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 a test-bed of how precipitation is altered by extreme greenhouse gas warming. Here, we use paleosol bulk geochemistry methods to quantify changes in precipitation during the PETM in the Uinta Basin, Utah. We find no change in mean annual precipitation 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 continued for 105-106 years after the PETM while paleosol geochemistry returned to pre-PETM conditions almost immediately at the boundary, 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.

Reconstructing water availability in terrestrial ecosystems is key to understanding past climate and landscapes, but there are few proxies for aridity that are available for use at terrestrial sites across the Cenozoic. The isotopic composition of tooth enamel is widely used as paleoenvironmental indicator and recent work suggests the potential for using the triple oxygen isotopic composition of mammalian tooth enamel (∆’17Oenamel) as an indicator of aridity. However, the extent to which ∆’17Oenamel values vary across environments is unknown and there is no framework for evaluating past aridity using ∆’17Oenamel data. Here we present ∆’17Oenamel and δ18Oenamel values from 50 extant mammalian herbivores that vary in physiology, behavior, diet, and water-use strategy. Teeth are from sites in Africa, Europe, and North America and represent a range of environments (humid to arid) and latitudes (34S to 69N), where mean annual δ18O values of meteoric water range from -26.0‰ to 2.2‰ (VSMOW). ∆’17Oenamel values from these sites span 146 per meg (-283 to -137 per meg, where 1 per meg = 0.001‰). The observed variation in ∆’17Oenamel values increases with aridity, forming a wedged-shape pattern in a plot of aridity index vs. ∆’17Oenamel that persists regardless of region. In contrast, the plot of aridity index vs. δ18Oenamel for these same samples does not yield a distinct pattern. We use these new ∆’17Oenamel data from extant teeth to provide guidelines for using ∆’17Oenamel data from fossil teeth to assess and classify the aridity of past environments. ∆’17Oenamel values from the fossil record have the potential to be a widely used proxy for aridity without the limitations inherent to approaches that use δ18Oenamel values alone. In addition, the data presented here have implications for how ∆’17Oenamel values of large mammalian herbivores can be used in evaluations of diagenesis and past pCO2.