Paleosol‐Based Reconstruction Indicates Decoupling of Mean Annual Precipitation and Precipitation Intensity During the Paleocene‐Eocene Thermal Maximum in the Uinta Basin, Utah.

The Earth is transitioning to a state unprecedented in human history. This transition poses a challenge for predicting the future, as climate models require testing and calibration with real‐world data from high greenhouse gas climates. Despite significant progress in climate modeling, changes in the precipitation remain highly uncertain. The Paleocene‐Eocene Thermal Maximum (PETM) was the warmest period of the Cenozoic Era, and thus serves as an analog for a hydrological cycle altered by extreme greenhouse gas warming. Here, we use paleosol‐based geochemical proxies to quantify changes in mean annual precipitation (MAP) during the PETM in the Uinta Basin, Utah. We find no change in MAP during this warming event. However, paleosol mass balance results track increased translocation of carbonates, increased clay illuviation, and increased accumulation of redox‐sensitive elements. These results, along with shifts in fluvial stratigraphy, provide evidence for increased intensity and intermittency of extreme precipitation events that may be related to changes in the transport direction, seasonality, and moisture transport capability of the North American Monsoon. Surprisingly, changes in fluvial stratigraphy, clay illuviuation, and redoximorphy continued for 105–106 years after the PETM, suggesting persistent changes in precipitation intensity despite a decrease in global temperature. These findings provide further support for an intensification of the hydrological cycle during and after the PETM, provide evidence for a decoupling between mean and extreme precipitation, and indicate the importance of multi‐proxy, regional studies for understanding the complexities of climate change. Plain Language Summary: Warmer air can hold more moisture, which will lead to increased precipitation and extreme rainfall events with human‐induced global warming. However, this moisture will not be distributed evenly spatially or temporally, leading to large uncertainties in our predictions of the future. To help resolve these questions, we turn to the geological record where changes in response to dramatic warming events are stored. One of these dramatic global warming events, the Paleocene‐Eocene Thermal Maximum (PETM), occurred about 56 million years ago and provides an analogue for future greenhouse gas induced warming. Here, we used the chemical composition of ancient soils to study changes in precipitation in central Utah during the PETM. We find that there was no change in MAP, but there is evidence for more intense rainfall. Despite a decrease in global temperatures after the PETM, extreme rainfall events persisted. Based on climate model results and a regional comparison to other locations during the PETM, these changes may be related to changes in the path and seasonality of the North American Monsoon in a warmer world. These findings have implications for understanding of how moisture will be distributed in the coming decades and centuries. Key Points: Soil geochemistry indicates no change in mean annual precipitation in the Uinta Basin during the Paleocene‐Eocene Thermal Maximum (PETM)The PETM was marked by more intense and intermittent extreme rainfall events, possibly connected to changes in the North American MonsoonClimate proxies track persistent change in precipitation intensity and intermittency for 105–106 years after the PETM [ABSTRACT FROM AUTHOR]