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Antecedent hydrological conditions are recorded through the evolution of dissolved lithium isotope signatures (δ7 ${\delta }^{7}$Li) by juxtaposing two storm events in an upland watershed subject to a Mediterranean climate. Discharge and δ7 ${\delta }^{7}$Li are negatively correlated in both events, but mean δ7 ${\delta }^{7}$Li ratios and associated ranges of variation are distinct between them. We apply a previously developed reactive transport model (RTM) for the site to these event‐scale flow perturbations, but observed shifts in stream δ7 ${\delta }^{7}$Li are not reproduced. To reconcile the stability of the subsurface solute weathering profile with our observations of dynamic stream δ7 ${\delta }^{7}$Li signatures, we couple the RTM to a distribution of fluid transit times that evolve based on storm hydrographs. The approach guides appropriate flux‐weighting of fluid from the RTM over a range of flow path lengths, or equivalently fluid residence times. This flux‐weighted RTM approach accurately reproduces dynamic storm δ7 ${\delta }^{7}$Li‐discharge patterns distinguished by the antecedent conditions of the watershed. Plain Language Summary: Storm events often cause characteristic shifts in stream solute chemistry. Interpreting these signals offers insight into the water‐rock interactions occurring within watersheds. Here, we use lithium stable isotopes and reactive transport modeling to relate how long water spends in a catchment, or how deep water infiltrates through a catchment, to the extent of chemical weathering. We show that the first significant storm after a dry season exports more chemically evolved water, while a wet season storm releases less evolved, shallower, and younger water. Our results indicate that stream flow δ7 ${\delta }^{7}$Li in small watersheds offers a sensitive record of hydrological conditions prior to the storm, reflecting subtle shifts in the efficiency of the Critical Zone to generate, transport, and ultimately export solutes. Key Points: Stream lithium stable isotope ratios (δ7Li) recorded at high frequency over storm events are sensitive to antecedent conditionsA reactive transport model cannot produce observed shifts in stream chemistry through variations in flow rate aloneFlux‐weighting of model fluid outputs based on time‐varying fluid transit time distributions describes stream δ7Li over storm hydrographs [ABSTRACT FROM AUTHOR]

Lithium isotope ratios (δ7Li) of rivers are increasingly serving as a diagnostic of the balance between chemical and physical weathering contributions to overall landscape denudation rates. Here, we show that intermediate weathering intensities and highly enriched stream δ7Li values typically associated with lowland floodplains can also describe small upland watersheds subject to cool, wet climates. This behavior is revealed by stream δ7Li between +22.4 and +23.5‰ within a Critical Zone observatory located in the Cévennes region of southern France, where dilute stream solute concentrations and significant atmospheric deposition otherwise mask evidence of incongruence. The water‐rock reaction pathways underlying this behavior are quantified through a multicomponent, isotope‐enabled reactive transport model. Using geochemical characterization of soil profiles, bedrock, and long‐term stream samples as constraints, we evolve the simulation from an initially unweathered granite to a steady state weathering profile which reflects the balance between (a) fluid infiltration and drainage and (b) bedrock uplift and soil erosion. Enriched stream δ7Li occurs because Li is strongly incorporated into actively precipitating secondary clay phases beyond what prior laboratory experiments have suggested. Chemical weathering incongruence is maintained despite relatively slow reaction rates and moderate clay accumulation due to a combination of two factors. First, reactive primary mineral phases persist across the weathering profile and effectively "shield" the secondary clays from resolubilization due to their greater solubility. Second, the clays accumulating in the near‐surface profile are relatively mature weathering byproducts. These factors promote characteristically low total dissolved solute export from the catchment despite significant input of exogenous dust. Plain Language Summary: Chemical weathering of silicate minerals by meteoric water serves as a long‐term regulator of atmospheric CO2. The extent to which these reactions produce secondary minerals dictates the chemical composition of soils and rivers. The isotope composition of metals is a potent tracer of this chemical weathering and significant progress has been gained in application of lithium stable isotopes for this purpose. Here, we demonstrate that water draining a mountainous landscape subject to a wet, temperate climate in Southern France produces isotopically heavy lithium in stream water. These signatures suggest an environment in which chemical weathering rates keep pace with the rates of soil erosion. To reveal the underlying causes of this behavior, we develop a numerical model that simulates the flow of water and its interaction with rock in the subsurface using measured data from the field site, including soil, bedrock, and water chemistry. We show that new minerals formed in the subsurface resist dissolution and effectively retain lithium. This work leverages the sensitivity of lithium isotopes to analyze weathering conditions that typify upland watersheds subject to cool, wet climates. Key Points: Small, upland watersheds under Mediterranean climate display characteristically high lithium stable isotope ratios (δ7Li)In a watershed in southern France subject to strong atmospheric inputs, weathering incongruence leads to high stream δ7Li valuesIsotope‐enabled reactive transport modeling indicates that at this site soil clays retain lithium and are resilient to (re‐)solubilization [ABSTRACT FROM AUTHOR]

