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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. 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. 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 Stream lithium stable isotope ratios (δ7Li) recorded at high frequency over storm events are sensitive to antecedent conditions A reactive transport model cannot produce observed shifts in stream chemistry through variations in flow rate alone Flux‐weighting of model fluid outputs based on time‐varying fluid transit time distributions describes stream δ7Li over storm hydrographs

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. 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. 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 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 values Isotope‐enabled reactive transport modeling indicates that at this site soil clays retain lithium and are resilient to (re‐)solubilization

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‐Qdata 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‐Qrelationships resulting from data acquisition over multiple flood events, lumping all trends together can lead to biases in characteristic C‐Qparameters. We then reframe C‐Qrelationships 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‐Qrelationships 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. 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 We combine high‐frequency (every 40 min) stream chemistry data and stream discharge recession analysis We propose a conceptual model for the relationship between water storage and solute export at the Orgeval Critical Zone Observatory (CZO), France Three classes of solute behavior are identified at the Orgeval CZO, related to the role of rock dissolution and amendments in their budget

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 (C‐Q) 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‐Qrelationships 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‐Qrelationships 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 Qand major solute (Si, Ca2+, Mg2+, K+, Na+, Cl‐, SO42−) Cand fluxes (F) for 13 gauging stations located throughout the Amazon basin. In agreement with the model predictions, C‐Qrelationships 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‐Qrelationship) toward the system mouth. More prominent C‐Qhysteresis 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‐Qrelationships of large‐scale hydrological systems. Modeling predicts that tributary mixing should affect river concentration‐discharge relationships, especially in large, heterogeneous basinsIn the Amazon basin river concentration‐discharge relationships of heterogeneous basins are the least “chemostatic”Concentration‐discharge relationships in large systems are strongly affected by mixing between waters following different pathways

The isotopic composition of dissolved lithium (δ7Li) near the Congo River mouth varied from 14‰ to 22‰ in 2010 and was negatively correlated to discharge. From the relationship between dissolved δ7Li and strontium isotopes, we suggest that this large variation is due to mixing of waters from two contrasting continental weathering regimes. One end‐member (high δ7Li ≈ 25‰) represents waters sourced from active lateritic soils covering the periphery of the basin (Li highly sequestered into secondary mineral products) and another representing blackwater rivers (low δ7Li ≈ 5.7‰) derived from the swampy central depression where high organic matter content in water leads to congruent dissolution of the Tertiary sedimentary bedrock. This suggests that the lithium isotopic signature of tropical low‐relief surfaces is not unique and traces the long‐term, large‐scale vertical motions of the continental crust that control geomorphological settings. This evolution should be recorded in the oceanic secular δ7Li curve. Dissolved Li isotope composition at mouth of the Congo River is controlled by hydrologic mixingTwo contrasting dissolved δ7Li values in the Congo reveal two distinct weathering regimesLow late Paleocene seawater δ7Li value is not unequivocally generated by flat continents

Throughout the Quaternary period, the Earth’s surface has been subject to large changes in temperature and precipitation associated with fluctuations between glacial and interglacial states that have affected biogeochemical cycling. However, the effect of these climate oscillations on weathering is debated, with climate modelling efforts using empirical relationships between measures of climate and weathering suggesting minimal changes in global weathering rates between these two climate states. The ratio of the cosmogenic isotope 10Be, which is produced in the atmosphere and deposited to the oceans and the land surface, to 9Be, which is introduced to the oceans by the riverine silicate weathering flux, can be used to track relative weathering fluxes. Here we apply this proxy to marine sediment beryllium records spanning the past two million years, and find no detectable shifts in inputs from global silicate weathering into the oceans. Using climate model simulations of the Last Glacial Maximum along with a model for silicate weathering, we find that there was large regional variability in runoff between glacial and interglacial periods, but that this regional variability was insufficient to shift global weathering fluxes. We suggest that this stability in weathering explains the observation that the removal of CO2from the atmosphere by silicate weathering has been in approximate balance with CO2degassing over the past 600,000 years.

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. 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. 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 Silicon isotopes in lateritic weathering profiles combined with dating tools record past weathering conditions Silicon isotope signatures of clays reflect the rate of water infiltration during laterite formation The duration and intensity of weathering episodes can be constrained using a reactive transport model

Gallium exhibits weak metallic properties owing to its proximity to non‐metals in the periodic table, yet is volatile in extra‐terrestrial bodies and fairly reactive in nature. It has been used extensively to elucidate the Solar System evolution, planet interior differentiation, and terrestrial processes. However, Ga speciation and transformation in various planetary compartments and the dynamics of its trans‐reservoir pathways remain to be fully resolved. Although recent studies and the development of modern analytical techniques for Ga isotopes have markedly improved our understanding of Ga geochemistry, a systematical summary of state‐of‐the‐art knowledge appears to be long overdue. Here we provide an overview of the geochemical properties of Ga in different reservoirs, including meteorites and Earth's interior and exterior compartments, and a timely review of Ga isotopic geochemistry. We also provide a first tentative estimate of total masses of Ga in different compartments and the trans‐reservoir Ga flux, based on the published data. This compilation reveals clearly the lack of geochemistry data of Ga in Earth's interior, the imbalance of oceanic Ga budget and the potential implications of Ga isotopes, stimulating future systematic studies of Ga and its isotopes in geosciences and related fields.