Late Paleocene Warming and Floral Change in the Bighorn Basin

Oxygen isotope measurements of deep-sea benthic foraminifera show the lowest global temperatures of the Paleocene occurring near the end of the middle Paleocene about 59 Ma, followed by a rise of 2-3 °C during the last ~3 million years of the epoch (Zachos et al. 2001). This increase was followed by sudden warming of ~5 °C at the PETM. The late Paleocene warming has been correlated to the mid-Tiffanian through Clarkforkian land mammal biozones through radiometric dating and magnetic polarity stratigraphy (Secord et al. 2006).

Tiffanian through Clarkforkian floras from the Bighorn Basin of Wyoming demonstrate warming in the interior of North America coincident with that in the marine oxygen isotope record. Reconstruction of mean annual temperatures by leaf margin analysis shows a rise from ~10 °C in the late Tiffanian to ~12 °C in the early Clarkforkian to 15 °C in the late Clarkforkian. The same method implies a further increase to 20 °C during the PETM. It is interesting that the magnitude of late Paleocene warming in Bighorn Basin leaf margin analyses is approximately twice the magnitude recorded by the marine oxygen isotope record, even though the magnitude of warming at the PETM is the same in both records. This may reflect a true difference between the northern Rockies and the global record, or indicate that the leaf margin percentage increased in response to drying as well as warming. There is independent evidence from paleosols that parts of the Bighorn Basin were becoming better drained during this interval.

Floral turnover during the late Tiffanian and Clarkforkian is characterized by a large number of first appearances, a few of which are long-ranging taxa that remain in the region throughout the early Eocene.

Zachos, J. et al., 2001. Science 292:686-693.

Secord, R. et al., 2006. American Journal Science 306:211-245.