PEDOGENESIS DURING THE INITIAL EOCENE THERMAL MAXIMUM: BIG BEND NATIONAL PARK, TEXAS

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 δ¹³C values is from -8.1 to -13.2⁰/₀₀. 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 paleosol B horizons have an average CIA of approximately 70, and IETM paleosol B horizons have an average CIA of 69. The small difference between the CIA's of these two groupings of paleosols suggests that during the IETM, climatic conditions did not change enough to cause an increase in 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. There is a notable increase in the amount of kaolinite in one paleosol horizon that is associated with the carbon excursion but it is uncertain if this is a direct result of the IETM or due to the acid phase of an overlying acid sulfate paleosol. Although the CIA data do not suggest any change in chemical weathering during the IETM, the increase in kaolinite, translocation of clays and an increase in the leaching of calcite and plagioclase suggests that there was a change in pedogenesis associated with this ancient global warming event. An increase in hydrolysis reactions caused by an increase in rainfall and or an increase of carbonic acid in the soil due to elevated CO₂ levels during the IETM is the interpreted mechanism for increased weathering.