The Biogeographical Extent and Global Synchronicity of the Late Miocene Reticulofenestra pseudoumbilicus Paracme

Sammanfattning: Previous biostratigraphic studies in the Indian Ocean, equatorial Pacific, Atlantic Ocean and the Caribbean Sea document the temporal disappearance of Reticulofenestra pseudoumbilicus in the late Miocene (Tortonian age). This interval has been subsequently defined as the R. pseudoumbilicus paracme. While the cause of the event remains unknown, the geographical extent and global synchronicity of the interval represent an open subject for research. Answers to these questions could help establish the biostratigraphic reliability of the interval and its use for global correlation. In this study, 49 deep-sea drilling sites spread across all major ocean basins were compiled into a biostratigraphic database and studied to evaluate the global extent and timing of the R. pseudoumbilicus paracme. To ensure accurate comparison of the studied sites, the timing of the event was estimated at all sites using the age estimates generated through Undatable MatLab software, which made use of established nannofossil bioevents as age-depth tie points for the analysis. Ages of all nannofossil bioevents were updated to the latest geological time scale. This systematic analysis reveals a prominent distribution of the paracme in the tropical and subtropical regions. The timing of the event in the tropical region appears to be synchronous, with the base around 8.8 Ma and the top of the event around 7.2 Ma. Sites in the equatorial Pacific, central Atlantic and the Indian Oceans record a similar duration of the event. The paracme had a shorter duration towards the southern higher latitudes. Although timings in southern high latitudes are diachronous compared to the tropics, the duration is consistent between sites (Sites 1088 and 1264) in the southern South Atlantic, thereby establishing a new time frame for the event in the region. This study narrows down on ODP Site 806 for a critical evaluation of the nannofossil assemblages, accumulation rates and Reticulofenestra size variation in the late Miocene, including the paracme. Thirty-four samples were collected and analysed for absolute abundance and nannofossil accumulation rates (NAR). Absolute abundance and NAR reveal three distinct stages, herein described as generally low, high, and “crashed” export production of nannofossils. The period of high NAR is characteristic of the late Miocene biogenic bloom interval (ca. 9–4.4 Ma). Importantly, the R. pseudoumbilicus paracme does not only coincide with the start of the biogenic bloom but it was also associated with remarkable changes in Reticulofenestra assemblages. Just before the base of the paracme, all except the smallest (<3 µm in coccolith size) Reticulofenestra groups experienced a sharp decline in abundance. Subsequently, the relative abundance of the Reticulofenestra morphospecies indicates the dominance of bloom-forming small and medium Reticulofenestra during the late Miocene biogenic bloom interval. The end of the biogenic bloom interval is concurrent with a decrease in the contribution of small Reticulofenestra. The exact biotic and abiotic processes that led to these major shifts in plankton composition are yet to be discovered, but the findings presented herein show that the mechanisms behind the temporary disappearance of R. pseudoumbilicus in the tropics most likely involved the interplay between a broad range of ecological changes and evolutionary processes affecting the Reticulofenestra assemblages.