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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]

We provide a new generalized framework accounting for the effects of water and nutrient uptake by roots in the weathering zone on broad patterns of solute concentration - discharge relationships in rivers. As plants withdraw water from the subsurface, they decrease the area-normalized fluid drainage rate. This results in a discharge flux to springs and streams which is a function of water use by plants, leading to ecosystem regulation of the characteristic timescales of chemical weathering reactions and fluid flow rate. Our predictive framework quantifies the pore water and river solute chemistry generated from water-rock interactions, regulated by the effects of vegetation on water flow rates. To this we add the capacity for vegetation to enrich, passively admit, or discriminate against a given dissolved solute based on ecophysiological needs, thus further modifying the local concentration of a given chemical species in solution across a weathering profile. This uptake flux is not lost to the atmosphere in the same manner as water is removed from the subsurface via evapotranspiration. Rather, these extracted chemical species become incorporated into the topsoil as litter which may be partially or fully resolubilized, creating an upper boundary condition which depends on the uptake and recycling characteristics of the ecosystem. These processes are united through a new solution to the advection-reaction equation which allows vegetation to regulate broad patterns of stream concentration - discharge relationships. We show that ecosystems are capable of both accelerating and impeding water-rock reaction rates across the weathering profile, ultimately leading to both thermodynamic and ecological controls on the baseflow concentration of rock-derived solutes in rivers. • We offer a new predictive model for the water and nutrient demands of ecosystems. • Plants to both enhance and impede mineral weathering reactions. • Plants modulate the relationship between river chemistry and flow rate. • River chemistry reflects both thermodynamic and ecological controls. [ABSTRACT FROM AUTHOR]

This study makes use of a highly instrumented active landslide observatory (9 years of data) in the French Alps, the Séchilienne slope. Here, we use a combination of major element chemistry and isotopes ratios (87 Sr / 86 Sr, δ34 S) measured in different water types of the stable and unstable part of the Séchilienne instability to assess the contribution of the different lithologies of the slope and the chemical weathering mechanisms. Chemical and isotopic ratios are used to characterize weathering processes and the origin of waters and their flow paths through the massif. A mixing model allows us to allocate the different major elements to different sources, to identify secondary carbonate formation as a major process affecting solutes in the subsurface waters of the instability, and to quantify the involvement of sulfuric and carbonic acids as a source of protons. We show that the instability creates favorable and sustained conditions for the production of sulfuric acid by pyrite oxidation, by opening new fractures and supplying fresh reactive surfaces. We clearly identify the contribution of the dissolution of each mineral phase to the chemistry of the waters, with a clear role of remote gypsum dissolution to the sulfate budget in the sampled waters. We are also able to refine the preexisting hydrogeological views on the local water circulation and water flow paths in the instability by showing the hydrological connectivity of the different zones. Overall, our results show that the Séchilienne landslide, despite its role in accelerating rock chemical and physical weathering, acts as a geological source of CO 2 to the atmosphere. If generalizable to other large instabilities in mountain ranges, this study illustrates the complex coupling between physical and chemical erosion and their impact on the carbon cycle and global climate. The study also highlights the importance of distinguishing between sulfite oxidation and gypsum dissolution as a source of sulfate ions to rivers, particularly in mountain ranges. [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]

Lithium (Li) isotopes are a promising tracer of chemical weathering processes for both modern and ancient times. In order to improve the use of Li isotopes in the sedimentary record, here we calibrate the relationship between weathering intensity and detrital Li isotope composition (δ7Li) using the fine fraction of modern large river sediments. Through independent estimates for sediment provenance to calculate the Li isotope signature of the rock from which the sediments derive through weathering, we show that source rock variability (in particular the relative contribution of sedimentary versus igneous rocks) must be corrected for before using Li isotopes as a weathering proxy. We also show that for rivers draining mountain ranges, the contribution to river sediments of particles derived from sedimentary rocks is correlated to their Li/Al ratio, making it possible to use Li contents to estimate the average source rock composition. Once corrected for bedrock variability, the Li isotope signature defines a negative relationship with the weathering intensity (ratio between silicate weathering rate and total denudation rate), with highest Li isotope fractionation for the highest weathering intensity. Altogether, we propose a set of new relationships between weathering, erosion, provenance, and Li isotopes that can be used to quantify present-day and paleo-weathering using detrital sediment. [ABSTRACT FROM AUTHOR]

