FOCUSING ON THE FLOODPLAIN: CYCLING OF CARBON AND STRUCTURE OF FORESTS IN WESTERN NORTH AMERICA DURING THE LATE CRETACEOUS

Extensive terrestrial deposits of Late Cretaceous age provide a window into this greenhouse world that has allowed for detailed descriptions of past environments and ecosystems. In western North America for example, low-relief river and floodplain environments were located in basins to the east of Sevier Orogeny highlands and west of the Western Interior Seaway (WIS), with these environments extending north-south from Alaska to Mexico. Living in these river and coastal areas was a very diverse assemblage of plants and animals, including the most diverse dinosaur ecosystems described to date.

Despite all that is known about the Late Cretaceous in western North America, there are some critical questions that remain unanswered. Considering climate, there is the question of how floodplains and coastal environments are linked to the global carbon cycle, and what role they play in maintaining greenhouse climate conditions. Focusing on ecosystems, a major question is how these forests were able to support such diverse associations of animals, particularly large herbivorous dinosaurs, given the absence of evidence for migratory behaviors.

In this study, stable isotope ratios (δ¹³C and δ¹⁸O) of hadrosaurid dinosaur teeth collected from a number of Campanian-aged localities along the WIS are used to investigate these questions. Isotope ratios of tooth dentine are altered by diagenetic processes taking place in soils/sediments, and thus provide information on these processes, including those involving carbon. In contrast, isotope ratios of tooth enamel preserve primary biological signals, and can be used to study animal diets, which in the case of hadrosaurids are trees of the floodplain forest.

High δ¹³C of dentine (> +5 ‰) indicates that CH₄ production took place in certain soils on the floodplain and that this CH₄ was emitted to the atmosphere. Such emission likely played an important role as a climate feedback that helped maintain greenhouse conditions during the Late Cretaceous. Offsets in δ¹³C of enamel for co-existing populations of hadrosaurids, including populations with high δ¹³C (> -6 ‰), provide evidence for both dietary niche partitioning and for the recycling of soil-derived carbon by overlying vegetation. In turn, the latter indicates that closed-forest canopies were present on the floodplain.