Search

[This study implemented a source‐to‐sink approach for the Great Salt Lake (GSL) hydro‐geosystem of the calcium dynamics, highlighting the dissolved pool in both the waterbody and the watershed. Major and minor pathways of calcium inputs were evaluated in order to quantify calcium delivered to the GSL during the Holocene. The value resulting from the hydrologic data was compared to the amount of calcium stored in the water column and sediments of the lake during the same period. The similarity between these two estimates is consistent with the premise of the source‐to‐sink approach: a mass balance between eroded and transported compounds and the sinks. The amount of calcium deposited in the basin can therefore be predicted indirectly from the different inputs. Alternative properties can be used to estimate the inputs in the fossil record when monitoring is unavailable. , Abstract Most source‐to‐sink studies typically focus on the dynamics of clastic sediments and consider erosion, transport and deposition of sediment particles as the sole contributors. Although often neglected, dissolved solids produced by weathering processes contribute significantly in the sedimentary dynamics of basins, supporting chemical and/or biological precipitation. Calcium ions are usually a major dissolved constituent of water drained through the watershed and may facilitate the precipitation of calcium carbonate when supersaturating conditions are reached. The high mobility of Ca2+ ions may cause outflow from an open system and consequently loss. In contrast, in closed basins, all dissolved (i.e. non‐volatile) inputs converge at the lowest point of the basin. The endoreic Great Salt Lake basin constitutes an excellent natural laboratory to study the dynamics of calcium on a basin scale, from the erosion and transport through the watershed to the sink, including sedimentation in lake's waterbody. The current investigation focused on the Holocene epoch. Despite successive lake level fluctuations (amplitude around 10 m), the average water level seems to have not been affected by any significant long‐term change (i.e. no increasing or decreasing trend, but fairly stable across the Holocene). Weathering of calcium‐rich minerals in the watershed mobilizes Ca2+ ions that are transported by surface streams and subsurface flow to the Great Salt Lake (GSL). Monitoring data of these flows was corrected for recent anthropogenic activity (river management) and combined with direct precipitation (i.e. rain and snow) and atmospheric dust income into the lake, allowing estimating the amount of calcium delivered to the GSL. These values were then extrapolated through the Holocene period and compared to the estimated amount of calcium stored in GSL water column, porewater and sediments (using hydrochemical, mapping, coring and petrophysical estimates). The similar estimate of calcium delivered (4.88 Gt) and calcium stored (3.94 Gt) is consistent with the premise of the source‐to‐sink approach: a mass balance between eroded and transported compounds and the sinks. The amount of calcium deposited in the basin can therefore be predicted indirectly from the different inputs, which can be assessed with more confidence. When monitoring is unavailable (e.g. in the fossil record), the geodynamic context, the average lithology of the watershed and the bioclimatic classification of an endoreic basin are alternative properties that may be used to estimate the inputs. We show that this approach is sufficiently accurate to predict the amount of calcium captured in a basin and can be extended to the whole fossil record and inform on the storage of calcium.]

TRANSKARST is an interdisciplinary research project with scientists and regional water resource administrators. This 3 years project (2019-2022) is set up on Arcier's karstic watershed used for the drinking water supply of 60 000 inhabitants of Besançon, France. Using this instrumental basin, as a part of Jurassic Karst French observation system, the project aims at defining pathways of mineral, organic and microbiological contamination in karstic system.The methodology used in the project, combines field expertise with the implementation of analytical tools related to conventional - dissolved phase chemistry (major ions, miners, traces, organic carbon), isotopes (oxygen-18, deuterium, carbon-13) – and prospective analysis - emerging pollutants such as pharmaceuticals and ETM, dissolved organic matter, suspended matter and microbiology (bacteria, fungal species) with a special focus on antibiotic resistance. The TRANSKARST project thus brings together a consortium of researchers from various disciplines: hydrogeology, chemistry, sedimentology, microbiology, geology and geophysic. The project is also highly associated to water managers as Arcier’s spring is used for drinking water supply.The three main under-objectives following by TRANSKARST could be summarized as follows. First, through geological and geophysical investigation, a conceptual and numerical geological model will be established under geomodeler and will be further used to constrain the pathways of karstic groundwater. The second point focus on the evaluation of karst media contamination: chemical (including emerging molecules) and microbiological. Then we expect to discriminate, by the use of ETM, dissolved, colloidal and particulate pathways of contamination. All the observations will be linked together to identify actions and feedback of different parameters and contaminants within karst hydrosystems.

Mixed cyanobacteria-dominated biofilms, enriched from a tributary of the M&eacute, rantaise (France) were used to conduct laboratory experiments in order to understand the relationship between the morphology of carbonate precipitates and the biological activity (e.g., cyanobacterial exopolymeric substances (EPS) production, photosynthetic pH increases). DNA sequencing data showed that the enriched biofilm was composed predominantly of two types of filamentous cyanobacteria that belonged to the Oscillatoriaceae and Phormidiaceae families, respectively. Microscopic analysis also indicated the presence of some coccoid cyanobacteria resembling Gloeocapsa. Analysis of carbonate precipitates in experimental biofilms showed three main morphologies: micro-peloids with different shapes of mesocrystals associated with Oscillatoriaceae filaments and theirs EPS, lamellae of carbonate formed directly on Phormidiaceae filaments, and rhombic sparite crystals wrapped in EPS. All crystals were identified by FT-IR spectroscopy as calcite. Similar structures as those that formed in laboratory conditions were observed in the microbial-tufa deposits collected in the stream. Microscopic and spectroscopic analysis of laboratory and natural samples indicated a close proximity of the cyanobacterial EPS and precipitated carbonates in both. Based on the laboratory experiments, we conclude that the microbial tufa in the stream is in an early stage of formation.

