Caractérisation des flux d'altération des contextes orogéniques en milieu tropical. Cas des bassins andins et d'avant pays de l'Amazone

L'évaluation du bilan d'altération des chaînes orogéniques et l'identification des paramètres de contrôle de ces processus est une question majeure dans l'étude du cycle du carbone sur le long terme. L'objectif de cette recherche est de mesurer les flux d'altération des bassins andins (deuxième chaîne mondiale après l'Himalaya) et d'avant-pays de l'Amazone (premier fleuve mondial) et de discuter de leurs variabilités en fonction des caractéristiques géographiques de ces bassins (climat, lithologie, couverture végétale, érosion). Ces flux d'altération ont été estimés à partir de l'acquisition à haute résolution temporelle et spatiale de données hydrochimiques (élément majeurs) et hydrologiques dans les réseaux de mesure PHICAB, HIBAM et ORE-HYBAM. Les bassins andins et d'avant-pays contrôlent près de 70% de la production du flux dissous de l'Amazone. L'altération des carbonates et des évaporites dans les bassins du Marañón et de l'Ucayali expliquent près de 70% de cette contribution. La variabilité spatiale des flux dissous est au premier ordre contrôlée par la variabilité de l'écoulement et par la nature lithologique du bassin considéré (carbonates / silicates). A l'exception des produits de dissolution des évaporites, la variabilité temporelle du flux de production de matières dissoutes est au premier ordre contrôlée par la variabilité hydrologique. La variabilité spatiale des taux d'altération des silicates n'est pas associée à celle des taux d'érosion dans les domaines andins. En normalisant par l'écoulement, l'intensité de production de matières dissoutes des zones de sédimentation actuelles (correspondant aux bassins d'avant-pays en subsidence du Madeira) est plus élevée que celle des zones d'avant-pays en érosion (Napo). La consommation de CO2 atmosphérique par altération des silicates (CO2 sil) des bassins andins et d'avant-pays représente 50-73% de celle de l'Amazone. La plaine du Solimões constitue un puits de CO2 sil significatif à l'échelle du bassin. Le flux de consommation de CO2 sil des bassins andins et d'avant-pays est équivalent à celui de l'Himalaya pour une surface deux fois plus faible et pour un débit spécifique deux fois plus important. Ce résultat met en évidence le rôle prédominant du climat dans la consommation de CO2 sil des chaines de montagnes tropicales. Par ailleurs, sur 80% du bassin de l'Amazone, l'intensité d'altération des silicates, indépendamment de leur teneur en éléments mobiles, est contrôlée par la variabilité spatiale de l'écoulement.The weathering flux estimation of orogenic belts and the identification of critical parameters that control these processes is a major question in the study of the long term carbon cycle. The objective of this research is to estimate the weathering of the Andean (the second largest orogenic belt after Himalaya) and foreland basins of the Amazon (the world's largest river) and to discuss variability according to geographical characteristics (climate, vegetation cover, lithology, erosion). The weathering fluxes of the Andean and foreland parts of the Amazon basin were estimated at high time and spatial resolution scales in the PHICAB, HIBAM and ORE-HYBAM frameworks. The Andean and foreland basins control the production of around 70% of the Amazon dissolved load. The carbonates and evaporites weathering in Marañón and Ucayali basins contribute approximately 70% of the total. The first order dissolved load spatial variability is controlled by the runoff and lithology (carbonates/silicates) variability. With the exception of the evaporites dissolution load products, the temporal variability of the dissolved load is firstly controlled by the hydrological variability. The silicate weathering variability in the Andean domain is not associated with the erosion rates variability. The dissolved rates are somewhat higher in the recent sedimentation areas (which correspond to Madeira subsidence foreland areas) than those calculated in foreland areas subjected to erosion if we normalize by the runoff (Napo). The atmospheric CO2 consumption derived from silicate weathering (CO2 sil) of the Andes and Foreland part represents 50-73% of the total CO2 sil budget of the Amazon. The Solimões plain contributes significantly to the Amazon basin CO2 sil. The Upper Amazon's silicate CO2 consumption flux is commensurate with the Himalaya's basins for an area two times lower and a runoff value two times higher. This result points out the importance of the climate for the CO2 sil consumption by tropical orogenic belts. 80% of the Amazon basin silicate weathering intensity is controlled by the runoff spatial variability, independently of the mobil cation content