Shifts in water fluxes and chemical heterogeneity through catchments combine to dictate stream solute export from the Critical Zone. The ways in which these factors emerge in resultant concentration‐discharge (C‐Q) relationships remain obscure, particularly at the timescale of individual precipitation and discharge events. Here we take advantage of a new high‐frequency, multi‐element and multi‐event stream C‐Q data set. The stream solute concentrations of seven major ions were recorded every 40 min over five flood events spanning one hydrologic year in a French agricultural watershed (Orgeval) using a lab‐in‐the‐field deployment we refer to as a "River Lab." We focus attention on the recession periods of these events to consider how geochemical heterogeneity within the catchment translates into dynamic stream solute concentrations during shifts in water storage. We first show that for C‐Q relationships resulting from data acquisition over multiple flood events, lumping all trends together can lead to biases in characteristic C‐Q parameters. We then reframe C‐Q relationships using a simple recession curve analysis to consider how hydrological processes produce chemical mixing of distinct solute pools immediately following discharge events. We find three distinct classes of behavior among the major solutes, none of which can be interpreted based on water storage changes alone. The shape of C‐Q relationships for each solute can then be related to their vertical zonation in the subsurface of Orgeval, and to the capacity for subcomponents of these distributions to be readily mobilized during a discharge event. Key Points: We combine high‐frequency (every 40 min) stream chemistry data and stream discharge recession analysisWe propose a conceptual model for the relationship between water storage and solute export at the Orgeval Critical Zone Observatory (CZO), FranceThree classes of solute behavior are identified at the Orgeval CZO, related to the role of rock dissolution and amendments in their budget [ABSTRACT FROM AUTHOR]

The Manguruyú Zungaro jahu, endemic to the La Plata Basin, is one of the largest neotropical fish species whose life history is virtually unknown. The objectives of this study were to determine if Z. jahu is a large‐scale migratory species and to describe habitat use throughout ontogeny using the otolith Sr isotope (87Sr/86Sr) ratio. Surface water samples were collected in 2020 at four sites representing the main water courses of the Great Pantanal in Brazil (upper Paraguay, Cuiabá, São Lourenço and Vermelho rivers). Eighteen fish individuals were collected from the Paraná River (Argentina). New water 87Sr/86Sr data were incorporated into previously established statistical models to infer the fish habitat use through the otolith 87Sr/86Sr ratios. The water 87Sr/86Sr ratio of the upper Paraguay tributaries varied (0.7209 and 0.7229) and were the highest in the La Plata Basin. The dispersal area of Z. jahu was restricted to the surroundings of the confluence of the Paraná and Paraguay rivers. Fish showed chemical signatures compatible with those of the middle Paraná (88%), Paraguay‐Pilcomayo‐Bermejo (50%) and the lower/upper Paraná (66%). The discriminant analysis based on otolith core indicated natal origins from the middle Paraná (37%), upper Paraná (32%), and Paraguay sub‐basin (32%). The dispersal area of Z. jahu migrations could be restricted to the surroundings of the confluence of Paraná and Paraguay, which highlights the need to review regional fishing regulations, especially between latitudes 30°–34° S. [ABSTRACT FROM AUTHOR]

