Philippe Négrel

Chemical and physical denudation in the Amazon River Basin

We present major and trace element data on the suspended and dissolved phases of the Amazon River and its main tributaries. The Sr isotopic composition of the dissolved load is also reported. Special attention is paid to the abundances of REE and to their fractionation between the dissolved and suspended phase. The rivers of the Amazon Basin are among the richest in dissolved REE and are similar to the rivers of the Congo system. However a greater range of fractionation between LREE and HREE is reported here. At a global scale the rivers have intermediate patterns between those of the Congo system and those of high pH rivers such as the Indus and Mississippi rivers. Only few elements (Rb, U, Ba, K, Na, Sr and Ca) are mobilized by silicate weathering. These elements are strongly depleted in the suspended phase with respect to upper continental crust. In the dissolved load, these elements are controlled by atmospheric inputs and the weathering of the main lithologies. We propose a model based on mass budget equations, that allow the proportions derived from the different sources to be calculated. As a consequence silicate, carbonate and evaporite weathering rates can be estimated as well as the consumption of CO2 by weathering of each of these lithologies. Physical weathering rates can be estimated by two complementary approaches. On the one hand, the multi-year average of suspended sediments yields can be used to estimate physical denudation. On the other hand, we have developed a steady-state model of erosion that allows us to calculate physical erosion rates on the basis of the dissolved load of rivers. A mean crustal composition is assumed in this model for the rock sources of the drainage basins. Comparison of the rates predicted by the model to the observed rates shows good agreement for the lowland rivers, but a strong discrepancy for the rivers derived from the Andes. Andean rivers (Solimoes, Madeira and Amazon) have observed sediment yields much greater than those predicted according to the steady-state model of chemical and physical weathering. Two interpretations can account for this inconsistency. The first is that these rivers are not in steady state and hence that the soils are being destroyed. The second requires that the local continental crust is different from the average continental crust of Taylor and McLennan, and contains a large proportion of sedimentary rocks. Using the measured sediment yields, and assuming a steady state, we can estimate the amount of sediment recycling for each drainage basin. For the Amazon at Santarem, we find that at least 25% of the mass of the upper continental crust of the Amazon drainage basin is constituted of recycled material.

Erosion sources determined by inversion of major and trace element ratios and strontium isotopic ratios in river water: The Congo Basin case

Dissolved and suspended load river material represents the integrated products of the erosion of drainage basins. To enlarge the study of erosion processes we have determined87Sr/86Sr ratios and the Cl, Na, Mg, Ca and Sr contents for the main tributaries of the Congo River Basin, both for water and suspended sediment. We have also analyzed 30 streams draining monolithological terranes. A systematic study of precipitation has permitted the estimation of a good rain correction factor. Sr isotopic ratios have shown that the seawater input correction based on riverine Cl content is not valid in the Congo Basin because a large part of the Sr, Ca and Mg come from a terrestrial source. The conventional atmospheric input correction by reference to the marine ratios underestimates the real atmospheric input because of the crustal elements carried by rainwaters. Different erosion source parameters have been obtained for carbonates, evaporites and silicates. An inversion scheme has been developed to compute the multimixing equations and allows the quantification of the input of each main reservoir (atmosphere, carbonates, evaporites and silicates) for each tributary and each element. For Ca and Mg, rainfall and carbonate dissolution are the main inputs. For Sr, the input is mainly controlled by rains and silicate weathering. By using Sr isotopic systematics we have calculated the Sr isotopic composition of the silicate weathered crust for each of the main tributaries of the Congo Basin. We obtain uniform values for the main tributaries ranging between87Sr/86Sr= 0.7195 ± 0.001 and 0.7251 ± 0.005. These results allow the calculation of strontium model agesTSr, which differ from neodymium model agesTNd. UsingTNd, we have calculated the87Rb/86Sr of the silicate weathered crust. We obtain homogeneous values close to 0.75, which is in agreement with estimates for the average silicate crust. The discrepancy betweenTSr andTNd may be linked to the vegetation impact which fractionates Rb and Sr.