Modelling the riverine δ7Li variability throughout the Amazon Basin

International audienceThe present study investigates the processes controlling the elementary and isotopic cycle of the lithium over the Amazon basin. A numerical model is developed to simulate two major processes that have been proposed as key controls of the river lithium isotopic composition: weathering reactions inside the regolith, accounting for secondary phase formation, and interactions between riverine water and secondary phases in floodplain. Both processes generate fractionation of lithium isotopes ("batch" fractionation and "Rayleigh" distillation respectively) that potentially control the riverine isotopic composition of the Amazon and its tributaries. A study of the model parameters shows that two different regimes are impacting the lithium isotopic composition of the rivers within the Amazon catchment. In the South (Madeira and its tributaries), the lithium isotopic signature of river waters can be explained by lithium release and fractionation during weathering reactions in the regolith, followed by "Rayleigh distillation" in the floodplain increasing progressively the lithium isotopic composition, in agreement with a previously published hypothesis. In contrast, the lithium isotopic composition of rivers located in the northern part of the Amazon watershed (Solimoes and tributaries) cannot be simulated by the model assuming the same processes than in the southern part. Model optimization suggests than the nature of the material being eroded and weathered is important. In the North, fresh source rocks of volcanic origin releases large amount of Li and promotes rapid smectite precipitation, allowing the riverine δ 7 Li to rise before flowing through floodplains. This result suggests that the environments able to generate high riverine δ 7 Li (higher than 25‰) are complex and not firmly identified yet

Mountain ranges, climate and weathering. Do orogens strengthen or weaken the silicate weathering carbon sink?

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Climate control on silicate weathering and physical erosion rates in youngorogenic belts: Case study along a runoff gradient in Pacific andAmazonian Andean basins based on SNO-HYBAM Monitoring Programdata

International audienceAt the global scale and on geological time scales, mechanical erosion and chemical weathering budgets are linked.Together, these processes contribute to the formation and the degradation of the Earth’s critical zone and to thebiogeochemical cycles of elements. In young orogenic belts, climate and tectonic subsidence control together therate of these matter balance budget and their relationships. The climate gradient observed along the Andean basinin both the Pacific and the Atlantic slopes offers the opportunity to explore the role of the climate variability onthe erosion and weathering budgets and on their reciprocal relationships.Based on the SNO-HYBAM Monitoring Program database (Geodynamical, hydrological and Biogeochemi-cal control of erosion/weathering and material transport in the Amazon, Orinoco and Congo basins), we explorethe relationship between climate, the lithology, silicate weathering rates and physical erosion rates along a runoffgradient in Andean basins of the Amazon River (13 gauging stations) and Pacific drainage rivers (5 gaugingstations).No homogenous relationship between erosion rates (E) and chemical weathering rate (W) is observed overthe monitored basins. Only the volcanic basins respond to a global relationship defined in the literature whilethe other basins budget may depend on anthropogenic interferences on erosion/sedimentation budget, a lithologydependence of the W-E relationship parameters or/and on the existence of a threshold in this relationship.The results presented here contribute to better understanding the role of mountains belt formation in thebiogeochemical cycles and in particular in the long-term carbon cycle.Your presentation type preference

Evidence of long term biogeochemical interactions in carbonate weathering: The role of planktonic microorganisms and riverine bivalves in a large fluviokarst system

International audienceThe infiltration of organic-rich surface waters towards groundwaters, is known to play a significant role in carbonate weathering and in contributing to the atmospheric continental carbon sink. This paper investigated biogeochemical interactions in karst critical zones, with strong surface water /groundwater interactions, and in particular the role of planktonic microorganisms and riverine bivalves through the analysis of particulate organic matter (OM) oxidation on carbonate weathering. In the large Val d'Orléans fluviokarst aquifer (France), a 20-year monthly dataset of Nitrates, Dissolved Oxygen (DO), dissolved inorganic and organic Carbon (DIC and DOC) fluxes was gathered. The surface water-groundwater comparison of geochemical trends showed that planktonic microorganisms had drastically decreased in surface waters, related to the proliferation of Corbicula bivalves spreading and a decrease in nutrients. This decrease in planktonic microorganisms was followed by a DO increase and an DIC decrease at the karst resurgence. The degradation of planktonic microorganisms consumes DO and produces NO3, dissolved inorganic carbon (DIC) and a proton that in turn, dissolves calcite and produces DIC. Without the input from planktonic microorganisms, the fluviokarst has lost 29 % of this nitrification and 12 % of the carbonate dissolution capacities. Thus, the oxidation of particulate organic matter of planktonic microorganisms, which is part of heterotrophic respiration, appears to be a significant source of the inorganic carbon flux in riverine ecosystems. This shows how weathering can remain active under waters saturated versus calcite and suggests that the oxidation of organic matter can be a more appropriate mechanism than autotrophic respiration to explain the relationship between global warming and DIC flux change in rivers. Through the consumption of plankton, the animal life in rivers thus influences the inorganic carbon in groundwaters, creating a negative feedback in the carbon cycle

