Long-Term Decoupling of Precipitation Extremes from Mean Annual Precipitation During Repeated Early Paleogene Hyperthermals in the North American Mid-Latitudes

Abstract. The early Paleogene hyperthermals, including the Paleocene-Eocene Thermal Maximum and the hyperthermals of the Early Eocene Climatic Optimum, were the warmest periods of the Cenozoic Era. Due to the similar continental configuration and drivers of extreme warmth, this period serves as an analogue for how precipitation is altered by extreme warming driven by greenhouse gases. Through high resolution geochronology and construction of a bulk organic carbon isotope curve, we identify up to 11 different hyperthermals in the Uinta Basin, Utah, adding to the small number of terrestrial sites where the lower magnitude Paleocene and Eocene hyperthermals have been recognized. We use paleosol bulk geochemistry methods to quantify changes in precipitation during these extreme warming events. We find no significant changes in mean annual precipitation during the warming events. However, paleosol mass balance results track increased clay illuviation, accumulation of redox-sensitive elements, and carbonate leaching during many of these events. These results, along with shifts in fluvial stratigraphy, provide evidence for increased intensity and seasonality or intermittency of precipitation that may be related to poleward shifts in global circulation. These results are compared to the state-of-the-art DeepMIP model ensemble, composed of the same models used for future climate simulations. The model ensemble overestimates mean annual precipitation and underestimates the seasonality or intermittency of precipitation compared to this proxy record. These differences may be a function of the coarse model resolution, missing processes, or incorrect boundary conditions that should be investigated further.