for paleo-redox reconstructions.

International audience; Many forest soils are acidic and have very low plant-available pools of magnesium. Past and present sylvicultural, nutritional and/or climatic pressures endured by forest ecosystems can result in net losses of nutrients and ecosystem function losses. Liming with a carbonate product is an alternative to counteract these degradations but the effects of liming on the biogeochemical cycling of nutrients over time and the dynamics of Mg released from liming products are still unclear. We studied the Mg isotopes composition in four paired-treatment experimental beech forest ecosystems in northern France. At the sites where dolomitic lime was applied, the variation in exchangeable and foliar δ26Mg demonstrated the direct contribution of dolomite-derived Mg to the replenishment of topsoil exchangeable pools and to tree nutrition improvement: dolomite-derived Mg was incorporated into the biological cycling which allows its retention on the mid to long term in the soil–plant system. At the sites limed with calcium carbonate, the changes in exchangeable and foliar Mg contents and δ26Mg observed on the long term suggest that the applied product contained a small amount of Mg and/or that Mg cycling changed after liming, to cope in particular with the low Mg availability. Lastly, our results highlight the high δ26Mg of the organic layer (humus): fractionation processes occurring within this layer (mineralization/ageing of organic matter, preferential retention of 26 Mg) could explain these singular signatures that could greatly influence the topsoil Mg exchangeable pools.

We determined the bromine isotope compositions of magmatic and hydrothermal sodalite (Na8Al6Si6O24Cl2) and tugtupite (Na8Al2Be2Si8O24Cl2) from the Ilímaussaq intrusion in South Greenland, in order to constrain the Br isotope composition of the melt and hydrothermal fluids from which these minerals were formed. Early formed magmatic sodalite has high Br contents (138 ± 10 µg/g, n = 5) and low δ81Br values (+0.23 ± 0.07‰). Late stage hydrothermal sodalite has lower Br contents (53±10 µg/g, n = 5) and higher δ81Br values (+0.36 ± 0.08‰). Tugtupite that forms at even later stages shows the lowest Br contents (26 ± 2 µg/g, n = 2) and the highest δ81Br values (+0.71 ± 0.17‰). One hydrothermal sodalite has a Br concentration of 48 ± 9 µg/g and an exceptionally high δ81Br of 0.82 ± 0.12‰, very similar to the δ81Br of tugtupites. We suggest that this may be a very late stage sodalite that possibly formed under Be deficient conditions. The data set suggests that sodalite crystallises with a negative Br isotope fractionation factor, which means that the sodalite has a more negative δ81Br than the melt, of −0.3 to −0.4‰ from the melt. This leads to a value of +0.5 to +0.6‰ relative to SMOB for the melt from which sodalite crystallises. This value is similar to a recently published δ81Br value of +0.7‰ for very deep geothermal fluids with very high R/Ra He isotope ratios, presumably derived from the mantle. During crystallisation of later stage hydrothermal sodalite and the Be mineral tugtupite, δ81Br of the residual fluids (both melt and hydrothermal fluid) increases as light 79Br crystallises in the sodalite and tugtupite. This results in increasing δ81Br values of later stage minerals that crystallise with comparable fractionation factors from a fluid with increasingly higher δ81Br values.

In semi-arid to arid regions, both anthropogenic sources (urban and agriculture) and deeper Critical Zone (groundwater with long flow paths and water residence times) may play an important role in controlling chemical exports to rivers. Here, we combined two anthropogenic isotope tracers: uranium isotope ratios (234U/238U) and boron isotope ratios (δ11B), with the 87Sr/86Sr ratios to identify and quantify multiple solute (salinity) sources in the Rio Grande river in southern New Mexico and western Texas. The Rio Grande river is a major source of freshwater for irrigation and municipal uses in southwestern United States. There has been a large disagreement about the dominant salinity sources to the Rio Grande and particularly significant sources are of anthropogenic (agriculture practices and shallow groundwater flows, groundwater pumping, and urban developments) and/or geological (natural groundwater upwelling) origins. Between 2014 and 2016, we collected monthly river samples at 15 locations along a 200-km stretch of the Rio Grande river from Elephant Butte Reservoir, New Mexico to El Paso, Texas, as well as water samples from agricultural canals and drains, urban effluents and drains, and groundwater wells. Our study shows that due to the presence of localized and multiple salinity inputs, total dissolved solids (TDS) and isotope ratios of U, B, and Sr in the Rio Grande river show high spatial and temporal variability. Several agricultural, urban, and geological sources of salinity in the Rio Grande watershed have characteristic and distinguishable U, Sr, and B isotope signatures. However, due to the common issue of overlapping signatures as identified by previous tracer studies (such as δ18O, δD, δ34S), no single isotope tracer of U, Sr, or B isotopes was powerful enough to distinguish multiple salinity sources. Here, combining the multiple U, Sr, and B isotope and elemental signatures, we applied a multi-tracer mass balance approach to quantify the relative contributions of water mass from the identified various salinity end members along the 200-km stretch of the Rio Grande during different river flow seasons. Our results show that during irrigation (high river flow) seasons, the Rio Grande had uniform chemical and isotopic compositions, similar to the Elephant Butte reservoir where water is stored and well-mixed, reflecting the dominant contribution from shallow Critical Zone in headwater regions in temperate southern Colorado and northern New Mexico. In non-irrigation (low flow) seasons when the river water is stored at Elephant Butte reservoir, the Rio Grande river at many downstream locations showed heterogeneous chemical and isotopic compositions, reflecting variable inputs from upwelling of groundwater (deeper CZ), displacement of shallow groundwater, agricultural return flows, and urban effluents. Our study highlights the needs of using multi-tracer approach to investigate multiple solutes and salinity sources in rivers with complex geology and human impacts.