Prod 2018-56 EA BIOmE AGROSUP; Many studies about climate change impacts assessment are published every year. These studies commonly use a hydroclimatic modelling chain, whose principle is to feed impact models with climate models outputs. An important step in this process is to test the validity of impact models in a climate change context. However, this step is not frequently applied. The aim of this study is to test the robustness of two hydrological models with distinct conceptualizations: a global and empirical model (GR4J) and a semi-distributed and physically-based model (SWAT). They both have been calibrated and validated over climate contrasted periods. Despite a higher decrease of performance between calibration and validation for the GR4J model, both of them show relative robustness. Moreover, the stability of parameters between the two calibration periods shows that their value are not much influenced by the climate of the calibration period, and consequently remains valid during the entire projection period.

National audience; En réponse à la dégradation qualitative de la ressource en eau, les politiques publiques ont mis en place de plus en plus de mesures agro-environnementales (Bouleau and Richard, 2008). L'amélioration constante des modèles agronomiques et/ou hydrologiques a permis d'atteindre une reproduction cohérente des interactions sol/plante/atmosphère et ainsi de prévoir les impacts de ces mesures agro-environnementales. Le modèle SWAT permet de prendre en compte des pratiques agricoles détaillées et d'en déterminer les impacts sur la ressource en eau (Xie et al., 2015). En France, des projets tels que ECCOTER (Vernier and Rousset, 2014) ont montré l'efficacité du modèle SWAT pour tester l'efficacité des mesures agro-environnementales. L'objectif de cette étude est de vérifier si une chaîne de modélisation hydroclimatique incluant le modèle SWAT représente être un outil capable d'aider à la prévision des impacts des mesures agro-environnementales en contexte de changement climatique, et donc s'avérer utile aux politiques publiques. Pour répondre à ce questionnement, nous avons testé la sensibilité de SWAT et la cohérence de ses sorties face à plusieurs scénarios de gestion du territoire.

The characteristics of dissolved organic matter (DOM) in soils are often determined through laboratory experiments. Many different protocols can be used to extract organic matter from soil. In this study, we used five air-dried soils to compare three extraction methods for water-extractable organic matter (WEOM) as follows: (i) pressurised hot-water-extractable organic carbon (PH-WEOC), a percolation at high pressure and temperature; (ii) water-extractable organic carbon (WEOC), a 1-hour end-over shaking; and (iii) leaching-extractable organic carbon (LEOC), a leaching of soil columns at ambient conditions. We quantified the extraction yield of organic carbon; the quality of WEOM was characterized by UV absorbance, potential biodegradability (48-day incubation) and parallel factor analysis (PARAFAC) modelling of fluorescence excitation emission matrices (FEEMs). Biodegradation of dissolved organic carbon (DOC) was described by two pools of organic C. The proportions of labile and stable DOC pools differed only slightly between the WEOC and LEOC methods, while PH-WEOC contains more stable DOC. The mineralization rate constants of both labile and stable DOC pools were similar for the three methods. The FEEMs were decomposed into three components: two humic-like fluorophores and a tryptophan-like fluorophore. The effect of extraction method was poorly discriminant and the most similar procedures were PH-WEOC and LEOC while WEOC extracts were depleted in humic-like fluorophores. This study demonstrates that WEOM quality is primarily determined by soil characteristics and that the extraction method has a smaller, but still significant, impact on WEOM quality. Furthermore, we observed considerable interaction between extraction procedure and soil type, showing that method-induced differences in WEOM quality vary with soil characteristics.

The relationship between environmental conditions and the development, mineralization and preservation of modern tufa microbialites was investigated in a 1.1 km long freshwater stream in Villiers-le-B&acirc, cle, a tributary of M&eacute, rantaise river. Detailed mapping of the tufa microbialite distribution combined with sedimentological, petrographical and mineralogical analyses were coupled with chemical measurements. Six organosedimentary structures were identified, their distribution appears heterogeneous along the stream and responds to physicochemical conditions of water and specific biological components (e.g., microorganism, exopolymeric substance). Two of the organosedimentary structures show evidence of mineralization and only one is lithified. Based on field observations and in-situ deployment of mineralization markers (bricks), three zones with increasing mineralization intensities are defined, ranging from no mineralization to thick mineralized crusts forming riverine tufa. Both biotic and abiotic processes were proposed for the tufa microbialite formation. We explained changes in mineralization intensities by the specific physicochemical conditions (e.g., calcite saturation index (SIcalc) and partial pressure of CO2 (pCO2) and a closed proximity of the cyanobacterial biofilm and carbonates precipitates. The physical and chemical composition of substrate impact development of microbial communities, mineralization potential of tufa microbialite. Even though the physicochemical and biological conditions were optimal for mineral precipitation, the potential of lithification depended on the presence of a suitable (physical and chemical) substrate.

11 pages; International audience; Campanian sediments from two French sedimentary basins were studied, using clay mineralogy and stable isotope (δ13C and δ18O) geochemistry, in order to investigate the Late Campanian Event. The clay fraction of the Campanian sediments from the Tercis-les-Bains section (Aquitaine Basin) and from the Poigny borehole (Paris Basin) is mainly composed of smectite. This background sedimentation was, however, interrupted during the Upper Campanian in the two basins by a substantial increase in detrital inputs, including illite, kaolinite, and chlorite at Tercis-les-Bains, and illite at Poigny. This detrital event, resulting from the enhanced erosion of nearby continental areas triggered by increasing runoff, has also been recognized in the Tethys and South Atlantic oceans. It coincided with a global negative carbon isotope excursion, the Late Campanian Event (LCE). Carbon isotope stratigraphy was used to correlate the two basins with previously studied sections from distant areas. Spectral analysis of the bulk δ13C from Tercis-les-Bains suggests a duration of ca. 400 kyr for a pre-LCE negative excursion and ca. 800–900 kyr for the LCE sensu stricto. The detrital event, as characterized by clay mineralogy, spans the interval that comprises the pre-LCE and the LCE, with a duration of 1.3 Myr. Intensification of continental erosion during the LCE may have resulted either from the Late Campanian polyplocum regression and/or from a regional tectonic pulse that triggered the emersion of previous submerged shelf areas and the increase of silicate erosion. As the LCE seems to be recorded at a large geographic scale, it is proposed here that enhanced chemical weathering and an associated decrease in atmospheric pCO2 levels could have contributed to the long-term Late Cretaceous cooling trend.

