A fossil-rich rock formation at the Cretaceous-Paleogene Boundary in Mississippi, USA indicates environmental changes before mass extinction

A fossiliferous spherule-rich bed at the Cretaceous–Paleogene (K–Pg) boundary in Mississippi, USA: Implications for the K–Pg mass extinction event in the Mississippi Embayment and Eastern Gulf Coastal Plain

James D.Witts, Neil H.Landman, Matthew P. Garb, Caitlin Boas, Ekaterina Larina, Remy Rovelli, Lucy E. Edwards, Robert M.Sherrell, J. KirkCochran

Summarized by Mckenna Dyjak. Mckenna Dyjak, who is an environmental science major with a minor in geology at the University of South Florida. She plans to go to graduate school for coastal geology; once she earns her degree, she plans on becoming a research professor at a university. Mckenna spends her free time playing the piano and going to the gym.

What data were used? A fossil and spherule-rich rock formation in Union County, Mississippi exposed by construction. The formation contains the Cretaceous-Paleogene (K-Pg) boundary, which marks the end of the Cretaceous and the beginning of the Paleogene, estimated at ~66 million years ago. This boundary is characterized by a thin layer of sediment with high levels of iridium which is uncommon in Earth’s crust, because it is almost exclusively from extraterrestrial sources.  The K-Pg boundary is associated with a mass extinction: a significant, widespread increase in extinction (ending of a lineage) of multiple species over a short amount of geologic time. The iridium indicates that the extinction was likely caused by an extraterrestrial impact; the spherules found support this idea as well, as spherules are formed from ejecta after an impact. 

Stratigraphic (ordered) section of the rock formation showing the rock units, type of
sedimentology (sand, silt, clay), and fossil type. The K-Pg boundary is marked by the horizontal
dashed line. The black arrows point to calcareous nannofossils and the white arrows point to
dinoflagellate cysts.

Methods: The fossils present in the rock formation were identified and compiled into a complete list. In order to find out the composition of the rock formation. 14 sediment samples were collected; these samples were used to construct a biostratigraphic analysis: corresponding relative rock ages of different rock layers to the fossils found within them. The mineral composition and grain size were determined to construct this analysis. The mineral composition (mineral percentages present) of the sediment samples were determined by using a Scanning Electron Microscope (SEM) and a Diffractometer (type of X-ray). The grain size analysis of the sediment samples was determined by using a sieve (mesh strainer) to sort into different sizes. The Carbon-13 levels of the sediment samples were analyzed: Carbon-13 can be used to determine the amount of plants that were present at the time.The data collected was used to construct the stratigraphic section shown in the figure below.

Results: There was a significant decrease in the amount of micro and macro fossils present. Along with the decrease of fossils there was a positive shift of Carbon-13. The positive shift of Carbon-13 indicates that there was an increase in plant matter buried in the rock record. Sedimentary structures such as weak cross-bedding and laminations (indicates flowing water and fluctuating energy levels) An important layer was analyzed: 15–30 cm thick muddy, poorly sorted sand containing abundant spherules (sphere pieces) that were likely a product of  the Chicxulub impact event.

Why is this study important? The findings suggest that there was a quick, local change in sediment supply and possibly sea level due to the significant variation in facies (body of sediment), fossil changes, and different geochemical data that coincided with the extinction event. 

Big Picture: This study helps us understand how different areas were affected locally before the mass extinction event, which can help us understand how recovery from mass extinctions take place. 

Citation: Witts, James, et al. “A Fossiliferous Spherule-Rich Bed at the Cretaceous-Paleogene (K-Pg) Boundary in Mississippi, USA: Implications for the K-Pg Mass Extinction Event in the MS Embayment and Eastern Gulf Coastal Plain.” 2018, doi:10.31223/osf.io/qgaj