STABLE ISOTOPES AND PEDOGENESIS DURING THE PALEOCENE/EOCENE TRANSITION: BIG BEND NATIONAL PARK TEXAS

Outcrops in Big Bend National Park are North America’s most southern exposure of Paleogene continental deposits in which the Paleocene/Eocene boundary is constrained by terrestrial mammal localities. The exposures are mostly red and black mudstones, that have been interpreted as paleosols. A chemostratigraphic section using the stable isotopes of carbon from pedogenic carbonate nodules has been developed with the goal of pinpointing the Paleocene/Eocene Thermal Maximum (PETM) and thus the Paleocene/Eocene boundary. A negative carbon excursion has been identified within C24r and is bracketed by a late Tiffanian or Clarkforkian fossil mammal site and an early Wasatchian (Wa-1) site.

Due to the condensed nature of the section as well as pedogenic overprinting the interpretation of the negative carbon excursion is complex, as is its effects on pedogenic processes. The range in d ¹³C values is from -14.08⁰/₀₀ to -8.14⁰/₀₀. Small differences in mean d ¹⁸O values within individual nodules and within single paleosol horizons, as well as the small difference in mean excursion and nonexcursion d ¹⁸O values indicate that the data have not been affected by diagenesis. The pattern of the excursion is different than that reported for the PETM in the Big Horn Basin. Instead of a smooth continuous curve, the interval is punctuated by three individual spikes. This results from single paleosol horizons recording both nonexcursion and excursion d ¹³C values (i.e. pedogenic overprinting), as well as the presence of interspersed paleosols that do not contain carbonate nodules.

During this global warming event, changes in pedogenesis did occur in the study area and resulted in a significant increase in the translocation of clays and the leaching of calcite and plagioclase. This increase in the weathering profile may have been caused by an increase in mean annual temperature and rainfall.