10 pages; International audience; During the last few decades, Europe has seen a faster increase of observed temperature than that simulated by models. The air temperature over Western Europe showed an abrupt shift at the end of the 1980s, still insufficiently documented. The aim of this study is to assess the characteristics of this shift and its potential impacts on the hydrological cycle over France. Such an assessment is essential for a better understanding of past and future climatic changes and their impact on water resources.A subset of 119 temperature, 122 rainfall, and 30 hydrometric stations was studied, over the entire French metropolitan territory. Several change-point detection tests were applied to temperature, rainfall and runoff time series.A shift in annual mean air temperature was detected in 1987/88, for more than 75% of the stations, and for both minimum and maximum temperatures. An abrupt increase of about 1 °C in minimum and maximum temperature provides evidence of this shift, which shows strong seasonality, with significant increases for DJF, MAM and JJA. Its detection is not affected by the length of the time series or any potential artefacts associated to the conditions of measurement.Cluster analysis of the rainfall stations was used to take account of regional variability in rainfall evolution. Two climate areas were obtained from this analysis: Mediterranean and temperate. No shift was detected in rainfall for either area. However, at annual and quarterly scales, several changes in runoff were observed between the periods 1969–87 and 1988–09. The significant changes occurred from January to July, in agreement with maximum increases in temperature. Evapotranspiration could well play a key role in these changes in the hydrological cycle, as a response to temperature increases in the watersheds studied.

10 pages; International audience; This paper presents the assessment and mapping of the Ribeira Seca catchment, an insular Sahelian mountain region sensitive to desertification, located on the island of Santiago, Cabo Verde. Desertification is a threat to the global environment, representing a serious ecological problem in Cabo Verde. To successfully combat desertification, an evaluation of desertification consequences is required and the building of cartography of the sensitivity for arid and semi-arid ecosystems is required as a first step. The MEDALUS model was the basis for this study in which six quality indicators were used: climate, soil, vegetation, land management, erosion and social factors. Several parameters were defined for each indicator with weights varying between 1 (very low) and 2 (very high). The geometric mean of each of the six quality indicators was employed to produce a map of areas sensitive to desertification. The results of this study show that more than 50% of the watershed show clear evidence of becoming a desertified area.

10 pages; International audience; This study seeks to determine to what extent trace metals resulting from past mining activities are transferred to the aquatic ecosystem, and whether such trace metals still exert deleterious effects on biota. Concentrations of Cd, Cu, Pb and Zn were measured in streambed sediments, transplanted bryophytes and wild brown trout. This study was conducted at two scales: (i) the entire Morvan Regional Nature Park and (ii) three small watersheds selected for their degree of contamination, based on the presence or absence of past mining sites. The overall quality of streambed sediments was assessed using Sediment Quality Indices (SQIs). According to these standard guidelines, more than 96% of the sediments sampled should not represent a threat to biota. Nonetheless, in watersheds where past mining occurred, SQIs are significantly lower. Transplanted bryophytes at these sites consistently present higher trace metal concentrations. For wild brown trout, the scaled mass and liver indices appear to be negatively correlated with liver Pb concentrations, but there are no obvious relationships between past mining and liver metal concentrations or the developmental instability of specimens. Although the impact of past mining and metallurgical works is apparently not as strong as that usually observed in modern mining sites, it is still traceable. For this reason, past mining sites should be monitored, particularly in protected areas erroneously thought to be free of anthropogenic contamination.

International audience; In the early Aptian, the Oceanic Anoxic Event (OAE) 1a is well defined by a negative ?13C excursion followed by a positive δ13C excursion, spanning the Deshayesites deshayesi and Dufrenoya furcata ammonite biozones. A cyclostratigraphic approach is performed in the Vocontian Basin, France, to estimate the time required for the carbon cycle recovery following the major disturbance associated to OAE1a and to provide durations of ammonite and foraminifer biozones. The Serre Chaitieu section, which consists of hemipelagic blue-grey marls with occasional marker limestone horizons and encompassing the Deshayesites deshayesi Zone to the end of the Epicheloniceras martini Zone, was used as a reference section in the Vocontian Basin. Using field Spectral Gamma Ray (SGR), 450 measurements were performed throughout the section, and a sample of each measured sediment was collected to further perform calcimetry, clay mineralogy, and magnetic susceptibility (MS) measurements. Detrital clay mineral assemblages consist of illite, illite/smectite mixed-layers (I-S), kaolinite and chlorite. Fluctuations of clay minerals are mainly driven by climate change, progradation/drowning of peri-vocontian platforms and sea-level changes. The proportions of illite and kaolinite covary and fluctuate in opposition with I-S. Cyclic fluctuations of relative proportions of clay minerals are particularly well recorded by the kaolinite/chlorite ratio (K/C). Spectral analyses, using the multi-taper and the amplitude spectrogram methods, were performed on SGR, MS, CaCO3 and K/C signals to detect sedimentary cycles related to an orbital forcing throughout the series. The geochronometer 405-kyr eccentricity cycle well expressed and significant (up to 99% confidence level) is used to provide a robust temporal framework. More than five 405-kyr eccentricity cycles are recognised, providing a total duration of at least 2.49 myr for the whole sedimentary succession. The minimum duration of the D. furcata Zone is assessed at 0.42 myr, and the duration of the E. martini Zone at 1.52 myr. Amplitude spectrograms show a strengthened signal of obliquity during the D. furcata Zone, which is coherent with the global cooling that has been depicted for this interval, and which could have favored the development of lowextension polar ice and thus the lowering of the sea level. Durations of C-isotope zones, worldwide correlated, are also calculated. From these results, the duration of the return to equilibrium in the carbon cycle in the aftermath of OAE1a could be calculated at 1.35 myr.