The long-term fate of uranium-contaminated sediments, especially downstream former mining areas, is a widespread environmental challenge. Essential for their management is the proper understanding of uranium (U) immobilization mechanisms in reducing environments. In particular, the long-term behavior of noncrystalline U(IV) species and their possible evolution to more stable phases in subsurface conditions is poorly documented, which limits our ability to predict U long-term geochemical reactivity. Here, we report direct evidence for the evolution of U speciation over 3,300 y in naturally highly U-enriched sediments (350–760 µg ⋅ g(−1) U) from Lake Nègre (Mercantour Massif, Mediterranean Alps, France) by combining U isotopic data (δ(238)U and ((234)U/(238)U)) with U L(3)-edge X-ray absorption fine structure spectroscopy. Constant isotopic ratios over the entire sediment core indicate stable U sources and accumulation modes, allowing for determination of the impact of aging on U speciation. We demonstrate that, after sediment deposition, mononuclear U(IV) species associated with organic matter transformed into authigenic polymeric U(IV)–silica species that might have partially converted to a nanocrystalline coffinite (U(IV)SiO(4)·nH(2)O)-like phase. This diagenetic transformation occurred in less than 700 y and is consistent with the high silica availability of sediments in which diatoms are abundant. It also yields consistency with laboratory studies that proposed the formation of colloidal polynuclear U(IV)–silica species, as precursors for coffinite formation. However, the incomplete transformation observed here only slightly reduces the potential lability of U, which could have important implications to evaluate the long-term management of U-contaminated sediments and, by extension, of U-bearing wastes in silica-rich subsurface environments.

The use of the boron content and isotopic composition of secondary silicate minerals and siliceous organisms to trace weathering reactions and past ocean pH requires characterizing the fundamental reactions that govern the incorporation and subsequent isotope fractionation of this element in these materials. Toward this goal we have investigated boron adsorption on the surface of amorphous silica (SiO2·0.32H2O) and quantified its isotopic fractionation. Boron adsorption envelopes and corresponding isotope fractionation factors were measured in dilute aqueous solutions (0.01 M) of NaCl and CaCl2. B maximum adsorbed fraction was found to be about 2 times higher in CaCl2 solutions than in NaCl. The modelling of chemical and isotopic data in NaCl solutions allowed to identify the formation of two main B surface species at the SiO2(am)/water interface: a neutral trigonal (B3) inner-sphere complex, >SiOB(OH)20, characterized by a fractionation factor of ~−16‰ relative to aqueous boric acid, and a negatively charged tetrahedral (B4) inner-sphere complex, >SiOB(OH)3–, with fractionation factors of −5‰ with respect to aqueous borate. In CaCl2 solutions the data modelling indicates the presence of B4 inner-sphere complexes with a fractionation factor of −6.5‰ relative to aqueous borate, but excludes the presence of trigonal surface species and suggests instead the formation of a Ca-B(OH)4– complex that could partly account for the observed increase of boron adsorption in these solutions. These observations indicate that changes in the surface charge density and interfacial water structure due to different background electrolytes can induce changes in concentration and both chemical and isotopic composition of B adsorbed on silica surfaces. Although this study suggests that the B adsorption reaction on SiO2(am) plays a minor role in the B incorporation and isotopic signature of clay minerals and siliceous cements forming during weathering reactions, the acquired data should allow for an improved knowledge of the biomineralization reactions. The observed B isotopic fractionations between adsorbed and aqueous B species should help determining the relative contribution of various processes, such as cellular transport, biological mediation and silicification reactions, on the amount of B incorporated in siliceous microorganisms and its isotopic signatures. In addition, the presence in NaCl solutions of both trigonal and tetrahedral boron complexes at the silica surface could explain the observed weak pH dependence of the boron isotopic composition of marine diatoms.