In the Sri Lankan Highlands erosion and chemical weathering rates are among the lowest for global mountain denudation. In this tropical humid setting, highly weathered deep saprolite profiles have developed from high-grade metamorphic charnockite during spheroidal weathering of the bedrock. The spheroidal weathering produces rounded corestones and spalled rindlets at the rock–saprolite interface. We used detailed textural, mineralogical and chemical analyses to reconstruct the sequence of weathering reactions and their causes. The first mineral attacked by weathering was found to be pyroxene initiated by in situ Fe oxidation. Volumetric calculations suggest that this oxidation leads to the generation of porosity due to the formation of micro-fractures allowing for fluid transport and subsequent dissolution of biotite and plagioclase. The rapid ensuing plagioclase weathering leads to formation of high secondary porosity in the corestone over a distance of only a few cm and eventually to the final disaggregation of bedrock to saprolite. The first secondary phases are oxides or amorphous precipitates from which secondary minerals (mainly gibbsite, kaolinite and goethite) form. As oxidation is the first weathering reaction, the supply of O 2 is a rate-limiting factor for chemical weathering. Hence, the supply of O 2 and its consumption at depth connects processes at the weathering front with those at the Earth's surface in a feedback mechanism. The strength of the feedback depends on the relative weight of advective versus diffusive transport of O 2 through the weathering profile. The feedback will be stronger with dominating diffusive transport. The low weathering rate is explained by the nature of this feedback that is ultimately dependent on the transport of O 2 through the whole regolith, and on lithological factors such as low bedrock porosity and the amount of Fe-bearing primary minerals. Tectonic quiescence in this region and low pre-development erosion rate (attributed to a dense vegetation cover) minimize the rejuvenation of the thick and cohesive regolith column, finally leading to low denudation rates. [ABSTRACT FROM AUTHOR]

• An intense storm was sampled as it infiltrated a steep canyon hillslope and stream. • Rock moisture water isotopes track mixing of storm water into shallow subsurface. • Stream δ 7 Li tracks delivery of solutes from further upslope at higher flow rates. • Reactive transport modeling of stream solute stable isotope ratios during storms. Long-term relationships between stream chemistry and discharge are regulated by watershed subsurface structure and biogeochemical functioning. The extent to which these mechanisms are expressed and may be explored in the geochemical response of streams during storm events remains an open question. Here, we monitor an intense storm as it infiltrated an upland hillslope draining into a small steep canyon stream that is typified by chemostatic concentration-discharge relationships in rock-derived solutes. Our approach couples a high-frequency record of stable lithium isotope ratios (δ 7 Li) in the stream with novel sampling of rock moisture within the hillslope. At peak discharge, lithium-sodium ratios (Li/Na) increased from 0.58 μM/mM to 0.82 μM/mM and δ 7 Li decreased from + 28.9 ± 0.1 ‰ to + 26.4 ± 0.4 ‰ in the stream. Hillslope hydrologic monitoring reveals that the rainwater infiltrated the subsurface, yet attenuated breakthrough of the heavily depleted δ D signal of this storm (as low as −86‰) only reached the upper 3-4 meters of the vadose zone. These δ D data show that the storm water mixed with previously stored rock moisture and displaced stored fluid to deeper depths, causing an observable rise in the water table. Groundwater 87Sr/86Sr and δ 7 Li demonstrate consistency in the fluid-rock interactions that occur below the water table prior to and during the storm. In total, these observations indicate that the transfer of fluid and generation of solutes through the interior of the hillslope produce the variability of Li/Na and δ 7 Li within the stream during the storm, and support application of a previously established 1-D reactive transport model framework developed for the evolution of lithium within the hillslope to this extreme hydrologic event. Based on the model, both Li/Na and δ 7 Li versus discharge relationships reflect an overall shorter transit time of fluid through the interior of the hillslope. These model results are consistent with our hydrologic observations and indicate that Li from further upslope (where the vadose zone becomes thicker) contributes to stream solute chemistry at the height of the storm. We conclude that in this system, stream lithium isotope signatures record the routing of water and generation of solutes within the hillslope even during intense storm events. [ABSTRACT FROM AUTHOR]