The transfers and origins of dissolved inorganic carbon (DIC) were studied for a year in a soil–spring–stream system in the Strengbach catchment, Vosges mountains, France. This 80 ha experimental research basin is located on the eastern side of the mountains, at an altitude ranging from 883 to 1146 m.a.s.l. and is mainly covered by spruce (80%). Brown acid and podzolic soils developed on a granitic basement, and, as a result, the DIC originates solely from CO2 generated by oxidation of soil organic matter. The ( δ 13 C DIC ) in catchment waters is highly variable, from about −22‰ in the springs and piezometers to about −12‰ in the stream at the outlet of the catchment. In the springs, pronounced seasonal variations of δ 13 C DIC exist, with the DIC in isotopic equilibrium with the soil CO2 that has estimated δ 13 C of about −24‰ in winter and −20‰ in summer. These seasonal variations reflect an isotopic fractionation that seems only induced by molecular diffusion of soil CO2 in summer. In stream water, seasonal variations are small and the relatively heavy DIC (−12‰ on average) is a result of isotopic equilibration of the aqueous CO2 with atmospheric CO2.

International audience

15 pages; International audience; In the context of land use change, the dynamics of the water extractable organic carbon (WEOC) pool and CO2 production were studied in soil from a native oak-beech forest and a Douglas fir plantation during a 98-day incubation at a range of temperatures from 8°C to 28°C. The soil organic carbon, water contents and mineralisation rates of soil samples from the 0–5 cm layer were higher in the native forest than in the Douglas fir plantation. During incubation, a temperature-dependent shift in the δ13C of respired CO2 was observed, suggesting that different carbon compounds were mineralised at different temperatures. The initial size of the WEOC pool was not affected by forest type. The WEOC pool size of samples from the native forest did not change consistently over time whereas it decreased significantly in samples from the Douglas plantation, irrespective of soil temperature. No clear changes in the δ13C values of the WEOC were observed, irrespective of soil origin. The fate of the WEOC, independent of soil organic carbon content or mineralisation rates, appeared to relate to forest types. Replacement of native oak-beech forest with Douglas fir plantation impacts carbon input to the soil, mineralisation rates and production of dissolved organic carbon.

National audience; L'étude porte sur l'adaptation au changement climatique en Bourgogne. Elle vise à définir la sensibilité de la ressource en eau des territoires Bourguignons à ces évolutions climatiques et à en estimer les incidences par secteurs géographiques et d'activité. Ce projet pluri-disciplaire a été mis en place par Alterre Bourgogne dont la tache a été de fédérer un réseau d'acteurs régionaux aux compétences variées : climatologie, pédologie et hydrologie. La première étape, présentée ici, a consisté à estimer la capacité des sols à stocker de l'eau, c'est-à-dire à calculer la taille de leur réserve utile. Ces calculs ont été réalisés à partir du référentiel pédologique de Bourgogne aux 1/250000ème. La seconde étape consiste en la réalisation d'un bilan hydrique, spatialisé sur la Bourgogne et modélisé au pas de temps mensuel et à l'échelle spatiale kilométrique (Lamy et al., GFHN 2009). Cette étude consiste à élaborer différentes représentations cartographiques de la réserve utile en eau des sols de Bourgogne. Cette demande s'inscrit dans un projet plus vaste visant à définir la sensibilité de la ressource en eau en bourgogne au regard des changements climatiques de demain, et de zoner les secteurs pouvant être impactés par la sécheresse à l'échelle des unités de paysage. Les cartes de la Réserve Utile des sols de Bourgogne a été élaborée à partir du référentiel pédologique au 1/250000ème. La structuration de la base de donnée des sols (IGCS) impose un mode de calcul par agrégation ascendante. De même, la spatialisation de cette réserve à l'échelle régionale épouse les contours des Unités Cartographiques de Sol (UCS) du référentiel pédologique de Bourgogne. Des limites au modèle ont également été définies vis à vis l'échelle spatiale considérée et de l'utilisation faite des résultats.

10 pages; International audience; One of the main environmental changes caused by human activities is that of land use. These changes influence the quantity and quality of the dissolved organic matter (DOM) fluxes through the vegetation–soil–stream system. The aim of this work is to evaluate the influence of the substitution of native deciduous forests by well managed coniferous forests on dissolved organic carbon (DOC) fluxes and their associated carbon isotopic composition (d13CDOC). DOC fluxes and d13CDOC were monitored for 2 years in the streams of four similar upland forested catchments in the Morvan Mountains (France). Mean annual DOC concentrations and fluxes were 2–4 times lower in streams in catchments with predominant coniferous vegetation than in those with deciduous vegetation. d13CDOC values were lower by 0.05–0.1% in the stream under deciduous vegetation (2.83% to 3.0%) than in the other streams (-2.71% to -2.95%). Furthermore, stream DOC was 13C depleted compared to the isotopic composition of the soil organic matter (SOM). This 13C depletion was higher under deciduous cover where SOM was enriched in 13C with increasing depth, producing 13C depleted solute fraction. Under coniferous cover, no significant isotopic differentiation was observed between the solute and solid phase in the soil. Our results show that the d13CDOC in stream is marked by the vegetation cover because of differences in the degradation pathway and dynamics of SOM. Besides, it means that the substitution of deciduous forests by coniferous plantations may significantly modify the chemical composition of DOC in streams.

