High-resolution quantitative biostratigraphy of planktonic foraminifera across the Aquitanian/Burdigalian boundary at low latitudes (ODP Leg 154 Site 926)

Biostratigraphic resolution of the Early Miocene planktonic foraminiferal zonations is relatively low compared to younger Neogene intervals, and high-resolution quantitative records of planktonic foraminifera are scarce. Additionally, there is an ongoing debate about which bioevents best approximate the Aquitanian/Burdigalian boundary, although the Base of the calcareous nannofossil Helicosphaera ampliaperta (20.43 Ma) has been provisionally adopted to mark the base of the Burdigalian Stage. Here, the results of a high-resolution quantitative analysis of planktonic foraminiferal assemblages from Site 926 (ODP Leg 154, Ceara Rise, equatorial Atlantic Ocean) for the interval from 21.5 to 17.6 Ma are presented. Following the astronomically calibrated composite section of Site 926, 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 s.l., B Globigerinatella sp. and B Globorotalia praescitula, is nearly consistent with the current biochronology. Conversely, T P. kugleri, which defines the M1b/M2 zonal boundary, is younger than what is reported in literature and postdates B H. ampliaperta. Notably, the age calibration of T P. kugleri and B G. altiaperturus s.l. is close to 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 Paragloborotalia 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 neoparawoodi/G. joli i gr., reveal details of their distribution patterns within the studied interval. Biostratigraphic correlation of Site 926 with other low-latitude (I)ODP Sites is hindered 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) at some of the considered sites. Overall, the high-resolution planktonic foraminiferal quantitative record of Site 926 obtained in this study may contribute to improving biostratigraphic resolution and enhancing the accuracy of Early Miocene standard zonation and biochronology.