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.
Bibliographical noteFunding Information:
This research was partially funded by the Colombian Association of Geologists and Petroleum Geophysicists (Asociación Colombiana de Geólogos y Geofísicos del Petroleo) through the Research Grant Corrigan and Promotion of Young Investigators-ARES Found . The Institute for Rock Magnetism is supported by a grant from the Instruments and Facilities Program, Earth Science Division, of the U.S. National Science Foundation. The Unidad Administrativa Especial de Servicios Públicos kindly provided access to the outcrops located in the Doña Juana sanitary landfill. Special thanks to the residents of the Mochuelo town for their logistical support. Thanks to L. Quiroz, F. Lamus, O. Montenegro, L. Jimenez, M. C. Ruiz, G. Jimenez, J. Moreno, and the ARES team for their help in the field. Thanks to Dr. J. H. Escobar and Dr. J. Curtis for analyzing the carbon isotopes at the University of Florida. Thanks to Drs. K. Kleinsphen and E. Nater for their discussions about the paleosols and to Dr F. McInerney for her discussion about the manuscript. This manuscript was improved by reviews from P. Lippert, P. Hesse, and an anonymous reviewer.
- Elemental composition
- Environmental magnetism