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238U–234U–230Th–226Ra radioactive disequilibria in an Amazon lateritic profile (Manaus, Brazil)
Goldschmidt Conference Abstract 2006
U–234U–230Th–226Ra radioactive disequilibria in an Amazon lateritic profile (Manaus, Brazil) F. CHABAUX1, E. PELT1, Y. LUCAS2, T. ALLARD3, E. FRITSCH3, E. BALAN3, M. SELO4, C. INNOCENT5 1
CGS-EOST, University of Strasbourg, France LEPI, University of Toulon-Var, France 3 LMC, University of Paris, France 4 LEME-MNHN, Paris, France 5 BRGM, Orle´ans, France 2
A powerful tool to understand processes and timescales of chemical weathering controlling geochemical dynamics and mineralogical evolution of weathering profiles is to combine study of tracers recording processes under different timescales (Dequincey et al., 2002; Chabaux et al., 2003). Here we propose to apply this approach to an old lateritic profile in Manaus (Brazil), developed on quartzo-kaolinitic sediments, using major elements, trace elements and 238U–234U–230Th–226Ra disequilibria. Variation of major and trace elements within the profile highlights the occurrence of two main geochemical fractionation processes: (1) strong chemical weathering in the sediment and nodular horizons, inducing quartz dissolution and transformation of kaolinite into gibbsite; (2) biogeochemical cycling by vegetation in the latosol, this process retaining Si and stabilize kaolinite in this surface horizon (Lucas et al., 1993). Both the variations and the intensity of the 238 U–234U–230Th–226Ra disequilibria remain quite limited within this profile. This is certainly ascribed to the specific location of U in strongly refractory minerals such as zircon or Ti-oxides (Balan et al., 2005). Nevertheless, the variations of 238 U–234U–230Th–226Ra disequilibria along the profile point out the following informations:
Variations with depth of (230Th/232Th) and (238U/232Th) activity ratio suggest that U–Th fractionation is quite old (older than 400–600 ka), whereas the occurrence of radioactive disequilibria among U-series nuclides—especially between 226Ra and 230Th—indicates that these fractionation processes are still active. 238 In the latosol, U–234U–230Th disequilibria— (230Th/234U) 1, (230Th/238U) < 1 and (234U/238U) < 1—imply that U enrichment by biological cycling and U depletion by chemical weathering would be close to a steady state. In the intermediate and deepest horizons, the (226Ra–230Th) disequilibria—Ra loss in the nodular zone, and Ra gain in the sediments—could be ascribed to a recent downward flux of Ra (<8 ka) related to the evolution of oxide nodules in the profile.
References Dequincey et al., 2002. Geochim. Cosmochim. Acta, 1197–1210. Chabaux et al., 2003. C. R. Acad. Sci., 1219–1231. Lucas et al., 1993. Science, 521–523. Balan et al., 2005. Geochim. Cosmochim. Acta, 2193–2204. doi:10.1016/j.gca.2006.06.098
Se-soil organic matter interactions: Direct or indirect association? C. CHABROULLET1, F. COPPIN1, A. MARTIN-GARIN1, M. FLORIANI1, E. TINSEAU2, J.-P. GAUDET3 1
IRSN/DEI/SECRE/LRE, BP 3, 13115 St. Paul-lez-Durance, France ([email protected]) 2 IRSN/DEI/SARG/LETS, BP 17, 92262 Fontenay aux roses, France 3 LTHE, University Grenoble I, BP 53, 38041 Grenoble, France Although soil organic matter (SOM) has an important role on oxyanions retention in soils (e.g. As, Se), the nature of interactions between those elements with SOM is not clearly known. In this study, Se (IV) was used to artificially contaminate a grassland soil (‘‘Roth2’’, Rothamsted Institute, UK). Physical and chemical extractions were then used to determine the solid partition of Se. In addition, transmission electronic microscopy (TEM) and scanning electronic microscopy (SEM) observations (Fig. 1), both coupled with EDX analyses, were realised on some isolated fractions. The Particulate Organic Matter (POM)—organic debris >50 lm– which represented 29.8% of the total soil organic carbon, was particularly responsible for more than 11% of total Se retention, although it represented only 5.6% of the total soil weight Coppin et al., 2006. Moreover, selenium sorption experiments, performed on isolated fractions (Fig. 2), revealed that the POM fraction (50–200 lm) was the most Se reactive fraction of the soil. In addition, we measured that POM50–200lm contained the highest iron concentration (4 times more Fe than POM>200lm and 2 times more than soil). As iron oxides represent an important Se carrier phase, it was expected that Se retention to POM was favoured by iron oxides surface precipitates. TEM and SEM observations showed that Se was spread onto the whole POM surface and in its core, as Fe was. Thus, Se diffused into the POM matrix and was uniformly accumulated. In addition, Se hot spots were located in the vicinity of Fe spots. Although direct Se-POM links were not excluded, microscopic observations confirmed that Se-POM associations were favored by the presence of Fe. Observations of few clay-size impurities on the POM surface did not reveal any other Se hot spots. 100% 75% Sorbed Se
238
A93
50% 25% 0% POM POM >200µm 50-200µm
min >50µm
Frac˚ <50µm
Soil "Roth2"
Fig. 1. SEM (top) and TEM (bottom) images of POM between 50 and 200 lm. Fig. 2. Selenite sorption on soil, POM (>200 lm and 50–200 lm), mineral >50 lm and fraction <50 lm; [Se] = 10 3M; t = 48 h; m/V = 1/5 (except for POM>200lm 1/20).
