PALEOPRECIPITATION TRENDS ACROSS THE PALEOCENE-EOCENE THERMAL MAXIMUM OF CABIN FORK, BIGHORN BASIN, WYOMING

The Paleocene-Eocene Thermal Maximum (PETM) was a transient period of global warming associated with a negative carbon-isotope excursion (CIE) that has been documented in marine and terrestrial records. Temperature shifts associated with the PETM have been well documented, but effects on precipitation are poorly understood on all geographic scales. One approach to clarifying paleoprecipitation during this episode is to study alluvial paleosols in a continental PETM section. Not only do paleosols record climatic setting during their formation, but, because they are vertically stacked, they provide a continuous record of paleoclimate and paleoclimate change. Using the climofunction developed by Sheldon et al. (2002), Mean Annual Precipitation (MAP) has been estimated for a vertical succession of paleosols in the Cabin Fork area of the southern Bighorn Basin, Wyoming, where the PETM interval is about 45 m thick.

Onset of the PETM at Cabin Fork is characterized by the appearance of better drained soils, as determined by MAP estimates and soil morphologies. The lowest MAP values are found 15 m above the beginning of the PETM. Pedogenic carbonate nodules, characteristic of drier soils, are also more prevalent at this level than elsewhere at Cabin Fork. Above the 15 m level and throughout the remainder of the PETM interval, the MAP values are higher and paleosol morphologies indicate less well drained paleosols. However, later PETM soils indicate drier conditions than pre-PETM soils from the same area. The general trend of initial drying at the onset of the PETM, followed by a peak in dryness about a third of the way through the PETM interval, and a return to wetter conditions in the upper PETM interval is consistent with observations from paleofloral estimates from Cabin Fork. This pattern is also similar to that determined for Polecat Bench, in the northern Bighorn Basin. However, comparison of MAP values indicates that the Cabin Fork area was wetter overall than Polecat Bench during the PETM.