PALEOHYDROLOGIC RESPONSE TO CONTINENTAL WARMING: PALEOSOLS ACROSS THE PALEOCENE-EOCENE THERMAL MAXIMUM, BIGHORN BASIN, WYOMING (Invited Presentation)

Geologically rapid global warming occurred during the Paleocene-Eocene Thermal Maximum (PETM) ~56 Ma. 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. Here we report major drying during the body of the PETM using a paleosol record in the southeastern Bighorn Basin. Paleosol changes also suggest more prolonged climate change with transitions that both precede and follow the PETM. The CALMAG method (Nordt and Driese, 2010) and a morphologic soil index were used to establish a high resolution record of changes in mean annual precipitation (MAP) and soil moisture through a ~70 m interval of paleosols. Those changes are compared with relative changes in mean annual temperature determined from δ¹⁸O values of tooth enamel from the mammal Coryphodon. In addition, mean annual precipitation (MAP) was estimated using floras from three stratigraphic levels. Paleosols of latest Paleocene age are characterized by features indicating very wet conditions. A distinct shift to drier soils occurred just prior to the onset of the carbon isotope excursion (CIE) associated with the PETM. The observation that climate change directly preceded the CIE is consistent with some previous marine and continental studies. Up-section changes in the morphologic and geochemical properties of the paleosols show a progressive drying trend into the lower part of the PETM. Even drier conditions are indicated by calcareous paleosols in the upper part of the PETM body, when temperatures were warmest. Purple-red, non-calcareous paleosols appear during the recovery phase of the PETM and indicate wetter soils and higher MAP, although the purple-red paleosols are better drained than those just below the PETM onset. The purple-red paleosols continue for ~15 m 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.