USING CARBON ISOTOPE RATIOS TO STUDY FOREST SOILS AND CANOPIES OF THE LATE CRETACEOUS IN SOUTHERN UTAH

Terrestrial ecosystems of the Late Cretaceous were both complex and fundamentally different than those of the present day. Those occupying wet coastal floodplains of western North America were characterized by a diverse association of vertebrates, including a number of herbivorous dinosaurs, and by plant communities that included a mix of ferns, cycads, conifers and angiosperms. The density and height of vegetation on the landscape, and the role these forest characteristics may have played in supporting these ecosystems, is not well known.

In this study, carbon isotope-based proxies are used to study soil processes and to infer forest structure over a late Cretaceous landscape in southern Utah. It is found that methanogenesis took place in water-logged soils and resulted in the formation of CO₂ with very high carbon isotope ratios. Furthermore, vegetation in some parts of the forest was dense enough and tall enough to allow for recycling of this CO₂ into lower canopy leaves. Lastly, carbon isotope ratios of hadrosaurid tooth enamel suggests these animals had variable habitat preferences, and were restricted in their movement.

Recognition that methane formation can take place in soils associated with high water tables has paleoclimatic implications, as methane plays an important role in maintaining of hothouse climate conditions. Growth of a mix of open and closed canopy forests in these soils points to the influence of surface hydrology on forest structure. Similarly, the common occurrence of angiosperms in these forests reflects their role in the development of this type of vegetation structure as far back as the late Cretaceous. These forests appear to have provided a wide variety of ecological niches for vertebrates, with some of the larger taxa (e.g. hadrosaurid dinosaurs) having specific habitat preferences.