B. Dupre

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.

Isotopic and Chemical Effects Produced in a Continuously Differentiating Convecting Earth Mantle

Trace element variations have established the present concept of chemical heterogeneity of the Earth’s mantle. A continuous range of variations is observed for the mantle source of basalts, from mantle depleted in incompatible trace elements to mantle enriched in such elements, in particular light rare earth elements. This heterogeneity is supported by the study of orogenic lherzolites and ultramafic nodules from volcanoes and kimberlite pipes. Radiogenic isotopes of Sr, Pb and Nd confirm this heterogeneity and show that it results from ancient chemical fractionations. This trace element and radiogenic isotope heterogeneity is identifiable in the Precambrian mantle. Pb isotopes are the clearest tracers of this past heterogeneity. Correlations are observed between variations in incompatible element ratios, radiogenic isotope ratios and radiogenic isotope and trace element ratios. These correlations define large domains (source of alkali basalts, source of mid-oceanic ridge basalts) and more restricted domains (e.g. the Canary Islands). Thus, they show that the mantle is heterogeneous on very large scales (ocean scales) as well as in more limited areas. Radiogenic data on ultramafic nodules also show that these mantle materials have isotopic heterogeneities at the mineral scale. These data also allow an estimation of the time scale of creation of these heterogeneities: they are of the order of some 109 years (= 1 Ga). The mechanisms involved in the chemical evolution of the mantle are discussed. Heterogeneity is created by chemical fractionation during petrogenetic processes; essentially oceanic lithosphere formation, storage into the continental crust and also recycling of sediments during lithospheric plate subduction. This heterogeneity tends to be erased in the mantle by convection and diffusion. The former efficiently mixes the mantle at large scales. The latter is very inefficient in solid mantle conditions, but can homogenize the radiogenic isotopes during partial melting. All these processes have been continuously active through geological time. A mathematical model is proposed which describes the chemical evolution of a continuously differentiating convecting mantle. The correlations, and in particular mantle isochrons, appear as artefacts without time significance. Evolution of both trace element ratios and isotopic compositions can be described simultaneously if isotopic homogenization is easier than chemical mixing. Variations of lead isotopes tend to indicate an ancient (initial) heterogeneity of the mantle which can be possibly attributed to loss of lead from mantle to core. The rate of chemical fractionation of the mantle cannot have been constant with time but was faster during the Archaean than at present.

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.

Pb isotopic compositions of Archean komatiites and sulfides

Abstract Pb isotopic compositions were measured in komatiites and other volcanic rocks from several Archean greenstone belts, and in magmatic and exhalative sulfides associated with these volcanics. Thin komatiite flows generally yielded PbPb isochron ages consistent with other geochronological or geological information, but thicker basaltic flows gave ages that are too young. These results suggest that the thin komatiite flows were hydrothermally altered during eruption and that this event is recorded by the PbPb age. Thicker basaltic flows resisted alteration during eruption but were affected by later events: their PbPb ages probably correspond to the time of metamorphism, and not to the time of emplacement. Magmatic and exhalative sulfides from the Abitibi Belt in Canada fall on the same trend in the 207 Pb/ 204 Pb 206 Pb/ 204 Pb diagram as komatiites, basalts, sedimentary rocks and K-feldspars from granites. The calculated μ 1 -value is 7.8±0.1. A similar suite of rocks and minerals from the Kambalda area in Australia has distinctly higher 207 Pb 204 Pb and a higher μ 1 of 8.3. Nd isotopic data demonstrate that some Kambalda rocks are highly contaminated with older continental crust. The differences in initial Pb isotopic compositions are attributed to the presence or absence, and the age, of underlying granitoid basement. Kambalda greenstones are underlain and contaminated by old (> 3.0 Ga) granitoid crust but significantly older granitoid basement appears absent from the Abitibi Belt. The Pb isotopic compositions of Kambalda greenstones are dominated by the crustal component and do not represent the composition of the mantle source of these rocks. The Abitibi greenstones are not highly influenced by crustal contamination and their Pb isotopic compositions may reflect those of Archean mantle.

Chemical aspects of the formation of the core

The interpretation of data on lead isotopes in mantle material indicates that some of the lead has been pumped into the core during geological time. Such a continuous lead loss yields an increase of the U /Pb ratio in the mantle. Quantitative modelling of this effect permits an evaluation of the core’s growth curve. In the preferred model, 85% of the core would have been formed in the early period of the Earth’s history (50-200 Ma after 4.55 Ga ago) and 15% later on, and its accretion continues. Some of the consequences of such a process are reviewed.

The Cretaceous-Tertiary boundary problem: An assessment from lead isotope systematics☆

Abstract The presence of high concentrations of siderophile elements, particularly of Ir in clay-rich layers corresponding to the end of the Cretaceous, and thus correlated with many mass extinctions, has given rise to various studies and theories. To provide new isotopic arguments, we have used UPb systematics in four famous Ir-rich locations where the Cretaceous-Tertiary (KT) boundary is exposed. Three were marine sites: Stevns Klint (Denmark), Caravaca (Spain) and a site just near Woodside Creek (New Zealand); and one was a continental site: Raton Basin (Colorado-New Mexico, U.S.A.). In all sites, the Pb isotopic ratio is characteristic of local conditions without any change at the KT boundary. These Pb isotopic ratios are different from those of carbonaceous chondrites, iron meteorites and Deccan basalts. In Stevns Klint, as in Caravaca, the amount of Pb increases very suddenly at the KT boundary. Because this Pb enrichment occurred only in two boundary beds which contain pyrites or iron hydroxides, we suggest that Pb and Ir might have been concentrated in the clay level by precipitation of sulfides produced by anaerobic bacterial activity. The enrichment remained even after alteration processes destroyed the sulfides leaving iron hydroxides in the boundary levels. These Pb data imply also a possible terrestrial explanation for other volatile-element enrichments in KT boundary layers.