Work

Lateritic Cenozoic paleoenvironmental and paleoclimatic conditions in the Central Amazon basin, Brazil, inferred from mineralogy, geochemistry and geochronology

Year: 2024

Type: article

Abstract: Lateritic profiles are natural archives recording the weathering history in the tropics over million-year timescales. This study combines oxygen, hydrogen, and silicon isotope data with mineralogical, geochronological, and geochemical analyses of lateritic secondary mineral assemblages to constrain the environmental and climatic conditions prevailing in the central Amazon Basin during the Cenozoic. The studied lateritic profile was developed over the sedimentary succession of the Alter do Chão Formation in Manaus, Brazil. Three distinct weathering episodes were identified, each constrained by (U Th)/He and EPR ages. Dated to the Oligocene , the earliest (Oligocene) and long-lasting (duration >10 Ma) weathering episode involves the formation of well-ordered kaolinites through in situ chemical weathering of parent minerals under well-drained conditions, indicative of a tropical climate with restricted seasonality . The second episode, occurring in the mid-Miocene (∼16 ± 3 Ma), involves the formation of a ferruginous duricrust . Most probably resulting from lateral iron migration and precipitation at the oxidizing front of an oscillating water table, the duricrust developed in a context of rising sea levels . The third episode, during the Upper Miocene (∼10 Ma), led to the replacement of the kaolinite initially precipitated at the top of the profile by lower-crystallinity kaolinite formed at faster kinetics. This episode coincides with the final phase of the Andean uplift and the onset of the transcontinental Amazon River, reflecting enhanced water drainage under a “monsoon-type” climate.

Source: Palaeogeography Palaeoclimatology Palaeoecology

DOI: 10.1016/j.palaeo.2024.112674