Trace Element and Stable Isotope Records Reveal Seasonal and Episodic Environmental Fluctuations in the Upper Cretaceous Coon Creek Formation

The Upper Cretaceous Coon Creek Formation (CCF) of western Tennessee is renowned for its diverse assemblage of exceptionally well-preserved shallow marine fossils, particularly shelled mollusks. Microfossils, including calcareous nannoplankton and dinoflagellates, confirm a late Campanian age for the site and indicate both warm- and cool-water taxa, suggesting shifting climatic conditions recorded within the sedimentary sequence. The formation, consisting of approximately 8 meters of glauconitic, clayey sands, was deposited in a shallow marine environment with significant terrestrial input, as evidenced by stable carbon isotope studies of arthropod fossils, fossilized wood, and abundant clay. However, the variability in the strength or influence of this terrestrial input over seasonal or longer timescales remains unclear. Recent investigations have revealed that some mollusk shells from the CCF may preserve evidence of harmful toxins, such as brevetoxin and saxitoxin, which are commonly associated with harmful algal blooms (HABs) in modern aquatic environments.

To further explore the relationship between paleoenvironmental parameters and HABs, we collected mollusk shells, sediments, and microfossils from multiple horizons within the CCF for geochemical and algal toxin analyses. SEM imaging confirmed the exceptional preservation of primary aragonite prisms and fibers in the fossilized shells. Using LA-ICP-MS, trace elements such as phosphorus, barium, magnesium, and strontium were measured along shell growth axes. Multiple transects were conducted at different points in the same shell to assess the reproducibility of trace element time series within a single specimen. Additionally, stable isotope analyses (δ¹⁸O and δ¹³C) were performed parallel to the trace element analyses to correlate these records within each shell.

The trace element and stable isotope records within CCF bivalves exhibit cyclic variations, suggesting seasonal environmental changes within this warm coastal ecosystem. Temperature estimates derived from δ¹⁸O are concordant with previous estimates and align with other regional records. Some trace elements, including barium and phosphorus, display transient spikes that may reflect episodes of increased terrestrial input and/or algal blooms, although direct evidence for these events remains limited. By investigating the chemistry of these ancient mollusks and comparing them to modern environments, this study aims to shed light on the potential occurrence of HABs during a prior greenhouse climate and evaluate the role of terrestrial runoff in influencing nutrient cycles and marine ecosystem health. These results provide valuable insights into the environmental and climatic conditions of the Cretaceous and contribute to a broader understanding of how terrestrial input may have driven nutrient dynamics and the development of algal blooms in shallow marine settings.