TRANSIENT DRYING DURING THE PALEOCENE-EOCENE THERMAL MAXIMUM (PETM): ANALYSIS OF PALEOSOLS IN THE BIGHORN BASIN, WYOMING

Alluvial paleosols in the Bighorn Basin that span the PETM interval contain a continuous and highly resolved record of climate including information on precipitation. Paleosols in sections at McDermotts Butte and Polecat Bench show a significant but transient drying at the onset of the PETM and a gradual return to pre-PETM moisture levels by the end of the interval.

The presence of carbonate-bearing, red paleosols and soil weathering indices indicate that soils became markedly drier shortly after the PETM began. At Polecat Bench, where temporal resolution is better, a 6 meter interval that just follows the peak isotope excursion appears to have been driest. This interval nearly coincides with the first definite Wa0 faunas. A change to less well drained yellow-brown and purple paleosols in the upper parts of both sections indicates a return to more humid climates as temperatures declined at the end of the PETM. Wetter soils are also indicated by the disappearance of carbonate accumulations at McDermotts Butte and their lesser abundance at Polecat Bench. Soil weathering indices also indicate a return to wetter soils.

The fact that both sections show similar vertical changes confirms that a change in regional climate was the major control on up-section changes in soil moisture rather than local influences on soil wetness such as floodplain topography. Despite the primary control of climate, differences between the two study sites suggest that a secondary control on soil drainage was permeability of the parent material. Paleosols at Polecat Bench, which developed on coarser parent materials than those at McDermotts Butte, have morphologic and geochemical features indicating that they were somewhat drier than those at McDermotts Butte.

This study counters interpretations of wetter climates for Wyoming at this time and shows the importance of detailed case studies of continental strata to test climatic generalizations and models that have been developed for PETM precipitation patterns. In addition, because the paleosols are vertically stacked, a relatively transient regional drying that is recorded by only ~8 m of section can be discerned as can the gradual return to more humid conditions by the end of the PETM.