Extinction of larger benthic foraminifera at the Eocene/Oligocene boundary

A number of long-ranging and widespread larger benthic foraminiferal taxa are known to have become extinct during the Eocene–Oligocene transition. However, detailed records through the transition are rare, and few complete sections are known. Here we study an apparently complete section from Tanzania from three drill cores across the Eocene/Oligocene boundary in a hemipelagic clay succession. Within these, larger benthic foraminifera occur dispersed in the clays and concentrated in limestones which occur as secondary debris flow deposits. These cores allow the larger benthic foraminiferal stratigraphy to be tied to planktonic foraminifera and nannofossil biostratigraphy and stable isotope (δ18O, δ13C) stratigraphy. These records show that all of the larger benthic foraminifera extinction events occur close to the Eocene/Oligocene boundary as recognised by the extinction of the planktonic foraminiferal Family Hantkeninidae, rather than at the prominent oxygen isotope excursion in the lower Oligocene that signifies maximum ice growth and global sea-level fall. This correlation raises questions about the mechanism responsible for the extinctions and has implications for global larger benthic foraminiferal stratigraphy.

► Larger benthic foraminifera suffer extinctions at the Eocene–Oligocene transition.
► Well-correlated, continuous boundary drill core records from Tanzania are examined.
► Extinction of LBF found to be co-incident with the extinction of Hantkeninidae.
► Precedes the sea-level drop (δ18O shift) previously thought to be responsible.
► Alternative extinction mechanisms are discussed.