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Título

PEDOGENIC PROCESSES AND CLIMATIC CONDITIONS FROM CRETACEOUS PALEOSOLS OF THE ITAPECURU FORMATION, (PARNAÍBA BASIN, BRAZIL)

Texto do resumo

On the Earth's surface, physical, chemical, and biological processes act on exposed rocks to produce soils. Once buried and incorporated into the sedimentary record, these soils preserve a wealth of information about past ecosystems, climate conditions, and atmospheric CO2 levels. The Early Cretaceous Itapecuru Formation, deposited within the Parnaíba Basin, Brazil, is a 600-meter-thick succession of interbedded very fine- to fine-grained sandstone and mudstone, containing a rich fossil record in which paleosols are stratigraphically common. Previous paleogeographic and paleoclimate studies propose that the Itapecuru Formation was deposited under predominantly tropical equatorial conditions. However, investigating the formative conditions of such tropical paleosols and their use as paleoclimatic proxies has not been previously attempted in Brazil. In this paper, the macro and microscopic pedogenic features of a hydromorphic paleosol profile in the Itapecuru Formation are described in detail and assigned as the Prata pedotype. Analyses of clay mineralogy and whole-rock geochemistry are used to define pedogenic processes, paleoclimate proxies, paleohumidity conditions, and paleovegetation. The two stacked profiles of compound paleosols were grouped into one pedotype (the Prata pedotype) based on chemical and physical similarities. Pedogenic processes recognized in the horizons included translocation, gleization, kaolinitization, lateritization, and biological activity. Estimates of paleoprecipitation and paleotemperature from the studied paleosols using climofunctions, chemical index of alteration without potassium, and paleosol weathering index proxy (PWI) showed values ranging from 673 to 1042 mm/yr and 10.4 to 11.3 °C, respectively. Based on climofunction values, the Kӧppen aridity index suggest a paleoclimate of subhumid to humid during weathering processes in soil formation. The climate data also suggests that during the Albian, the Prata pedotype was geographically within the Equatorial Humid belt with vegetation consisting of moist tropical forests.

Palavras Chave

Ultisols; Redoximorphic features; Tropical paleosols