Isotope paleo-analysis of freshwater bivalves and implications on paleoenvironment bounding the K-Pg extinction

The K-Pg extinction is the most documented of the five major extinction events and its cause is well understood to be a large extraterrestrial impact. Despite the numerous studies, several questions still remain, such as whether climatic changes prior to the extinction event weakened the ecosystem, and how long it took the environment to recover after the impact. Additional independent proxies could provide clues to past environments and help gain a better understanding of extinction events. Freshwater bivalves utilize dissolved inorganic carbon (DIC) to form their shells and are therefore a great marker of environmental information which can be garnered from carbon isotope values. A large-scale literature study reveals that the carbon isotopic value of DIC in modern streams somewhat correlates to climate, although the signal is also influenced by other factors. This study describes a new approach which allows for the first ever minimum estimate of δ13CDIC in ancient streams by setting the portion of metabolic carbon in DIC to zero. A total of 11 shells collected in Montana, five from the Late Cretaceous Hell Creek formation as well as six from the Early Paleogene Fort Union formation, were analyzed for carbon isotopes. Calculated δ13C DIC values ranged from -11.44 to 2.06‰, which suggests arid or temperate climatic conditions. Currently, based on past atmospheric CO2 concentrations, the climatic conditions of this region are assumed to be tropical, yet our values disagree. Directly interpreting these values has its limits however, since processes such as carbonate weathering, atmospheric equilibration, and high productivity could result in higher values. Despite these issues which need to be resolved, this method provides the possibility to learn more about the paleoenvironmental conditions bounding the K-Pg extinction than previously possible.