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See detailSensitivity of carbonate weathering to soil CO2 production by biological activity along a temperate climate transect
Calmels, D.; Gaillardet, J.; François, Louis ULg

in Chemical Geology (2014), 390

We investigated the controls on carbonate weathering in a well-drained pure carbonate area subject to strong environmental gradients, the Jura Mountains, Western Europe. The water chemistry of sampled ... [more ▼]

We investigated the controls on carbonate weathering in a well-drained pure carbonate area subject to strong environmental gradients, the Jura Mountains, Western Europe. The water chemistry of sampled springs and resurgences is dominated by Ca2+ (87 to 96Eq% of the cationic charge) and HCO3 - (90 to 97Eq% of the anionic charge), reflecting the overwhelming imprint of calcium carbonate dissolution by atmospheric/biogenic CO2. Ca2+ concentration, which directly gives access to the amount of calcium carbonate dissolved per unit of water runoff, shows a gradual two-fold decrease (from 3000 to 1400μmol/L) along the elevation gradient (from 300 to 1200m). After discussing the possible influence of each environmental parameter on the observed water chemistry gradient, a decreasing soil pCO2 (the main source of acidity) with increasing altitude appears as the most likely explanation. As no spatial and temporal record of soil pCO2 are available for the Jura Mountains, we performed soil pCO2 modeling using the ecological and hydrological ASPECTS model that allows reconstructing carbon and water exchange fluxes between the vegetation, soil and atmosphere. Modeling results suggest that soil pCO2 decreases with altitude in response to both the change in vegetation species from deciduous-dominated forest in the lowlands to evergreen-dominated forest above 800m (responsible for 65% of the variation) and the change in climate and soil properties (responsible for 35% of the variation). Carbonate weathering would thus be strongly sensitive to the type of vegetation, which drives both temporal and spatial variations of soil carbon and water budgets. Based on field observations, we show that carbonate weathering rates are 20-30% higher under deciduous vegetation cover than under conifers (at a given water runoff value), in agreement with modeling results. Chemical denudation rates of carbonate in the Jura Mountains vary from 152 to 375t/km2/yr, corresponding to 60-150mm/ka of carbonate being removed. Carbonate weathering within the 10,000km2 of the study area accounts for an atmospheric CO2 consumption of 0.3 TgC/yr, showing that carbonate rocks have an enhanced capacity of atmospheric CO2 neutralization at least transiently. This study demonstrates that carbonate weathering is sensitive to the ecosystem dynamics, a conclusion that might be much more general, and suggests that carbonate weathering and associated CO2 consumption fluxes quickly react to any global change or land use modification. [less ▲]

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See detailMagma chamber processes in the Tellnes ilmenite deposit (Rogaland Anorthosite Province, SW Norway) and the formation of Fe-Ti ores in massif-type anorthosites
Charlier, Bernard ULg; Duchesne, Jean-Clair ULg; Vander Auwera, Jacqueline ULg

in Chemical Geology (2006), 234(3-4), 264-290

The origin of igneous Fe-Ti oxide ores associated with massif-type anorthosites is investigated through a detailed study of the world-class Tellnes ilmenite deposit, part of the late-Proterozoic (930-920 ... [more ▼]

