FLORAL TURNOVER AND CLIMATIC CHANGES ASSOCIATED WITH THE CRETACEOUS/PALEOGENE (K/T) BOUNDARY, NORTH DAKOTA, USA

In order to improve understanding of terrestrial events at the end of the Mesozoic, fossil plants have been collected from 118 sampling horizons in a 183 m composite section that is continuous across and calibrated to the K/T boundary in the Williston Basin of North Dakota. The section covers an estimated 2.2 my and includes the predominantly Cretaceous Hell Creek and predominantly Paleocene Fort Union formations. The boundary is recognized locally by an iridium anomaly and shocked quartz, thought to be derived from the Chicxulub impact, which are associated with a palynological extinction and the loss of dinosaurs. Our data set contains two, partially overlapping components: 12,284 identified museum vouchers from 157 quarries and 11,045 identified, field-censused specimens from 40 of the quarries. The collections include 382 plant organ types, which represent an estimated 349 species and are dominated by dicot leaves, with 309 types. Of 297 Cretaceous species, 268 are not found in the Paleocene, yielding a coarse, maximum estimate of a 90% extinction.

Both ordination and stratigraphic range data show changing floral composition within the Cretaceous, with the highest significant turnover 15 m below the boundary. However, the loss of plant diversity and change in floral composition associated with the K/T boundary significantly exceeds earlier turnovers and is consistent with a single, major extinction event. There are 80 species with more than one stratigraphic occurrence in the section as a whole that either occur or range through the interval between the K/T boundary and 15 m below. Of these 80, 51 do not occur in the Paleocene, and 29 occur last at the highest well-sampled level in the Cretaceous, which is 3.6 m below the boundary, or even higher in the Cretaceous. Confidence intervals of 50% on the range tops of the 51 "victims" bracket the K/T boundary, highlighting the boundary as the most likely extinction horizon for these taxa and suggesting a Signor-Lipps smear on the range tops of the less well-sampled fraction that mostly have last appearances further below the K/T boundary. The 51 victims therefore are the most likely to have suffered a real extinction at the K/T boundary, yielding a conservative extinction estimate of 64% (51/80). Only 10 species with multiple stratigraphic occurrences have first appearances during the Paleocene.

Leaf-margin analyses based on range-through taxa estimate a cooling of 2-3 deg. C of mean annual temperature near the base of the section; warming of ~5 deg. C to ~15 deg. C into the upper 35 m of the Hell Creek; a peak of warmth near -15 m, within chron 29R, of ~18 deg. C; possible cooling in the uppermost 3 m; and a reversion to cool temperatures in the basal Paleocene of 9-12 deg. C. The temperature trends correspond well to foraminiferal data from several, latitudinally widely spaced cores, implying a linkage of terrestrial and marine climates. Paleocene plant diversity is significantly lower than Cretaceous samples from similar paleotemperatures, such that climate change is not a sufficient explanation for the plant extinctions, nor, by extension, dinosaur extinctions.