EVIDENCE FOR ABRUPT PALYNOFLORAL CHANGE IN CONTINENTAL SETTINGS DURING THE PALEOCENE-EOCENE THERMAL MAXIMUM

Although the impact of the Paleocene–Eocene Thermal Maximum (PETM, ~56 Ma) is relatively well constrained in marine settings, the effect on terrestrial ecosystems remains ambiguous. Megafloras from fluvial sediments in the Bighorn Basin of northern Wyoming demonstrate nearly complete extirpation of late Paleocene floodplain species at the onset of the PETM, accompanied by the appearance of numerous new taxa. These immigrants in turn disappeared from the record at the end of the PETM, and many latest Paleocene plants reappeared. In contrast to the pattern in the megaflora, palynofloras in the same sections have shown a high degree of continuity in composition, with just a few immigrant taxa. The contradictory pattern of change in palynological and megafloral remains has not been explained. Here we report on new palynological samples and analyses demonstrating that the pattern of palynofloral turnover is consistent with the megaflora. Late Paleocene palynofloral assemblages dominated by fern spores, and relatives of bald cypress, walnut, elm and birch, all temperate deciduous forest lineages, are rapidly replaced during the initial phase of the PETM body with low diversity assemblages that include palms and legumes, suggesting seasonally dry subtropical to tropical forests. With the exception of palm and legume pollen, palynofloras recovered from the later phase of the PETM body are poorly preserved. This observation, together with higher abundances of Mesozoic dinoflagellates, is consistent with an increase in reworked material likely resulting from more seasonal and extreme rainfall events and possibly an increase in autochthonous organic material degradation. The PETM recovery is characterized by an abrupt change in the palynofloras, with the return of walnut, elm and birch pollen, in addition to linden and plane tree pollen. The co-occurrence of these temperate taxa with megathermal palms and legumes suggests that substantial cooling coincided with onset of the PETM recovery phase. The return of temperate deciduous forests within the first million years of the Eocene is attributed to further cooling and an increase in precipitation. Vegetation changes, as recorded from palynofloras, therefore represent an important high-resolution archive of terrestrial ecosystem response to the PETM.