Examination of the behavior of oxygen and hydrogen during weathering reactions shows that river dissolved load, although widely used, is an imperfect tracer of chemical denudation. At the current state of knowledge, none of the metrics for river total dissolved loads (such as the silicate-derived total dissolved solids, TDSsil = Ca2+ + Mg2+ + Na+ + K+ + SiO2, converted or not to equivalent oxides) account, in a mechanistic manner, for the transfer of oxygen and hydrogen between the solid and fluid phase during weathering reactions. We assess that chemical denudation derived from TDSsil will significantly overestimate the true chemical denudation for weathering of Ca-feldspar to kaolinite, whereas weathering of water-rich sedimentary rocks will be characterized by an underestimation of chemical denudation by TDSsil. For a handful of field sites, we estimate that the bias is lower than ±10%. The sign and extent of the bias depends on the nature of bedrock and on weathering conditions. Altogether, our analysis questions the broadly accepted concept of chemical denudation rate. [ABSTRACT FROM AUTHOR]

When rock is converted to weathering products, the involved processes can be fingerprinted using the stable isotope ratios of metals (for example Li, Mg, Ca, Fe, Sr) and metalloids (B, Si). Here we construct a framework for interpreting these "novel" stable isotope ratios quantitatively in the compartments of the weathering zone in a geomorphic context. The approach is applicable to any novel stable isotope system and is based on a simple steady-state mass balance model that represents the weathering zone from the scale of a soil column to that of entire continents. Our model is based on the assumption that the two main processes associated with isotope fractionation are formation of secondary precipitates such as clays, and uptake of nutrients by plants. The model results show that the isotope composition of a given element in the weathering zone compartments depends on (1) the ratio between the release flux to water through primary mineral dissolution and the erosion flux of isotopically fractionated solid material, consisting of secondary precipitates and organic matter; (2) the isotope fractionation factors associated with secondary mineral precipitation and uptake by plants. A relationship is established between isotope ratios, isotope fractionation factors, and indexes for chemical weathering [such as chemical depletion fractions (CDF) and elemental mass transfer coefficients (7)] derived from simple elemental concentration measurements. From this relationship, isotope fractionation factors can be calibrated from chemical and isotope data measured on field material. Furthermore, we show how the ratio of solid export to dissolved export of a given element from the weathering system can be estimated from the comparison of the isotope composition between bedrock, water, and sediment. This calculation can be applied to samples from soils, from rivers, and from the sedimentary record, and does not require knowing the isotope fractionation factors involved in the reactions. Finally, we apply the model to the oceanic Li isotope record reconstructed from marine carbonate sediments in order to discuss changes in global geomorphic regimes through the Cenozoic. [ABSTRACT FROM AUTHOR]

Abstract: Large river sediments are mostly derived from tectonically active mountain belts, but then undergo a series of sedimentation, temporary storage and reworking episodes on their journey to the ocean. The long transfer time of these sediments through active floodplains might result in a significant chemical maturation via weathering reactions, which is of critical importance for biogeochemical cycles at the Earth surface. This study reports the chemical composition of river sediments from different locations throughout the courses of the main tributaries of the Amazon Basin. Sampling along river depth-profiles yields access to the whole grain size and chemical composition range of river sediment. Here, weathering intensities (i.e. losses of Na, K, Mg and Ca) associated with chemical weathering in floodplains are (1) examined as a function of grain size, and (2) integrated over the whole grain size range, for three selected long river reaches flowing through the foreland and the lowland of the Amazon Basin: the upper Marañon, the Beni and the lower Madeira rivers. A relatively small Na loss through plagioclase dissolution is observed in the Madeira reach, while an important Ca loss due to carbonate dissolution occurs in the two foreland reaches (Marañon and Beni). No significant loss of K and Mg is observed in any of the reaches, showing the low alterability of primary K and Mg-bearing minerals and suggesting retention of K and Mg in the particulate phase by secondary minerals. The combination of these findings with previously reported data on the downstream change of dissolved Na, K, Mg and Ca fluxes suggests that chemical weathering of “stable” alluvial deposits could also significantly contribute to theweathering flux generated in foreland and lowland areas. The comparison between the three Amazon reaches and the Gangetic plain tends to show that the features observed in the Amazon are valid on a global scale. Finally, we show that, although resulting in a relatively small change in the chemical composition of the river sediment, silicate weathering in the lower Madeira floodplain can lead to a CO2 drawdown equivalent to ca. 10% of the total CO2 consumption flux of the whole Madeira basin. [Copyright &y& Elsevier]