SrNdPb isotope systematics in Amazon and Congo River systems: constraints about erosion processes

Abstract 87 Sr 86 Sr , 143 Nd 144 Nd , 206 Pb 204 Pb , 207 Pb 204 Pb and 208 Pb 204 Pb isotopic ratios and Rb, Sr, Sm, Nd, U, Pb and Th concentrations have been measured in the suspended loads of the Congo and Amazon rivers and their tributaries. In the dissolved load, 87 Sr 86 Sr , Rb, Sr, Nd, Sm, U, Pb and Th concentrations are also reported. These results show that Nd, Sm, Th and Pb are almost insoluble and that their mass balance is controlled by particulates whereas Rb, Sr and U are fractionated between soluble and particulate phases. The 87 Sr 86 Sr ratios can only be interpreted after computing the amount of carbonate recycling and the partitioning for silicates between soluble and insoluble. This paper presents a method based on the lead isotopic system that can be used to test the steady state of erosion which is tacitly assumed in many river and erosion studies. The results presented show that the steady state is validated in each river of the Congo Basin and in the lowland rivers of the Amazon Basin, but in not verified in the rivers from the Andes (Rio Solimoes and Rio Madeira). 87 Sr 86 Sr , 206 Pb 204 Pb and 208 Pb 204 Pb ratios are positively correlated and negatively correlated with 143 Nd 144 Nd ratios. A number of arguments indicate that these correlations are mixing lines, the end-members being orogenic and shield components, respectively. For the Amazon Basin, analysis of the river loads shows that the orogenic zone is favoured some 5 times more than the shield on an equal area basis. This leads to a reinterpretation of SmNd data for shales and casts some doubts upon the proposition of secular variations in the Sm Nd ratio of the continental crust.

Chemistry of rainwater in the Massif Central (France): a strontium isotope and major element study

Atmospheric aerosols (sea salt, crustal dust, and biogenic aerosols) are the primary source of dissolved species in rainwater as well as one of the sources of dissolved species in river water. Chemical weathering studies require quantification of this atmospheric input. The crustal component of atmospheric input can have various origins, both distant and local. The proportions of the various inputs (marine, distant or local) are determined in this study. Strontium isotope ratios and Ca, Na, K, Mg, Al, Cl, SO4, NO3 and Sr concentrations were measured in rainwater samples collected in the Massif Central (France) over a period of one year. Each sample, collected automatically, represents a monthly series of rain events. Chemical composition of the rainwater samples varied considerably and the 87Sr/86Sr ratios ranged between 0.709198 and 0.713143. Using Na as an indicator of marine origin, and Al for the crustal input in rain samples, the proportion of marine and crustal elements was estimated from elemental ratios. A marine origin of 4 to 100% of Cl, of 0.6 to 20% of the SO4, of <1 to 10% of Ca, <1 to 40% of K, 4 to 100% of Mg and 1 to 44% of Sr was determined. Strontium isotopes were used to characterize the crustal sources. The 87Sr/86Sr ratios of the crustal sources varied considerably from 0.7092 to 0.71625 and indicate the occurrence of multiple sources for the crustal component in the analysed rainwaters.

Strontium isotope systematics used to decipher the origin of groundwaters sampled from granitoids: the Vienne Case (France)

Sr isotope data from surface, shallow and deep groundwaters from the granitoids of the Vienne District (France) are presented in this paper. In surface waters, the Sr contents in the rocks and groundwaters agree with previous data for groundwaters sampled from granitic and sedimentary rocks in France where a large range in 87Sr/86Sr ratios is observed. After correction for the Sr input from rain, the surface water samples plot within a mixing field that can be explained by three end-members, one anthropogenic (low 87Sr/86Sr and high Cl/Sr ratio) and two end-members characterised by low Cl/Sr ratios and a large range in 87Sr/86Sr ratios (from around 0.707–0.720). For deep groundwaters, the 87Sr/86Sr ratios and Sr contents are also determined by applying a correction to account for the influence of cleaning waters during drilling operations. The results are scattered amongst five different groups and the lack of a direct linear relationship between any of the samples implies that, as found for the shallow groundwaters, the results are due to mixing between more than two end-members. A model to determine the 87Sr/86Sr ratio (Irf) of groundwater after interaction with an actively weathering granite is developed. The results yield a low Irf value for waters associated with weathering of the tonalite (0.70463) and a higher one for waters associated with weathering of the monzogranite (0.70704). Given the much higher Irf values derived from the deep groundwater samples, these results indicate that the deep groundwaters analysed within the Vienne hydrosystem cannot be directly related to weathering of either tonalite or monzogranite. It is speculated that this high 87Sr/86Sr source originated from marine incursions during the Jurassic and have been diluted by mixing with former groundwaters produced by water–rock interaction (WRI) with the granites.