One of the main factors controlling the behavior of uranium (U) in water bodies is bottom water oxygenation, which enables the use of U isotopic ratios and concentration in oceanic sedimentary records as paleo-redox proxies [1]. Here, we investigated the mechanisms governing U accumulation in the sediments of Lake Nègre (Mediterranean Alps, South-East France) over the past 9200 years. These sediments display exceptional natural U concentrations (300–1500 µg.g-1), allowing the use of high-resolution X-Ray Fluorescence core-scanning for the measurement of U. Chemical proxies (Ti content, Zr/K and K/Ti ratios) indicate that erosional sedimentary inputs were controlled by Holocene climatic variations. After a period of low erosion during the Holocene Climatic Optimum, a major regime shift was recorded at 4.2 kyr BP when terrigenous fluxes increased until present with high sensitivity to centennial-scale climatic events. The temporal evolution of carbon to nitrogen (C/N) and bromine to organic carbon (Br/TOC) ratios of sedimentary organic matter (OM) indicate that it was mostly of terrigenous origin from the catchment soils until 2.4 kyr BP. From 2.4 kyr BP to present, lake primary production and soils equally contributed to sedimentary OM. The Fe/Mn core profile points out that the lake bottom water oxygenation remained constant over the past 9200 years. Uranium depositional conditions were also constant over this period as attested by the 238U/235U ratio (expressed as δ238U) core profile. However, U fluxes to the sediments varied substantially and were correlated to terrigenous OM fluxes from 7 kyr BP to present. This correlation highlights that U supply to the lake was controlled by U scavenging in the soils of the watershed followed by transport of U bound to detrital organic particles. The fluctuations of U sedimentary inputs thus appear to be independent of bottom water oxygenation and rather controlled by climate-driven variations in terrigenous OM production and erosion. This finding confirms that the use of U (and potentially other metals with high affinity to OM) concentrations alone should be used with caution for paleo-redox reconstructions.

In sedimentary basins, clay compaction by burial can lead to fluid overpressure and is suspected to also generate fresh waters, but few geochemical tracers are available to assess this process both qualitatively and quantitatively. Our objective was to carry out experiments on the chemical - and halogen (Cl, Br) isotope evolutions of pore water expelled during clay compaction. For this, the smectite-rich bentonite MX80 and an illite-rich marl (Sainte-Suzanne) were equilibrated with ocean water. During two compaction experiments under high fluid pressure (45 MPa), mechanical stress (up to 150 MPa) and temperature (up to 150 °C), it was found that the chemistry and isotope behavior is considerably different between the swelling clay and the non-swelling clay. We saw a general decrease of the cat- and anion concentrations in the expelled water, for swelling clay while its concentration slightly increases in the remaining pore fluid. This was not the case for non-swelling clay. More freshening of the expelled water occurred during the compaction at higher temperature. We also observed a larger range of isotopic variation for Br (δ81Br from 0.9‰ up to 1.5‰) than for Cl (δ37Cl from −0.5‰ to −0.1‰) in the compaction experiments. During the compaction of illite-rich marl, no significant variation of Cl isotope (δ37Cl close to 0‰) was observed while the δ81Br value of the expelled water showed the same general increase (from 0.9‰ up to 1.5‰) as during the compaction of smectite-rich bentonite. We observed limited mineralogical transformations in terms of dissolution/precipitation processes. Therefore the surface chemistry of the clay in combination with decreasing porosity, in part, drive the anion and isotope evolution. We propose that significant retention of Cl and Br in the pore water of the compacted smectite-rich bentonite is indicative of ultrafiltration and that Cl and Br isotopes are promising tracers to consider when tracking the origin of low-salinity formation waters in sedimentary basins.

JWBR received funding from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme (Grant Agreement No. 805246) and from the Natural Environmental Research Council (NERC) grant NE/N011716/1. Boron isotopes in marine carbonates are increasingly used to reconstruct seawater pH and atmospheric pCO2 through Earth’s history. While isotope ratio measurements from individual laboratories are often of high quality, it is important that records generated in different laboratories can equally be compared. Within this Boron Isotope Intercomparison Project (BIIP), we characterised the boron isotopic composition (commonly expressed in δ11B) of two marine carbonates: Geological Survey of Japan carbonate reference materials JCp‐1 (coral Porites) and JCt‐1 (giant clam Tridacna gigas). Our study has three foci: (a) to assess the extent to which oxidative pre‐treatment, aimed at removing organic material from carbonate, can influence the resulting δ11B; (b) to determine to what degree the chosen analytical approach may affect the resultant δ11B; and (c) to provide well‐constrained consensus δ11B values for JCp‐1 and JCt‐1. The resultant robust mean and associated robust standard deviation (s*) for un‐oxidised JCp‐1 is 24.36 ± 0.45‰ (2s*), compared with 24.25 ± 0.22‰ (2s*) for the same oxidised material. For un‐oxidised JCt‐1, respective compositions are 16.39 ± 0.60‰ (2s*; un‐oxidised) and 16.24 ± 0.38‰ (2s*; oxidised). The consistency between laboratories is generally better if carbonate powders were oxidatively cleaned prior to purification and measurement. Publisher PDF

Measurement of nanoparticle (NP) concentration and size by single-particle inductively coupled plasma mass spectrometry (spICP-MS) usually requires the use of a NP reference material to determine the loss of NPs and/or ions during their transport from the sample solution to the detection system. The determination of this loss, qualified as nebulization efficiency (ηNebulization) and/or transport efficiency (ηTransport), is time-consuming, costly and lacks reliability. Nebulization of the NPs directly into the plasma (without a spray chamber) results in ηNebulization = 100% and is thus a promising strategy to avoid these calibration steps. In this work, we used the μ-dDIHEN introduction system: a demountable direct injection high-efficiency nebulizer (dDIHEN) hyphenated to a flow-injection valve and a gas displacement pump. For the first time with a continuous flow nebulizer, complete transport efficiency was reached (i.e. ηTransport = 100%). Operated at a very low uptake rate (as low as 8 μL min−1), the μ-dDIHEN accurately and reproducibly determined average diameters of Au-, Ag- and Pt-NPs, in full agreement with their reference values. It was also successfully tested for Au-NPs in complex matrices, such as surface waters. spICP-MS analyses with the μ-dDIHEN sample introduction system only require a dissolved standard calibration to determine NP average diameter (dNPs in nm) and number concentration (NNPs) from the simplified set of equations: $$ \overline{d_{NPs}}=\sqrt[3]{\frac{6.\kern0.5em \left(\overline{I_{NPs}}-{I}_{bckgd}\right).\kern0.5em {t}_d.{q}_{liq}}{S.\kern0.5em {f}_a.\kern0.5em \pi .\kern0.5em \rho }} $$ and $$ \overline{N_{NPs}}=\frac{\overline{D_{NPs}}}{q_{liq}.\kern0.5em D.\kern0.5em {t}_{d.}} $$ Graphical abstract