Barium and its isotopes are receiving growing interest in their capacity to record the variation of the oceanic biological pump through geological timescales. Nonetheless, only little is known about the continental sources and processes that can drive the Ba flux and isotope composition to the ocean. Whereas the role of processes such as secondary phase formation, sorption and biological uptake on Ba isotopes has been recently investigated, the potential of various rock sources in producing a range of Ba isotope compositions remains poorly known. In particular, black shales often exhibit enrichment in Ba that might potentially be associated with various Ba isotope signatures. In this study, we report the isotope composition of Ba in rivers with different relative contribution of black shale rocks in their catchments, located in the Mackenzie Basin (Northwest Canada). Both the dissolved and solid loads of the Mackenzie tributaries are enriched in heavy Ba isotopes with respect to the continental crust, which is consistent with an additional source to the weathering of igneous and siliciclastic rocks. Although the dissolved Ba abundance is partially driven by weathering processes, the river dissolved Ba isotope composition rather reflects a binary mixing between 1) a "classical" siliciclastic source (with a Ba isotope composition close to that of the upper continental crust δ 138Ba ∼ 0.15 ± 0.05‰) and 2) an isotopically heavier source especially present in the Mackenzie Mountains (with a δ 138Ba ∼ 0.40‰). The positive relationships between dissolved δ 138Ba and radiogenic osmium isotope ratios, as well as with sulphuric acid production, indicate that the heavy Ba source is tightly linked to the weathering of black shale. Although the exact reason for such heavy Ba isotope signatures remains elusive, this study emphasises the potential of black shale weathering in shifting the Ba isotope composition of the dissolved flux to the ocean towards heavier values. [ABSTRACT FROM AUTHOR]

• The riverine dissolved Li budget is mainly controlled by silicate weathering. • The variations of dissolved δ7Li values reflect the change of weathering intensity. • River management by damming insert a control on the river sediment geochemistry. • The Li isotopic fingerprint of weathering is preserved in river sediments. Lithium (Li) isotopes signatures in sedimentary archives are promising proxies to reconstruct paleo-weathering rates and paleo-seawater composition. Nonetheless, the interpretation of the seawater Li isotope composition evolution over the Cenozoic is still debated. In this aim, the study of large rivers can provide constraints for continental weathering flux to the ocean. However, in the recent past, human activities like flow regulation via reservoir construction, have significantly modified the hydrodynamic of riverine systems, impacting the transfer of matter from the continent to the ocean. Hence, these effects need to be considered and investigated when unravelling the signature of natural processes from river borne materials. We report Li concentrations and Li isotope compositions (δ7Li) for the dissolved and sediment loads of subtropical river waters from the Xijiang basin, the largest tributary of the Pearl River (also known as the Zhujiang), Southern China. These rivers display low dissolved Li concentrations. Although they are potentially impacted by carbonate dissolution in the upstream karst terrains, the Li budget of these rivers is dominated by silicate weathering. The dissolved Li isotope compositions of rivers vary over a range from +16‰ to +27‰. These values are high compared to the estimated composition of the bedrock (~0‰), and are explained by the preferential incorporation of the light 6Li in solid weathering products formed during the incongruent dissolution of silicate rocks. We show that the variations of the dissolved δ7Li values across the basin reflect changes in silicate weathering intensity which controls the formation of secondary phases. The high δ7Li values in the headwaters reflect greater degree of secondary phase formations associated with the higher erosion rates in the mountainous part of the basin, whereas the lower δ7Li values in the lowlands characterized by low soil erosion rates and more abundant rainfall reflect a more intensive weathering. This interpretation is supported by the positive relationship between soil erosion rates and the Li dissolved fluxes. The isotope compositions of the suspended particulate material (SPM) transported by rivers in the Xijiang Basin (from −3.8‰ to −0.7‰) are systematically lower than the composition of the bedrock. We observe that the δ7Li of most SPM samples collected during the wet season are lower than those of samples collected during the dry season. We interpret this seasonality as a consequence of seasonal changes in the sediment properties due to water regulation. In spite of the influence induced by river managements on the river sediments, our data also highlight that the imprint of weathering and erosion processes is still preserved in the sedimentary load of the Xijiang as suggested by the relationship between the SPM δ7Li values and the weathering intensity defined as the ratio between the chemical weathering rates and the total denudation rates. Altogether, our study confirms the robustness of Li isotopes as a proxy of weathering and erosion processes, even in this heavily-managed river system. [ABSTRACT FROM AUTHOR]

