Paleogene larger benthic foraminifera of Tanzania and the Eocene - Oligocene Transition

Mass extinctions are important events for the evolution of life on Earth but often the mechanisms behind them are poorly understood. The Eocene - Oligocene Transition (EOT) had a profound and lasting influence on global climate and, though not one of the “big 5” mass extinction events, widespread extinctions in many fossil groups have been recognised. Larger benthic foraminifera (LBF), one of the most conspicuous and widespread fossil groups during the Eocene, are known to have experienced a rapid global overturning during the EOT, including the extinction of long-ranging families such as the Discocyclinidae, Orbitoclypeidae, Pellatispiridae and a number of species in the Nummulitidae. However, detailed records through the transition are rare, and few complete sections are known; the timing and causes of extinctions therefore remain uncertain. Extensive field samples from the southern Tanzanian coastal region along with Tanzania Drilling Project (TDP) samples are used to give an overview of Eocene to Miocene LBF and to produce high resolution stratigraphic records of LBF events across the EOT. Two further important EOT sites, Fuente Caldera, Spain and Melinau Gorge, Sarawak, both with unresolved questions relating to LBF at the EOT were studied for comparison. The field samples combined with the TDP data cover an area of approximately 200 km from Kilwa to the Mozambique border. Initial comparisons with Tethyan and Indo-Pacific faunas show perhaps an intermediate assemblage in the Eocene and some similarities with the Indo-Pacific in the Miocene. However in both epochs there are features distinctive of an East African or western Indo-Pacific fauna. Additionally, this work identifies many previously unreported localities and provides a solid basis for future work. Three of the TDP sites span the EOT and contain abundant LBF. Extensive calcareous micro-, nannofossil and stable isotope studies of these cores allow the LBF stratigraphy to be tied to global stratigraphy. These records show that the LBF extinction event occurs close to the Eocene/Oligocene boundary (EOB), as defined by the extinction of the planktonic foraminiferal Family Hantkeninidae, rather than at the prominent oxygen isotope excursion in the early Oligocene that signifies maximum ice growth and global sea-level fall. New bulk isotope data from the Melinau Limestone of Sarawak further support this conclusion. In Fuente Caldera, where the extinction level was previously reported to be within the Oligocene, extensive reworking means that an EOB extinction is also likely at this site. This correlation raises new questions about the cause of the extinctions and has important implications for global larger benthic foraminiferal stratigraphy