Reference Coppin, F., Chabroullet, C., Martin-Garin, A., Balesdent, J., Gaudet, J.P., 2006. BFS, 1–8. doi:10.1016/j.gca.2006.06.099
U–234U–230Th–226Ra radioactive disequilibria in an Amazon lateritic profile (Manaus, Brazil) F. CHABAUX1, E. PELT1, Y. LUCAS2, T. ALLARD3, E. FRITSCH3, E. BALAN3, M. SELO4, C. INNOCENT5 1
CGS-EOST, University of Strasbourg, France LEPI, University of Toulon-Var, France 3 LMC, University of Paris, France 4 LEME-MNHN, Paris, France 5 BRGM, Orle´ans, France 2
A powerful tool to understand processes and timescales of chemical weathering controlling geochemical dynamics and mineralogical evolution of weathering profiles is to combine study of tracers recording processes under different timescales (Dequincey et al., 2002; Chabaux et al., 2003). Here we propose to apply this approach to an old lateritic profile in Manaus (Brazil), developed on quartzo-kaolinitic sediments, using major elements, trace elements and 238U–234U–230Th–226Ra disequilibria. Variation of major and trace elements within the profile highlights the occurrence of two main geochemical fractionation processes: (1) strong chemical weathering in the sediment and nodular horizons, inducing quartz dissolution and transformation of kaolinite into gibbsite; (2) biogeochemical cycling by vegetation in the latosol, this process retaining Si and stabilize kaolinite in this surface horizon (Lucas et al., 1993). Both the variations and the intensity of the 238 U–234U–230Th–226Ra disequilibria remain quite limited within this profile. This is certainly ascribed to the specific location of U in strongly refractory minerals such as zircon or Ti-oxides (Balan et al., 2005). Nevertheless, the variations of 238 U–234U–230Th–226Ra disequilibria along the profile point out the following informations:
Variations with depth of (230Th/232Th) and (238U/232Th) activity ratio suggest that U–Th fractionation is quite old (older than 400–600 ka), whereas the occurrence of radioactive disequilibria among U-series nuclides—especially between 226Ra and 230Th—indicates that these fractionation processes are still active. 238 In the latosol, U–234U–230Th disequilibria— (230Th/234U) 1, (230Th/238U) < 1 and (234U/238U) < 1—imply that U enrichment by biological cycling and U depletion by chemical weathering would be close to a steady state. In the intermediate and deepest horizons, the (226Ra–230Th) disequilibria—Ra loss in the nodular zone, and Ra gain in the sediments—could be ascribed to a recent downward flux of Ra (<8 ka) related to the evolution of oxide nodules in the profile.
References Dequincey et al., 2002. Geochim. Cosmochim. Acta, 1197–1210. Chabaux et al., 2003. C. R. Acad. Sci., 1219–1231. Lucas et al., 1993. Science, 521–523. Balan et al., 2005. Geochim. Cosmochim. Acta, 2193–2204. doi:10.1016/j.gca.2006.06.098
Se-soil organic matter interactions: Direct or indirect association? C. CHABROULLET1, F. COPPIN1, A. MARTIN-GARIN1, M. FLORIANI1, E. TINSEAU2, J.-P. GAUDET3 1
IRSN/DEI/SECRE/LRE, BP 3, 13115 St. Paul-lez-Durance, France ([email protected]) 2 IRSN/DEI/SARG/LETS, BP 17, 92262 Fontenay aux roses, France 3 LTHE, University Grenoble I, BP 53, 38041 Grenoble, France Although soil organic matter (SOM) has an important role on oxyanions retention in soils (e.g. As, Se), the nature of interactions between those elements with SOM is not clearly known. In this study, Se (IV) was used to artificially contaminate a grassland soil (‘‘Roth2’’, Rothamsted Institute, UK). Physical and chemical extractions were then used to determine the solid partition of Se. In addition, transmission electronic microscopy (TEM) and scanning electronic microscopy (SEM) observations (Fig. 1), both coupled with EDX analyses, were realised on some isolated fractions. The Particulate Organic Matter (POM)—organic debris >50 lm– which represented 29.8% of the total soil organic carbon, was particularly responsible for more than 11% of total Se retention, although it represented only 5.6% of the total soil weight Coppin et al., 2006. Moreover, selenium sorption experiments, performed on isolated fractions (Fig. 2), revealed that the POM fraction (50–200 lm) was the most Se reactive fraction of the soil. In addition, we measured that POM50–200lm contained the highest iron concentration (4 times more Fe than POM>200lm and 2 times more than soil). As iron oxides represent an important Se carrier phase, it was expected that Se retention to POM was favoured by iron oxides surface precipitates. TEM and SEM observations showed that Se was spread onto the whole POM surface and in its core, as Fe was. Thus, Se diffused into the POM matrix and was uniformly accumulated. In addition, Se hot spots were located in the vicinity of Fe spots. Although direct Se-POM links were not excluded, microscopic observations confirmed that Se-POM associations were favored by the presence of Fe. Observations of few clay-size impurities on the POM surface did not reveal any other Se hot spots. 100% 75% Sorbed Se
238
A93
50% 25% 0% POM POM >200µm 50-200µm
min >50µm
Frac˚ <50µm
Soil "Roth2"
Fig. 1. SEM (top) and TEM (bottom) images of POM between 50 and 200 lm. Fig. 2. Selenite sorption on soil, POM (>200 lm and 50–200 lm), mineral >50 lm and fraction <50 lm; [Se] = 10 3M; t = 48 h; m/V = 1/5 (except for POM>200lm 1/20).
Reference Coppin, F., Chabroullet, C., Martin-Garin, A., Balesdent, J., Gaudet, J.P., 2006. BFS, 1–8. doi:10.1016/j.gca.2006.06.099