Associazioni a foraminiferi planctonici del Burdigaliano superiore-Langhiano dell’Oceano Atlantico (ODP Leg 154 Site 925, ODP Leg 208 Site 1264): biostratigrafia ad alta risoluzione e implicazioni paleoceanografiche-climatiche

ABSTRACT The early–middle Miocene represents a key step in the Cenozoic global climate evolution. It is characterized by the Middle Miocene Climatic Optimum (MMCO, 17 -15 Ma), followed by a thermal decrease during the Middle Miocene Climate Transition (MMCT, 15 - 14 Ma) culminating in the marked Mi3b cooling event at 13.8 Ma. A high-resolution (≈20 kyr) quantitative planktonic foraminiferal analysis was carried out on equatorial and (sub)tropical Atlantic Ocean sedimentary successions (respectively, ODP Leg 154 Site 925 and ODP Leg 208 Site 1264) from about 18 to about 14 Ma, not available yet for low latitudes. These sites were selected for their almost continuous successions, good preservation of foraminiferal assemblages and age model obtained through astronomical calibration. For each selected sample 200-300 specimens of planktonic foraminifera were classified at supraspecific and specific level (about 56 species) and counted to elaborate abundance curves against meter composite depth (mcd) and/or age. Carbonate dissolution, diversity, surface paleotemperature and paleoproductivity and water masses stratification indices (existing in literature or elaborated for the studied time interval) were plotted along the stratigraphic record and correlated with abundance curves. The obtained results could contribute to improve the recent biostratigraphic schemes for (sub)tropical latitudes (Wade et al., 2011 modified by Berggren et al., 2018) through a more accurate positioning of the biozonal events (e.g. T of Catapsydrax dissimilis, B of Globorotalia praemenardii, B of Orbulina suturalis; for both site) previously defined in Shipboard Scientific Party (Pearson & Chaisson, 1997 for Site 925; Zachos et al., 2004 for Site 1264) and additional events identification (e.g. random to sinistral coiling change of Paragloborotalia siakensis at Site 925; Tc of Globorotalia zealandica, B of Globorotalia miozea, AB/AE of P. siakensis and Parabloborotalia bella at Site 1264). Moreover, this study highlighted the rarity (and discontinuity only at Site 925) of the phylum Praeorbulina / Orbulina’s species along the record. The lack of agreement on the Praeorbulina datum concept (Praeorbulina curva in this study; Praeorbulina sicana in the standard zonation) limits the application of low latitudine (sub)tropical biozonation. Furthermore, this research has highlighted that planktonic foraminifera assemblages at both site were already differentiated from the base of the studied stratigraphic interval (upper Burdigalian). The quantitative planktonic foraminiferal record from Site 1264 was compared with the stable oxygen and carbon isotope records of Beddow (2016) to point out relationships between faunal and paleoclimatic variations. The statistical treatment of the planktonic foraminiferal dataset (Principal Component Analysis and Cluster Analysis) allowed to speculate about the abiotic parameters that controlled planktonic foraminiferal assemblages. The PC1 curve does not show a clear response to the paleoclimatic-oceanographic evolution from MMCO to MMCT, highlighted on a global scale in several benthic foraminifera isotopic records for the studied stratigraphic range (e.g. Zachos et al., 2001; Diester-Hass et al., 2009; Holbourn et al., 2015; Beddow, 2016). It seems to show a local response to the oceanographic changes linked to the paleoclimatic evolution of the southern hemisphere (alternating climatic deterioration / amelioration probably due to the instability of the Antarctic Ice Sheet). Although both sites are located at sub(tropical) latitudes, in the studied stratigraphic range they show different paleoclimatic-oceanographic settings characterized by switch between intervals of strengthening / weakening of the equatorial downwelling at the Site 925 and intervals of absence / presence of the cold eutrophic Benguela Outer Current at the Site 1264, respectively, in the phases of climatic deterioration / amelioration.