High-resolution quantitative record of planktonic foraminifera at low latitudes (ODP Leg 154 Site 926): biostratigraphy and paleoecology of the Aquitanian-Burdigalian interval

Very few high-resolution quantitative records of planktonic foraminifera have been established for the Aquitanian to Burdigalian interval, particularly at low latitudes. In addition, the Burdigalian GSSP has not yet been defined, and there is still a debate about which bioevents provide the best approximation of the boundary. Provisionally, the Base of the calcareous nannofossil Helicosphaera ampliaperta (calibrated at Ceara Rise at 20.43 Ma) has been chosen for the recognition of the base of Burdigalian Stage (e.g., Raffi et al., 2020). However, other events were proposed to define the boundary, such as Top Paragloborotalia kugleri (21.12 Ma), Base Globigerinoides altiaperturus (19.97 Ma), Top of Chron C6An (19.97 Ma), Base of calcareous nannofossil Sphenolithus belemnos (19.01 Ma) (e.g., Raffi et al., 2020). The aim of this work was to improve the resolution of planktonic foraminiferal record of the Aquitanian – Burdigalian interval, refine the stratigraphic position of bioevents and their age calibration, possibly identifying additional events, and obtain a paleoecological interpretation of the studied assemblages. The first part of this work deals with a high-resolution quantitative analysis of the planktonic foraminiferal assemblages of Site 926 (ODP Leg 154, Ceara Rise, Equatorial Atlantic Ocean), in the interval from 21.5 to 17.6 Ma (Aquitanian-Burdigalian). Following the composite section of Site 926, astronomically calibrated by Shackleton et al. (1999) and Pälike et al. (2006a), 137 samples were selected from pale levels of nannofossil-foraminiferal chalk with a time resolution of ~30 kyr. The high-resolution quantitative analysis provided the quantitative distribution patterns of the 55 taxonomic categories identified in the assemblages. Particular focus was given to biostratigraphic marker species of this interval, typical of the low-latitude zonation (Wade et al., 2011; King et al., 2020), such as Top (T) Paragloborotalia pseudokugleri, Top P. kugleri, Top Tenuitella munda, Base (B) Globigerinoides altiaperturus, Base Globigerinatella sp. and Base Globorotalia praescitula. In the second part of this study, the high-resolution quantitative analysis was extended downwards in Hole 926B, selecting 44 samples after a qualitative survey and focusing on the Paragloborotalia pseudokugleri/P. kugleri group. This choice was made to obtain the quantitative distribution pattern of the group from the first occurrence of P. kugleri, close to the Oligo-Miocene boundary, to its last occurrence in the Aquitanian - Burdigalian interval, to observe the gradual morphological transition between the end members of this plexus and to better understand the evolution of P. kugleri. The last part of this study deals with paleoecological evaluations on the planktonic foraminifera of the Aquitanian – Burdigalian interval, considering specific diversity, preservation state, depth habitat, temperature preference and productivity.