SHE ANALYSIS OF MIOCENE BENTHIC FORAMINIFERA, CALVERT CLIFFS, MARYLAND

The Miocene Climatic Optimum (17–14.8 Ma; MCO) was a period in Earth’s climate history when global mean temperatures rose 2–8 °C. The subsequent Middle Miocene Climate Transition (14.7–13.8 Ma; MMCT) was a cooling period when global mean temperatures fell 2–8 °C. Miocene-aged sediments spanning both climatic events are exposed along Calvert Cliffs, Maryland. They consist of the basal Calvert, Choptank, and St. Marys formations and are subdivided into 24 beds known as Shattuck Zones, based on lithology and faunal content. This study uses benthic foraminifera from beds 4–9 (considered as a single unit) through bed 18 from the Calvert and Choptank formations to document changes in communities across the MCO and MMCT to better understand the impact of these climatic events on shallow shelf environments. SHE analysis (number of species, S, the information function, H, and the measure of evenness, E) of the benthic foraminifera identified four biofacies. Biofacies 1 is composed of samples from beds 4–9 through 11 and is dominated by Valvulineria floridana (24.8%), Bulimina elongata (13.6%), and Cibicides americanus (9.6%). Biofacies 2 is composed of samples from beds 12 and 13 and are dominated by Valvulineria floridana (38.2%), Bulimina elongata (15.1%), and Hanzawaia concentrica (7.1%). Biofacies 1 and 2 were deposited within the MCO and are interpreted to have been deposited in a middle to inner shelf environment. Biofacies 3 is composed of samples from bed 14 and the lower part of bed 15 and is dominated by Cibicides americanus (17.9%), Valvulineria floridana (15.6%), and Trochulina bassleri (13.6%). Biofacies 4 is composed of samples from the upper part of bed 15 through bed 17. This unit is dominated by Cibicides americanus (29.4%), Bolivina paula (22.0%), and Buliminella elegantissimia (10.0%). Biofacies 3 and 4 were deposited within the MMCT and the benthic foraminiferal assemblages record a shallower shelf environment than those deposited during the MCO. Thus SHE analysis differentiates benthic foraminifera communities from the MCO and MMCT and shows that there were environmental changes within each of these climatic episodes. These changes are most likely due to relative (and likely global) sea-level rise across the MCO and subsequent fall during the MMCT.