Chemical weathering and atmospheric/soil CO2 uptake in the Andean and Foreland Amazon basins

International audienceThis study is a geochemical investigation of the Andean and Foreland basins of the Amazon River at high spatial and time resolution, carried out within the framework of the HYBAM research program (Hydro-geodynamics of the Amazon Basin). Monthly sampling was carried out at 27 gauging stations located in the upper tributaries of the Amazon Basin (from north to south: the Napo, Marañon, Ucayali, Madre de Dios-Beni and Mamore Rivers). The aim of this paper is to estimate the present-day chemical weathering rate (CWR), as well as the flux of CO2 consumption from total and silicate weathering in the Andes and Foreland Amazon basins, and to discuss their distribution as a function of geomorphic and structural parameters. Based on the forward method, the Napo and other Ecuadorian basins present high silicate weathering rates in comparison with the other basins. We confirm that the Marañon and Ucayali Rivers control the Amazon hydrochemistry due to the presence of salt rocks and carbonates in these basins. The Madre de Dios, Beni and Mamore basins do not contribute much to the Amazon dissolved load. This north to south CWR gradient can be explained by the combination of decreasing weatherable lithology surface and decreasing runoff rates from the north to the south. The foreland part of the basins (or Mountain-Lowland transition) accounts for nearly the same proportion of the Amazon silicate chemical weathering and carbonate chemical weathering fluxes as the Andean part. This result demonstrates the importance of the sediment accumulation areas in the Amazon Basin weathering budget and can be explained by the occurrence of a higher temperature, the deposition of fresh sediments from Andean erosion and a higher sediment residence time than in the upper part of the basin. With a total CO2 consumption rate of 744.103 moles km−² year− 1 and a silicate CO2 consumption rate of 300.103 moles km−² year− 1, the Upper Amazon River (Andes + Foreland part) is the most intense part of the Amazon Basin in terms of atmospheric CO2 consumption by weathering processes. It is an important CO2 sink by weathering processes but accounts for only somewhat more than half of the CO2 consumption by silicate weathering of the Amazon Basin. This result points out the importance of the Lowland part of the basin in the inorganic C silicate budget. The Upper Amazon accounts for 2-4% of the world's silicate CO2 consumption, which is the same proportion as for the southern and southern-east Himalaya and Tibetan plateau

Geoquímica de rios de água preta do sudeste do Amazonas: origem, fluxo dos elementos e consumo de CO2 Geochemistry of black water rivers from the southeastern of the Amazonas State: source, element fluxes and consumption of CO2

Neste trabalho foi examinada a composição química das águas dos afluentes da margem direita do rio Madeira que drenam as rochas do Escudo Brasileiro. O estudo foi realizado no município de Apuí no sudeste do estado do Amazonas, Brasil. Foram analisados pH, condutividade, as concentrações de SiO2 e de íons dissolvidos (Na+, K+, Mg2+, Ca2+, HCO3-, Cl-, NO3- e SO4(2-)) em quatro épocas do ano segundo o índice pluviométrico: chuvosa, transição para a estiagem, estiagem e transição para a chuvosa. As águas são diluídas, mais concentradas em HCO3- e SiO2 e representam o típico ambiente de intensa lixiviação que afeta as rochas na Amazônia. Apesar da química das águas terem influência da sazonalidade, foram identificadas variações em função da litologia. As taxas de exportação de cátions e de erosão química das rochas são baixas e refletem a estabilidade tectônica da região e o manto intempérico que dificulta a interação da água com o substrato rochoso.This work examined the chemical composition of waters from tributaries of the right bank of the Madeira River which drain silicaterocks of the Brazilian Craton. The study was conducted at the municipalityof Apui, in the southeastern of the Amazonas State, Brazil. There were analyzed pH, conductivity, SiO2 and dissolved ions (Na+, K+, Mg2+ e Ca2+, HCO3-, Cl-, NO3- and SO4(2-)) in the four pluviometric seasons of the Amazon region: rainy, transition to the dry, dry and transition to the rainy. The waters are diluted, have high HCO3-and SiO2 concentration and represent the typical environment of intense leaching that affects the rocks in the Amazon. Although the chemical composition reveals seasonal influences, it was identified variation due to lithological composition. The rate of exportation of cations and the chemical erosion are low in consequence of the tectonic stability of the region and the weathering mantle, which hinders the interaction of water with the bedrock

Hydrological control, fractionation, and fluxes of dissolved rare earth elements in the lower Orinoco River, Venezuela