Large rivers integrate the products of physical erosion and chemical weathering in different forms (dissolved, fine to coarse particles). The geochemical composition of these products carry important informations on the mechanisms controlling erosion and weathering at the basin scale : sources of the products, vegetation, climate, anthropogenic and tectonic factors, timescale involed. In the framework of a large franco-chinese project, the four largest rivers in China (Changjiang, Huanghe, Zhujiang, Heilongjiang) were sampled : vertical profiles, surface tributaries, bedload sediments and surface monthly time series over a year were collected at different locations along the stream of these rivers. We report major and trace elements concentrations and Nd-Sr isotopes in the suspended particulate matter and bedloads. The four river basins cover a large diversity of climatic, lithologic, tectonic and anthropic settings, that our results put in perspective as controls on erosion-weathering mechanism and its associated timescales. [ABSTRACT FROM AUTHOR]

Chemical weathering of silicate rocks sustains long term solute cycling through water-rock interactions in the terrestrial subsurface that transforms pristine bedrock into the diverse landscapes that characterize the Earth's surface (i.e. the Critical Zone). The drawdown of CO2 via these weathering reactions serves as a major component in the long term cycling of carbon and is thought to play a key role in the regulation of Earth's climate over geologic timescales. Thus, understanding the silicate weathering feedback is critical for assessing regional and global scale responses to a rapidly changing climate. However, to accurately quantify silicate weathering rates, a better understanding of solute generation and transport in the subsurface is required. Stream solute concentrations and discharge are highly coupled, integrating various reaction pathways and fluid residence times and thus can provide key insight into the hydrogeochemical controls within the critical zone. Strong responses of solute export with "storm" (flood) events characterized by large variations in discharge over a short time, suggests that transient hydrologic processes could exert a predominant control on weathering reactions in the critical zone. To address this, we utilize stable isotopes of Si (designated δ30Si), a highly sensitive silicate weathering tracer, to gain insight into the hydrogeochemical processes taking place under transient hydrological conditions.Our study focuses on Si isotope analyzes performed on storm events from several small catchments spanning a range of different climates and lithologies as part of an international collaboration between U.S and French Critical Zone Observatory networks to better understand concentration-discharge relationships (SAVI). Our preliminary findings suggest that δ30Si – discharge relationships could provide unique insight into the balance between primary mineral dissolution and secondary mineral precipitation in the weathering zone and, thus, be a powerful complementary tool to frequently used C-Q relationships. [ABSTRACT FROM AUTHOR]

The evolution of boron isotope compositions of seawater (δ 11 B sw) over the Cenozoic has important implications for reconstructions of atmospheric CO 2 and is tightly linked to boron input from the Himalaya-Tibetan Plateau. However, controls on evolution in δ 11 B sw remain elusive. We report geochemical measurements of the Yarlung Tsangpo River draining the Tibetan Plateau and observe exceptionally high riverine boron concentrations and extremely low δ 11 B values. Calculation indicates that >50% of riverine boron is sourced from geothermal waters. Combined with global datasets, we show that global riverine boron input to the ocean is largely influenced by geothermal input. Mass-balance calculations indicate that an averaged 1.5-fold decrease in global geothermal inputs is sufficient to introduce 3‰ increase in Cenozoic δ 11 B sw. Therefore, geothermal waters significantly affect global riverine and thus oceanic boron budgets. The increased δ 11 B sw since the Cenozoic may be resulted partly from declining global geothermal activity. • Exceptionally high boron and low δ 11 B values are found in Yarlung Tsangpo River. • More than 50% of riverine boron is sourced from geothermal waters. • Geothermal water significantly influences global riverine and oceanic boron budgets. • Increased δ 11 B sw since Cenozoic is resulted partly from declining geothermal activity. [ABSTRACT FROM AUTHOR]

