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Reading the climate signals hidden in bauxite

Abstract : The dynamics of tropical weathering through time with the formation and long-term evolution of laterite associated with climatic and geodynamic forcing is still a poorly explored issue. To better access this, we focused on lateritic-bauxitic duricrusts from the well-explored, 300 m high Kaw mountain in northeastern French Guiana. Macroscopically homogeneous Fe (oxyhydr)oxides-rich subsamples were separated from 10 bulk samples. Bulk-and subsamples were investigated by mineralogical and geochemical analyses and (U-Th)/He geochronology. The samples show a large heterogeneity on the macroand microscopic scale reflecting different stages of duricrust formation and evolution through processes of dissolution and (re)precipitation of Fe (oxyhydr)oxides. The 284 (U-Th)/He ages obtained for goethite a-(Fe, Al)OOH and hematite a-Fe 2 O 3 range from 30.5 ± 3.1 to < 0.8 Ma and show large variability within a sample. The oldest hematite and Al-poor goethite subsamples precipitated since 30 Ma and formed while kaolinite was stable. Precipitation of Fe-minerals increased since $ 14-12 Ma but still happened under ferruginous lateritic, non-bauxitic conditions. The dissolution and (re)precipitation of Fe minerals, Al-substitution in goethite, and the overall enrichment in Th, as well as gibbsite precipitation indicate an intensification of weathering and a shift towards bauxitic conditions since the end of the Miocene. The Th-, U-, and Al-rich Fe phases formed in this late episode of intense weathering partially replace the oldest, often Th-and U-poor phases leading to a considerable age spread. We claim that this episode of intensified weathering, which had its peak at ca. 6-2 Ma, corresponds to the bauxitization of the lateritic cover of Kaw mountain. Our proposed model of lateritization since at least 30 Ma and bauxitization since the late Miocene provides new constraints on the timescale and intensity of weathering at Kaw mountain. The onset of lateritization in French Guiana (Kaw region) is potentially older than ca. 30 Ma and was possibly synchronous with the development of bauxites in neighboring countries, and differences in the Paleogene weathering intensity might be due to different drainage capacities of the parental material. Late Neogene bauxitization suggests a regional increase in precipitations and drainage potentially linked to increased incision owing to local uplift.
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Soumis le : samedi 4 juin 2022 - 08:40:30
Dernière modification le : mercredi 3 août 2022 - 04:06:13

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Beatrix Heller, Silvana Bressan Riffel, Thierry Allard, Guillaume Morin, Jean-Yves Roig, et al.. Reading the climate signals hidden in bauxite. Geochimica et Cosmochimica Acta, Elsevier, 2022, 323, pp.40 - 73. ⟨10.1016/j.gca.2022.02.017⟩. ⟨hal-03665730⟩

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