Opfergelt,S., de Bournonville,G., Cardinal,D., Andre(eの頭に´),L., Delstanche,S. and Delvaux,B.(2009): Impact of soil weathering degree on silicon isotopic fractionation during adsorption onto iron oxides in basaltic ash soils, Cameroon. Geochimica et Cosmochimica Acta, 73, 7226-7240.

『カメルーンの玄武岩質火山灰土壌中の鉄酸化物への吸着時におけるケイ素同位体分別作用に与える土壌風化度の影響』


Abstract
 The sequestration of silicon in soil clay-sized iron oxides may affect the terrestrial cycle of Si. Iron oxides indeed specifically adsorb aqueous monosilicic acid (H4SiO4o), thereby influencing Si concentration in soil solution. Here we study the impact of H4SiO4o adsorption on the fractionation of Si isotopes in basaltic ash soils differing in weathering degree (from two weathering sequences, Cameroon), hence in clay and Fe-oxide contents, and evaluate the potential isotopic impact on dissolved Si in surrounding Cameroon rivers. Adsorption was measured in batch experiment series designed as function of time (0-72 h) and initial concentration (ic) of Si in solution (0.61-1.18 mM) at 20℃, constant pH (5.5) and ionic strength (1 mM). After various soil-solution contact times, the δ30Si vs. NBS28 compositions were determined in selected solutions by MC-ICP-MS (Nu Plasma) in medium resolution, operating in dry plasma with Mg doping with an average precision of ±0.15‰ (±2σSEM). The quantitative adsorption of H4SiO4o by soil Fe-oxides left a solution depleted in light Si isotopes, which confirms previous study on synthetic Fe-oxides. Measured against its initial composition (δ30Si = +0.02±0.07‰ (±2σSD), the solutions were systematically enriched in 30Si reaching maximum δ30Si values ranging between +0.16‰ and +0.95‰ after 72 h contact time. The enrichment of the solution in heavy isotopes increased with increasing values of three parameters: soil weathering degree, iron oxide content, and proportion of short-range ordered Fe-oxide. The Si-isotopic signature of the solution was partly influenced by Si release, possibly through mineral dissolution and Si desorption from oxide surfaces, depending on soil type, highlighting the complex pattern of natural soils. Surrounding Cameroon rivers displayed a mean Si-isotopic signature of +1.19‰. Our data imply that in natural environments, H4SiO4o adsorption by soil clay-sized Fe-oxides at least partly impacts the Si-isotopic signature of the soil solution exported to water streams.』

1. Introduction
2. Materials and methods
 2.1. Environmental setting
 2.2. Sampling
 2.3. Sample characterization
  2.3.1. Soil and clay properties
  2.3.2. Silicon in river waters
 2.4. Adsorption experiments
 2.5. Silicon isotopes analyses
3. Results
 3.1. Soil weathering degree and implications for soil properties
 3.2. Silicon adsorption in soils
 3.3. Si-isotopic signatures induced by adsorption of monosilicic acid in soils
 3.4. Dissolved and particulate Si in river waters
4. Discussion
 4.1. Quantitative Si adsorption and soil components
 4.2. Si-isotopic variations during Si adsorption as influenced by soil weathering degree
 4.3. Potential impact of desorption and mineral dissolution
 4.4. Comparison with Si-isotopic fractionation induced by synthetic Fe-oxides
 4.5. Implications on soil solution and surrounding rivers
5. Conclusions
Acknowledgments
References


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