INVESTIGATING ANCIENT FORESTS USING CARBON ISOTOPE RATIOS OF FOSSIL LEAVES FROM CORRAL BLUFFS, COLORADO, USA

The bolide impact at Chicxulub 66.02 Ma resulted in a mass extinction event (the K–Pg event) that included the loss of non-avian dinosaurs and other vertebrates larger than ~10 kg in mass, as well as the loss of ~57%-75% of all plant species. The result was total ecosystem collapse in both marine and terrestrial environments. A lack of high-resolution sedimentary records of the earliest Paleogene Period (i.e., the Paleocene Epoch) has made it difficult to study how terrestrial ecosystems recovered after the K–Pg event, however discovery of a fossil-rich record of the latest Cretaceous and early Paleogene at Corral Bluffs, Colorado, USA, has provided a new window into this time period that reveals changes in in the taxonomic composition of forest ecosystems over the first 1 Ma of the Paleogene. What is less clear is the three-dimensional structure of the resulting forests (i.e., their density and relative heights) and the relation of forests composition and structure to climate conditions.

Here carbon isotope ratios (δ13C) of fossilized plant leaves from four sites at Corral Bluffs —one in the Late Cretaceous and three from the early Paleocene – are measured in order to investigate forest structure and mean annual precipitation (MAP). Several common patterns in isotopic data are observed: for each plant morphotype, δ13C ranges by 0.5 to 1.0‰; different morphotypes from the same locality have different average d13C values such that the total range in δ13C for a single locality can be as large as 4.5‰; and morphological differences in average d13C are consistent over time. The overall average d13C for fossil leaves from Corral Bluffs is -27.1‰. These results indicate MAP was over 3 m/year, and that post-extinction forests had 'non-open’ canopy structures characterized by a range in light and moisture conditions within the canopy. d13C of Corral Bluffs leaves, and estimates of MAP, are similar to those observed for tropical rainforest forests of the same age, suggesting rainforest biomes were not restricted to tropical latitudes at this time. More generally the rapid reorganization of forests after the K–Pg event to include non-open canopy structures provides essential context for understanding overall ecosystem recovery in western North America at this time.