Maastrichtian atmospheric pCO2 and climatic reconstruction from carbonate paleosols of the Marília Formation (southeastern Brazil)

Abstract

Paleosols are the product of ancient physical, chemical, and biological processes on the Earth's surface and, as such, may record information that can be used to reconstruct the paleoatmospheric and paleoenvironmental conditions under which they formed. In Brazilian continental sedimentary successions, few studies using ancient soils have focused on the relationship between paleopedogenesis and paleoclimate. The Marilia Formation is a 160-m-thick section of the Bauru Basin in which ∼ 66% of the deposits show some evidence of pedogenic modification as paleosols. In this paper, paleosol profiles in the Marilia Formation containing thick calcrete intervals are described and attributed to three pedotypes: Avencas, Monte Alto, and Garça. Macro and microscopic pedogenic features of each pedotype are described in detail. Moreover, the analysis of clay mineralogy, whole-rock geochemistry, and stable-isotope composition are used to define pedogenic processes, paleoclimate proxies, and atmospheric pCO2 estimates. The Avencas pedotype is composed of six polygenetic profiles with different phases of carbonate precipitation, clay illuviation, and biogenic actions. The Monte Alto pedotype is moderately developed and composed of calcic horizons formed mainly by rhizoliths, with higher degrees of calcification and oxidation compared to Avencas profiles. The Garça pedotype is well developed with five polygenetic profiles presenting high carbonate content and low accumulation of clay minerals (CIA-K) and leaching. Estimates of paleoprecipitation and paleotemperature from the studied paleosols using climofunctions of molar ratio of base to alumina, depth of carbonate accumulation, salinization, oxygen composition, and paleosol weathering index proxy (PWI) show values ranging from 242 to 718 mm/yr and 11° to 14°, respectively. Climofunction values suggest a paleoclimate of semiarid to subhumid during deposition of the Marília Formation. The climate data also suggests that during the Maastrichtian, the Bauru Basin was geographically within the Southern Hot Arid Belt zone, though showing strong influence of the lower latitudinal Equatorial Humid belt. Furthermore, atmospheric pCO2 values calculated from pedogenic carbonates may correlate with a cooling interval during the latest Maastrichtian (68.5–66.25 My).