A CLUMPED ISOTOPE HYDROCLIMATOLOGIC RECORD THROUGH THE PALEOCENE EOCENE THERMAL MAXIMUM IN SOUTHWESTERN NORTH AMERICA

One of the key unresolved questions regarding early Paleogene hyperthermals concerns the response of the hydrologic cycle to abrupt global warming. Previous studies have produced conflicting evidence for increased or decreased moisture, or possibly that precipitation became more strongly seasonal in some areas during the PETM. To address the subject of PETM hydrologic change, we conducted clumped isotope measurements of carbonate nodules in a paleosol sequence spanning the Paleocene-Eocene boundary, in the North Horn Formation of central Utah. This analysis provides soil temperatures during nodule formation, while also allowing for determination of the oxygen isotope ratios of soil water with which the nodules equilibrated. Temperature data show soil warming of ~15-20°C during the carbon isotope excursion, compared to pre-PETM minima. Temperatures only partially recover post-PETM, to ~10°C cooler than the maximum. Pre- and post-PETM soil water oxygen isotope ratios are more similar, with PETM soil water d¹⁸O enriched by 5‰. Common to both the temperature and oxygen data are a distinct double-peaked profile, with a secondary maximum occurring roughly 40 meters below the PETM maximum. Based on long-term accumulation rates derived from magnetostratigraphy, this secondary climate event preceded the PETM by 0.5-1.2 million years.