International audienceThe monthly variation of dissolved rare earth elements (REEs) was assessed in the lower Orinoco River during a two year period (2007-2008) to determine the seasonal variability of REE concentrations, to identify the variables that exert the main control in their concentrations and fractionation, and to quantify the annual fluxes of dissolved REEs to the Orinoco estuary. Overall, the abundance of dissolved REEs is dominated by hydrological variations in the water discharge, wherein the lowest concentrations and greater fractionation occur during low water-discharge periods. The pH and Al-and Fe-mineral colloids are identified as the main variables that control both the abundance and fractionation of dissolved REEs. An enrichment of heavy REEs (HREEs) relative to light REEs (LREEs) occurs at circumneutral and alkaline pH values. However, the logarithmic relationships between the Yb UCC /Nd UCC ratios and Al and Fe concentrations indicate that Al-and Fe-mineral colloids are responsible for the progressive enrichment of LREEs relative to HREEs under acidic conditions. The Ce and Eu anomalies are also dominated by variations in the water discharge. Negative Ce-anomalies are observed during low flow periods. This is probably due to the signature of the Andean host rocks and/or the oxidation and co-precipitation of Ce (III) to CeO 2 at alkaline pH. However, the lesser Ce fractionated values during flood/high-water periods may indicate less oxidized/more reduced source conditions during these periods. Conversely, positive Eu anomalies are observed during low-water periods because of the preferential weathering of plagioclase in shield terranes and Eu-bearing minerals in the Andes. The fluxes of dissolved REEs from the lower Orinoco River to the Orinoco estuary display strong inter-annual variations, which range from 45.6% for Lu to 56.5% for Gd. These results highlight the importance of performing monthly and inter-annual REE time series in order to develop a more precise quantification of the annual REE fluxes from large rivers to the oceans

Supply-limited weathering regime in a tropical shields basin (Ogooué River basin, Gabon)

International audienceAt the global scale and on geological time scales, mechanical erosion and chemical weathering budgets are linked. Together, these processes contribute to the formation and the degradation of the Earth's critical zone and to the biogeochemical cycles of elements. While the weathering of hot and humid shields areas exhibit low weathering rates because of the depth of the mature depleted soil mantle there, shields areas dominate the continents areas over intertropical regions and, therefore, represent a significant proportion of the global delivery of dissolved matter to the oceans. In addition, these environments are under supply-limited conditions (the weathering rate is limited by the low rates of the erosion) and thus particularly sensitive to long-term variability erosion rates. Despite this importance, weathering-erosion budgets and rates estimation in these environments is sparse, and generally performed at a local scale (soil profiles) or, when performed at a larger catchment scale, the intra cratonic characteristics variabilities (e. g. the diversity of mechanical erosional regimes) are usually not singled out.In the present study, we explored the variability of the weathering intensity of the Ogooué sub-basins (Western central Africa, Gabon) as a function of their geomorphologic, tectonic and lithological setting variability. We analyzed major and trace elements concentration and the strontium and neodymium isotopes of water, suspended matter sediments and bedload sampled in 24 Ogooué tributaries (September 2017 campaign). Our results show that shield areas exhibit a high variability of chemical weathering intensity, which follows the erosional regime characteristics of the studied sub-basins, likely related to their tectonic activity. Three regions can be distinguished: The Bateke plateau (East sub-basins - PB), is composed of pure sandstones (quartz) and is inert in term of tectonic activity and therefore in term of erosion and weathering budget; the northern sub-basins (NB) are subjected to low tectonic activity and exhibit slightly higher erosion and weathering intensity than PB region and, by comparison, southern sub-basins (SB) exhibits uplift activity which is traduced by more intensive erosion and weathering processes.The annual dissolved solid budget of the Ogooué basin is ~2.52 t.yr-1 for a rate of 11.7 t.km-2.yr-1. According to the source discrimination method performed based on the geochemical analysis, the atmospheric inputs contributes to around 20% to the TDS, the silicate weathering contribution dominates the dissolved exports throughout 70% of its production while the carbonates weathering lowly contributes to the TDS production.By comparison to the other large shields rivers, this basin exhibit a lower range of chemical silicate weathering rate than most of the world's large rivers, with values similar to those of the Congo River. This new dataset provides a key information to complete the World River chemistry database, which is limited for inter-tropical regions, especially in tectonically quiescent environments. Moreover, this study provides new data for tropical shields contexts allowing for the exploration of the interactions between erosion rates and climate in the control of continental weathering rates, and their relationships with long-term carbon cycle and short-term biogeochemical cycles

Controls on the geochemistry of suspended sediments from large tropical South American rivers (Amazon, Orinoco and Maroni)

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