PALEOENVIRONMENTAL CONTEXT OF HARMFUL ALGAL BLOOMS IN THE CRETACEOUS – EVIDENCE FROM FOSSIL MOLLUSK GEOCHEMISTRY

The Upper Cretaceous Coon Creek Formation (CCF) of western Tennessee is renowned for containing a diverse assemblage of well-preserved shallow marine fossils. At the type locality, mollusk fossils are found at multiple horizons within an ~8 m exposure of glauconitic, clayey sands deposited in a shallow marine environment. Stable carbon isotope studies of arthropod fossils and the presence of fossilized wood, along with the abundant clay, demonstrate significant input from terrestrial runoff. It is currently unknown if the strength or influence of this terrestrial input varied seasonally or on longer time scales. Recent investigation of well-preserved mollusks from the CCF reveal that some shells preserve evidence of harmful toxins, including microcystin and saxitoxin, which are commonly associated with harmful algal blooms (HABs) in aquatic habitats today.

To investigate links between terrestrial input, nutrient flux, and HABs, we collected mollusk shells, sediments, and microfossils from multiple horizons within the CCF for geochemical and algal toxin analyses. Scanning electron microscope imaging of the shells confirmed that they are exceptionally well-preserved and consist of primary aragonite prisms and fibers. Shells were cut along their growth axes and polished to develop multi-year records of changes in shell chemistry using LA-ICP-MS, targeting elements associated with terrestrial influx, including phosphorus and barium. Further, we compare the results from Coon Creek shells with modern mollusk shells from sites known to be influenced by HABs. Microfossils confirm a Late Cretaceous (Campanian) age for the site and include a combination of warm and cool-water calcareous nannoplankton, as well as dinoflagellates. Terrestrial runoff and excess nutrients in modern coastal settings are associated with the development of HABs. Fossils from Coon Creek will shed light on the presence of HABs during a prior greenhouse climate and target whether terrestrial runoff and seasonal temperature fluctuations might have promoted or inhibited the development of HABs in the past.