Reading the climate signals hidden in bauxite
Résumé
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 macro- and 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 -(Fe, Al)OOH and
hematite -Fe2O3 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, non2
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.
Finally, we confirm that a detailed combination of geochronological results coupled to mineralogical and
geochemical analyses improves our understanding of tropical weathering processes and duricrust
formation by placing mineralogical and geochemical processes into a temporal framework.
Domaines
Sciences de l'environnementOrigine | Fichiers produits par l'(les) auteur(s) |
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