Jérôme Gaillardet

Global silicate weathering and CO2 consumption rates deduced from the chemistry of large rivers

Abstract The main problem associated with the study of silicate weathering using river dissolved load is that the main control of solute chemistry is lithology and that all rivers are influenced by carbonate and evaporite weathering. In this paper, newly compiled data on the 60 largest rivers of the world are used to calculate the contribution of main lithologies, rain and atmosphere to river dissolved loads. Technically, an inverse method is used to solve a model containing of a series of mass budget equations relating river concentrations to chemical weathering products and atmospheric inputs. New estimates of global silicate weathering fluxes and associated CO2 consumption fluxes are given. The role of basalt weathering on oceanic islands and volcanic arcs is emphasized. For each large river, an attempt is made to calculate chemical weathering rates of silicates per unit area. Only relative chemical weathering rates can be calculated. The relationships between the chemical weathering rates of silicates and the possible controlling parameters are explored. A combined effect of runoff-temperature and physical denudation seems to explain the variability of modern silicate chemical weathering rates. The results of this study highlight the coupling between the physical and the chemical processes of silicate weathering. Only an active physical denudation of continental rocks seems to be able to maintain high chemical weathering rates and significant CO2 consumption rates.

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 of Deccan Traps determined by river geochemistry: impact on the global climate and the 87Sr/86Sr ratio of seawater

Abstract The impact of the Deccan Traps on chemical weathering and atmospheric CO2 consumption on Earth is evaluated based on the study of major elements, strontium and 87Sr/86Sr isotopic ratios of the main rivers flowing through the traps, using a numerical model which describes the coupled evolution of the chemical cycles of carbon, alkalinity and strontium and allows one to compute the variations in atmospheric pCO2, mean global temperature and the 87Sr/86Sr isotopic ratio of seawater, in response to Deccan trap emplacement. The results suggest that the rate of chemical weathering of Deccan Traps (21–63 t/km2/yr) and associated atmospheric CO2 consumption (0.58–2.54×106 mol C/km2/yr) are relatively high compared to those linked to other basaltic regions. Our results on the Deccan and available data from other basaltic regions show that runoff and temperature are the two main parameters which control the rate of CO2 consumption during weathering of basalts, according to the relationship: f=R f ×C 0 exp −Ea R 1 T − 1 298 where f is the specific CO2 consumption rate (mol/km2/yr), Rf is runoff (mm/yr), C0 is a constant (=1764 μmol/l), Ea represents an apparent activation energy for basalt weathering (with a value of 42.3 kJ/mol determined in the present study), R is the gas constant and T is the absolute temperature (°K). Modelling results show that emplacement and weathering of Deccan Traps basalts played an important role in the geochemical cycles of carbon and strontium. In particular, the traps led to a change in weathering rate of both carbonates and silicates, in carbonate deposition on seafloor, in Sr isotopic composition of the riverine flux and hence a change in marine Sr isotopic composition. As a result, Deccan Traps emplacement was responsible for a strong increase of atmospheric pCO2 by 1050 ppmv followed by a new steady-state pCO2 lower than that in pre-Deccan times by 57 ppmv, implying that pre-industrial atmospheric pCO2 would have been 20% higher in the absence of Deccan basalts. pCO2 evolution was accompanied by a rapid warming of 4°C, followed after 1 Myr by a global cooling of 0.55°C. During the warming phase, continental silicate weathering is increased globally. Since weathering of continental silicate rocks provides radiogenic Sr to the ocean, the model predicts a peak in the 87Sr/86Sr ratio of seawater following the Deccan Traps emplacement. The amplitude and duration of this spike in the Sr isotopic signal are comparable to those observed at the Cretaceous–Tertiary boundary. The results of this study demonstrate the important control exerted by the emplacement and weathering of large basaltic provinces on the geochemical and climatic changes on Earth.