International audience; The biogeochemical carbon cycle, which plays an undeniable role in global climate change, is defined both by the size of carbon reservoirs (such as the atmosphere, biomass, soil and bedrock) and the exchange between them of various mineral and organic carbon forms. Among these carbon forms, fossil organic carbon (FOC) (i.e., the ancient organic matter stored in sedimentary rocks) is widely observed in modern environments but is not included in the supergene carbon budget. Using a digitized map of the world and an existing model of CO2 consumption associated with rock weathering, we establish the global distribution of FOC stored in the first meter of sedimentary rocks and a first estimation of annual FOC delivery to the modern environment resulting from chemical weathering of these rocks. Results are given for the world's 40 major river basins and extended to the entire continental surface. With a mean value of 1100 109 t, mainly controlled by shale distribution, the global FOC stock is significant and comparable to that of soil organic carbon (1500 109 t). The annual chemical delivery of FOC, estimated at 43 106 t yr− 1 and controlled by the areal distribution of shales and runoff, is of the same order of magnitude as the FOC output flux to oceans. Chemical weathering of bedrock within the Amazon basin produces one-quarter of the total global flux of FOC derived from chemical weathering, and thus is expected to govern FOC release on a global scale. These results raise important questions concerning the role of FOC in the modern carbon cycle as well as the origin and the budget of carbon in soils and rivers

[1] The silicate rock weathering followed by the formation of carbonate rocks in the ocean, transfers CO2 from the atmosphere to the lithosphere. This CO2 uptake plays a major role in the regulation of atmospheric CO2 concentrations at the geologic timescale and is mainly controlled by the chemical properties of rocks. This leads us to develop the first world lithological map with a grid resolution of 1° × 1°. This paper analyzes the spatial distribution of the six main rock types by latitude, continents, and ocean drainage basins and for 49 large river basins. Coupling our digital map with the GEM-CO2 model, we have also calculated the amount of atmospheric/soil CO2 consumed by rock weathering and alkalinity river transport to the ocean. Among all silicate rocks, shales and basalts appear to have a significant influence on the amount of CO2 uptake by chemical weathering.

International audience; Controversy surrounds the role of the ocean in interhemispheric transport of carbon. On one hand, observations in the atmosphere and in the ocean both seem to imply that the preindustrial ocean transported up to 1 Pg C yr($^{-1}$) from the Northern to the Southern Hemisphere. On the other hand, three dimensional (3-D) ocean models suggest that global interhemispheric transport of carbon is near zero. However, in this debate, there has been a general neglect of the river carbon loop. The river carbon loop includes (1) uptake of atmospheric carbon due to inorganic and organic erosion on land, (2) transport of carbon by rivers, (3) subsequent transport of riverine carbon by the ocean, and (4) loss of riverine carbon back to the atmosphere by air-sea gas exchange. Although carbon fluxes from rivers are small compared to natural fluxes, they have the potential to contribute substantially to the net air-sea fluxes of CO$_2$. For insight into this dilemma, we coupled carbon fluxes from a global model of continental erosion to a 3-D global carbon-cycle model of the ocean. With rivers, total southward interhemispheric transport by the ocean increases from 0.1 to 0.35 $\pm$0.08 Pg Cyr($^{-1}$), in agreement with oceanographic observations. Resulting air-sea fluxes of riverine carbon and uptake of CO$_2$ by land erosion were installed as boundary conditions in a 3-D atmospheric model. The assymetry in these fluxes drives a preindustrial atmospheric gradient of CO$_2$ at the surface of -0.6 $\pm$ 0.1 $\mu$atm for the North Pole minus the South Pole and longitudinal variations that exceed 0.5 $\mu$atm. Conversely, the gradient for Mauna Loa minus South Pole is only -0.2 $\pm$ 0.1 $\mu$atm, much less than the -0.8 $\mu$atm gradient extrapolated linearly from historical atmospheric CO$_2$ measurements from the same two sites. The difference may be explained by the role of the terrestrial biosphere. Regardless, the river loop produces large gradients both meridionally and zonally. Accounting for the river carbon loop changes current estimates of the regional distribution of sources and sinks of CO$_2$, particularly concerning partitioning between natural and anthropogenic processes.

Knowledge of the impact of N-fertilizers on the weathering-erosion processes of soils in intensively cultivated regions is of prime importance. Nitrification of NH4− fertilizers produces HNO3 in the basin of the Garonne river, enhancing soil degradation. Their influence on the weathering rates was determined by calculating the consumption rate of atmospheric/soil CO2 by soil weathering and erosion, and its contribution to the total dissolved riverine HCO3−. This contribution was found to be less than 50% which corresponds normally to a complete carbonate dissolution by carbonic acid, suggesting that part of the alkalinity in the river waters is due to carbonate dissolution by an acid other than carbonic acid, probably HNO3.

It has been proposed that increased rates of chemical weathering and the related drawdown of atmospheric CO2 on the continents may have at least partly contributed to the low CO2 concentrations during the last glacial maximum (LGM). Variations in continental erosion could thus be one of the driving forces for the glacial/interglacial climate cycles during Quaternary times. To test such an hypothesis, a global carbon erosion model has been applied to a LGM scenario in order to determine the amount of CO2 consumed by chemical rock weathering during that time. In this model, both the part of atmospheric CO2 coming from silicate weathering and the part coming from carbonate weathering are distinguished. The climatic conditions during LGM were reconstructed on the basis of the output files from a computer simulation with a general circulation model. Only the predicted changes in precipitation and temperature have been used, whereas the changes in continental runoff were determined with an empirical method. It is found that during the LGM, the overall atmospheric CO2 consumption may have been greater than today (by about 20%), mainly because of greater carbonate outcrop area related to the lower sea level on the shelves. This does not, however, affect the atmospheric CO2 consumption by silicate weathering, which alone has the potential to alter atmospheric CO2 on the long-term. Silicate weathering and the concomitant atmospheric CO2 consumption decreased together with a global decrease of continental runoff compared to present-day (both by about 10%). Nevertheless, some uncertainty remains because the individual lithologies of the continental shelves as well as their behavior with respect to chemical weathering are probably not well enough known. The values we present refer to the ice-free continental area only, but we tested also whether chemical weathering under the huge ice sheets could have been important for the global budget. Although glacial runoff was considerably increased during LGM, weathering under the ice sheets seems to be of minor importance.