Abstract Forest ecosystem nutrition and rock weathering are intimately linked through the supply of mineral nutrients and recycling (re-utilisation of nutrients released from plant litter during mineralisation). Here, we report on novel applications of Mg stable isotopes in combination with mass balance calculations to quantify nutrient supply (from bedrock, atmospheric wet and dry deposition) and recycling in a tropical ecosystem, characterised by a supply-limited weathering regime in the Central Highlands of Sri Lanka. We measured Mg stable isotopes, radiogenic Sr isotope ratios, and element concentrations in the different compartments of the Earth's Critical Zone (bedrock, saprolite, soil, soil pore water, stream water and vegetation) as tracers for element mass fluxes and pathways. We find that in the clay-rich saprolite Mg isotopes are fractionated towards isotopically heavy Mg during secondary mineral formation. Soil pore water and stream water are the complementary isotopically light reservoirs. The Mg isotope composition of above-ground vegetation is on average identical to soil pore water. Based on these observations we suggest that a tight near-surface mineral nutrient loop between plants and soil sustains the tropical forest ecosystem. This loop is disconnected from the rock weathering nutrient source in the saprolite beneath soil. External atmospheric wet and dry inputs (mainly Mg and Ca) strongly contribute to the plant-available nutrient budget that the nutrient depleted regolith does not supply. These findings, based on Mg stable isotopes, agree with independently-obtained information on bio-element (P, K, Ca, Mg, Si) recycling. This nutrient supply setting contrasts with that reported for the Southern Sierra Nevada (USA), where a kinetically limited weathering regime prevails. Our study illustrates the application of new flux estimators based on elemental and metal stable isotope measurements across a range of climate and weathering regimes. [ABSTRACT FROM AUTHOR]

The erosion of major mountain ranges is thought to be largely cannibalistic, recycling sediments that were deposited in the ocean or on the continents prior to mountain uplift. Despite this recognition, it has not yet been possible to quantify the amount of recycled material that is presently transported by rivers to the ocean. Here, we have analyzed the Li content and isotope composition (δ7Li) of suspended sediments sampled along river depth profiles and bed sands in three of the largest Earth's river systems (Amazon, Mackenzie and Ganga-Brahmaputra rivers). The δ7Li values of river-sediments transported by these rivers range from +5.3 to -3.6‰ and decrease with sediment grain size. We interpret these variations as reflecting a mixture of unweathered rock fragments (preferentially transported at depth in the coarse fraction) and present-day weathering products (preferentially transported at the surface in the finest fraction). Only the finest surface sediments contain the complementary reservoir of Li solubilized by water-rock interactions within the watersheds. Li isotopes also show that river bed sands can be interpreted as a mixture between unweathered fragments of igneous and sedimentary rocks. A mass budget approach, based on Li isotopes, Li/Al and Na/Al ratios, solved by an inverse method allows us to estimate that, for the large rivers analyzed here, the part of solid weathering products formed by present-day weathering reactions and transported to the ocean do not exceed 35%. Li isotopes also show that the sediments transported by the Amazon, Mackenzie and Ganga-Brahmaputra river systems are mostly sourced from sedimentary rocks (>60%) rather than igneous rocks. This study shows that Li isotopes in the river particulate load are a good proxy for quantifying both the erosional rock sources and the fingerprint of present-day weathering processes. Overall, Li isotopes in river sediments confirm the cannibalistic nature of erosion and weathering. [ABSTRACT FROM AUTHOR]

In this study, to better understand the factors controlling the concentration and isotope composition of lithium (Li) in the ocean, we investigated the behaviour of Li during interaction of kaolinite with artificial seawater. Dissolution of kaolinite in Li-free seawater at acidic conditions (exp. 1) results in a strong preferential release of light Li isotopes, with △7Li aq-kaol ∼ −19‰, likely reflecting both the preferential breaking of 6Li O bonds over 7Li O bonds and the release of Li from the isotopically lighter AlO 6 octahedral sites. Sorption experiments on kaolinite (exp. 2) revealed a partition coefficient between kaolinite and fluid of up to 28, and an isotopic fractionation of −24‰. Thermodynamic calculation indicates authigenic smectites formed from the dissolution of kaolinite in seawater at pH 8.4 (exp. 3). The formation of authigenic phase strongly removed Li from the solution (with a partition coefficient between the solid and the fluid equal to 89) and led to an increase of ca. 25‰ in seawater δ7Li. This fractionation can be described by a Rayleigh fractionation model at the early stage of the experiment during rapid clay precipitation, followed, at longer reaction time, by equilibrium isotope fractionation during the much slower removal of aqueous Li via co-precipitation and adsorption. Both processes are consistent with a fractionation factor between the solid and the aqueous solution of ∼−20‰. These experiments have implications for interpreting the Li isotopic composition of both continental and marine waters. For instance, the preferential release of 6Li observed during kaolinite far-from-equilibrium dissolution could explain the transient enrichments in 6Li observed in soil profiles. With regard to the evolution of seawater δ7Li over geological time scales, our experimental results suggest that detrital material discharged by rivers to the ocean and ensuing "reverse chemical weathering" have the potential to strongly impact the isotopic signature of the ocean through the neoformation of clay minerals. [ABSTRACT FROM AUTHOR]