The global control of silicate weathering rates and the coupling with physical erosion: new insights from rivers of the Canadian Shield

Abstract The chemical evolution of the surface of the Earth is controlled by the interaction of rainwaters, the atmosphere and the continental crust. That is the main reason why the knowledge of the parameters that control chemical denudation on Earth is of crucial importance. We report chemical and isotopic analyses for river waters from the Canadian Shield in order to estimate chemical weathering fluxes. We present a comparison of the chemical composition and Sr isotopic composition of a set of rivers sampled in the Slave Province (Northwest Territories, Canada) and in the Grenville Province (Quebec, Canada). The surface waters of these high latitude catchments are very dilute, with the Slave rivers about three times more concentrated than the Grenville rivers. A detailed analysis of the Sr isotopic composition and chemical signature of these rivers shows that silicate weathering reactions are not the only mechanisms that control solute concentrations. An atmospheric component, constituted by the dissolution of evaporite and carbonate aerosols, is necessary to explain the dispersion of chemical ratios such as Ca/Na, Mg/Na, Sr/Na and Cl/Na. These aerosols probably have a local origin. Chemical denudation rates for the Slave Province are four times lower than those found in the Grenville Province (0.35 and 1.55 tons/km 2 /yr respectively). Compared to a panel of surface waters from other shield areas of the world, the Slave Province appears to have the lowest chemical denudation rate in the world. In a chemical weathering rate vs. temperature plot, shield rivers define a triangular relationship, hot climate being able to produce the most variable denudation rates. But no simple relationship between chemical weathering rates and temperature or runoff is observed, in contrast to rivers draining basaltic areas. We show that a global power law (0.66 exponent) exists between chemical denudation rates and physical denudation rates, indicating that the shield areas with low mechanical denudation (such as the Slave Province or Cameroon) have also low chemical denudation rates. These results give importance to physical denudation in determining the chemical weathering rates of silicates. We think that any further modeling on Earth’s long term climate will have to take into account this fundamental coupling between mechanical and chemical weathering fluxes.

Major and trace elements of river-borne material: The Congo Basin

The Congo river Basin is the second largest drainage basin in the world, after the Amazon. The materials carried by its main rivers provide the opportunity to study the products of denudation of a large fraction of the upper continental crust of the African continent. This paper presents the chemical composition of the different phases carried in the Congo rivers and is followed by a companion paper, devoted to the modelling of major and trace elements. The Congo river between Bangui and Brazzaville as well as its main tributaries, including a few organic-rich rivers, also called Black rivers, were sampled during the 1989 high water stage. The three main phases (suspended load, dissolved load, and bedload) were analysed for twenty-five major and trace elements. Concentrations normalized to the upper continental crust show that in each river, suspended sediments and dissolved load are chemical complements for the most soluble elements (Ca, Na, Sr, K, Ba, Rb, and U). While these elements are enriched in the dissolved loads, they are considerably depleted in the corresponding suspended sediments. This is consistent with their high mobility during weathering. Another type of complementarity is observed for Zr and Hf between suspended sediments and bedload, related to the differential velocity of suspended sediments and zircons which are concentrated in bedloads. Compared to other rivers, absolute dissolved concentrations of Ca, Na, Sr, K, Ba, Rb, and U are remarkably low. Surprisingly, high dissolved concentrations are found in the Congo waters for other trace elements (e.g., REEs), especially in the Black rivers. On a world scale, these concentrations are among the highest measured in rivers and are shown to be pH dependent for a number of dissolved trace elements. The dissolved loads are systematically normalized to the suspended loads for each river, in order to remove the variations of the element abundances owing to source rock variations. Normalized diagrams for REEs are presented and extended to the other elements. They strongly support the argument that the apparent higher solubility of trace elements in the Congo waters is due to the presence in the dissolved load of a colloidal phase (as a result of 0.2 μm filtration). An important result is that these colloids are strongly depleted in Fe and Al with respect to the other elements. Finally, the comparison of the dissolved, suspended, and sandy transport fluxes of each element in the Congo Basin rivers shows that, although the proportions of, for example, the REEs in the dissolved loads of the majority of the Congo Basin rivers is close to 10% of the total transport flux, up to 80% of the REEs are transported by the so-called “dissolved”load in the Black rivers.