The origin of igneous Fe-Ti oxide ores associated with massif-type anorthosites is investigated through a detailed study of the world-class Tellnes ilmenite deposit, part of the late-Proterozoic (930-920 Ma) AMC series of the Rogaland Anorthosite Province (SW Norway). More than 100 samples from drill cores reveal significant petrographical and compositional variations within the ore body. Four zones are defined, based on variations in modal proportions and cumulus mineral assemblages: the Lower and Upper Central Zones and the Lower and Upper Marginal Zones. Plagioclase and whole-rock compositions discriminate the zones and display patterns interpreted as a result of mixing of either plagioclase-ilmenite or plagioclase-ilmenite-orthopyroxene-olivine cumulates with a melt of ferrodioritic (jotunitic) composition with a content decreasing from 80 to 20% from the margins to the central part of the ore body. Phase diagrams for a jotunitic parental magma reproduce the crystallization sequence at 5 kb. The orthopyroxene-olivine liquidus boundary is a peritectic in the Bjerkreim-Sokndal layered intrusion and a cotectic in Tellnes and this explains the differences in the sequence of crystallization of the two intrusions. The high concentration of ilmenite, well above cotectic proportions, resulted from gravity-sorting in the Tellnes ore body, which represents the lower part of a larger magma chamber. Uniform Sr isotope ratios do not support magma mixing. The cryptic layering of the ore body precludes injection as a crystal mush but favours in situ crystallization from an evolving magma in a sill-like magma chamber. The present trough-shape and mineral orientations result from deformation during gravity-induced subsidence and by up-doming of the anorthosite. Fractional crystallization of a TiO2-rich magma with ilmenite as an early liquidus mineral and plagioclase buoyancy are the principal mechanisms responsible for the formation of Fe-Ti deposits in Proterozoic massif-type anorthosites. (c) 2006 Elsevier B.V. All rights reserved. [less ▲]

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See detailBasalt weathering laws and the impact of basalt weathering on the global carbon cycle
Dessert, C.; Dupre, B.; Gaillardet, J. et al

in Chemical Geology (2003), 202(3-4), 257-273

This study attempts to characterise the chemical weathering of basalts and to quantify the flux of carbon transferred from the atmosphere to the ocean during this major process at the surface of the Earth ... [more ▼]

This study attempts to characterise the chemical weathering of basalts and to quantify the flux of carbon transferred from the atmosphere to the ocean during this major process at the surface of the Earth. To this aim, we have compiled different published chemical compositions of small rivers draining basalts. Basaltic river waters are characterised by relatively high Na-normalized molar ratios (Ca/Na: 0.2-3.9; HCO3/Na: 1-10; Mg/Na: 0.15-6) in comparison with those usually observed for river draining silicates. The data also show the climatic influence on basalt weathering and associated CO2 consumption. Runoff and temperature are the main parameters controlling the chemical weathering rate and derived CO2 consumption during basaltic weathering. From these relationships and digital maps, we are able to define the contribution of basalts to the global silicate flux. Taking account of this result, we estimate that the CO2 flux consumed by chemical weathering of basalts is about 4.08 x 10(12) mol/year. The fluxes from the islands of Indonesia and regions of central America represent around 40% of this flux. The global flux of CO2 consumed by chemical weathering of basalts represents between 30% and 35% of the flux derived from continental silicate determined by Gaillardet et al. [Chem. Geol. 159 (1999) 3]. Finally, it appears that volcanic activity not only acts as a major atmospheric CO2 source, but also creates strong CO2 sinks that cannot be neglected to better understand the geochemical and climatic evolution of the Earth. (C) 2003 Elsevier B.V. All rights reserved. [less ▲]

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See detailDirect effect of ice sheets on terrestrial bicarbonate, sulphate and base cation fluxes during the last glacial cycle: minimal impact on atmospheric CO2 concentrations
Tranter, Martyn; Huybrechts, Philippe; Munhoven, Guy ULg et al

in Chemical Geology (2002), 190(1-4), 33-44

Chemical erosion in glacial environments is normally a consequence of chemical weathering reactions dominated by sulphide oxidation linked to carbonate dissolution and the carbonation of carbonates and ... [more ▼]

