ABSTRACT Deciphering the time milestones of clay formation is crucial for modeling the evolution of major geological systems near the Earth's surface. In the present study, kaolinite samples collected along four vertical profiles within a Gleysol‐Ferralsol‐Podzol soil toposequence near São Gabriel da Cachoeira, NW Amazon Basin (Brazil) are dated using electron paramagnetic resonance (EPR) spectroscopy. Owing to the large number of samples, only selected kaolinite samples were artificially irradiated and used as models for other samples sharing similar EPR spectra. The radiation dose rate was assessed using two contrasting models corresponding to a “linear” or “early” evolution of U, Th concentration during the profile formation. It was also assumed that Th is homogeneously distributed in the clay matrix. Both models reveal close and episodic histories of weathering based on average profile kaolinite ages. The average ages increase with elevation from Quaternary to Miocene and are discussed in terms of toposequence evolution and possible relation with formerly recognized paleoclimatic major episodes of kaolinite formation in the Amazon Basin. Discrete weathering episodes at 1–3 and 6–12 Ma are identified, aligning with known climatic and tectonic events in the Amazon Basin, such as the onset of monsoonal climate patterns and Andean uplift phases. Kaolinite from the podzol horizon of the top hill profile (P1) likely occurs due to inheritance and relative dissolution of the most disordered small grains rather than in situ formation while underlying kaolinic horizons display the oldest ages of the toposequence. Kaolinite samples in deep horizons from the bottom profile (P4) are the youngest and contain vanadyl (VO 2+ ) impurities, suggesting reducing conditions, which indicates in situ formation. Translocation from the upslope soils could have affected the topsoil of the P4 profile. Kaolinite ages in the intermediate profiles (P2 and P3) could result from a mixture of older and more recent populations. It is thus inferred that the soil sequence exhibits a dissolution/loss regime at the top and a formation regime on the slope and at the bottom. The study underscores the importance of considering specific environmental and geochemical contexts in interpreting EPR dating results in tropical soils.
Montes et al. (Mon,) studied this question.