Geochemistry of large river suspended sediments: Silicate weathering or recycling tracer?

This study focuses on the major and trace element composition of suspended sediments transported by the world’s largest rivers. Its main purpose is to answer the following question: is the degree of weathering of modern river-borne particles consistent with the estimated river dissolved loads derived from silicate weathering? In agreement with the well known mobility of elements during weathering of continental rocks, we confirm that river sediments are systematically depleted in Na, K, Ba with respect to the Upper Continental Crust. For each of these mobile elements, a systematics of weathering indexes of river-borne solids is attempted. A global consistency is found between all these indexes. Important variations in weathering intensities exist. A clear dependence of weathering intensities with climate is observed for the rivers draining mostly lowlands. However, no global correlation exists between weathering intensities and climatic or relief parameters because the trend observed for lowlands is obscured by rivers draining orogenic zones. An inverse correlation between weathering intensities and suspended sediment concentrations is observed showing that the regions having the highest rates of physical denudation produce the least weathered sediments. Finally, chemical and physical weathering are compared through the use of a simple steady state model. We show that the weathering intensities of large river suspended sediments can only be reconciled with the (silicate-derived) dissolved load of rivers, by admitting that most of the continental rocks submitted to weathering in large river basins have already suffered previous weathering cycles. A simple graphical method is proposed for calculating the proportion of sedimentary recycling in large river basins. Finally, even if orogenic zones produce weakly weathered sediments, we emphasize the fact that silicate chemical weathering rates (and hence CO2 consumption rates by silicate weathering) are greatly enhanced in mountains simply because the sediment yields in orogenic drainage basins are higher. Hence, the parameters that control chemical weathering rates would be those that control physical denudation rates.

GEOCHEMISTRY OF DISSOLVED AND SUSPENDED LOADS OF THE SEINE RIVER, FRANCE :ANTHROPOGENIC IMPACT, CARBONATE AND SILICATE WEATHERING

This study focuses on the chemistry of the Seine river system, one of the major rivers in Europe, and constitutes the first geochemical investigation of both suspended and dissolved loads of this river. The Seine river drains a typical Mesozoic-Cenozoic sedimentary basin: the Paris basin, constituted of limestones mixed or interbedded with terrigenous sediments derived from the paleoreliefs bordering the Mesozoic and Cenozoic seas. In the context of quantifying the global influence of carbonate and silicate weathering on atmospheric CO2 consumption, the Seine river offers the possibility of examining weathering rates in a flat sedimentary environment, under temperate climatic conditions. One of the major problems associated with the Seine river, as with many temperate rivers, is pollution. We propose, in this paper, 2 approaches in order to correct the dissolved load of the Seine river for anthropogenic inputs and to calculate weathering rates of carbonates and silicates. The first uses the dissolved load of rivers and tries to allocate the different solutes to different sources. A mixing model, based on elemental ratios, is established and solved by an inversion technique. The second approach consists in using the suspended load geochemistry. Under steady state conditions, we show that the geochemistry of suspended sediments makes it possible to estimate the amount of solutes released during the chemical weathering of silicates, and thus to calculate weathering rates of silicates. The total dissolved load of the Seine river at Paris can be decomposed into 2% of solutes derived from natural atmospheric sources, 7% derived from anthropogenic atmospheric sources, 6% derived from agriculture, 3% derived from communal inputs, and 82% of solutes derived from rock weathering. During high floods, the contribution of atmospheric and agriculture inputs predominates. The weathering rate of carbonates is estimated to be 48 t/km2/yr (25 mm/1000 yr). Only 10% of carbonates are transported in a solid form, the rest being transported in solution. CO2 consumption by carbonate weathering approaches 400 × 103 mol/km2/yr. In the Seine river at Paris, about 2–3 mg/l of dissolved cations are found to originate from the chemical weathering of silicates. By taking dissolved silica into accounts, the total dissolved load derived from silicate weathering is about 6–7 mg/l. This value is minimal because biological uptake of silica probably occur in the Seine river. The chemical weathering rate of aluminosilicates is estimated to be 2 t/km2/yr . The ratio of physical over chemical weathering of silicates range between 1 and 3 and the total (chemical and physical) erosion rates of sedimentary silicates are about 2–3 mm/kyr. The CO2 consumption by silicate weathering 15–24 × 103 mol/km2/yr and is independent of dissolved silica concentration. Silicate consumption is thus 20 times less than carbonate consumption in the Paris basin. Compared to the neighboring granitic areas, the sedimentary region drained by the Seine river has 2 to 3 times lower CO2 consumption rates. We attribute this difference to the cation-depleted nature of the Seine basin aluminosilicates, which are of sedimentary origin. At a world scale, the chemical denudation rates found for the Seine basin are very low and comparable to those given for tropical lowland rivers draining silicates, such as the rivers of the Congo and Amazon basins, in spite of huge climatic differences. We attribute this similarity to the low mechanical denudation that characterizes these two types of regions.

