Effects of Volcanic Ash Deposition and the Manson Impact on Marine Paleoredox and Paleoproductivity: Geochemical Evidence from the Cretaceous Pierre Shale

Effects of Volcanic Ash Deposition and the Manson Impact on Marine Paleoredox and Paleoproductivity: Geochemical Evidence from the Cretaceous Pierre Shale Isabella Cross-Najafi The Pierre Shale Group is a massive black shale deposited in the Late Campanian during the Cretaceous Period. There is limited research on organic carbon content of the Pierre Shale in South Dakota. Frequent volcanic eruptions combined with climate change resulted in an increase in carbon dioxide in the atmosphere, leading to decreases in marine oxygen content. Decreasing marine oxygen has been attributed to higher amounts of preserved organic matter in marine sediment. Impact of volcanic ash deposition in the Cretaceous Interior Seaway has not been thoroughly studied. The Pierre Shale also contains the Crow Creek Member, a 5 foot thick layer of unconsolidated sand and rip-up clasts which may indicate a high-energy depositional event. Some hypothesize that it was deposited by a tsunami generated by the Manson impact. Others believe the Crow Creek Member is evidence of a marine low-stand that occurred before the Bearpaw Cyclothem. It is possible that the depositional event that deposited the Crow Creek Member may have led to increases in organic carbon preservation depending on the burial rates and amount of organic carbon preserved. To investigate the connection between volcanic ash deposition, the Crow Creek Member deposition, and organic matter preservation of the coastal Cretaceous Interior Seaway, stable isotope geochemistry, trace element geochemistry, and total organic carbon analyses were performed on a 500 foot core drilled near Fort Pierre, South Dakota. Ash beds were identified using X-ray diffraction analysis. Core sampling was driven by location of the Crow Creek Member (above below and within one foot) and by location of ash beds (above below and within one inch), but samples were also taken based on highest and lowest gamma ray values for each five foot (1.52m) core segment. Core sampling was restricted because every other five foot (1.52 meter) section of the Treedam core segment was available for sampling. Statistical T-tests and Ztests were performed on sample data to determine if there was a significant difference in geochemical signatures between core deposited before and after ash bed deposition and Crow Creek Member deposition. Results and T and Z statistical analyses show no significant changes in stable isotopes nor trace elements as a result of ash bed deposition nor the Crow Creek Member depositional event. Results also indicate that variability of the coastal brackish marine system made any significant trends harder to isolate on such a small scale. Overall δ C 13 org signatures ( -27 to -26 ‰) indicate that the Cretaceous Interior Seaway was deposited in a brackish shallow marine environment and that there were no drastic changes in sea level throughout the deposition of the Pierre Shale Group that was sampled (Gregory Member up through Virgin Creek Member). The δ N 15 data range (-6 to +1 ‰) show that fixed nitrogen was scarce during the deposition of the Pierre Shale and that most of the available marine nitrogen was likely fixed by cyanobacteria.