Abstract Solutes derived from the dissolution of silicate minerals play a key role in Earth's climate via the carbon and other biogeochemical cycles. Silicon (Si) is a unique constituent of silicate minerals and a biologically important nutrient, so tracing its behavior in near-surface environments may provide important insights into weathering processes. However, Si released by weathering is variably incorporated into secondary mineral phases and biota, obscuring signals derived from primary weathering processes. Due to chemical similarities, Germanium (Ge) may help better understand the Si cycle and its relationship to chemical weathering. With this aim, we report new measurements of the concentration and isotopic composition of Ge for both the dissolved and particulate phases of a variety of global rivers. These measurements are combined with analyses of concentration and isotopic ratio of Si on the exact same sample set in order to make direct comparisons of the behavior of these two elements in natural river systems. With this dataset, we develop a new modeling framework describing the full elemental and isotopic systems of these solutes in rivers (i.e., Ge/Si, δ 74 Ge , and δ 30 Si). This multi-proxy approach allows us to ascertain the relative importance of biological versus mineral uptake in modulating the fluxes of these elements delivered to the modern ocean. Dissolved δ 74 Ge composition of rivers studied thus far range from 0.9 to 5.5‰ with a discharge-weighted global average of 2.6 ± 0.5‰. The Ge isotope composition of riverine suspended and bedload sediments is indistinguishable from silicate source rocks, which is consistent with mass balance expectations. The multi-proxy modeling suggests that, among the watersheds studied here, the isotopic fractionation of Si during secondary mineral phase precipitation (Δ 30 Si s e c) ranges from −2.7 to −0.2‰, which removes between 19–79% of the initial dissolved Si, while between 12–54% is incorporated by biota. For Ge, modeling indicates that 79–98% of the dissolved load is incorporated into secondary mineral phases with a Δ 74 Ge s e c ranging from −4.9 to −0.3‰. The fractionation induced by biological uptake is calculated to range from −2.6 to −1.3‰ for Δ 30 Si b i o and −0.7 ± 0.7‰ for Δ 74 Ge b i o. In addition to improving our understanding of the coupled Ge and Si cycles, our study provides a framework for using multiple isotopic tracers to elucidate the chemical behavior of solutes in natural waters. Highlights • Ge and Si isotope composition of water and sediments in various rivers is presented. • Dissolved δ 74 Ge range 0.9–5.5‰, river sediment δ 74 Ge range 0.5–0.7‰. • Dissolved δ 74 Ge is strongly fractionated due to low chemical Ge mobility. • Ge/Si-isotope multi-proxy provides quantitative constraints on secondary vs biological fractionation. [ABSTRACT FROM AUTHOR]

In order to better understand how stable metal isotope signals in large rivers can be used to constrain present and past weathering, the seasonal riverine Mg Sr isotopic pattern in the middle Yellow River was systematically investigated based upon weekly collected samples for the whole year of 2013. The results demonstrate that Mg is mainly transported in the dissolved form (65%) in this river system and that 45% of the total dissolved Mg is transported during the monsoon seasons, with 2% exported over 4 days during a single storm event. Dissolved Mg in the middle Yellow River is dominantly derived from both silicate and carbonate (82–89%) in this semi-arid region, with limited evaporite contribution (∼7%). Lithological mixing is the first order control on riverine dissolved Mg and Sr isotopes, with a contribution from ∼40% carbonate dissolution and ∼ 60% from silicate dissolution in the dry seasons, and ∼ 50% carbonate and ∼ 50% silicate during the monsoon seasons according to δ26Mg signals. Furthermore, a significant role of prior calcite precipitation (PCP) can be quantified, which fractionates Mg isotopes by about 0.17‰ to 0.39‰ positively depending on the choice of elemental and isotope partition of Mg in secondary carbonates. Clay formation following the PCP further fractionates riverine Mg isotopes to the negative side. An ∼0.2‰ decrease of riverine Mg isotopes is attributable to (1) a single storm event causing carbonate dissolution and (2) delayed delivery of depleted waters to rivers (∼3 months after the storm event) because of subsurface hydrological circulation. Annually, the weighted average riverine δ26Mg (−1.05‰) in the middle Yellow River is identical to the global average (−1.09‰). Despite the significant impact of lithology on the riverine dissolved Mg isotope signature, the mixing proportions of different Mg sources remain virtually constant, even when there are huge contrast of temperature, hydrology, and precipitation seasonally along the year, providing a basis for dissolved δ26Mg response to climatic forcing on the continental scale. This means that significant changes in the sedimentary Mg isotope records would reflect extreme conditions in deep time. [ABSTRACT FROM AUTHOR]