A Pb isotope and trace element study of rainwater from the Massif Central (France)

Lead isotope ratios and Zn, Pb, Cu, Cd, Sb and Rb contents were measured in samples of rainwater collected over a period of 15 months from the Massif Central (France). Each sample, collected automatically at monthly intervals, represents a series of rainfall events. Rainwater chemistry was interpreted in terms of the chemical contributions from wet deposition and from different source regions for dust in the centre of France. Trace element concentrations in rainwater samples showed a wide range, particularly for Pb (1.30-465 microg/l), with variations decreasing for Cd (0.07-1.70 microg/l), Zn (1.00-54.00 microg/l), Cu (0.20-25.00 microg/l), Sb (approximately 0-0.33 microg/l) and Ni (approximately 0-15.00 microg/l). Trace element contents do not correlate with rainfall amount and no inter-element correlations are evident in the data. Lead is the most common trace metal found in the rainwater (mean value = 996 microg/m2/y) while Sb is the least common element measured (mean value = 1.12 microg/m2/y). The composition of rainwater collected from the Massif Central shows a range in Pb isotope ratios from 17.935 to 19.22 (206Pb/204Pb), 15.578 to 15.73 (207Pb/204Pb) and 37.559 to 38.606 (208Pb/204Pb). A five-component mixing model involving contributions from the natural background, gasoline inputs from industrial and agricultural activity and a source resulting from mining waste may be used to explain both the Pb isotope signature and the fluctuations in trace metal contents of Massif Central rainwater.

Exotic stable isotope compositions of saline waters and brines from the crystalline basement

Abstract The overwhelming majority of groundwaters reported in literature show isotopic compositions that place them on or right of the meteoric water line (MWL) in a δ2H vs. δ18O diagram. A closer look to the steadily increasing data from the deeper crystalline basement reveals nevertheless that points left of the MWL are not so unusual as has been suggested during the last decades. Their supposed rareness seems to be due to a bias of sampling: The concerned fluids develop only at greater depths and in low permeability environments, which makes access to them so difficult. Paradoxically, “exotic” stable isotope compositions may be the rule in a restricted depth zone in all crystalline basements with slow groundwater movements but not too high temperatures. The present study takes stable isotope data on saline fluids from the deep French granitic basement as a starting point. In order to situate the French data from the Vienne region in a more general context, a literature review has been undertaken assembling more than 1300 stable isotope analyses of waters and brines in crystalline rocks from stable cratons and mountain belts world-wide. The French fluids show some particular features with respect to the majority of other studies as their major ion composition close to seawater or a significant isotopic shift at relatively low salinities. Detailed knowledge on the hydrological and chemical history of the site allowed to develop a conceptual model of isotopic evolution by interaction of fracture fluids with fracture minerals. This model considers interactions of the last fluids flushing the system (marine water, unaltered, evaporated or diluted) with previously precipitated mineral phases and the neoformation of the last generation of fracture minerals. The model has been tested quantitatively for several combinations of dissolution–precipitation reactions involving formation of carbonates, clay minerals and Fe-hydroxides. Most of the modelled scenarios would leave the residual liquid phase depleted in 18O and either enriched or depleted in 2H and produce waters with isotopic signatures corresponding to the observed δ2H vs. δ18O correlation. It is therefore suggested that the general phenomenon of an isotopic shift in deep fissured silicate rocks, which seems to be rather independent of lithology, may result from a combination of different reaction mechanisms implying not solely the rock matrix but also the different generations of fracture minerals.