The export of carbon from land to sea by rivers represents a major link in the global carbon cycle. For all principal carbon forms, the main factors that control the present-day fluxes at the global scale have been determined in order to establish global budgets and to predict regional fluxes. Dissolved organic carbon fluxes are mainly related to drainage intensity, basin slope, and the amount of carbon stored in soils. Particulate organic carbon fluxes are calculated as a function of sediment yields and of drainage intensity. The consumption of atmospheric/soil CO2 by chemical rock weathering depends mainly on the rock type and on the drainage intensity. Our empirical models yield a total of 0.721 Gt of carbon (Gt C) that is exported from the continents to the oceans each year. From this figure, 0.096 Gt C come from carbonate mineral dissolution and the remaining 0.625 Gt C stem from the atmosphere (FCO2). Of this atmospheric carbon, 33% is discharged as dissolved organic carbon, 30% as particulate organic carbon, and 37% as bicarbonate ions. Predicted inorganic carbon fluxes were further compared with observed fluxes for a set of 35 major world rivers, and possible additional climatic effects on the consumption of atmospheric CO2 by rock weathering were investigated in these river basins. Finally, we discuss the implications of our results for the river carbon fluxes and the role of continental erosion in the global carbon cycle during the last glacial maximum.

Continental erosion of organic and inorganic carbon is estimated using the carbon transport by major world rivers to the oceans. A geochemical modeling allows to calculate, for each large river basin, the flux of atmospheric/soil CO₂ consumed by chemical erosion of continental rocks and exported as bicarbonates by rivers. Drainage intensity and carbonate rock outcrops appear to be the major factors controlling CO₂ fluxes and river transport of inorganic carbon. A global erosion model (GEM-CO₂) has been developed and validated for 3 large river basins (Congo, Amazon and Garonne). GEM-CO₂ allows to simulate CO₂ fluxes for the different continents. Carbon contents in soils, steepness of morphology and drainage intensity of river basins are controlling DOC river fluxes, while РОС river fluxes mainly depend on suspended sediment transport, which could be modeled from drainage intensity, seasonal rainfall variability, steepness of morphology and softness of outcropping rocks in river basins. Modeling of organic carbon river fluxes (GEM-Corg) allows to simulate the organic carbon erosion over the continents. This empirical modeling (GEM-CO₂ and GEM-Corg) is coupled with a river routing file in order to determine the spatial distribution of river carbon inputs (alkalinity, dissolved and particulate organic carbon) to the world's oceans in grid point resolutions of 2 x 2.5° and 4x5° latitude/longitude., Modélisation à l'échelle globale des flux de CO₂ consommé par l'érosion continentale et des transports fluviaux de carbone vers les océans L'érosion continentale du carbone organique et inorganique est estimée à partir des transports de carbone par les grands fleuves du monde vers les océans. Un modèle géochimique permet de calculer pour chaque grand bassin fluvial le flux de CO₂ atmosphérique/biosphérique consommé par l'érosion chimique des roches continentales et exporté principalement sous forme de bicarbonates par les fleuves. L'intensité de l'écoulement et la surface occupée par les affleurements de roches carbonatées sont les principaux facteurs de contrôle des flux de CO₂ et des transports fluviaux de carbone inorganique. Un modèle global d'érosion (GEM-CO₂) a été développé et validé pour trois grands bassins fluviaux (Congo, Amazone et Garonne). GEM-C02 permet de simuler à l'échelle globale les flux de CO₂ consommé par l'altération sur les différents continents et les flux d'alcalinité exportés vers les océans. Les transports fluviaux de COD sont contrôlés à l'échelle globale par la teneur en carbone organique des sols, la morphologie des pentes des bassins versants et l'intensité des transports fluviaux de sédiments qui ont pu être modélisés à partir de l'intensité de l'écoulement, de la variabilité saisonnière des pluies, de la morphologie des bassins et de la dureté des roches. La modélisation des flux de carbone organique (GEM-Corg) permet de simuler l'érosion continentale des matières organiques. Cette modélisation empirique (GEM-CO₂ et GEM-Corg) est couplée avec un schéma de routage des eaux fluviales à la surface des continents afin de déterminer la distribution spatiale des apports fluviaux de carbone aux océans (alcalinité, carbone organique dissous et particulate) avec des résolutions de 2 x 2,5° et 4 x 5° de latitude et de longitude., Probst J.L., Ludwig Wolfgang, Amiotte Suchet Philippe. Global modeling of CO2 uptake by continental erosion and of carbon river transport to the oceans. / Modélisation à l'échelle globale des flux de CO2 consommé par l'érosion continentale et des transports fluviaux de carbone vers les océans. In: Sciences Géologiques. Bulletin, tome 50, n°1-4, 1997. The global carbon cycle in the terrestrial biosphere, sous la direction de Gérard Dedieu et Jean-Luc Probst. pp. 131-156.

The flux of atmospheric/soil CO2 consumed by chemical weathering of the continents (FCO2) can be estimated from river fluxes of bicarbonates. Using published data for 232 small monolithologic watersheds, empirical relationships between FCO2 and runoff (Q) have been determined for the major rock types outcropping on the continents. For validation, the models fitted to these relationships are applied to the Garonne, Congo and Amazon river basins to calculate the average CO2 consumption on these large river basins; the model results are close to previous estimates based on field measurements. The model (GEM-CO2: Global Erosion Model for CO2 fluxes) is then applied at the global scale and allows the determination of latitudinal variations of the consumption of atmospheric/soil CO2 by chemical erosion. The main results show that the consumption of CO2 is mainly localized in the Northern hemisphere, because of large continental area with a high proportion of carbonate rocks, and in equatorial regions which are very humid. Finally, the mean annual CO2 consumption for the whole continents amount to 0.26 Gt C y−1, and 72 % of this flux is removed in the Northern hemisphere

