High-resolution quantitative biostratigraphy of planktonic foraminifera from the Early Miocene (ODP Leg 154 Site 926, Ceara Rise, Equatorial Atlantic Ocean)

During the Early Miocene, the biostratigraphic resolution of planktonic foraminiferal zonations is relatively low, compared to the younger intervals of Neogene, and high-resolution quantitative records of planktonic foraminifera are scarce, especially at low latitudes. In addition, there is an ongoing debate regarding which bioevents best approximate the Aquitanian/Burdigalian boundary. In absence of a formal definition, the Base of the calcareous nannofossil Helicosphaera ampliaperta (dated at Ceara Rise at 20.43 Ma) has been provisionally adopted to mark the base of the Burdigalian Stage (e.g., Raffi et al., 2020). Here, the results of a high-resolution quantitative analysis of planktonic foraminiferal assemblages from Site 926 (ODP Leg 154, Ceara Rise) for the interval from 21.5 to 17.6 Ma are presented. Following the astronomically calibrated composite section of Site 926 (Shackleton et al., 1999; Pälike et al., 2006), samples were selected from nannofossil-foraminiferal chalk levels with a time resolution of ~30 kyr. This study provides the quantitative distribution patterns of the biostratigraphic marker species of the studied interval and allows the refinement of the stratigraphic position and age of the main bioevents. Age calibration of some identified bioevents, such as Top (T) Paragloborotalia pseudokugleri, Base (B) Globigerinoides altiaperturus, B Globigerinatella sp. and B Globorotalia praescitula, is in good agreement with the current biochronology (Raffi et al., 2020). Conversely, T P. kugleri appears younger than what reported in literature, postdating B H. ampliaperta. Notably, the age calibration of T P. kugleri and B G. altiaperturus closely aligns with the age of the top of magnetic polarity Chron C6An (19.97 Ma), possibly offering new perspectives on the selection of criteria for defining the base of the Burdigalian Stage. Further identified events, Top common (Tc) Paragloborotalia kugleri, Tc Tenuitella munda, T Globorotalia mendacis and T Ciperoella fariasi, enhance the biostratigraphic resolution of the studied interval. Finally, quantitative records of other biostratigraphic markers, such as Catapsydrax dissimilis, Paragloborotalia siakensis, P. birnageae, P. nana, Trilobatus trilobus gr. and Globigerinoides parawoodi gr., reveal details of their distribution pattern within the studied interval. Biostratigraphic correlation of Site 926 with other low-latitude (I)ODP Sites is hampered by inconsistencies in the relative stratigraphic positions of T P. pseudokugleri, T P. kugleri and calcareous nannofossil events, partly due to different resolution of the biostratigraphic analysis and partly due to the absence of important events (e.g., B Globigerinatella sp., B H. ampliaperta, Helicosphaera euphratis/H. carteri crossover) at some of the considered sites. Overall, these results may contribute to improving the biostratigraphic resolution and the accuracy of Early Miocene standard zonation.