Climate change during the Early Paleogene in the Bogotá Basin (Colombia) inferred from paleosol carbon isotope stratigraphy, major oxides, and environmental magnetism

Paleosols in the Bogotá Basin, Colombia, record an increase in chemical weathering across the Paleocene-Eocene (P-E) transition. Rock magnetic properties and major element geochemistry exhibit an abrupt change in an interval identified as including the P-E boundary, as established by previously published biostratigraphy and a U/Pb date on volcanic zircons (56.2±1.6Ma). During the stratigraphic interval that contains the P-E transition, magnetic susceptibility increases significantly, with an order of magnitude higher concentration of magnetite/maghemite and hematite. The preponderance of pure stoichiometric magnetite in the paleosols of this interval indicates that the increase in magnetic susceptibility is due to changes in the rate of pedogenesis, rather than an increase in the erosion of nearby volcanic rocks, which would contribute titanomagnetite. Pedogenic structures preserved within the paleosols, a lack of iron oxides as cement, friability of the sandstones, and previously published thermochronologic data are not consistent with burial diagenesis as an explanation for the origin of the magnetic mineral assemblage. These enhanced pedogenesis and related chemical weathering is also confirmed by increases in the concentration of Fe₂O₃ and Al₂O₃ and loss on ignition values. An increase in mean precipitation across this interval is inferred by the lack of carbonate nodules and a decrease in SiO₂ in paleosols. Thus, we hypothesize that there was an intensification of chemical weathering during the interval that contains the P-E transition.