Peatlands provide a large panel of socio‐ecosystemic services such as biodiversity, water and carbon storage and amenities. Hydrological and geochemical interactions between peats and their surroundings are expected to be favoured in mountainous areas, which are nowadays increasingly sensitive to climate changes. In order to provide an integrated scheme of potential interactions, this study evaluates spatio‐temporal patterns of environmental tracers (87Sr/86Sr, δ18O/δ2H, elemental ratios) during high‐ and low‐flow periods in the largest peatland complex of the Jura Mountains (France). Systematically depleted δ18O/δ2H values in the deepest peat pore waters suggest contrasted dynamics and origins, both compatible with either preferential winter recharge or supply from adjacent high‐elevation areas. Combined with strontium isotopes (87Sr/86Sr), we show that these water fluxes are purveyors of solutes derived from water‐rock interactions, modified by mixing, evapotranspiration and dilution with local meteoric inputs. An end member mixing analysis of the peat pore water solute composition is consistent with a major contribution of carbonates from the regional Cretaceous limestone formations, located beneath fluvio‐glacial Quaternary deposits underlying the peat. This contribution implies a significant upward water flux from the underlying syncline that could reach a sufficient hydraulic head thanks to recharge from an adjacent regional anticline. These multiscale (anticline‐syncline, syncline‐peatland, peatland‐surface) constraints allow us to propose a relevant scheme for the hydrogeochemical functioning of the peatland, enabling an improved understanding of the current high socio‐ecosystemic value of the area, and the potential future evolution of the related services. [ABSTRACT FROM AUTHOR]

Under tropical climates, mineral assemblages composing lateritic weathering profiles offer precious records of past weathering conditions. Silicon (Si) isotope signatures in the clay fractions of a deep lateritic profile in central Amazonia, Brazil, in combination with previously determined kaolinite ages, suggest that the surrounding region underwent two major weathering episodes, distinct in their intensity. The first episode (ca. 35–20 Ma) of moderate intensity produced well‐crystallized kaolinites from the parent sediment with limited Si isotope fractionation. A more recent (8–6 Ma) and shorter phase caused the replacement, from top to bottom of the profile, of the first kaolinite generation by a new population characterized by higher crystallographic disorder and stronger Si isotope fractionation, suggesting weathering under conditions of rapid water percolation. These inferences are supported by results from an isotope‐enabled reactive transport model, and they are consistent with paleoclimatic and paleogeographic evidences recorded over the Amazon Basin. Plain Language Summary: Tropical landscapes are often covered by nutrient‐depleted soils overlying lateritic weathering profiles, that result from a complex set of chemical reactions between air, water, rocks, and living organisms. Mineral phases hosted by these deep weathering profiles, such as clays, have formed from reactions between primary minerals and elements present in subsurface waters, and thus hold the potential to record local past environmental conditions. We use numerical models to show that the distribution of the various isotopes of the major clay‐forming element silicon (Si) depends on the rapidity at which rainwater percolates downward through soil porosity. We demonstrate that clays present in a lateritic profile in central Amazonia formed during two distinct episodes. The first episode occurred between 35 and 20 million years ago at a time when rain in Amazonia was much less abundant that it is today. A second, shorter episode occurred between 8 and 6 million years ago under significantly wetter conditions than at present. Our study thus shows that silicon isotope analyses of lateritic profile can reveal the past evolution of climate in the tropics, which can in turn illuminate the future changes that will inevitably affect this region of our planet. Key Points: Silicon isotopes in lateritic weathering profiles combined with dating tools record past weathering conditionsSilicon isotope signatures of clays reflect the rate of water infiltration during laterite formationThe duration and intensity of weathering episodes can be constrained using a reactive transport model [ABSTRACT FROM AUTHOR]