The boron isotopic composition (δ11B) of benthic foraminifera provides a valuable tool to reconstruct past deep-water pH. As the abundance of monospecific species might be limited in sediments, microanalytical techniques can help to overcome this problem, but such studies on benthic foraminiferal δ11B are sparse. In addition, microanalytics provide information on the distribution of δ11B at high spatial resolution to increase the knowledge of e.g. biomineralization processes. For this study, we investigated the intra- and inter-shell δ11B variability of the epibenthic species Cibicidoides wuellerstorfi, which is widely used in paleoceanography, by secondary ion mass spectrometry (SIMS) and femtosecond laser ablation multicollector inductively coupled plasma mass spectrometry (LA-MC-ICPMS). While the average δ11B values obtained from these different techniques agree remarkably well with bulk solution values to within ± 0.1 ‰, a relatively large intra-shell variability was observed. Based on multiple measurements within single shells, the SIMS and LA data suggest median variations of 4.8 ‰ and 1.3 ‰ (2σ), respectively, where the larger spread for SIMS is attributed to the smaller volume of calcite being analyzed in each run. When analytical uncertainties and volume-dependent differences in δ11B variations are taken into account for these methods, the intra-shell variability is presumably in the order of ~ 3 ‰ and ~ 0.4 ‰ (2σ) on a ~ 20 µm and 100 µm scale, respectively. In comparison, the δ11B variability between shells exhibits a total range of ~ 3 ‰ for both techniques, suggesting that several shells need to be analyzed for accurate mean δ11B values. Based on a simple resampling method, we conclude that ~ 7 shells of C. wuellerstorfi must be analyzed using LA-MC-ICPMS to obtain an accurate average value within ± 0.5 ‰ (2σ) to resolve pH variations of ~ 0.1. Based on our findings, we suggest to prefer the conventional bulk solution MC-ICPMS over the in-situ methods for e.g. paleo-pH studies. However, SIMS and LA provide powerful tools for high-resolution paleoreconstructions, or for investigating ontogenetic trends in δ11B, possibly due to vital effects during chamber formation.

Uranium (U) is a toxic radionuclide which environmental dissemination must be limited. In this regard, understanding U immobilization mechanisms in reducing environments is essential for improving the management of radioactive waste and the remediation of contaminated sites. In particular, determining the long-term behavior of non-crystalline U(IV) species in (sub-)surface conditions is of growing importance, as these environmentally-relevant species have been recently showed to play a major role in U mobility. For this purpose, we investigated the evolution of U speciation over a pluri-millennial period in naturally U-enriched sediments from Lake Nègre (alt. 2354 m, Mercantour, France) as an analogue of contaminated systems. Several sediment cores were sampled at 24 m of water depth and preserved under anoxic conditions. Bottom sediments were dated back to 8700 cal BP. These organic- and Si-rich sediments display increasing U concentration with depth, from 350 to more than 1000 µg/g. Sequential ultrafiltration of surface waters and uranium isotopic ratios (238U/235U and (234U/238U)) of sediments and waters suggest that the deposition mode of U did not vary significantly with time, thus giving the opportunity to follow the effect of diagenesis on U speciation over more than 1000 years. Uranium LIII-edge X-Ray Absorption Near-Edge Structure (XANES) analysis shows that U is rapidly reduced in the upper sediment layers and is fully reduced at depth. Preliminary Extended X-Ray Absorption Fine Structure (EXAFS) spectroscopy data at the U LIII-edge reveals that U speciation evolved with depth in the sediment core, suggesting an effect of diagenesis in anoxic conditions on U solid speciation. Our results may help to design long-term storage conditions that are able to enhance the formation of poorly soluble U species in U-contaminated soils and sediments.

