Reduced mean annual precipitation immediately after the onset of the Paleocene-Eocene Thermal Maximum in the Bighorn Basin (Wyoming, USA)

The Paleocene-Eocene Thermal Maximum (PETM) is used as a geological analogue for contemporary global warming driven by anthropogenic greenhouse gas emissions. Changes in the hydrological cycle during the PETM are debatable with large differences among geographical regions and as a function of proximity to the oceans. Here, we analyze multiple paleosol profiles through the PETM in the Bighorn Basin, Wyoming, located at mid-latitudes in the continental interior of North America. Temporally uniform detrital proxies (i.e., TiO₂/Al₂O₃, Zr/Hf, Nb/Ta and rare earth elements) indicate a stable source of detrital material before and during this event. However, paleosol profiles immediately following onset of the carbon isotope excursion (CIE) at the beginning of the PETM have significantly lower mean annual precipitation values. The chemical index of alteration (CIA) and K₂O/Al₂O₃ ratio indicate that chemical weathering during the early PETM phase was weaker than that in pre-PETM period and during the PETM recovery stage. Increasing coexistence of beidellite with montmorillonite as the main authigenic clay mineral points to more alkaline conditions, consistent with reduced precipitation and decreased intensity of chemical weathering. Our results make clear that precipitation was reduced with the onset of the CIE. Lower precipitation at the beginning of the PETM may have been a regional climatic feature specific to the Bighorn Basin or a more widespread response to contemporaneous global warming.