Paleohydrologic response to continental warming during the Paleocene-Eocene Thermal Maximum, Bighorn Basin, Wyoming

Geologically rapid global warming occurred during the Paleocene-Eocene Thermal Maximum (PETM) ~56Ma. Several studies have argued that important changes occurred in the hydrological cycle during the PETM, but results have been inconsistent, ranging from global increases in humidity to drier conditions. Changes in paleosols during the PETM in the southeastern Bighorn Basin document major drying during the body of the event. Paleosol changes also suggest transitional episodes of climate change that both preceded and followed the PETM. Qualitative, semi-quantitative, and fully quantitative analyses of a ~70m thick interval of paleosols provide a high-resolution record of changes in soil moisture and precipitation. Those changes are compared to changes in temperature determined from δ¹⁸O values of tooth enamel from the mammal Coryphodon. A distinct shift to drier soils occurred just prior to the PETM, a conclusion that is consistent with previous observations that warming began before the onset of the negative carbon isotope excursion associated with the PETM. Paleosols show a progressive drying trend into the lower part of the PETM and become even drier in the upper part of the body of the PETM. Purple-red paleosols that appear during the recovery phase of the PETM indicate wetter soils, although they are better drained than paleosols below the onset. The purple-red paleosols continue for ~15m above the recovery and indicate that wetter soil conditions persisted after the recovery. It is not clear whether changes in the paleosols that preceded and followed the PETM reflect global forcing factors like orbital cycles or release of carbon that lacks an isotopic label; however, such mechanisms would provide a unifying explanation for shifts seen in continental and marine environments.