Chemical erosion in glacial environments is normally a consequence of chemical weathering reactions dominated by sulphide oxidation linked to carbonate dissolution and the carbonation of carbonates and silicates. Solute fluxes from small valley glaciers are usually a linear function of discharge. Representative glacial solute concentrations can be derived from the linear association of solute flux with discharge. These representative glacial concentrations of the major ions are similar to 25% of those in global river water. A 3-D thermomechanically coupled model of the growth and decay of the Northern Hemisphere ice sheets was used to simulate glacial runoff at 100-year time steps during the last glacial cycle (130 ka to the present). The glacially derived fluxes of major cations, anions and Si over the glaciation were estimated from the product of the glacial runoff and the representative glacial concentration. A second estimate was obtained from the product of the glacial runoff and a realistic upper limit for glacial solute concentrations derived from theoretical considerations. The fluxes over the last glacial cycle are usually less than a few percent of current riverine solute fluxes to the oceans. The glacial fluxes were used to provide input to an oceanic carbon cycling model that also calculates changes in atmospheric CO2. The potential change in atmospheric CO2 concentrations over the last glacial cycle that arise from perturbations in glacial solute fluxes are insignificant, being < 1 ppm. [less ▲]

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See detailCarbon stocks and isotopic budgets of the terrestrial biosphere at mid-Holocene and last glacial maximum times
François, Louis ULg; Godderis, Y.; Warnant, Pierre ULg et al

in Chemical Geology (1999), 159(1-4), 163-189

The carbon fluxes, stocks and isotopic budgets of the land biosphere at mid-Holocene (6 ka BP) and last glacial maximum (21 ka BP) times are reconstructed with the CARbon Assimilation In the Biosphere ... [more ▼]

The carbon fluxes, stocks and isotopic budgets of the land biosphere at mid-Holocene (6 ka BP) and last glacial maximum (21 ka BP) times are reconstructed with the CARbon Assimilation In the Biosphere (CARATB) model forced with two different sets of climates simulated by the European Centre-HAMburg (ECHAM) and LMD general circulation models. It is found that the trends predicted on the basis of both sets of GCM climatic fields are generally consistent with each other, although substantial discrepancies in the magnitude of the changes may be observed. Actually, these discrepancies in the biospheric results associated with the use of different GCM climatic fields are usually smaller than the differences between biospheric runs performed while considering or neglecting the CO2 fertilization effect (which might, however, be overestimated by the model due to uncertainties concerning changes in nutrient availability). The calculated changes with respect to the present of the biosphere carbon stock range from - 132 to + 92 Gt C for the mid-Holocene and from -710 to +70 Gt C for the last glacial maximum. It is also shown that the relative contribution of the material synthesized by C-4 plants to the total biomass of vegetation, litter and soils was substantially larger at mid-Holocene and last glacial maximum times than today. This change in the relative importance of the C-3 and C-4 photosynthetic pathways induced changes in the C-13 fractionation of the land biosphere. These changes in the average biospheric fractionation resulting from the redistribution of C-3 and C-4 plants were partly compensated for by changes of opposite sign in the fractionation of C-3 plants due to the modification of the intercellular CO2 pressure within their leaves. With respect to present times, the combination of both processes reduced the C-13 discrimination (i.e., less negative fractionation) of the land biosphere by 0.03 to 0.32 parts per thousand during the mid-Holocene and by 0.30 to 1.86 parts per thousand at the last glacial maximum. (C) 1999 Elsevier Science B.V. All rights reserved. [less ▲]

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See detailIsotopic constraints on the Cenozoic evolution of the carbon cycle
François, Louis ULg; Godderis, Y.

in Chemical Geology (1998), 145(3-4), 177-212

In the last few years, several models have been built to explore the Cenozoic evolution of the carbon and strontium cycles. Of particular interest is the study of the impact on the carbon cycle of major ... [more ▼]

