PEDOGENESIS AND GLOBAL WARMING: A PERSPECTIVE FROM THE ANCIENT

Paleocene and Eocene rocks in Big Bend National Park, Texas, can provide data on the effects of global warming on pedogenesis. Chemostratigraphic methods using the stable isotopes of carbon and oxygen extracted from pedogenic carbonate are being used to pinpoint a well-documented global warming event, the Paleocene/Eocene Thermal Maximum (PETM). Detailed columnar sections are being measured to identify changes in pedogenesis, and XRD analysis is being used to identify the clay mineralogy of these ancient soils.

The first occurrence of Hyracotherium (Mammalia: Perissodactyla) is close to the contact between the fluvial Black Peaks and overlying Hannold Hill Members of the Tornillo Formation and is used in this study to approximate the Paleocene/Eocene boundary. Black Peaks Member paleosols are either red or black vertisols. Pedogenic features of red paleosols include prominent redoximorphic features, slickensides, and abundant carbonate nodules. These red vertisols can be classified as Aquic Hapluderts or possibly Aeric Calciaquerts that formed under cycles of wetting and drying. The black vertisols are typified by large slickensides and desiccation peds; carbonate nodules in them are rare. These ancient soils formed under very wet to saturated conditions. Soils that formed during the deposition of the Eocene Hannold Hill Member are also classified as vertisols based on the presence of slickensides, but carbonate nodules are less abundant and reduced in size. The black vertisols that are present in the Paleocene Black Peaks Member are absent in the Eocene. The decrease in the abundance and size of carbonate nodules and the lack of soils that formed under very wet conditions appear to indicate a change to a warmer and drier climatic regime during the P/E transition.