Understanding the atmosphere-continent-ocean carbon cycle and its associated oceanic carbon system is one of the keystones to face the Anthropocene’s climate change. Since the 1990s the isotopic ratio of boron (δ11B) in calcitic shells of planktonic foraminifera has proven to be a powerful geochemical proxy to determine the oceanic paleo-pH and its link to atmospheric CO2 level over geological times1, whereas the ratio B/Ca as proxy of the seawater carbonate chemistry is still questionable2,3.However, the use of planktonic foraminifera in paleoclimatic reconstructions requires calibrations of the pH – δ11B relationships to correct what is known as « vital effect »4: each species controls differently its calcification process and consequently slightly modifies the seawater chemistry during biomineralization5,6. Moreover, shell size effect on δ11B has been reported for some symbiont-bearing species due to photosynthetic increase of pH7,8.Calibrations for the symbiont-barren Globigerina bulloides have been already determined9,10 but sparse data have been reported so far for the test size effect on δ11B 11.Here we measured the δ11B of three different fractions (250-315, 315-400 and >400 μm) of G. bulloides sampled along the coretop PS97-122 from the Chilean margin (54.10°S, 74.91°W), by using a new protocol developed at IPGP and dedicated to small samples which couple a microsublimation technique and a micro-direct injection device (μ-dDIHEN12). Our preliminary results show significantly higher δ11B values for the large fractions compared to the small ones, as found for symbiont-bearing planktonic species such as Globigerinoides sacculifer7 and Globigerinoides ruber8. (1) Pearson & Palmer, 2000, Nature 406, 695-699 (2) Yu et al., 2007, Paleoceanography 22, PA2202 (3) Allen et al., 2012, EPSL 351-352, 270-280 (4) Urey et al., 1951, Soc. Am. Bull. 62, 399-416 (5) Erez, 2003, Rev. in Min. and Geochem. 54 (1), 115-149 (6) de Nooijer et al., 2014, Earth-Science Reviews 135, 48-58 (7) Hönisch & Hemming, 2004, Paleoceanography 19, PA4010 (8) Henehan et al., 2013, EPSL 364, 111-122 (9) Martínez-Botíet al., 2015, Nature 518, 219-222 (10) Raitzsch et al., 2018, EPSL 487, 138-150 (11) Henehan et al., 2016, EPSL 454, 282-292 (12) Louvat et al., 2019, JAAS 8, 1553-1563

The role of hydrothermal fluids in the dissolution of primary minerals in volcanic areas, hotspots of silicate weathering at the global scale, is not well quantified. This study aims to determine the hydrothermal contribution to the lateral export of solutes from the Aso caldera, southern Japan. A set of indicators for the hydrothermally related contribution is systematically discussed. Spring, groundwater and river water samples were collected along the two main rivers of the caldera, and their major and trace element concentrations, as well as the stable isotopic compositions of water (δ2Η and δ18O), dissolved inorganic carbon (δ13C), and sulphate (δ34S and δ18O) were analysed. Hot springs in the study area are associated with a substantial sulphate contribution of magmatic origin. In contrast, the hydrochemistry of most cold springs reflects surface water-soil interactions. In particular, non-hydrothermal waters have significantly higher selenium to sulphate ratios than hydrothermal waters. The selenium to sulphate ratios allow to distinguish two different sulphur sources in the Aso catchment: hydrothermal waters impacted by the oxidation of sulphides or controlled by disproportionation reactions of SO2, and non-hydrothermal sulphate. Overall, the results show that hydrothermal waters strongly influence the sulphur budget, accounting for 67 to 91% of the total sulphate flux at the caldera outlet. The dissolution of magmatic CO2 and SO2 contribute to >60% of the observed weathering fluxes in this volcanic area. Therefore, magmatic gases and hydrothermal fluids should be considered for the estimation of biogeochemical budgets at the regional and global scale using and their products must be parameterized. However, the temporal variation of the magmatic activity and its effect on the chemistry of waters has to be investigated in future work.

International audience

The behaviour and fate of copper (Cu) and zinc (Zn) at river confluences is poorly understood, although chemical and physical processes during mixing of compositionally different tributaries might condition metal availability and fluxes to the ocean. To identify and quantify the effect of such processes in river mixing zones, particulate Cu and Zn isotope signatures (delta(65) Cu-SPM and delta Zn-66(SPM)) were measured along cross sections and vertical profiles at the largest river confluence in the world, the "Encontro das Aguas" mixing zone of the Amazon River, where the organic-rich Negro River meets the sediment-rich Solimoes River. The Negro River suspended sediments, with highly variable Cu and Zn concentrations as well as delta Cu-65(SPM) and delta(66) Zn-SPM, are mostly influenced by organic matter and by the lateritic cover of the watershed. The Solimoes River suspended sediments, more homogeneous in Cu and Zn concentration and isotope composition across the river section, reflect the signature of weathered silicate-rich sediments derived from the Andes. The Solimoes River supplies the majority of the suspended Cu and Zn to the Amazon River, and despite important flux losses across the confluence (-35% for Cu and -27% for Zn), delta(65) Cu-SPM and delta(66) Zn-SPM show a conservative behaviour during the mixing. In the dissolved load, Cu concentrations and delta(65) Cu-diss, mostly supplied by the Solimoes River, behave conservatively whereas Zn, derived mainly from the Negro River, suffers an important loss ( -58%) that can be attributed to Zn adsorption onto the suspended sediments from the Solimoes River. This transfer does not induce a significant delta(66) Zn-SPM shift in the Amazon River suspended sediments. Therefore, Cu and Zn isotope ratios in the suspended sediments behave conservatively through this confluence, which mixes two very chemically-contrasted rivers. Our findings thus suggest that the riverine isotopic information on the sources of particulate Cu and Zn is preserved during tributary mixing.

