ABRUPT PALEOENVIRONMENTAL CHANGE DURING THE PALEOCENE-EOCENE THERMAL MAXIMUM IN THE BIGHORN BASIN, WYOMING

Interbedded alluvial paleosols and avulsion deposits were analyzed through a PETM section at Polecat Bench, WY. The section begins ~20 below the carbon isotope excursion (CIE) associated with the PETM and ends ~20 m above onset of the recovery phase of the CIE. Thick, red paleosols in the PETM interval have been attributed to increased weathering and, thus, increased humidity during the global warming event.

Based on matrix color and redoximorphic properties, paleosol profiles can be assigned to a particular position along a paleosol continuum that ranges from well-drained to poorly-drained end members. Red profiles are the well-drained end member in the spectrum, and purple profiles are the more poorly drained end member. Transitional profiles have intermediate features and a vertical progression within the profile from red to purple matrix. Using this qualitative approach to soil moisture regime, paleosols in the main PETM interval developed under drier conditions than pre-PETM paleosols. In addition, the paleosols show that seasonality increased during the PETM. Dramatically lower water tables and increased seasonality during the global warming event are also indicated by the stratigraphic distributions of manganiferous rhizoliths and crayfish burrows, both of which are absent or less common in the PETM part of the section.

Paleosol complexity also varies through the section. In the main body of the CIE, grain size variations and complicated pedogenic features show that paleosols are dominated by multiple horizons, and profiles show evidence for overprinting of pedogenic features. In contrast, profiles directly above and below the PETM interval are relatively simple, with few subdivisions of the B horizon(s) and fewer signs of pedogenic overprinting. Avulsion deposits, which usually separate successive paleosols from one another, are less well developed in the main body of the PETM. The thin, relatively fine-grained nature of the avulsion deposits produced welded paleosols and, thus, thick red paleosols in the PETM interval. These differences suggest that sediment accumulation was slower and more episodic during the main body of the PETM interval than in deposits directly below and above. This pattern of deposition is consistent with reduced precipitation during the PETM.