Evaluation of paleosol paleoprecipitation proxies during the Paleocene-Eocene Thermal Maximum, Bighorn Basin, Wyoming, U.S.A

The Paleocene-Eocene Thermal Maximum (PETM) was a short-lived period of dramatic global warming. Because the cause of warming has been attributed to a rapid, massive addition of greenhouse gases to the atmosphere, the PETM serves as an analogue to current warming. Although the amount of warming during the PETM has been well-documented, changes of other climate factors such as precipitation are less resolved. Estimates of precipitation during the PETM vary widely and may be contradictory. Resolving potential precipitation changes is critical in the context of modern global warming because precipitation changes may have a greater impact on human welfare than temperature.;PETM paleosols exposed in the Bighorn Basin of Wyoming contain morphologic and geochemical features that reflect changes in paleohydrology. Although these sediments represent an enormous amount of data from the PETM, further testing of quantitative proxies is necessary to ensure accurate estimates of paleoprecipitation. Analysis of the Chemical Index of Alteration minus Potassium (CIA-K) paleoprecipitiation proxy indicates that this method should only be used in vertic paleosols that had reached steady-state with respect to base cation leaching during pedogenesis. Recognition of steady-state base cation conditions also indicates that use of the CIA-K method may be limited by an upper precipitation level. A field sampling strategy for CIA-K is also introduced.;Results from CIA-K are compared to results from another paleosol-based paleoprecipitiation proxy that utilizes iron-manganese nodules and precipitation estimates derived from the only existing PETM paleobotanical collections. Up-section changes in soil morphology and proxy results at Cabin Fork are similar to other areas of the Bighorn Basin. These results suggest that the basin-wide drying observed at the beginning of the PETM is driven by climate change, not local drainage variations. The northern end of the basin was dryer than the southern. Inconsistent agreement between quantitative proxies indicates that further testing of proxies in modern soils is necessary to provide more exact estimates of paleoprecipitiation. With further evaluation of quantitative methods, paleosols will be able to provide paleoclimatological data at higher spatial and temporal resolution than other sources.