International audience; The complex behaviour of Zn and Cu at the soil-river interface was investigated in soil and riverine water samples from the Rio Negro basin, a secondary tributary of the Rio Amazonas, using their stable isotope compositions. This acidic and organic river drains two types of intensely weathered terrains: podzols in its upstream part, and lateritic soils downstream. Bulk soil particles, suspended particulate matter (SPM) as well as colloidal fractions were sampled across the whole basin during low and high water stages.In the basin, Zn and Cu are mostly exported from lateritic soils and transported by organic colloids where significant losses are observed in the downstream part of the river. The use of delta Zn-66 and delta Cu-66 measurements reveals distinct stories for these two metals in suspended sediments and colloids. In the colloids, the constant delta Zn-66(coll) across the basin is induced by the same weak association mode between Zn and organic ligands, regardless of the origin of the water. By contrast, in SPM, the speciation of Zn and thus delta Zn-66(SPM) differ according to the type of drained soils. Zn is associated with organic complexes in particles exported with water draining podzol whereas Zn2+ is incorporated in the structure of the remaining kaolinite clays in lateritic output.The stronger reactivity of Cu than Zn with organic ligands induces its complete complexation. Copper is controlled by refractory particulate organic matter (POM) and by reactive colloidal organic matter; the latter being enriched in delta Cu-65 due to stronger binding interactions than in POM. While the Cu content remains constant in the upstream part of the Rio Negro, downstream, the decrease of SPM and colloidal Cu fluxes is associated with a constant delta Cu-65(SPM) and with an increase of delta Cu-65(coll) at the Rio Negro outlet. Geochemical mass balance modelling, based on SPM, Cu and Zn fluxes in SPM and their associated isotopic signatures, confirms distinct host phases for Zn and Cu, and identifies the most probable places where losses of these two metals occur. In colloids, the observed Cu isotope fractionation (from 0.24 to 0.45%) superimposed on the significant Cu-coll loss is assumed to result from a new isotopic equilibrium in a low velocity and high productivity zone: Cu-rich colloids enriched in Cu-63 aggregate and settle down, whereas the remaining heavy Cu is partially complexed on strong organic ligands secreted by phytoplankton, forming new Cu-colloids

International audience; The mu-dDIHEN allows direct nebulization of small samples into the plasma at a flow rate down to 5 mu L min(-1). It is composed of a demountable direct injection high efficiency nebulizer (d-DIHEN), a flow injection analysis (FIA) valve and a gas displacement pump (GDP) connected to a mu-mass-flow meter delivering very stable liquid flow. The system was successfully tested with sample loops of 10 and 50 mu L and for liquid flow rates between 5 and 50 mu L min(-1) on two different inductively coupled plasma mass spectrometry (ICP-MS) instruments, a sector-field-ICP-MS (SF-ICP-MS) and a multi-collector-ICP-MS (MC-ICP-MS), and for three different applications: boron isotopic ratio measurement of geological samples, multi-element analyses of natural water samples and gold nanoparticle characterization by single particle ICPMS (spICP-MS). Signal sensitivity was increasing with the liquid uptake rate of the mu-dDIHEN (and the GDP gas flow rate), but stopped increasing above 30 mu L min(-1), due to bad spray quality and poor ionization under extra-wet conditions. We successfully measured boron isotopic ratios (delta B-11) for reference material solutions with B concentrations between 10 and 200 mu g L-1; the reproducibility of the measurements depending on both the B concentration of the sample and the uptake rate: B-10 signals below 20-30 mV (with 10(11) ohms amplifiers) gave 2SD reproducibility >= 0.5 parts per thousand (n = 5). The best measurement conditions were not necessarily with the lowest uptake rate but rather at 25 mu L min(-1), allowing for higher signals. We also tested transient versus continuous measurement modes for a series of natural samples (with 10 and 50 mu L sample loops). The two modes gave fully comparable results but the transient mode allows triplicate delta B-11 measurements and is recommended for very small samples (

To further develop boron isotopes as a tool for understanding shale weathering, we explored patterns of boron concentrations and isotopes across the forested Susquehanna Shale Hills Critical Zone Observatory (CZO). We present boron measurements for all watershed components that provided a foundation for examining water-rock interactions in a shale dominated watershed, including water compartments (e.g., precipitation, stream water, groundwater) and solid compartments (e.g., soil, bedrock, stream sediments, suspended load, and leaf litter). Results show boron isotopes (δ11B) in the bedrock (− 4.6‰) and soil (− 5.9 to - 4.2‰) were very similar. All waters were enriched in 11B by comparison: precipitation (7.2 to 22.6‰), stream (10.3 to 15.5‰), and groundwater (2.2 to 17.4‰). Modeling revealed that isotopic fractionation observed in the surface water and groundwater could mainly be explained by water-rock interactions including clay mineral dissolution (e.g., chlorite) and coprecipitation/adsorption processes (e.g., coatings on illite particles), likely in the near surface soils (~2 m deep). We found that leaching, the loss of boron from vegetation to stream water, plays a secondary role. Specifically, such leaching likely contributes the equivalent of 10 to 26% of the B fluxes from the watershed outlet. Boron mass balance between bedrock and precipitation inputs and the exported flux of dissolved and solid pools identified a “missing” isotopically light solid flux (δ11B of −12.2 ± 5.3‰ at ~4.4 ± 3.8 mol/ha/y of B; uncertainty reported as 2 SD). We did not sample any pool with this isotopic signature. Here our data suggest the composition of this pool is more likely related to precipitation of secondary clays rather than adsorption or (co)precipitation on Fe oxides. We propose two hypotheses to explain the missing light B pool: 1) a significant portion of the particles carrying the missing 10B are not sampled because they enter groundwater at depth and are transported out of the catchment under the stream; and/or 2) the inputs and outputs of boron are not operating at steady state in the catchment today, suggesting that the missing boron particles were lost in the past in proportions higher than today. When this B budget is paired with studies of δ26Mg and δ56Fe from Shale Hills, both of which also show missing isotopic pools, the pattern indicates a fundamental gap in understanding of shale weathering. We concluded that light B particles, presumably generated in the upper soils, are likely transported deep beneath the surface in the groundwater system or episodically in the past through riverine fluxes.