Strontium isotopes (87Sr/86Sr) were evaluated as a potential method for studying the geographical origin of populations and movements patterns of migratory fish from the La Plata Basin (Paraná, Uruguay and Bermejo Rivers and Río de la Plata Estuary, South America). Surface water samples were collected at 43 sites during austral summer and winter (2018) while, Surubí (Pseudoplatystoma corruscans), Patí (Luciopimelodus pati), dorado (Salminus brasiliensis) and sábalo (Prochilodus lineatus) fishes were collected at seven locations. Water 87Sr/86Sr ratio was analysed by MC‐ICP‐MS (N = 74) and otolith core‐to‐edge 87Sr/86Sr transects (N = 50) were measured by LAfs‐MC‐ICP‐MS. Several water bodies presented significantly different (p < 0.05) water 87Sr/86Sr values. A ~ 1:1 relationship was found between 87Sr/86Sr measured in edge otolith and water. Data provide novel perspectives about migratory behaviour for all species, such as potential cross‐border migrations between countries of more than 1,000 km recorded for L. pati, S. brasiliensis and P. lineatus. These species seem to move between the Paraná and Uruguay rivers, which imply using the delta or the estuary as a corridor between them. Discriminant analysis based on otolith core 87Sr/86Sr suggested that the four species studied originate in sub‐basins different from those that were collected and revelled the presence of several potential spawning/nursery areas. The otolith 87Sr/86Sr profiles also suggested movements between different environments such as reservoirs, large plain rivers, mountain streams, floodplain valleys and estuaries. These results show the potential of 87Sr/86Sr as a tool for tracking the life history of fishes, and depict for the first time a complex use of the La Plata basin by the fishes, providing new information for management plans at the regional level. [ABSTRACT FROM AUTHOR]

The demand for large and reliable data sets on isotopic composition has increased in geochemistry and environmental sciences over recent years. We present an automated ion chromatographic separation method using a robotic pipetting arm, termed 'ChemCobOne', to reduce sample separation time. Its performance was tested for lithium isotope separation in geological reference materials using a single‐step separation with HCl (0.2 mol l−1) and a 2 ml resin volume. This refined lithium purification method does not forfeit precision, accuracy or purity compared with manual sample processing. In addition, a δ7Li value for NASS‐6 of 30.99 ± 0.50‰ (2s) (95% CI = 0.14‰, n = 44) was determined and the first δ7Li values for the granite rock reference material GS‐N (−0.57 ± 0.25‰ (2s), 95% CI = 0.15‰, n = 15), and for the soil reference material NIST SRM 2709a (−0.37 ± 0.67‰ (2s), 95% CI = 0.15‰, n = 63) are proposed. [ABSTRACT FROM AUTHOR]

The impact of intensive farming on chemical weathering in the Critical Zone is still an open question. Extensively instrumented and monitored over the last 50 years, the Orgeval Critical Zone Observatory (CZO) in France is an observation site impacted by intensive farming since the 1960s. The Orgeval observatory represents an ideal place to study the response and resilience capability of the Critical Zone under agricultural stress. This paper investigates the chemical composition of different water bodies in two nested catchments of the Orgeval CZO, including rainfall, springs, rivers, and rocks, over one and half hydrological year. We show that elemental and strontium isotopic ratios are powerful to constrain the origin of the elements. The results show that the river chemistry at the outlet of the two nested catchments is dominated by rain inputs (particularly atmospheric dust dissolution) and the chemical weathering of limestone and gypsum. Fertilizer input is clearly visible, although the distinction between gypsum dissolution and fertilizer inputs needs more investigation. The mixtures of water masses inferred from our data are in good agreement with the hydrological context of the watershed, that is, a multilayered aquifer structure. At the main outlet of the CZO, we estimate that the input of ocean‐derived solutes through rainfall represents 7 t km−2 year−1, on the same order of magnitude as the net fertilizer input (10 t km−2 year−1), and that rock weathering releases 50 t km−2 year−1. Including previously published physical erosion rates, we estimate that the total denudation rate (physical and chemical) of the Orgeval CZO is 20 mm (1,000 year)−1, which, along with the entire Seine watershed, is among the lowest chemical denudation rates for carbonate terrains under temperate climate. Chemical denudation is about 10 times higher than physical erosion in the Orgeval CZO. The consumption of CO2 by rock weathering is estimated to be between 265.103 and 360.103 molC km2 year−1, similar for the two nested catchments. Compared with the rivers, the springs show a higher CO2 consumption rate that suggests, as pointed out earlier, a enhancement of carbonate dissolution linked to nitrification and thus fertilizer application. The hyporheic zone appears to be a hot spot in the carbon cycle at the Orgeval CZO. This study sheds light on the complex, anthropocenic, interplay between geology, climate, and human activities that characterize and that take place in intensive agriculture regions. [ABSTRACT FROM AUTHOR]