A global geochemical mass budget applied to the Congo basin rivers: Erosion rates and continental crust composition

Rivers carry the products of continental denudation either in a dissolved form (chemical erosion) or in a solid form (physical erosion). We focus in this paper on the relationship between physical erosion and chemical erosion. We establish the mass budget of the Congo Basin Rivers using chemical complementarities between river suspended sediments, sandy bedload, and dissolved load of the Congo Basin rivers reported in a previous paper (Dupre et al., 1995). A series of equations are presented, assuming that the physical and chemical erosion processes are in a steady state during one year. The total mass of river-borne material (dissolved and particulate) transported in the river over a given period of time should balance the mass of upper continental crust eroded during this time. We show that the local continental crust on each drainage basin can be estimated and solve our steady-state weathering model using an inversion procedure. The very good agreement between modelled and measured values of the river-suspended sediment concentrations validates the steady-state hypothesis in this wet tropical area. Consequently, in this area, the sediment yield provide a good estimate of the rates of mechanical denudation. This result also validates the calculation of the chemical and isotopic composition of the local continental upper crust using the bulk river load. Erosion rates for the silicate upper crust and thus independent of the lithological variability (silicates, evaporites, and carbonates) of the drainage basins are calculated. Mechanical erosion rates and chemical erosion rates for the Congo Basin at Brazzaville are 8 t/km2/y and 5 t/km2/y. The corresponding consumption of atmospheric CO2 by weathering process is estimated to 51 × 103 mol/km2/an. These weathering and consumption rates are low in spite of the severity of the weathering conditions, of the high soil temperature, and of the intensity of precipitations. These conclusions indicate the limiting influence the dynamic equilibrium of soils for silicate weathering. Finally, by estimating the local continental crust chemical composition before the onset of erosion processes, especially for the most soluble elements, we can test the model of Taylor and McLennan.

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.

The influence of rivers on marine boron isotopes and implications for reconstructing past ocean pH

Ocean pH is particularly sensitive to atmospheric carbon dioxide content. Records of ocean pH can therefore be used to estimate past atmospheric carbon dioxide concentrations. The isotopic composition of boron (δ11B) contained in the carbonate shells of marine organisms varies according to pH, from which ocean pH can be reconstructed. This requires independent estimates of the δ11B of dissolved boron in sea water through time. The marine δ11B budget, however, is still largely unconstrained. Here we show that, by incorporating the global flux of riverine boron (as estimated from δ11B measurements in 22 of the worlds main rivers), the marine boron isotope budget can be balanced. We also derive ocean δ11B budgets for the past 120 Myr. Estimated isotope compositions of boron in sea water show a remarkable consistency with records of δ11B in foraminiferal carbonates, suggesting that foraminifera δ11B records may in part reflect changes in the marine boron isotope budget rather than changes in ocean pH over the Cenozoic era.