The stable isotope stratigraphy and paleosols of North America's most southern exposure of late Paleocene/early Eocene fossiliferous continental deposits: Documenting the initial Eocene thermal maximum in Big Bend National Park, Texas.

摘要

A chemostratigraphic section across the Paleocene/Eocene boundary, using the stable isotopes of carbon and oxygen, has been developed for North America's most southern exposure of early Paleogene continental deposits in which the boundary is constrained by fossil mammals. A negative carbon excursion has been identified within C24r. The range in delta 13C values is from -8.1 to -13.2‰. Until the development of the chemostratigraphic section it was uncertain if the earliest Eocene was recorded in Big Bend. An early Wasatchian (Wa1) fossil site occurs stratigraphically higher than the carbon excursion and has yielded the stratigraphically lowest Hyracotherium in the Big Bend region. Based on the stable isotope stratigraphy, time equivalent to Wa0 is recorded in Big Bend but no Wa0 fossils have been found.; To examine the possible effects of the initial Eocene thermal maximum (IETM) on pedogenesis in the study area, the chemical index of alteration (CIA) was calculated for pre IETM paleosols and paleosols that occur within the negative carbon excursion. Pre IETM paleosols have CIA values that indicate moderate weathering. IETM paleosols have CIA values that indicate moderate to intense weathering. The clay mineralogy of pre IETM paleosols is dominated by smectite, and it is only within the carbon excursion that there is a change. Kaolinite increases from 2% to 17% in one paleosol horizon that is associated with the carbon excursion. Other notable differences are an increase in the translocation of clays and irons, an increase in base loss and a decrease in the amount of calcite in IETM paleosols. These changes suggest that the climate must have been moister during this time. An increase in hydrolysis reactions caused by an increase in humidity, rainfall and an increase in carbonic acid in the soil due to elevated CO2 levels during the IETM are the interpreted mechanisms for increased weathering.