REWORKED POLLEN REDUCES APPARENT FLORAL CHANGE DURING THE PALEOCENE-EOCENE THERMAL MAXIMUM

Paleobotanical research on Paleocene-Eocene Thermal Maximum (PETM) megafloras in the Bighorn Basin, USA, has emphasized that their composition is radically different from those immediately before and after the PETM. Palynofloral change in the same studied sections was, in contrast, subtle. To investigate this enigmatic discrepancy, we employ novel species-specific pollen stable carbon isotope and traditional palynological analysis on numerous PETM successions. The δ¹³C_pollen of PETM grains with temperate affinity are similar to pre-PETM grains of the same taxon, whereas the δ¹³C_pollen of PETM grains with tropical affinity are ~4‰ lighter than the same taxon before the PETM. Pollen taxa of temperate affinity are physically deteriorated during the PETM, whereas pollen taxa of tropical affinity are pristine. We propose that the deteriorated Cretaceous-Paleocene grains with consistent δ¹³C_pollen across the PETM are reworked. The profound increase in reworked palynofloras during the PETM is attributed to the sudden onset of more seasonal precipitation which led to enhanced erosion of terrestrial material. This previously unrecognized reworking reveals that turnover in palynofloras is higher than previously thought, consistent with megafloral evidence that temperate plants were locally extirpated throughout the Bighorn Basin during the body of the PETM. Extensive reworking may affect organic microfossils during other intervals of major climate change as well.