Large hydrological systems aggregate compositionally different waters derived from a variety of pathways. In the case of continental-scale rivers, such aggregation occurs noticeably at confluences between tributaries. Here we explore how such aggregation can affect solute concentration-discharge (CQ) relationships and thus obscure the message carried by these relationships in terms of weathering properties of the Critical Zone. We build up a simple model for tributary mixing to predict the behavior of C-Q relationships during aggregation. We test a set of predictions made in the context of the largest world's river, the Amazon. In particular, we predict that the C-Q relationships of the rivers draining heterogeneous catchments should be the most "dilutional" and should display the widest hysteresis loops. To check these predictions, we compute 10 day-periodicity time series of Q and major solute (Si, Ca2+, Mg2+, K+, Na+, Cl-, SO42- 4) C and fluxes (F) for 13 gauging stations located throughout the Amazon basin. In agreement with the model predictions, C-Q relationships of most solutes shift from a fairly "chemostatic" behavior (nearly constant C) at the Andean mountain front and in pure lowland areas, to more "dilutional" patterns (negative C-Q relationship) toward the system mouth. More prominent C-Q hysteresis loops are also observed at the most downstream stations. Altogether, this study suggests that mixing of water and solutes between different flowpaths exerts a strong control on C-Q relationships of large-scale hydrological systems. [ABSTRACT FROM AUTHOR]

We report here an optimisation of the demountable direct injection high efficiency nebuliser (d-DIHEN) for isotopic measurements with a Neptune (ThermoFisher Scientific, Bremen, Germany) multi-collector inductively coupled plasma-mass spectrometer (MC-ICP-MS) and describe a method for boron isotopic ratio determination. With direct injection nebulisation 100% of the analyte was introduced into the ICP-MS plasma and wash times were drastically reduced for elements such as boron and thorium. Compared to the classical stable introduction system (SIS: double Scott/cyclonic spray chamber), sensitivity for boron was 2-5 times higher with d-DIHEN and wash times up to ten times shorter. Repeatability of B/B sample-calibrator bracketing measurements reached 0.25‰ (2 s) for seawater and coral samples. Method accuracy and reproducibility were tested on mixed reference solutions having δB values in the ranges −90 to +40‰ and −2 to +2.5‰, demonstrating our ability to distinguish δB values with differences of only 0.25‰. The international seawater reference material NRCC NASS-5 (National Research Council, Ottawa, Canada), analysed in different sessions over a 10-month period, yielded an average δB value of +39.89 ± 0.25‰, in the upper range of previously published seawater values. A comparison between δB determined by d-DIHEN MC-ICP-MS and positive-TIMS (P-TIMS) for four modern corals showed an excellent agreement (with bias of less than 0.4‰). [ABSTRACT FROM AUTHOR]

The determination of total organic carbon content and composition in detrital sediments requires careful removal of their carbonate minerals. In detrital sediments containing large amounts of carbonates, including dolomite, this can only be achieved by liquid acid leaching that may solubilise a significant proportion of the organic carbon. For a set of detrital sediments from the Himalayan system and the Amazon River as well as five geological reference materials, we determined the proportion of organic carbon (Corg) solubilised during acid leaching. This proportion is significant for all analysed sediments and generally tends to increase with the organic carbon content. Compared to other types of sediments analysed, clay fractions extracted from river sediments and bed sediments with very low organic carbon content have high and low proportions of acid soluble Corg respectively. In Himalayan and Amazon river sediments, the proportion of Corg solubilised during acid leaching was relatively constant with average values of 14 and 19 % respectively. Thus, it is possible to correct the Corg content for the dissolved organic carbon content measured after decarbonation. Data presented here show that Corg dissolved during liquid acid leaching must be taken into account. After careful calibration, the method presented here should, therefore, be applied to any carbonate-rich detrital sediment. [ABSTRACT FROM AUTHOR]