13C enrichment within Cretaceous amber as an alternate C3 plant proxy to investigate anomalous carbon isotope values observed within Cretaceous food webs

Stable carbon isotope analysis of animal tissues has commonly been used in trophic studies to determine dietary preference. Bioapatite from Late Cretaceous animals has consistently demonstrated anomalously enriched δ 13 C values for organisms living in a predominantly C 3 plant-dominated ecosystem. The origin of such enrichment is poorly understood. Initially thought to be only occurring within Dinosauria, 13 C enrichment is now observed in a wide range of Cretaceous vertebrates and thought to be a result of an unknown ecosystem-wide shift in isotopic baseline. Several factors have been suggested as potential contributors to the observed enrichment, such as diagenesis, dietary preferences, aquatic-terrestrial resource exchange, and/or environmental pressures resulting in 13 C enrichment in C 3 plants. This study focuses on improving estimates of the average δ 13 C values for Cretaceous C 3 plants by using amber stable isotope geochemistry. δ 13 C values were measured from ambers within a Dinosaur Park Formation bonebed near Unity, Saskatchewan. Our results suggest that the average δ 13 C value for C 3 plants were +1.4‰ more enriched at Unity than previously described in studies using ambient organic carbon. In addition to reporting an enriched δ 13 C amber baseline, bioapatite from the site is notably enriched compared to amber (+19.6‰, σ = 1.41 for ceratopsians and +20.3‰, σ = 1.59 for hadrosaurs). While other factors could possibly be contributing to the carbon isotope enrichment seen in Cretaceous animals, the 13 C enrichment of C 3 plants suggests atmospheric factors and/or regional environmental inputs played a large role in the recently defined positive Carbon Isotope Anomaly in Cretaceous vertebrate enamel.