National audience; Le site-atelier de Breuil (Morvan), appartenant à l’Observatoire de Recherche sur l’Environnement F-ORE-T, est un support opérationnel des travaux actuels sur les cycles biogéochimiques en relation avec la diversité biologique du sol. De nombreux projets multidisciplinaires sont en cours sur ce site dont l’objectif commun est d’améliorer les connaissances sur les effets physiques, chimiques et biologiques des substitutions d’essences. Des travaux y sont menés à différentes échelles temporelles et spatiales, de l’instantané au pluriannuel, du microsystème fonctionnel jusqu’à l’écosystème complet. Les travaux développés à Breuil depuis 2001, montrent que les essences ont affecté les cycles clé du carbone et de l’azote avec les conséquences sur le fonctionnement des cycles de tous les autres éléments. Ces modifications sont entre autres liées à un contrôle des populations de micro-organismes du sol par les essences forestières qui les structurent. Les recherches en cours tentent d’élucider cette question pertinente d’écologie. Compte tenu de la longévité des essences forestières, ce site devra être suivi sur le long terme, vraisemblablement par une approche diachronique.

Numerous studies dealing with climate change impacts on water ressources have been done at global scale, but the moderated size watersheds scale is less studied. The Burgundy region is located over several hydrographical basins (Seine, Loire and Rhône river), and provides a great diversity of hydrogeological and climate contexts. For these reasons, impacts of climate change on water resources are expected to be very different in space, and require an entire hydroclimatic modelling chain at local scale to be assessed. The main objective of this thesis consisted of the implementation of a modelling tool at the moderated-size watersheds scale of Burgundy. In this way, several watersheds and two hydrological models have been chosen : a global and empirical rainfall-runoff model (GR4J) and a physical based and semi-spatialised model (SWAT). These two models allow us to assess every aspects of climate change impacts : quantitative (streamflow and spatialised ressource) and qualitative impacts.Models have been calibrated, including a cross calibration/validation test in climate contrasted periods, to confirm their robustness. A disaggregation of global climate data has been done in order to feed models during projection.Observations analysis highlighted a climate shift (in Burgundy as well as over the entire french territory), resulting in a modification of hydrological regime. Models projection over the XXIth century showed i) not much evolution of annual streamflow, but an accented seasonality, with an increase of winter streamflow, and a decrease of summer streamflow, linked with the rainfall regime and the evapotranspiration increase, ii) drought flow indices showing an increase of severity of the drought flow period, iii) a decrease in water quality. In order to explore the capacities of the modelling chain, several sensibility tests have been done, based on management practices scenarios.; De nombreuses études portant sur les impacts du changement climatique sur la ressource en eau ont été réalisées à l'échelle des grands bassins versants continentaux, celle des petits bassins versants demeurant peu étudiée. La région Bourgogne se situe à la croisée de plusieurs bassins hydrographiques (Seine, Loire, Rhône), et offre une grande diversité de contextes hydrogéologiques. Les impacts du changement climatique sont susceptibles de varier localement et leur détermination requiert l'implémentation d'une chaîne de modélisation hydroclimatique à l'échelle locale. L'objectif de cette thèse a consisté en la mise en place d'un tel outil de modélisation à l'échelle des petits bassins versants de Bourgogne. Un échantillon de bassins versants et deux modèles hydrologiques de conceptualisation différente ont été choisis : un modèle pluie-débit global (GR4J) et un modèle semi-distribué à base physique (SWAT). Ces modèles ont permis d'approcher les différents aspects des impacts sur la ressource en eau (débit à l'exutoire, ressource spatialisée, qualité de l'eau). Les modèles ont été calibrés, et leur robustesse a été testée en contexte de changement climatique, validant ainsi leur utilisation sur une période future. Une désagrégation spatiale des données climatiques a été nécessaire afin d'alimenter les modèles hydrologiques sur la période de projection. L'analyse de données observées a permis de mettre en évidence une rupture dans les températures de surface en 1987/88 (en Bourgogne comme en France métropolitaine), et une modification des régimes hydrologiques de part et d'autre de cette rupture. Les projections réalisées sur la période 1980-2100 ont permis de montrer : i) peu d'évolution des débits moyens annuels mais une accentuation de la saisonnalité, via une augmentation des débits hivernaux et une diminution des débits estivaux, en lien avec le régime des précipitations et avec l'augmentation de l'évapotranspiration. ii) des indicateurs d'étiages montrant une aggravation de leur sévérité et une emprise temporelle plus grande durant l'année hydrologique, iii) une qualité de l'eau dégradée. Afin d'ouvrir de nouvelles perspectives grâce à cette chaîne de modélisation (en termes aide à la décision), l'aptitude de la chaîne de modélisation à évaluer les impacts de divers scénarios de changement de pratiques agricoles a été testée.

