Palynofloral Change Through the Paleocene-Eocene Thermal Maximum in the Bighorn Basin, Wyoming

To better understand the effect of the Paleocene-Eocene Thermal Maximum (PETM) on continental ecosystems, we studied 40 new palynological samples from the Bighorn Basin (BHB), northwestern Wyoming, USA. We see palm and fern abundances increase in the last 20-40 ka of the Paleocene, then dramatically with the onset of the carbon isotope excursion (CIE) defining the base of the PETM. Palynomorphs of plant groups with modern temperate climate distributions are absent from the CIE body, and this is when tropical plants are most diverse and abundant. During the CIE recovery, pollen of mesophytic/wetland plants become more common while tropical taxa persist. In the post-CIE early Eocene tropical taxa are rare and temperate forms abundant, similar to the late but not latest Paleocene. Changes in the palynoflora are more easily detected if reworked palynomorphs are removed from analyses. We interpret palynofloral changes to indicate warming in the latest Paleocene, rapid warming and drying with the CIE onset, dry tropical climates through the CIE body, a return to wetter floodplains during a very warm CIE recovery, and cooler wet conditions in the post-PETM early Eocene. These inferences are consistent with geochemical and paleobotanical proxies. Strikingly similar patterns in the palynoflora and megaflora suggest changes in vegetation were a basin-wide phenomenon. These rapid, climatically forced changes in floral composition occurred without major extinction, perhaps indicating nearby refugia in which plants adapted to cooler and wetter climates persisted through the PETM. Bighorn Basin palynofloras began shifting toward hot/dry tolerant plants during the latest Paleocene "pre-onset" carbon isotope excursionsPalynofloras from early in the carbon isotope excursion (CIE)-body were dominated by ferns and palms indicating disturbance, then later by dry tropical lineagesWetland taxa regained abundance during the CIE recovery, indicating climate-forced changes in plant distributions without major extinction