The correlation between chemical weathering fluxes and denudation rates suggests that tectonic activity can force variations in atmospheric pCO 2 by modulating weathering fluxes. However, the effect of weathering on pCO 2 is not solely determined by the total mass flux. Instead, the effect of weathering on pCO 2 also depends upon the balance between 1) alkalinity generation by carbonate and silicate mineral dissolution and 2) sulfuric acid generation by the oxidation of sulfide minerals. In this study, we explore how the balance between acid and alkalinity generation varies with tectonic uplift to better understand the links between tectonics and the long-term carbon cycle. To trace weathering reactions across the transition from the Peruvian Andes to the Amazonian foreland basin, we measured a suite of elemental concentrations (Na, K, Ca, Mg, Sr, Si, Li, SO 4 , and Cl) and isotopic ratios ( 87 Sr/ 86 Sr and δ 34 S ) on both dissolved and solid phase samples. Using an inverse model, we quantitatively link systematic changes in solute geochemistry with elevation to downstream declines in sulfuric acid weathering as well as the proportion of cations sourced from silicates. With a new carbonate-system framework, we show that weathering in the Andes Mountains is a CO 2 source whereas foreland weathering is a CO 2 sink. These results are consistent with the theoretical expectation that the ratio of sulfide oxidation to silicate weathering increases with increasing erosion. Altogether, our results suggest that the effect of tectonically-enhanced weathering on atmospheric pCO 2 is strongly modulated by sulfide mineral oxidation. [ABSTRACT FROM AUTHOR]

• Reactive transport modeling of lithium in fluids draining a first-order hillslope. • Fluid δ 7 Li and Li/Na ratios distinguish compartments of the weathering profile. • Contemporaneous formation of multiple secondary phases governs fluid δ 7 Li. • Weathering continues below the water table as groundwater laterally drains to stream. The stable isotopes of lithium (Li) serve as a robust proxy of silicate weathering. The fate and transport of these isotopes in the dissolved load of major rivers have been characterized to infer changes in both contemporary weathering regimes and paleo-conditions. In this contribution, we deconvolve this integrated signal into the individual processes that fractionate Li at the inception of silicate weathering by directly measuring Li isotope ratios of waters (δ 7 Li) transiting through a rapidly eroding first-order hillslope. We use these data to develop a multicomponent reactive transport framework, which shows that net dissolution of weathered material generates light δ 7 Li signatures (as low as −9.2‰) in the shallow portion of the vadose zone. An increase in δ 7 Li deeper into the vadose zone (as much as +18‰) reflects an increasing contribution of secondary mineral formation. Below the water table, congruent weathering occurs and imparts elevated cation concentrations and bedrock δ 7 Li. Silicate weathering continues within the saturated zone as groundwater travels downslope (δ 7 Li = +13 to + 24‰) to the stream. The stream signatures (δ 7 Li = +28 to +29‰) reflect the terminus of this network of silicate weathering reactions and the relative magnitude of each contributing process (e.g., transitions in secondary mineral formation, dissolution of weathered material). We show that fluid progressing through the weathering profile of this first-order hillslope is distinguished by a sequence of characteristic Li isotope signatures, which can be reproduced in a forward, process-based model framework. This model development offers an improved quantitative basis for the use of metal(loid) stable isotopes in disentangling catchment-scale chemical weathering fluxes. [ABSTRACT FROM AUTHOR]