The archipelago of Cape Verde comprises 10 volcanic islands and belongs to the Sahelian zone stretching from the Atlantic to the Red Sea. Since several decades, Cape Verde is affected by desertification caused largely by climatic recession and soil erosion. These factors, coupled with high population pressure on resources, rugged topography and torrential tropical rains, cause serious losses of lands. However, since its independence in 1975, the Government conducted an extensive program of afforestation, soil and water conservation. Nevertheless very little research has been conducted to evaluate the actions of protection and conservation of soil and water. Therefore, there is no data on the problem of land degradation or budget. As part of this work, we studied the various factors that control soil erosion by water. Specifically, we sought to differentiate the effects of human activities including agriculture, those climatic factors such as rainfall and runoff generation. We also established the first mass budget of suspended and dissolved load in the context of the archipelago of Cape Verde. The study was conducted across three watersheds of the Santiago Island, Cape Verde. These three watersheds (Longueira, Grande and Godim) are located in the central part of the island and are representative of various types of land use and of the different climates of the island. There is a climatic gradient between the three watersheds. Indeed, Longueira which covers an area of 4.18 km2, an average slope of 47 % is localized in a humid zone and covered to 69 % by forest and the agricultural area is 17 %. Grande with an area of 1.87 km2, is localized in sub humid zone for an average slope of 50 %, it is mainly agricultural. Godim, with an area of 2.0 km2, is localized in semi arid, it is particularly agriculture and its average slope is 32 %. For these three watersheds, flood flows at the outlet were measured and sampled from 2004 to 2009. Watershed Longueira has been further monitoring, particularly in terms of sampling and monitoring of flood flows out. On each sample we determined the concentration of suspended sediment and we analyzed dissolved major elements. The results show that mechanical erosion in the three watersheds is characterized by high spatial and temporal variability. Over the period 2005-2009, the average annual budget for Longueira, Grande and Godim watersheds is: 4266, 157 and 10.1 t.km2.an-1 respectively. The wet season 2006 was the most erosive for all three watersheds, particularly in Longueira with 2 exceptional floods that have generated an average concentration of suspended sediment exceeding 100 g/l. In contrast, the seasons 2005 and 2008 have been generally low erosive because mean concentrations did not exceed 20 g/l. Moreover, there was no runoff for the seasons 2005 and 2007 for the watershed of Godim. In Longueira watershed, the study of hysteresis phenomena can characterize each flood and show that the temporal evolution of suspended sediment yield during the season is strongly influenced by agricultural activities. Indeed, the first flood caused the massive export of sediment available and located in the bed of the watercourse. Accordingly, the second exported less sediment. One month after the first rains, weeding activities decreased the density of vegetation cover and deconstruct the superficial soil, which again caused a very strong export of sediments during the third run off. The results of the chemical erosion in the watershed of Longueira indicate that the budget of chemical erosion rate is 45 t.km-2.an-1 with a strong temporal variability. Indeed, the wettest seasons of 2006 and 2007 are the most export of dissolved load, whereas 2005 had a low export. The use of model mixtures EMMA (End-Members Mixing Analysis) shows that the interflow and deep, feeding the streams in dissolved elements are the main factors of chemical erosion. etc, L’archipel du Cap Vert constitue de 10 iles volcaniques appartient a la zone sahelienne qui s’etend de l’atlantique jusqu’a la mer rouge. Depuis, plusieurs decennies le Cap Vert est affecte par la desertification causee en grande partie par la recession climatique et l’erosion des sols. Ces facteurs, associes a la forte pression anthropique sur les ressources, a l’orographie accidentee et a des pluies tropicales parfois diluviennes, provoquent de serieuses pertes du patrimoine foncier. Cependant, depuis son Independance en 1975, le Gouvernement a mene un vaste programme d’arborisation, de restauration des terres et d’amenagement des cours d’eau. Pourtant, tres peu de recherches ont ete menees pour evaluer les actions de protection et de conservation des sols et des eaux. Par consequent, il n’existe quasiment pas de donnees sur la problematique de la degradation des terres ni de bilans. Dans le cadre de ce travail, nous avons etudie les differents facteurs qui controlent l’erosion hydrique des sols. Nous avons plus particulierement cherche a differencier les effets des activites humaines, notamment agricoles, de ceux des facteurs climatiques comme les precipitations et la generation des ecoulements. Nous avons egalement etabli les premiers bilans d’exportations de matieres en suspension et en solution dans le contexte de l’archipel du Cap Vert. L’etude a ete menee a l’echelle de trois bassins versants de l’ile de Santiago Cap-Vert. Ces trois bassins versant (Longueira, Grande et Godim) sont localises dans la partie centrale de l’ile de Santiago et representatifs des divers modes d’occupation du sol et des differents climats de l’ile. Il existe un gradient climatique entre les trois bassins versants. En effet, Longueira qui presente une superficie de 4,18 km2, une pente moyenne de 47 %, se localise dans une zone humide couverte a 69 % par une foret et une surface agricole de 17 %. Grande avec une superficie de 1,87 km2, se localise en zone sub humide pour une pente moyenne de 50 %, il est essentiellement agricole. Godim, avec une superficie de 2,0 km2, se localise en zone semi aride, il est particulierement agricole et sa pente moyenne est de 32 %. Pour ces trois bassins versants, les ecoulements de crue a l’exutoire ont ete mesures et echantillonnes de 2004 a 2009. Le bassin versant de Longueira a fait l’objet d’un suivi plus pousse, notamment en termes de frequence d’echantillonnage et de suivi des ecoulements hors crue. Sur chaque echantillon nous avons procede a la determination de la concentration des matieres en suspension ainsi qu’a l’analyse des elements majeurs. Les resultats obtenus montrent que l’erosion mecanique dans les 3 bassins versants est caracterisee par une forte variabilite spatiale et temporelle. Sur la periode 2005-2009, le bilan moyen annuel pour les bassins versants de Longueira, Grande et Godim est de : 4266, 157 et 10,1 t.km2.an-1 respectivement. La saison humide 2006 a ete la plus erosive pour l’ensemble des trois bassins versants et particulierement dans Longueira avec 2 crues exceptionnelles qui ont genere une concentration moyenne de matieres en suspension superieure a 100 g/l. En revanche, les saisons 2005 et 2008 ont ete dans l’ensemble peu erosives car les concentrations moyennes ne depasserent pas 20 g/l. Par ailleurs, il n’y a pas eu de lames d’eau ecoulees pour les saisons 2005 et 2007 pour le bassin de Godim. Sur le bassin de Longueira, l’etude des phenomenes d’hysteresis permet de caracteriser chaque crue et de montrer que l’evolution temporelle des exportations de matieres en suspension au cours de la saison est fortement influencee par les activites agricoles. En effet, la premiere crue provoque l’exportation massive des sediments disponibles et localises dans le lit du cours d’eau. En consequence, la seconde est moins exportatrice de sediments. etc