In the last few years, several models have been built to explore the Cenozoic evolution of the carbon and strontium cycles. Of particular interest is the study of the impact on the carbon cycle of major mountain uplifts such as the Himalayan orogeny. To explain the Cenozoic increase in the measured seawater strontium isotopic ratio, it was recently proposed that the Himalyan uplift could be responsible for an enhanced consumption of atmospheric CO2 by continental silicate weathering. Here, a new model of the carbon cycle evolution over Cenozoic times is presented. It calculates the various fluxes involved in the organic and inorganic components of the carbon cycle from the seawater delta C-13, the biological isotopic fractionation in the ocean and the seafloor spreading rate. The model equilibrates the budgets of the carbon and alkalinity cycles on the million year timescale, assuming as many previous investigators that the system remains close to equilibrium. The validity of this equilibrium approximation is examined critically. Various sensitivity experiments are performed in order to test the impact of the model parameters on the results. The calculated history of the carbonate deposition rate is consistent with the available reconstruction. The continental silicate weathering rate calculated by the model appears to be widely insensitive to the model parameters, showing three distinct evolutions over the Cenozoic. The model indeed suggests a time of relative constancy of the silicate weathering flux before 40 Ma, followed by a period of slow decrease until 15 Ma and finally a marked increase up to the present. In a progressively cooler world, this evolution may be interpreted as a change from a 'chemically' controlled to a 'physically' controlled weathering regime. The evolution of continental silicate weathering thus partly appears decoupled from the increase in the observed seawater strontium isotopic ratio. For this reason, the evolution of the calculated riverine Sr-87/ Sr-86 ratio shows a strong increase over the Cenozoic, from about 0.710 to 0.712. However, this increase may largely be reduced by considering the recycling of a pelagic carbonate reservoir increasing over the Cenozoic or by assuming that seafloor basalt weathering is a CO2- or climate-dependent process. (C) 1998 Elsevier Science B.V. All rights reserved. [less ▲]

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See detailThe Cenozoic evolution of the strontium and carbon cycles: Relative importance of continental erosion and mantle exchanges
Godderis, Y.; François, Louis ULg

in Chemical Geology (1995), 126(2), 169-190

The past variations of the seawater Sr-87/Sr-86 isotopic ratio are related to changes in the relative contribution of the mantle Sr input to the ocean and the Sr supply from continental weathering ... [more ▼]

The past variations of the seawater Sr-87/Sr-86 isotopic ratio are related to changes in the relative contribution of the mantle Sr input to the ocean and the Sr supply from continental weathering. Recently, it has been postulated that the Cenozoic increase in the seawater Sr-87/Sr-86 isotopic ratio was associated with the uplift of the Himalayan and Andean mountains at that time. These orogenies may have changed the Sr isotopic ratio of the continental rocks undergoing weathering (as a result of extensive metamorphism), increased the river flux of Sr through enhanced weathering in these regions and possibly caused the global climatic cooling trend of the Cenozoic. A model of the major geochemical cycles coupled to an energy balance climate model is used to explore the possible causes of the Mesozoic-Cenozoic fluctuations in the seawater Sr-87/Sr-86 isotopic ratio. The contribution of the mantle exchanges at mid-ocean ridges, of the recycling of seafloor carbonates through plate margin volcanism and of the alteration of seafloor basalts to the fluctuations of the seawater Sr-87/Sr-86 isotopic ratio are studied. Finally, this model tentatively describes the impact of the Himalayan orogeny on the geochemical cycles of Sr and C. Some possible effects of the extensive metamorphism associated with the India-Asia collision and of the Himalayan uplift are modelled. The model reproduces the Cenozoic increase of the seawater Sr-87/Sr-86 isotopic ratio. However, the impact of the Himalayan orogeny on the C cycle appears to be limited and insufficient to generate the global climatic cooling of the Cenozoic. Rather, in the model, the Cenozoic cooling is mostly due to the reduction of the CO2 emission from mid-ocean ridge volcanism and to changes in the chemical weathering rates in the rest of the world excluding the Himalayas. [less ▲]

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See detailIsotopic constraints on the genesis of the anorthosite suite of rocks
Demaiffe, Daniel; Weis, Dominique; Michot, Jean et al

in Chemical Geology (1986), 57

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