An isotope record of magnetic bacteria Microorganisms have shaped Earth's oceans and atmosphere over billions of years. Ancient microbes left very little direct morphological evidence of their existence in the rock record, thereby requiring geochemical clues for evidence of their activity. Amor et al. show that magnetotactic bacteria impart a distinct isotopic signature to their internal iron nanoparticles. Cultures of a modern magnetic bacterium fractionated 57 Fe isotopes independent of their mass, in contrast to fractionation patterns often observed for other isotopes. Because this signature is not produced abiotically or by other iron-metabolizing bacteria, it could serve as a reliable biomarker of this ancient magnetic microbial lifestyle. Science , this issue p. 705

International audience; AbstractAbstract ImageWe describe a novel method for measuring stable bromine isotope compositions in saline solutions such as seawater, brines, and formation waters. Bromine is extracted from the samples by ion exchange chromatography on anion exchange resin AG 1-X4 with NH4NO3 and measured by MC-ICP-MS in wet plasma conditions. Sample introduction through a small spray chamber provided good sensitivity and stability of the Br signal compared to direct injection (d-DIHEN) and desolvation (APEX). NH4NO3 media allowed fast (3 years) reproducibility between ± 0.11 and ± 0.27‰ (2 SD) was obtained for the four HBr solutions, the international standard reference material NIST SRM 977 (δ81BrSMOB = −0.65 ± 1.1‰, 1 SD), and seawaters (synthetic and natural). The accuracy of the MC-ICP-MS method was validated by comparing the δ81Br obtained for these solutions with dual-inlet IRMS measurements on CH3Br gas. Finally, the method was successfully applied to 22 natural samples.

International audience; In this study, we quantify zinc isotope fractionation during its sorption onto kaolinite, by performing experiments under various pH, ionic strength, and total Zn concentrations. A systematic enrichment in heavy Zn isotopes on the surface of kaolinite was measured, with Delta Zn-66(adsorbed-solution) ranging from 0.11 parts per thousand at low pH and low ionic strength to 0.49 parts per thousand at high pH and high ionic strength. Both the measured Zn concentration and its isotopic ratio are correctly described using a thermodynamic sorption model that considers two binding sites: external basal surfaces and edge sites. Based on this modeling approach, two distinct Zn isotopic fractionation factors were calculated: Delta Zn-66(adsorbed-solution) = 0.18 +/- 0.06 parts per thousand for ion exchange onto basal sites, and Delta Zn-66(adsorbed-solution) = 0.49 +/- 0.06 parts per thousand for specific complexation onto edge sites. These two distinct factors indicate that Zn isotope fractionation is dominantly controlled by the chemical composition of the solution (pH, ionic strength)

Evidence for relationships between seismotectonic activity and dissolved weathering fluxes remains limited. Motivated by the occurrence of new springs emerging after the 2016 Kumamoto earthquake and supported by historical groundwater data, this study focuses on the long-term effect of near-surface structural deformation on the contribution of deep, highly saline fluids to the solute fluxes from the Aso caldera, Kyushu, Japan. Available hydrologic and structural data suggest that concentrated, over-pressured groundwaters migrate to the surface when new hydraulic pathways open during seismic deformation. These new springs have a hydrochemical fingerprint (including δDH2O, δ18OH2O, δ7Li, δ11B, δ18OSO4, and δ34SSO4) indistinguishable from long-established confined groundwater that likely reflects a mixture of infiltrated meteoric water with high-sulfate hydrothermal fluids. A comparison of historical hydrochemistry data and patterns of past seismicity suggests that discharge of deep fluids is associated with similar deformation structures to those observed during the Kumamoto earthquake, and that seismic activity plays an important role over historic timescales in delivering the majority of the solutes to the caldera outlet, sustaining fluxes that are amongst the world’s highest. This upwelling mechanism might be relevant for other systems too, and could contribute to the over-proportional share of active volcanic areas in global weathering fluxes.

International audience; Three sub-basins of the Seine River (France) under contrasted land uses (i.e., forested, agricultural, and urban) have been investigated in order to assess the origin and seasonal variation of trace metals, and evaluate their geochemical background and dynamics. Our results highlight a high anthropogenic impact on all elements for both the dissolved and particulate fractions. The main source for each element in the dissolved phase was determined and shows that transition and post-transition metals mainly originate from forested areas, while alkali and alkaline earth elements, metalloids, and halogens rather originate from agricultural land use. Conversely, for the particulate phase, most of the elements cannot be associated with a specific land use. Seasonal variation of elements was assessed according to the forested and agricultural land uses, and geochemical backgrounds were determined using average export rates, highlighting that the geochemical background for the forested land use is higher than the agricultural one for most of the elements. Finally, to confirm those results, Zn dynamics in the three characteristic sub-basins and between the different land uses was investigated using a combination of Zn speciation, Zn isotopic ratio, and Zn export rates.