Chemical and strontium isotope characterization of rainwater in France: influence of sources and hydrogeochemical implications

Strontium isotope ratios and Ca2+, Na+, K+, Mg2+, Cl−, , and Sr2+ concentrations were measured in rainwater samples collected in four stations in France (Brest, Dax, Orleans and Clermont-Ferrand) over a period of 1 year. Each sample represented a monthly series of rain events. The chemical composition and the 87Sr/86Sr ratios of the rainwater samples varied considerably. Using Na concentrations as an indicator of marine origin, the proportion of marine and crustal elements was estimated from elemental ratios. Strontium isotopes were used to characterize the different sources using data from the four stations and the literature. Such sources include sea salts, crustal sources (carbonates, silicates and volcanic rocks) and anthropogenic sources (fertilizers, automobile exhausts, incinerators and urban heating).

Geochemistry and water dynamics of a medium-sized watershed: the Hérault, southern France: 1. Organisation of the different water reservoirs as constrained by Sr isotopes, major, and trace elements

Abstract The aim of this study of the Herault watershed is to constrain the river–groundwater relations with a global geochemical investigation: major and trace elements, radiogenic isotopes (Sr, Pb) and stable isotopes (oxygen, deuterium). Among the six sampling campaigns in high and low flows, this paper focuses on the first one (March 1995) sampled during a low flow period for major and trace elements and Sr isotopes on both dissolved and particulate loads. A companion paper will focus on the river–karst relations in a dynamical scheme over a complete hydrological cycle. The objective of this paper is to constrain the spatial organisation of the different water reservoirs, to show the relations between surficial and groundwaters and to assess the mechanical and chemical weathering in this low-flow period. The Herault main stream successively drains a Palaeozoic basement, a karstified Mesozoic cover and a Tertiary and Quaternary alluvial plain, the different tributaries also drain these lithologies. This watershed is also impacted by ancient mining, and agricultural activities especially in the southern part. Major element concentrations generally reflect the drained lithologies. The chemistry of the Herault main stream is mainly controlled by silicate and carbonate endmembers, whereas the karstic springs show clearly a mixture of limestones and dolomitic limestones. Major elements point out some specific characteristics of some karstic springs, and Sr isotopes are used to check previous underground circulation hypotheses and sometimes reveal strong connections with overlying surface waters. Mixing phenomena between the Herault river and its tributaries can be quantified in the 87 Sr / 86 Sr vs. 87 Rb / 86 Sr diagram based on a simple two-component scheme and agree within 10% with the real discharge measurements. As in large basins, the suspended matter presents more variable and more radiogenic 87 Sr / 86 Sr than the dissolved load, due to the more important contribution of silicate particles which are less soluble than carbonate. Dissolved Ca fluxes were estimated to be 36, 51 and 38 kg km −2 day −1 in the Palaeozoic basement, the Mesozoic cover and the alluvial plain, respectively. This implies a chemical erosion estimated to be 20 μm/year in the Palaeozoic basement and 58 μm/year in the Mesozoic cover, similar to values calculated for large basins.

An Overview of Dissolved and Suspended Matter Fluxes in the Loire River Basin: Natural and Anthropogenic Inputs

The spatial and temporal distributions of major elements were investigated in the surface waters and in associated suspended matter at two sites of the upper Loire basin (Orleans and Brehemont) between 1995 and 1998.According to geochemical and isotopic patterns, the dissolved load appears to result from a process of mixing rainwater inputs, weathering processes of carbonate and silicate bedrock, and agricultural and urban inputs. Natural inputs influence 60% of water chemical composition at both sites. Annual dissolved fluxes were estimated to be 1300 103 t/y at Orleans and 1620 103 t/y at Brehemont. Major elements are transported mainly in the dissolved fraction. After correcting for atmospheric and anthropogenic inputs, the silicate specific export rate was calculated to be 11 t/y/km2 throughout the basin and the carbonate specific export rate to be from 47 t/y/km2 at Orleans to 23 t/y/km2 at Brehemont.The suspended load appears to result from at least two particle reservoirs: a silicate reservoir and a carbonate reservoir. The silicate reservoir has a detrital origin, mainly during periods of high flow, while the carbonate reservoir has a detrital origin during periods of high flow and an authigenic origin during periods of low flow. Of the total annual flow of suspended matter, this authigenic material represents 16% at Orleans, 25% at Brehemont and 37% in the fluvial part of the estuary. After correcting authigenic inputs, the specific export rate due to mechanical weathering was estimated to be 8 t/y/km2 throughout the Loire basin.