Chronic inflammation in end-stage renal disease (ESRD) is partly attributed to gut bacterial translocation (GBT) due to loss of intestinal epithelium integrity. Increased levels of circulating lipopolysaccharide (LPS) –a surrogate marker of GBT– contribute to maintain a chronic inflammatory state. However, circulating LPS can be neutralized by lipoproteins and transported to the liver for elimination. While ESRD-associated GBT has been widely described, less is known about its changes and impact on clinical outcome after kidney transplantation (KT). One hundred and forty-six renal transplant recipients with serum samples obtained immediately before and 1 year after transplantation (1-Year post KT) were included. Intestinal epithelium integrity (iFABP), total LPS (by measuring 3-hydroxymyristate), LPS activity (biologically active LPS measured by the LAL assay), inflammatory biomarkers (sCD14 and cytokines), lipoproteins and LPS-binding proteins (LBP and phospholipid transfer protein [PLTP] activity) were simultaneously measured. At 1-Year post KT, iFABP decreased but remained higher than in normal volunteers. Total LPS concentration remained stable while LPS activity decreased. Inflammation biomarkers decreased 1-Year post KT. We concomitantly observed an increase in lipoproteins. Higher sCD14 levels before transplantation was associated with lower incidence of acute rejection. Although GBT remained stable after KT, the contemporary increase in lipoproteins could bind circulating LPS and contribute concomitantly to neutralization of LPS activity, as well as improvement in ESRD-associated chronic inflammation. Chronic exposure to LPS in ESRD could promote endotoxin tolerance and explain why patients with higher pre-transplant sCD14 are less prompt to develop acute rejection after transplantation.

Although adsorption at the solid surface is the first step controlling boron incorporation in the crystal lattice during the standard growth mechanism of calcite and aragonite, little is known about the identity, structure and isotopic composition of the boron complexes formed at the CaCO3-solution interface. To generate this important information, we investigated experimentally the boron chemical and isotopic fractionation during adsorption at the calcite-water interface as a function of pH (6.5–11.7) at 4 and 20 °C in 0.01 and 0.1 M NaCl aqueous solutions. The surface complexation modeling of B adsorption and isotopic composition data showed that boron is sorbed at the calcite surface as a tetrahedral complex (>CaB(OH)40) formed by reaction of borate ions with Ca-protonated surface sites (log K i n t 0 = 1.54 ± 0.37 at 20 °C) and excluded the formation of trigonal B surface complexes (>CaB(OH)3+). The B isotopic composition of >CaB(OH)40 is ∼5‰ and 2‰ heavier than that of aqueous B(OH)4− in 0.01 and 0.1 M NaCl solution, respectively. Consistently, these values suggest that adsorbed borate ions have isotopic compositions intermediate between those of aqueous borate and structural tetrahedral species in calcite, which have been recently predicted to be ∼12‰ heavier than aqueous borate using quantum mechanical calculations ( Balan et al., 2018 ). The good agreement between the isotopic composition of adsorbed boron measured in this study and boron experimentally co-precipitated with calcite in the 8–9 pH range at close to equilibrium conditions (i.e. via ion-by-ion attachment at advancing steps) ( Noireaux et al., 2015 ) indicate that the isotopic composition of structural boron can be inherited from the boron surface complexes formed at the calcite/water interface. The results of this study contradict the assumption of no isotopic fractionation between tetrahedral boron in calcite and the borate ion that sustains the boron paleo-pH proxy, but confirm that trigonal B cannot be directly incorporated in the crystal structure during near equilibrium growth of calcite.

Rationale During transient signal acquisition by Multi-Collection Inductively Coupled Plasma Mass Spectrometry (MC-ICPMS), an isotope ratio increase or decrease (isotopic drift hereafter) is often observed which is related to the different time responses of the amplifiers involved in multi-collection. This isotopic drift affects the quality of the isotopic data and, in a recent study, a method of internal amplifier signal synchronization for isotope drift correction was proposed. In this work the determination of the amplifier time constants was investigated in order to validate the method of internal amplifier signal synchronization for isotope ratio drift correction. Methods Two different MC-ICPMS instruments, the Neptune and the Neptune Plus, were used, and both the lead transient signals and the signal decay curves of the amplifiers were investigated. Results Our results show that the first part of the amplifier signal decay curve is characterized by a pure exponential decay. This part of the signal decay was used for the effective calculation of the amplifier first-order time constants. The small differences between these time constants were compared with time lag values obtained from the method of isotope signal synchronization and were found to be in good agreement. Conclusions This work proposes a way of determining amplifier first-order time constants. We show that isotopic drift is directly related to the amplifier first-order time constants and the method of internal amplifier signal synchronization for isotope ratio drift correction is validated. Copyright © 2015 John Wiley & Sons, Ltd.